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Pub
1310
2001
RADAR NAVIGATION AND
MANEUVERING BOARD MANUAL
Seventh Edition
Prepared and published by the
NATIONAL IMAGERY AND MAPPING AGENCY
Bethesda, Maryland
© COPYRIGHT 2001 BY THE UNITED STATES GOVERNMENT
NO COPYRIGHT CLAIMED UNDER TITLE 17 U.S.C.
For sale by the U.S. Government Printing Office
Superintendant of Documents, Mail Stop: SSOP, Washington, DC 20402-9328
iii
PREFACE
The2001editionofPub.1310RadarNavigationandManeuveringBoard
ManualcombinesselectedchaptersfromthesixtheditionofPub.1310,
RadarNavigationManual,andthefourtheditionofPub.217,Maneuvering
Board Manual.
ThismanualhasbeencompiledbytheeditorialstaffoftheMaritime
SafetyInformationCenterattheNationalImageryandMappingAgency.It
isintendedtobeusedprimarilyasamanualofinstructioninnavigation
schoolsandbynavalandmerchantmarinepersonnel.Bycombiningthe
previouseditionsofPub.1310andPub.217intoonebookwehopethatwe
haveprovidedapracticalreferenceformarinersonboardshipand
instructorsashore.Itisalsointendedtobeofassistancetootherswhoare
concerned with marine radar in different and less direct ways.
Incombiningthetwomanuals,everyefforthasbeenmadetoretainthe
originalstyleandformatwhichhasproventobeclearandhelpfultothe
maritimecommunity.Mostoftheillustrationsandexampleshavebeen
carried forward into this edition.
ThechapteronARPAhasbeenexpandedandnowincludesasample
operatingmanualforamoderncommercialradarandARPA.Manyexcellent
otherpublicationsonARPAareavailableandshouldbeconsultedforamore
thorough understanding on this subject matter.
Usersshouldrefercorrections,additions,andcommentsforimproving
this product to:
MARITIME SAFETY INFORMATION CENTER
NATIONAL IMAGERY AND MAPPING AGENCY
ST D 44
4600 SANGAMORE ROAD
BETHESDA MD 20816-5003
ACKNOWLEDGEMENTS
Theinformationwhichwasusedinthebook’srecompilationhascome
fromawidevarietyofsources.ThestaffatNIMAwouldliketothankthe
manyindividualsfortheircontributions.Theseinclude;U.S.NavyRadar
TrainingFacilities,merchantmarineacademies,U.S.CoastGuardAcademy,
radar manufactures and a number of individual mariners.
ParticularthanksareduetoMr.EricK.Larsson,Director,Centerfor
MaritimeEducation,Seaman’sChurchInstitute,NewYorkandtheFuruno
ElectricCo.,LTD.forprovidingtheinstructionmanualfortheirlatestraster
scan radar and ARPA units.
v
TABLE OF CONTENTS
CHAPTER 1—BASIC RADAR PRINCIPLES AND
GENERAL CHARACTERISTICS
INTRODUCTION..........................................................................................1
A BRIEF HISTORY......................................................................................2
RADAR PROPAGATION CHARACTERISTICS....................................3
THE RADIO WAVE................................................................................3
THE RADAR BEAM...............................................................................4
Beam Width.......................................................................................4
EFFECT OF SEA SURFACE ON RADAR BEAM................................5
ATMOSPHERIC FACTORS AFFECTING THE
RADAR HORIZON.......................................................................................9
THE RADAR HORIZON.........................................................................9
DIFFRACTION........................................................................................9
REFRACTION.........................................................................................9
Standard Atmospheric Conditions......................................................9
Sub-refraction..................................................................................10
Super-refraction...............................................................................10
Extra Super-refraction or Ducting...................................................11
Ducting Areas..................................................................................11
WEATHER FACTORS AFFECTING THE RADAR HORIZON........13
Attenuation by rain, fog, clouds, hail, snow, and dust.....................13
Rain..................................................................................................13
Fog...................................................................................................13
Clouds..............................................................................................14
Hail..................................................................................................14
Snow................................................................................................14
Dust.................................................................................................14
Unusual Propagation Conditions....................................................14
A BASIC RADAR SYSTEM......................................................................15
RADAR SYSTEM CONSTANTS.....................................................15
Carrier Frequency........................................................................15
Pulse Repetition Frequency.........................................................15
Pulse Length.................................................................................15
Power Relation.............................................................................16
COMPONENTS AND SUMMARY OF FUNCTIONS.....................17
FUNCTIONS OF COMPONENTS....................................................18
Power Supply...............................................................................18
Modulator.....................................................................................18
Transmitter...................................................................................18
Transmitting and Receiving Antenna System..............................18
Receiver.......................................................................................20
Indicator.......................................................................................21
FACTORS AFFECTING DETECTION, DISPLAY, AND
MEASUREMENT OF RADAR TARGETS.............................................24
FACTORS AFFECTING MAXIMUM RANGE...............................24
Frequency.....................................................................................24
Peak Power...................................................................................24
Pulse Length.................................................................................24
Pulse Repetition Rate...................................................................24
Beam Width.................................................................................24
Target Characteristics..................................................................24
Receiver Sensitivity.....................................................................24
Antenna Rotation Rate.................................................................24
vi
FACTORS AFFECTING MINIMUM RANGE..........................................25
Pulse Length..............................................................................................25
Sea Return..................................................................................................25
Side-Lobe Echoes......................................................................................25
Vertical Beam Width.................................................................................25
FACTORS AFFECTING RANGE ACCURACY.......................................25
Fixed Error.................................................................................................25
Line Voltage..............................................................................................25
Frequency Drift..........................................................................................25
Calibration.................................................................................................25
Pip and VRM Alignment...........................................................................26
Range Scale...............................................................................................26
PPI Curvature.............................................................................................26
Radarscope Interpretation..........................................................................26
FACTORS AFFECTING RANGE RESOLUTION....................................26
Pulse Length..............................................................................................26
Receiver Gain............................................................................................28
CRT Spot Size...........................................................................................28
Range Scale...............................................................................................28
FACTORS AFFECTING BEARING ACCURACY...................................29
Horizontal Beam Width.............................................................................29
Target Size.................................................................................................29
Target Rate of Movement..........................................................................29
Stabilization of Display.............................................................................29
Sweep Centering Error...............................................................................29
Parallax Error.............................................................................................29
Heading Flash Alignment..........................................................................29
FACTORS AFFECTING BEARING RESOLUTION................................29
Horizontal Beam Width.............................................................................29
Range of Targets........................................................................................30
CRT Spot Size...........................................................................................31
WAVELENGTH.........................................................................................31
TARGET CHARACTERISTICS...............................................................34
Height........................................................................................................34
Size............................................................................................................34
Aspect.......................................................................................................34
Shape.........................................................................................................34
Texture......................................................................................................34
Composition..............................................................................................34
CHAPTER 2—RADAR OPERATION
RELATIVE AND TRUE MOTION DISPLAYS.....................................35
GENERAL..................................................................................................35
RELATIVE MOTION RADAR.................................................................35
Orientations of Relative Motion Display..................................................36
Stabilization..............................................................................................36
TRUE MOTION RADAR..........................................................................37
Stabilization..............................................................................................37
Radarscope Persistence and Echo Trails..................................................37
Reset Requirements and Methods.............................................................37
Modes of Operation..................................................................................38
Types of True Motion Display..................................................................38
PLOTTING AND MEASUREMENTS ON PPI......................................39
THE REFLECTION PLOTTER.................................................................39
Basic Reflection Plotter Designs..............................................................39
Marking the Reflection Plotter.................................................................39
Cleanliness................................................................................................39
PLOTTING ON STABILIZED AND UNSTABILIZED DISPLAYS.......39
Stabilized North-Upward Display............................................................39
vii
RANGE AND BEARING MEASUREMENT.......................................44
Mechanical Bearing Cursor.............................................................44
Variable Range Marker (Range Strobe)..........................................44
Electronic Bearing Cursor...............................................................44
Interscan...........................................................................................45
Off-Center Display..........................................................................45
Expanded Center Display................................................................46
RADAR OPERATING CONTROLS.........................................................47
POWER CONTROLS............................................................................47
Indicator Power Switch....................................................................47
Antenna (Scanner) Power Switch....................................................47
Special Switches..............................................................................47
PERFORMANCE CONTROLS—INITIAL ADJUSTMENTS.............48
Brilliance Control............................................................................48
Receiver Gain Control.....................................................................49
Tuning Control.................................................................................50
PERFORMANCE CONTROLS - ADJUSTMENTS ACCORDING
TO OPERATING CONDITIONS..........................................................50
Receiver Gain Control.....................................................................50
Fast Time Constant (FTC) Switch (Differentiator).........................50
Rain Clutter Control........................................................................50
Sensitivity Time Control (STC).......................................................52
Performance Monitor.......................................................................53
Pulse Lengths and Pulse Repetition Rate Controls..........................54
LIGHTING AND BRIGHTNESS CONTROLS....................................54
Reflection Plotter.............................................................................54
Heading Flash..................................................................................54
Electronic Bearing Cursor...............................................................54
Fixed Range Markers.......................................................................54
Variable Range Marker....................................................................54
Panel Lighting..................................................................................54
MEASUREMENT AND ALIGNMENT CONTROLS......................54
Range...........................................................................................54
Bearing.........................................................................................55
Sweep Centering..........................................................................55
Center Expansion.........................................................................55
Heading Flash Alignment............................................................55
Range Calibration........................................................................55
TRUE MOTION CONTROLS...........................................................56
Operating Mode...........................................................................56
Normal Reset Control..................................................................56
Delayed Reset Control.................................................................56
Manual Reset Control..................................................................56
Manual Override Control.............................................................56
Ship’s Speed Input Selector Control............................................56
Set and Drift Controls..................................................................56
Speed and Course Made Good Controls......................................57
Zero Speed Control......................................................................57
CHAPTER 3—COLLISION AVOIDANCE
RELATIVE MOTION................................................................................59
THE VECTOR TRIANGLE...............................................................63
VECTOR EQUATIONS.....................................................................64
MANEUVERING BOARD........................................................................66
MANEUVERING BOARD FORMAT..............................................66
PLOTTING ON MANEUVERING BOARD.....................................66
Relative Movement Problems......................................................71
THE LOGARITHMIC TIME-SPEED-DISTANCE NOMOGRAM.....74
NAUTICAL SLIDE RULES..............................................................76
viii
GRAPHICAL RELATIVE MOTION SOLUTIONS.............................76
RAPID RADAR PLOTTING.................................................................77
TRANSFER PLOTTING........................................................................77
SELECTION OF PLOTTING TECHNIQUES......................................77
RADAR PLOTTING SYMBOLS...............................................................81
GRAPHICAL SOLUTIONS ON THE REFLECTION
PLOTTER RAPID RADAR PLOTTING.................................................83
CLOSEST POINT OF APPROACH......................................................83
TRUE COURSE AND SPEED OF CONTACT....................................83
COURSE TO PASS AT SPECIFIED CPA............................................85
SPECIAL CASES...................................................................................86
CONSTRUCTING THE PLASTIC RULE USED WITH
RAPID RADAR PLOTTING.....................................................................88
EXAMPLES
e-r-m TRIANGLE
EXAMPLE 1—DETERMINATION OF CLOSEST POINT
OF APPROACH (CPA).......................................................90
EXAMPLE 2—COURSE AND SPEED OF A RADAR
CONTACT...........................................................................92
EXAMPLE 3—COURSE AND SPEED OF RADAR CONTACT
BY THE LADDER METHOD............................................94
EXAMPLE 4—COURSE TO PASS A SHIP AT A SPECIFIED CPA
(Own ship’s speed is greater than that of other ship)...........96
EXAMPLE 5—COURSE TO PASS SHIP AT A SPECIFIED CPA
(Own ship’s speed is less than that of other ship)..............98
EXAMPLE 6—VERIFICATION OF FIXED OBJECTS OR RADAR
CONTACTS DEAD IN THE WATER...........................100
EXAMPLE 7—AVOIDANCE OF MULTIPLE CONTACTS WITHOUT
FIRST DETERMINING THE TRUE
COURSES AND SPEEDS OF THE CONTACTS.........102
EXAMPLE 8—DETERMINING THE CLOSEST POINT OF APPROACH
FROM THE GEOGRAPHICAL PLOT..........................104
ALTERNATIVE RADAR PLOTTING SYMBOLS.............................106
STANDARD PLOTTING PERIOD.................................................108
SUMMARY OF ALTERNATIVE PLOTTING SYMBOLS
R-T-M TRIANGLE..........................................................................110
ALTERNATIVE GRAPHICAL SOLUTIONS ON THE
REFLECTION PLOTTER......................................................................112
CLOSEST POINT OF APPROACH................................................112
TRUE COURSE AND SPEED OF CONTACT...............................112
COURSE TO PASS AT SPECIFIED CPA......................................114
SPECIAL CASES.............................................................................115
BLACK LIGHT ILLUMINATION..................................................115
EXAMPLES
R-T-M TRIANGLE
EXAMPLE 9—DETERMINATION OF CLOSEST POINT OF
APPROACH (CPA)..........................................................118
ix
EXAMPLE 10—COURSE AND SPEED OF A RADAR
CONTACT.......................................................................120
EXAMPLE 11—COURSE AND SPEED OF RADAR CONTACT
BY THE LADDER METHOD........................................122
EXAMPLE 12—COURSE TO PASS A SHIP AT A SPECIFIED CPA
(Own ship’s speed is greater than that of other ship).......124
EXAMPLE 13—COURSE TO PASS SHIP AT A SPECIFIED CPA
(Own ship’s speed is less than that of other ship)............126
EXAMPLE 14—VERIFICATION OF FIXED OBJECTS OR
RADAR CONTACTS DEAD IN THE WATER............128
EXAMPLE 15—AVOIDANCE OF MULTIPLE CONTACTS
WITHOUT FIRST DETERMINING THE TRUE
COURSES AND SPEEDS OF THE CONTACTS.........130
PRACTICAL SOLUTION FOR CPA IN TRUE
MOTION MODE.......................................................................................132
SITUATION RECOGNITION.................................................................139
INTRODUCTION................................................................................139
RULES FOR SPEED CHANGE..........................................................140
Reduced Speed...............................................................................140
Increased Speed.............................................................................140
Speed of Relative Motion (SRM)..................................................140
SITUATION DISPLAYS.....................................................................140
APPLICATION....................................................................................140
RULES FOR MANEUVERING..........................................................145
CHAPTER 4—RADAR NAVIGATION
RADARSCOPE INTERPRETATION....................................................147
LAND TARGETS.............................................................................147
SHIP TARGETS...............................................................................149
RADAR SHADOW..........................................................................149
BEAM WIDTH AND PULSE LENGTH DISTORTION................149
SUMMARY OF DISTORTIONS.....................................................151
RECOGNITION OF UNWANTED ECHOES AND
EFFECTS..........................................................................................152
Indirect (False) Echoes..................................................................152
Side-lobe Effects...........................................................................153
Multiple Echoes.............................................................................153
Second-Trace (Multiple-Trace) Echoes........................................153
Electronic Interference Effects......................................................155
Blind and Shadow Sectors.............................................................155
Spoking..........................................................................................156
Sectoring........................................................................................156
Serrated Range Rings....................................................................156
PPI Display Distortion...................................................................156
Hour-Glass Effect..........................................................................156
Overhead Cable Effect..................................................................156
AIDS TO RADAR NAVIGATION..........................................................158
RADAR REFLECTORS...................................................................158
RADAR BEACONS.........................................................................158
Racon.........................................................................................159
Ramark.......................................................................................160
RADAR FIXING METHODS.................................................................161
RANGE AND BEARING TO A SINGLE OBJECT.......................161
TWO OR MORE BEARINGS.........................................................161
x
TANGENT BEARINGS.......................................................................161
TWO OR MORE RANGES.................................................................161
MIXED METHODS.............................................................................162
PRECONSTRUCTION OF RANGE ARCS........................................163
CONTOUR METHOD.........................................................................164
IDENTIFYING A RADAR-INCONSPICUOUS OBJECT...................165
FINDING COURSE AND SPEED MADE GOOD BY
PARALLEL-LINE CURSOR...................................................................166
USE OF PARALLEL-LINE CURSOR FOR ANCHORING................167
PARALLEL INDEXING..........................................................................169
THE FRANKLIN CONTINUOUS RADAR PLOT
TECHNIQUE.............................................................................................182
TRUE MOTION RADAR RESET IN RESTRICTED
WATERS....................................................................................................184
RADAR DETECTION OF ICE................................................................186
ICEBERGS...........................................................................................186
BERGY BITS.......................................................................................186
GROWLERS.........................................................................................186
RADAR SETTINGS FOR RADARSCOPE PHOTOGRAPHY...........187
NAVIGATIONAL PLANNING...............................................................188
SPECIAL TECHNIQUES....................................................................189
Identifying Echoes.........................................................................189
Fixing.............................................................................................189
CHAPTER 5—AUTOMATIC RADAR PLOTTING AIDS
(ARPA)
INTRODUCTION.....................................................................................191
STAND-ALONE AND INTEGRAL ARPA’S.........................................191
ARPA DISPLAY...............................................................................192
Raster-scan PPI..............................................................................192
Monochrome and Color CRT........................................................192
FEATURES AND OPERATING INSTRUCTIONS FOR
A MODERN RASTER SCAN RADAR AND ARPA............................193
INTRODUCTION............................................................................193
FEATURES......................................................................................193
General Features............................................................................194
ARPA Features..............................................................................194
DISPLAY CONTROLS.....................................................................196
Mode Panel....................................................................................196
Plotting Keypad.............................................................................197
OPERATION.............................................................................................198
TURNING ON POWER....................................................................198
TRANSMITTER ON.........................................................................198
CRT BRILLIANCE...........................................................................198
TUNING THE RECEIVER...............................................................199
Auto Tune......................................................................................199
Manual Tune..................................................................................199
Video Lockup Recovery................................................................199
DEGAUSSING THE CRT SCREEN................................................202
INITIALIZING THE GYRO READOUT.........................................202
PRESENTATION MODES...............................................................202
Relative Motion (RM)...................................................................202
True Motion (TM).........................................................................202
xi
SELECTING THE PRESENTATION MODE...................................202
Head-up Mode................................................................................203
Course-up Mode.............................................................................203
Head-up TB (True Bearing) Mode.................................................204
North-up Mode...............................................................................204
True Motion Mode..........................................................................205
SELECTING THE RANGE SCALE..................................................206
SELECTING THE PULSEWIDTH....................................................206
Selecting Pulsewidth 1 or 2............................................................206
Presetting Pulsewidths 1 and 2.......................................................206
ADJUSTING THE SENSITIVITY....................................................206
SUPPRESSING SEA CLUTTER.......................................................207
Automatic Anti-clutter Control.......................................................207
Manual Anti-clutter Control...........................................................207
SUPPRESSING PRECIPITATION CLUTTER.................................207
INTERFERENCE REJECTOR..........................................................207
MEASURING THE RANGE.............................................................208
MEASURING THE BEARING.........................................................208
COLLSION ASSESSMENT BY OFFSET EBL................................209
MEASURING RANGE AND BEARING BETWEEN
TWO TARGETS.................................................................................210
SETTING A GUARD ZONE (GUARD ALARM)............................210
SILENCING AUDIBLE ALARM, REACTIVATING
GUARD ALARM...............................................................................210
DISABLING GUARD ZONE (GUARD ALARM)...........................211
INWARD AND OUTWARD GUARD ALARMS............................211
OFF-CENTERING.............................................................................211
ECHO STRETCH...............................................................................211
ECHO AVERAGING.........................................................................211
ELECTRONIC PLOTTING (E-PLOT)..............................................212
Plotting a target...............................................................................212
True or Relative Vector..................................................................213
Vector Time...................................................................................213
Target Data....................................................................................213
Reading the Target Data............................................................... 213
Terminating Target Plotting..........................................................213
Entering Own Ship’s Speed...........................................................213
Automatic Speed Input..................................................................214
Manual Speed Input.......................................................................214
TARGET TRAILS (ECHO TRAILS)...............................................215
True or Relative Trails...................................................................215
Trail Gradations.............................................................................215
Displaying and Erasing Echo Trails..............................................215
Resetting Echo Trails.....................................................................215
PARALLEL INDEX LINES............................................................ 216
Displaying and Erasing the Index Lines........................................216
Adjusting Index Line Intervals......................................................216
ANCHOR WATCH...........................................................................216
Activating Anchor Watch..............................................................216
Alarm Range Setting......................................................................216
Showing Drag Lines......................................................................217
Anchor Watch in Standby or Transmit Status...............................217
Origin Mark.................................................................................. 217
Zoom..............................................................................................217
MARKERS........................................................................................218
Heading Marker.............................................................................218
Temporarily Erasing Heading Marker...........................................218
North Marker.................................................................................218
Stern Marker..................................................................................218
Menu Keys.................................................................................... 218
FUNCTION KEYS............................................................................219
Watch Alarm..................................................................................220
EPA Menu.................................................................................... 220
xii
NAVIGATION INFORMATION......................................................221
Menu and Navigation Data Display...............................................221
Suppressing Second-trace Echoes..................................................221
Adjusting Relative Brilliance Levels of Screen Data.....................222
Set and Drift (Set and Rate)............................................................222
OPERATION OF ARPA............................................................................223
GENERAL.........................................................................................223
PRINCIPAL SPECIFICATIONS.......................................................223
Acquisition and Tracking...............................................................223
Vectors............................................................................................223
ARPA MENU OPERATION..............................................................224
START UP PROCEDURE.................................................................224
Activating the ARPA......................................................................224
Entering Own Ship’s Speed............................................................224
Automatic Speed Input...................................................................224
Manual Speed Input........................................................................225
Target Based Speed........................................................................225
Cancelling Target Based Speed......................................................225
Deactivating the ARPA................................................................. 225
AUTOMATIC ACQUISITION..........................................................225
Enabling and Disabling Auto Acquisition......................................226
Setting Auto Aquisition Areas........................................................226
Terminating Tracking of Targets....................................................227
Individual Targets...........................................................................227
All Targets......................................................................................227
Discrimination Between Landmass and True Targets....................227
MANUAL ACQUISITION................................................................227
CHANGING PLOT SYMBOL SIZE.................................................227
ADJUSTING BRILLIANCE OF PLOT MARKS..............................227
DISPLAYING TARGET DATA........................................................230
MODE AND LENGTH OF VECTORS............................................231
True or Relative Vectors................................................................231
True Vector....................................................................................231
Relative Vector..............................................................................231
Vector Time...................................................................................231
PAST POSITIONS............................................................................231
Displaying and Erasing Past Positions..........................................231
Selecting the Number of Dots and Past Positions
Intervals........................................................................................ 232
SETTING CPA/TCPA ALARM RANGES......................................232
Silencing CPA/TCPA Aural Alarms.............................................232
Setting a Guard Zone.....................................................................232
Activating the Guard Zone............................................................233
Deactivating the Guard Zone.........................................................233
Silencing the Guard Zone Audible Alarm.....................................233
Operational Warnings................................................................... 233
CPA/TCPA Alarm.........................................................................234
Guard Zone Alarm.........................................................................234
Lost Target Alarm..........................................................................234
Target Full Alarm..........................................................................234
Manually Acquired Targets...........................................................234
Automatically Acquired Targets....................................................234
System Failure Alarm................................................................... 234
TRIAL MANEUVER........................................................................235
Dynamic Trial Maneuver...............................................................235
Static Trial Maneuver....................................................................235
Terminating Trial Maneuver..........................................................235
CRITERIA FOR SELECTING TARGETS FOR
TRACKING.......................................................................................236
Acquisition and Tracking..............................................................236
Quantization...................................................................................236
xiii
RADAR OBSERVATION......................................................................237
GENERAL..........................................................................................237
Minimum Range.............................................................................237
Maximum Range............................................................................237
X-Band and S-Band........................................................................237
Radar Resolution............................................................................237
Bearing Resolution........................................................................ 237
Range Resolution............................................................................237
Bearing Accuracy...........................................................................237
Range Measurement.......................................................................237
FALSE ECHOES................................................................................238
Multiple Echoes..............................................................................238
Sidelobe Echoes..............................................................................238
Virtual Image..................................................................................238
Shadow Sectors...............................................................................238
SEARCH AND RESCUE TRANSPONDER (SART).......................238
POST-IT NOTE METHOD OF RADAR CONTACT
THREAT AND ASPECT ASSESSMENT...............................................239
CHAPTER 6
—
MANEUVERING BOARD MANUAL
PART ONE: OWN SHIP AT CENTER.................................................243
PART TWO: GUIDE AT CENTER.......................................................309
APPENDICES
APPENDIX A.EXTRACT FROM REGULATION 12, CHAPTER V
OF THE IMO-SOLAS (1974) CONVENTION
AS AMENDED TO 1983 ...............................................367
APPENDIX B.GLOSSARY AND ABBREVIATIONS..........................373
APPENDIX C.RELATIVE MOTION PROBLEMS...............................381
APPENDIX D.BIBLIOGRAPHY............................................................398
xiv
INDEXOF MANEUVERING BOARD EXAMPLES
OWN SHIP
AT CENTER
GUIDE
AT CENTER
EXPageEXPage
TRACKING
Closest Point of Approach (CPA)....................................................................................................................1244——
Course and Speed of Other Ship......................................................................................................................2246——
Course and Speed of Other Ship using Relative Plot as Relative Vector........................................................3248——
CHANGE OF STATION
With Time, Course, or Speed Specified...........................................................................................................425029310
Three Ship Maneuvers.....................................................................................................................................525230312
PASSING
At Given Distance............................................................................................................................................625431314
Course and Speed to pass using Relative Plot as Relative Vector...................................................................7256——
At Maximum (Minimum) Distance.................................................................................................................825832316
At a Distance Required for Several Ships to Clear..........................................................................................926033318
WIND
Determination of True Wind............................................................................................................................10262——
Desired Relative Wind (three methods)...........................................................................................................11264——
PRACTICAL ASPECTS OF MANEUVERING BOARD SOLUTIONS
Advance, Transfer, Acceleration, and Deceleration........................................................................................1227034320
Maneuvering by Seaman’s Eye.......................................................................................................................——35322
COLLISION AVOIDANCE
Avoidance of Multiple Contacts......................................................................................................................13272——
Avoidance of Multiple Contacts Without First Determining the True Courses and Speeds of the
Contacts...........................................................................................................................................................14276——
Determining the Closest Point of Approach from the Geographical Plot.......................................................15278——
xv
AXIS ROTATION
F
ormation Axis Rotation—Guide in Center....................................................................................................——36324
Formation Axis Rotation—Guide out of Center (Replot Method)..................................................................——37326
Formation Axis Rotation—Guide out of Center (Parallel Offset Method).....................................................——38328
LIMITING RANGE
Remain in Range for Specified Time...............................................................................................................1628039330
Open Range in Minimum Time.......................................................................................................................1728240332
Close Range in Minimum Time.......................................................................................................................1828441334
Remain in Range for Maximum Time.............................................................................................................1928642336
Remain Outside Range for Maximum Time....................................................................................................2028843338
FICTITIOUS SHIP
One Ship Alters Course and/or Speed During Maneuver................................................................................2129044340
Both Ships Alter Course and/or Speed During Maneuver...............................................................................2229245342
SCOUTING
Out and In on Present Bearing at Given Speed...............................................................................................2329446344
Change Stations, Scouting Enroute.................................................................................................................2429647346
BEARINGS ONLY
Course, Speed, and Position derived from Bearings Only..............................................................................25298——
ANTI SUBMARINE WARFARE TECHNIQUES
Limited Lines of Approach..............................................................................................................................2630048348
Torpedo Danger Zone......................................................................................................................................——49350
Missle Danger Zone.........................................................................................................................................——50352
Cone of Courses (two methods)......................................................................................................................27302——
Evasive Action Against a Slow Moving Target...............................................................................................28306——
OWN SHIP
AT CENTER
GUIDE
AT CENTER
EXPageEXPage
1
CHAPTER 1 —BASIC RADAR PRINCIPLES AND GENERAL CHARACTERISTICS
INTRODUCTION
ThewordradarisanacronymderivedfromthephraseRAdioDetection
AndRangingandappliestoelectronicequipmentdesignedfordetectingand
trackingobjects(targets)atconsiderabledistances.Thebasicprinciple
behindradarissimple-extremelyshortburstsofradioenergy(travelingat
thespeedoflight)aretransmitted,reflectedoffatargetandthenreturnedas
an echo.
Radarmakesuseofaphenomenonwehaveallobserved,thatofthe
ECHOPRINCIPLE.Toillustratethisprinciple,ifaship’swhistlewere
soundedinthemiddleoftheocean,thesoundwaveswoulddissipatetheir
energyastheytraveledoutwardandatsomepointwoulddisappearentirely.
If,howeverthewhistlesoundednearanobjectsuchasacliffsomeofthe
radiated sound waves would be reflected back to the ship as an echo.
Theformofelectromagneticsignalradiatedbytheradardependsupon
thetypeofinformationneededaboutthetarget.Radar,asdesignedfor
marinenavigationapplications,ispulsemodulated.Pulse-modulatedradar
candeterminethedistancetoatargetbymeasuringthetimerequiredforan
extremelyshortburstofradio-frequency(r-f)energytotraveltothetarget
andreturntoitssourceasareflectedecho.Directionalantennasareusedfor
transmittingthepulseandreceivingthereflectedecho,therebyallowing
determination of the direction or bearing of the target echo.
Oncetimeandbearingaremeasured,thesetargetsorechoesare
calculatedanddisplayedontheradardisplay.Theradardisplayprovidesthe
operator a birds eye view of where other targets are relative to own ship.
Radarisanactivedevice.Itutilizesitsownradioenergytodetectand
trackthetarget.Itdoesnotdependonenergyradiatedbythetargetitself.
Theabilitytodetectatargetatgreatdistancesandtolocateitspositionwith
high accuracy are two of the chief attributes of radar.
Therearetwogroupsofradiofrequenciesallocatedbyinternational
standardsforusebycivilmarineradarsystems.ThefirstgroupliesintheX-
bandwhichcorrespondstoawavelengthof3cm.andhasafrequencyrange
between9300and9500MHz.ThesecondgroupliesintheS-bandwitha
wavelengthof10cm.andhasafrequencyrangeof2900to3100MHz.Itis
sometimesmoreconvenienttospeakintermsofwavelengthratherthan
frequency because of the high values associated with the latter.
Afundamentalrequirementofmarineradaristhatofdirectional
transmissionandreception,whichisachievedbyproducinganarrow
horizontalbeam.Inordertofocustheradioenergyintoanarrowbeamthe
lawsofphysicsprevailandthewavelengthmustbewithinthefew
centimeters range.
Theradio-frequencyenergytransmittedbypulse-modulatedradars
consistsofaseriesofequallyspacedpulses,frequentlyhavingdurationsof
about1microsecondorless,separatedbyveryshortbutrelativelylong
periodsduringwhichnoenergyistransmitted.ThetermsPULSE-
MODULATEDRADARandPULSEMODULATIONarederivedfromthis
method of transmission of radio-frequency energy.
Ifthedistancetoatargetistobedeterminedbymeasuringthetime
requiredforonepulsetotraveltothetargetandreturnasareflectedecho,it
isnecessarythatthiscyclebecompletedbeforethepulseimmediately
followingistransmitted.Thisisthereasonwhythetransmittedpulsesmust
beseparatedbyrelativelylongnontransmittingtimeperiods.Otherwise,
transmissionwouldoccurduringreceptionofthereflectedechoofthe
precedingpulse.Usingthesameantennaforbothtransmittingandreceiving,
therelativelyweakreflectedechowouldbeblockedbytherelativelystrong
transmitted pulse.
2
A BRIEF HISTORY
Radar,thedevicewhichisusedfordetectionandrangingofcontacts,
independentoftimeandweatherconditions,wasoneofthemostimportant
scientificdiscoveriesandtechnologicaldevelopmentsthatemergedfrom
WWII.It’sdevelopment,likethatofmostgreatinventionswasmotheredby
necessity.Behindthedevelopmentofradarlaymorethanacenturyofradio
development.
Thebasicideaofradarcanbetracedbacktotheclassicalexperimentson
electromagneticradiationconductedbythescientificcommunityinthe19th
century.Intheearly1800s,anEnglishphysicist,MichaelFaraday,
demonstratedthatelectriccurrentproducesamagneticfieldandthatthe
energyinthisfieldreturnstothecircuitwhenthecurrentisstopped.In1864
theScottishphysicist,JamesMaxwell,hadformulatedthegeneralequations
oftheelectromagneticfield,determiningthatbothlightandradiowavesare
actuallyelectromagneticwavesgovernedbythesamefundamentallawsbut
havingdifferentfrequencies.Heprovedmathematicallythatanyelectrical
disturbancecouldproduceaneffectataconsiderabledistancefromthepoint
oforiginandthatthiselectromagneticenergytravelsoutwardfromthe
source in the form of waves moving at the speed of light.
AtthetimeofMaxwell’sconclusionstherewasnoavailablemeansto
propagateordetectelectromagneticwaves.Itwasnotuntil1886that
Maxwell’stheoriesweretested.TheGermanphysicist,HeinrichHertz,set
outtovalidateMaxwell’sgeneralequations.Hertzwasabletoshowthat
electromagneticwavestravelledinstraightlinesandthattheycanbe
reflected from a metal object just as light waves are reflected by a mirror.
In1904theGermanengineer,ChristianHulsmeyerobtainedapatentfora
devicecapableofdetectingships.Thisdevicewasdemonstratedtothe
Germannavy,butfailedtoarouseinterestprobablydueinparttoitsvery
limitedrange.In1922,GuglielmoMarconidrewattentiontotheworkof
HertzandrepeatedHertz’sexperimentsandeventuallyproposedinprinciple
what we know now as marine radar.
Thefirstobservationoftheradareffectwasmadein1922byDr.Albert
TayloroftheNavalResearchLaboratory(NRL)inWashington,D.C.Dr.
Taylorobservedthatashippassingbetweenaradiotransmitterandreceiver
reflectedsomeofthewavesbacktothetransmitter.In1930furthertestsat
theNRLobservedthataplaneflyingthroughabeamfromatransmitting
antennacausedafluctuationinthesignal.Theimportanceofradarforthe
purposesoftrackingaircraftandshipsfinallybecamerecognizedwhen
scientistsandengineerslearnedhowtouseasingleantennafortransmitting
and receiving.
Duetotheprevailingpoliticalandmilitaryconditionsatthetime,the
UnitedStates,GreatBritain,SovietUnion,France,Italy,GermanyandJapan
allbeganexperimentingwithradar,withvaryingdegreesofsuccess.During
the1930s,effortsweremadebyseveralcountriestouseradioechofor
aircraftdetection.Mostofthesecountrieswereabletoproducesomeform
ofoperationalradarequipmentforusebythemilitaryatthestartofthewar
in 1939.
AtthebeginningofWWII,Germanyhadprogressedfurtherinradar
developmentandemployedradarunitsonthegroundandintheairfor
defenseagainstalliedaircraft.Theabilityofradartoserveasanearly
warningdeviceprovedvaluableasadefensivetoolfortheBritishandthe
Germans.
Althoughradarwasemployedatthestartofthewarasadefensive
weapon,asthewarprogressed,itcametobeusedforoffensivepurposestoo.
Bythemiddleof1941radarhadbeenemployedtotrackaircraft
automaticallyinazimuthandelevationandlatertotracktargets
automatically in range.
Alloftheprovenradarsystemsdevelopedpriortothewarwereinthe
VHFband.Theselowfrequencyradarsignalsaresubjecttoseveral
limitations,butdespitethedrawbacks,VHFrepresentedthefrontierofradar
technology.Latein1939,Britishphysicistscreatedthecavitymagnetron
oscillatorwhichoperatedathigherfrequencies.Itwasthemagnetronthat
mademicrowaveradarareality.Itwasthistechnologicaladvancethatmarks
the beginning of modern radar.
Followingthewar,progressinradartechnologyslowedaspostwar
prioritiesweredirectedelsewhere.Inthe1950snewandbetterradarsystems
begantoemergeandthebenefitstothecivilmarinerbecamemore
important.Althoughradartechnologyhasbeenadvancedprimarilybythe
military,thebenefitshavespilledoverintomanyimportantcivilian
applications,ofwhichaprincipalexampleisthesafetyofmarinenavigation.
Thesamefundamentalprinciplesdiscoverednearlyacenturyagoandthe
basicdatatheyprovide,namelytargetrangeandbearing,stillapplyto
today’s modern marine radar units.
3
RADAR PROPAGATION CHARACTERISTICS
THE RADIO WAVE
Toappreciatethecapabilitiesandlimitationsofamarineradarandtobe
abletouseittofulladvantage,itisnecessarytocomprehendthe
characteristicsandbehaviorofradiowavesandtograsptheprinciplesof
theirgenerationandreception,includingtheechodisplayasseenbythe
observer.Understandingthetheorybehindthetargetpresentationonthe
radarscopewillprovidetheradarobserverabetterunderstandingoftheart
and science of radar interpretation.
Radar(radio)waves,emittedinpulsesofelectromagneticenergyinthe
radio-frequencyband3,000to10,000MHzusedforshipbornenavigational
radar,havemanycharacteristicssimilartothoseofotherwaves.Likelight
wavesofmuchhigherfrequency,radarwavestendtotravelinstraightlines
orraysatspeedsapproximatingthatoflight.Also,likelightwaves,radar
waves are subject to refraction or bending in the atmosphere.
Radio-frequencyenergytravelsatthespeedoflight,approximately
162,000nauticalmilespersecond;therefore,thetimerequiredforapulseto
traveltothetargetandreturntoitssourceisameasureofthedistancetothe
target.Sincetheradio-frequencyenergymakesaroundtrip,onlyhalfthe
timeoftraveldeterminesthedistancetothetarget.Theroundtriptimeis
accounted for in the calibration of the radar.
Thespeedofapulseofradio-frequencyenergyissofastthatthepulsecan
circumnavigatetheearthattheequatormorethan7timesin1second.Itshould
beobviousthatinmeasuringthetimeoftravelofaradarpulseorsignalfrom
oneshiptoatargetship,themeasurementmustbeanextremelyshorttime
interval.Forthisreason,theMICROSECOND(µsec)isusedasameasureof
timeforradarapplications.Themicrosecondisone-millionthpartof1second,
i.e., there are 1,000,000 microseconds in 1 second of time.
Radiowaveshavecharacteristicscommontootherformsofwavemotion
suchasoceanwaves.Wavemotionconsistsofasuccessionofcrestsand
troughswhichfollowoneanotheratequalintervalsandmovealongata
constantspeed.Likewavesinthesea,radarwaveshaveenergy,frequency,
amplitude,wavelength,andrateoftravel.Whereaswavesintheseahave
mechanicalenergy,radarwaveshaveelectromagneticenergy,usually
expressedinwattunitsofpower.Animportantcharacteristicofradiowaves
inconnectionwithradarispolarization.Thiselectromagneticenergyhas
associatedelectricandmagneticfields,thedirectionsofwhichareatright
anglestoeachother.TheorientationoftheELECTRICAXISinspace
establisheswhatisknownasthePOLARIZATIONofthewave.Horizontal
polarizationisnormallyusedwithnavigationalradars,i.e.,thedirectionof
theelectricaxisishorizontalinspace.Horizontalpolarizationhasbeen
foundtobethemostsatisfactorytypeofpolarizationfornavigationalradars
inthatstrongerechoesarereceivedfromthetargetsnormallyusedwith
these radars when the electric axis is horizontal.
Eachpulseofenergytransmittedduringafewtenthsofamicrosecondor
afewmicrosecondscontainshundredsofcompleteoscillations.ACYCLE
isonecompleteoscillationoronecompletewave,i.e.,thatpartofthewave
motionpassingzeroinonedirectionuntilitnextpasseszerointhesame
direction(seefigure1.1).TheFREQUENCYisthenumberofcycles
completedpersecond.Theunitnowbeingusedforfrequencyincyclesper
secondistheHERTZ.Onehertzisonecyclepersecond;onekilohertz(kHz)
isonethousandcyclespersecond;onemegahertz(MHz)isonemillion
cycles per second.
WAVELENGTHisthedistancealongthedirectionofpropagation
betweensuccessivecrestsortroughs.Whenonecyclehasbeencompleted,
the wave has traveled one wavelength.
TheAMPLITUDEisthemaximumdisplacementofthewavefromits
mean or zero value.
Sincethespeedofradarwavesisconstantat300,000kilometersper
second, there is a definite relationship between frequency and wavelength.
Figure 1.1 - Wave.
4
TheCYCLEisacompletealternationoroscillationfromonecrest
through a trough to the next crest.
When the wavelength is 3.2 centimeters (0.000032 km),
THE RADAR BEAM
Thepulsesofr-fenergyemittedfromthefeedhornatthefocalpointofa
reflectororemittedandradiateddirectlyfromtheslotsofaslotted
waveguideantennawould,forthemostpart,formasinglelobe-shaped
patternofradiationifemittedinfreespace.Figure1.2illustratesthisfree
spaceradiationpattern,includingtheundesirableminorlobesorSIDE
LOBESassociatedwithpracticalantennadesign.Becauseofthelarge
differencesinthevariousdimensionsoftheradiationpattern,figure1.2is
necessarily distorted.
Althoughtheradiatedenergyisconcentratedorfocusedintoarelatively
narrowmainbeambytheantenna,similartoabeamoflightfromaflashlight,
thereisnoclearlydefinedenvelopeoftheenergyradiated.Whiletheenergyis
concentratedalongtheaxisofthebeam,itsstrengthdecreaseswithdistance
alongtheaxis.Thestrengthoftheenergydecreasesrapidlyindirectionsaway
fromthebeamaxis.Thepowerinwattsatpointsinthebeamisinversely
proportionaltothesquareofthedistance.Therefore,thepowerat3milesisonly
1/9thofthepowerat1mileinagivendirection.Thefieldintensityinvoltsat
pointsinthebeamisinverselyproportionaltothedistance.Therefore,the
voltageat2milesisonlyone-halfthevoltageat1mileinagivendirection.With
therapiddecreaseintheamountofradiatedenergyindirectionsawayfromthe
axisandinconjunctionwiththerapiddecreasesofthisenergywithdistance,it
followsthatpracticallimitsofpowerorvoltagemaybeusedtodefinethe
dimensions of the radar beam or to establish its envelope of useful energy.
Beam Width
Thethree-dimensionalradarbeamisnormallydefinedbyitshorizontal
andverticalbeamwidths.Beamwidthistheangularwidthofaradarbeam
betweenpointswithinwhichthefieldstrengthorpowerisgreaterthan
arbitrarily selected lower limits of field strength or power.
Therearetwolimitingvalues,expressedeitherintermsoffieldintensity
orpowerratios,usedconventionallytodefinebeamwidth.Oneconvention
definesbeamwidthastheangularwidthbetweenpointsatwhichthefield
strengthis71percentofitsmaximumvalue.Expressedintermsofpower
ratio,thisconventiondefinesbeamwidthastheangularwidthbetween
HALF-POWERPOINTS.Theotherconventiondefinesbeamwidthasthe
angularwidthbetweenpointsatwhichthefieldstrengthis50percentofits
maximumvalue.Expressedintermsofpowerratio,thelatterconvention
definesbeamwidthastheangularwidthbetweenQUARTER-POWER
POINTS.
Thehalf-powerratioisthemostfrequentlyusedconvention.Which
conventionhasbeenusedinstatingthebeamwidthmaybeidentifiedfrom
thedecibel(dB)figurenormallyincludedwiththespecificationsofaradar
set.Halfpowerand71percentfieldstrengthcorrespondto-3dB;quarter
power and 50 percent field strength correspond to -6 dB.
Figure 1.2 - Free space radiation pattern.
frequency
speed of radar waves
wavelength
--------------------------------------------------=
frequency
300000km,
ondsec
---------------------------------
0.000032km
cycle
-----------------------------------
÷
frequency9375megahertz
=
=
5
Theradiationdiagramillustratedinfigure1.3depictsrelativevaluesof
powerinthesameplaneexistingatthesamedistancesfromtheantennaor
theoriginoftheradarbeam.Maximumpowerisinthedirectionoftheaxis
ofthebeam.Powervaluesdiminishrapidlyindirectionsawayfromtheaxis.
Thebeamwidthinthiscaseistakenastheanglebetweenthehalf-power
points.
Foragivenamountoftransmittedpower,themainlobeoftheradarbeam
extendstoagreaterdistanceatagivenpowerlevelwithgreater
concentrationofpowerinnarrowerbeamwidths.Toincreasemaximum
detectionrangecapabilities,theenergyisconcentratedintoasnarrowa
beamasisfeasible.Becauseofpracticalconsiderationsrelatedtotarget
detectionanddiscrimination,onlythehorizontalbeamwidthisquitenarrow,
typicalvaluesbeingbetweenabout0.65˚to2.0˚.Theverticalbeamwidthis
relatively broad, typical values being between about 15˚ to 30˚.
Thebeamwidthisdependentuponthefrequencyorwavelengthofthe
transmitted energy, antenna design, and the dimensions of the antenna.
Foragivenantennasize(antennaaperture),narrowerbeamwidthsare
obtainedwhenusingshorterwavelengths.Foragivenwavelength,narrower
beam widths are obtained when using larger antennas.
Theslottedwaveguideantennahaslargelyeliminatedtheside-lobe
problem.
EFFECT OF SEA SURFACE ON RADAR BEAM
Withradarwavesbeingpropagatedinthevicinityofthesurfaceofthe
sea,themainlobeoftheradarbeam,asawhole,iscomposedofanumberof
separatelobesasopposedtothesinglelobe-shapedpatternofradiationas
emittedinfreespace.Thisphenomenonistheresultofinterferencebetween
radarwavesdirectlytransmittedandthosewaveswhicharereflectedfrom
thesurfaceofthesea.Theverticalbeamwidthsofnavigationalradarsare
suchthatduringnormaltransmission,radarwaveswillstrikethesurfaceof
theseaatpointsfromneartheantenna(dependinguponantennaheightand
verticalbeamwidth)totheradarhorizon.Theindirectwaves(seefigure1.4)
reflectedfromthesurfaceoftheseamay,onrejoiningthedirectwaves,
eitherreinforceorcancelthedirectwavesdependinguponwhethertheyare
inphaseoroutofphasewiththedirectwaves,respectively.Wherethedirect
andindirectwavesareexactlyinphase,i.e.,thecrestsandtroughsofthe
wavescoincide,hyperboliclinesofmaximumradiationknownasLINESOF
MAXIMAareproduced.Wherethedirectandindirectwavesareexactlyof
oppositephase,i.e.,thetroughofonewavecoincideswiththecrestofthe
otherwave,hyperboliclinesofminimumradiationknownasLINESOF
MINIMAareproduced.Alongdirectionsawayfromtheantenna,thedirect
andindirectwaveswillgraduallycomeintoandpassoutofphase,producing
lobesofusefulradiationseparatedbyregionswithinwhich,forpractical
purposes, there is no useful radiation.
Figure1.5illustratesthelowerregionoftheINTERFERENCE
PATTERNofarepresentativenavigationalradar.Sincethefirstlineof
minimaisatthesurfaceofthesea,thefirstregionofminimumradiationor
energy is adjacent to the sea’s surface.
Fromfigure1.5itshouldbeobviousthatifr-fenergyistobereflected
fromatarget,thetargetmustextendsomewhatabovetheradarhorizon,the
amountofextensionbeingdependentuponthereflectingpropertiesofthe
target.
AVERTICAL-PLANECOVERAGEDIAGRAMasillustratedinfigure
1.5isusedbyradardesignersandanalyststopredictregionsinwhichtargets
will and will not be detected.
Ofcourse,onthesmallpageofabookitwouldbeimpossibletoillustrate
thecoverageofaradarbeamtoscalewithantennaheightbeinginfeetand
thelengthsofthevariouslobesoftheinterferencepatternbeinginmiles.In
providinggreaterclarityofthepresentationofthelobes,non-linear
graduations of the arc of the vertical beam width are used.
Figure 1.3 - Radiation diagram.
Figure 1.4 - Direct and indirect waves.
6
Figure 1.5 - Vertical-plane coverage diagram (3050 MHz, antenna height 125 feet, wave height 4 feet).
7
Figure 1.6 - Vertical-plane coverage diagram (1000 MHz, vertical beam width 10˚, antenna height 80 feet, wave height 0 feet).
8
Thelengthsofthevariouslobesillustratedinfigures1.5and1.6shouldbe
givennospecialsignificancewithrespecttotherangecapabilitiesofa
particularradarset.Aswithothercoveragediagrams,thelobesaredrawnto
connectpointsofequalfieldintensities.Longerandbroaderlobesmaybe
drawn connecting points of equal, but lesser, field intensities.
Thevertical-planecoveragediagramasillustratedinfigure1.6,whilenot
representativeofnavigationalradars,doesindicatethatatthelower
frequenciestheinterferencepatternismorecoarsethanthepatternsfor
higherfrequencies.Thisparticulardiagramwasconstructedwiththe
assumptionthatthefreespaceusefulrangeoftheradarbeamwas50
nauticalmiles.Fromthisdiagramitisseenthattherangesoftheusefullobes
areextendedtoconsiderablygreaterdistancesbecauseofthereinforcement
ofthedirectradarwavesbytheindirectwaves.Also,theelevationofthe
lowestlobeishigherthanitwouldbeforahigherfrequency.Figure1.6also
illustratestheverticalviewoftheundesirablesidelobesassociatedwith
practicalantennadesign.Inexaminingtheseradiationcoveragediagrams,
thereadershouldkeepinmindthattheradiationpatternisthree-
dimensional.
Antennaheightaswellasfrequencyorwavelengthgovernsthenumberof
lobesintheinterferencepattern.Thenumberofthelobesandthefinenessof
theinterferencepatternincreasewithantennaheight.Increasedantenna
heightaswellasincreasesinfrequencytendstolowerthelobesofthe
interference pattern.
Thepitchandrolloftheshipradiatingdoesnotaffectthestructureofthe
interference pattern.
9
ATMOSPHERIC FACTORS AFFECTING THE RADAR HORIZON
THE RADAR HORIZON
Theaffectoftheatmosphereonthehorizonisafurtherfactorwhich
shouldbetakenintoaccountwhenassessingthelikelihoodofdetectinga
particular target and especially where the coastline is expected.
Generally,radarwavesarerestrictedintherecordingoftherangeoflow-
lyingobjectsbytheradarhorizon.Therangeoftheradarhorizondepends
ontheheightoftheantennaandontheamountofbendingoftheradarwave.
Thebendingiscausedbydiffractionandrefraction.Diffractionisaproperty
oftheelectromagneticwaveitself.Refractionisduetotheprevailing
atmospheric conditions. There is, therefore, a definite radar horizon.
DIFFRACTION
Diffractionisthebendingofawaveasitpassesanobstruction.Because
ofdiffractionthereissomeilluminationoftheregionbehindanobstruction
ortargetbytheradarbeam.Diffractioneffectsaregreateratthelower
frequencies.Thus,theradarbeamofalowerfrequencyradartendsto
illuminatemoreoftheshadowregionbehindanobstructionthanthebeamof
radar of higher frequency or shorter wavelength.
REFRACTION
Refractionaffectstherangeatwhichobjectsaredetected.The
phenomenonofrefractionshouldbewell-knowntoeverynavigationofficer.
Refractiontakesplacewhenthevelocityofthewaveischanged.Thiscan
happenwhenthewavefrontpassestheboundaryoftwosubstancesof
differingdensities.Onesubstanceoffersmoreresistancetothewavethanthe
otherandthereforethevelocityofthewavewillchange.Likelightrays,
radarraysaresubjecttobendingorrefractionintheatmosphereresulting
fromtravelthroughregionsofdifferentdensity.However,radarraysare
refractedslightlymorethanlightraysbecauseofthefrequenciesused.Ifthe
radarwavesactuallytraveledinstraightlinesorrays,thedistancetothe
horizongrazedbytheserayswouldbedependentonlyontheheightofthe
antenna,assumingadequatepowerfortheraystoreachthishorizon.
Withouttheeffectsofrefraction,thedistancetotheRADARHORIZON
would be the same as that of the geometrical horizon for the antenna height.
Standard Atmospheric Conditions
Thedistancetotheradarhorizon,ignoringrefractioncanbeexpressedin
thefollowingformula.Wherehistheheightoftheantennainfeet,the
distance,d,totheradarhorizoninnauticalmiles,assumingstandard
atmospheric conditions, may be found as follows:
Withthedistancestothegeometricalorordinaryhorizonbeing1.06
andthedistancetothevisibleoropticalhorizonbeing1.15.Weseethat
therangeoftheradarhorizonisgreaterthanthatoftheopticalhorizon,
whichagainisgreaterthanthatofthegeometricalhorizon.Thus,likelight
raysinthestandardatmosphere,radarraysarebentorrefractedslightly
downwards approximating the curvature of the earth (see figure 1.7).
Thedistancetotheradarhorizondoesnotinitselflimitthedistancefrom
whichechoesmaybereceivedfromtargets.Assumingthatadequatepower
istransmitted,echoesmaybereceivedfromtargetsbeyondtheradarhorizon
iftheirreflectingsurfacesextendaboveit.Notethatthedistancetotheradar
horizon is the distance at which the radar rays graze the surface of the earth.
Intheprecedingdiscussionstandardatmosphericconditionswere
assumed.Thestandardatmosphereisahypotheticalverticaldistributionof
atmospherictemperature,pressure,anddensitywhichistakentobe
representative of the atmosphere for various purposes.
Figure 1.7 - Refraction.
d1.22h=
h
h
10
Standard conditions are precisely defined as follows:
Pressure = 1013 mb decreasing at 36 mb/1000 ft of height
Temperature = 15˚C decreasing at 2˚C/1000 ft of height
Relative Humidity = 60% and constant with height.
Theseconditionsgivearefractiveindexof1.00325whichdecreasesat
0.00013units/1000ftofheight.Thedefinitionof“standard”conditions
relatestotheverticalcompositionoftheatmosphere.Marinersmaynotbe
abletoobtainapreciseknowledgeofthisandsomustrelyonamoregeneral
appreciationoftheweatherconditions,theareaoftheworld,andofthetime
of the year.
Whiletheatmosphericconditionsatanyonelocalityduringagiven
seasonmaydifferconsiderablyfromstandardatmosphericconditions,the
slightlydownwardbendingofthelightandradarraysmaybedescribedas
the typical case.
Whiletheformulaforthedistancetotheradarhorizon
isbaseduponawavelengthof3cm,thisformulamaybe
usedinthecomputationofthedistancetotheradarhorizonforother
wavelengthsusedwithnavigationalradar.Thevaluesodeterminedshould
beconsideredonlyasanapproximatevaluebecausethemarinergenerally
has no means of knowing what actual refraction conditions exist.
Sub-refraction
Thedistancetotheradarhorizonisreduced.Thisconditionisnotas
commonassuper-refraction.Sub-refractioncanoccurinpolarregionswhere
Arcticwindsblowoverwaterwhereawarmcurrentisprevalent.Ifalayerof
cold,moistairoverridesashallowlayerofwarm,dryair,aconditionknown
asSUB-REFRACTIONmayoccur(seefigure1.8).Theeffectofsub-
refractionistobendtheradarraysupwardandthusdecreasethemaximum
ranges at which targets may be detected.
Sub-refractionalsoaffectsminimumrangesandmayresultinfailureto
detectlowlyingtargetsatshortrange.Itisimportanttonotethatsub-
refractionmayinvolveanelementofdangertoshippingwheresmallvessels
andicemaygoundetected.Theofficerinchargeofthewatchshouldbe
especiallymindfulofthisconditionandextraprecautionsbeadministered
such as a reduction in speed and the posting of extra lookouts.
Super-refraction
Thedistancetotheradarhorizonisextended.Incalmweatherwithno
turbulencewhenthereisanupperlayerofwarm,dryairoverasurfacelayer
ofcold,moistair,aconditionknownasSUPER-REFRACTIONmayoccur
(seefigure1.9).Forthisconditiontoexist,theweathermustbecalmwith
littleornoturbulence,otherwisethelayersofdifferentdensitieswillmix
andtheboundaryconditionsdisappear.Theeffectofsuper-refractionwill
increasethedownwardbendingoftheradarraysandthusincreasetheranges
atwhichtargetsmaybedetected.Super-refractionfrequentlyoccursinthe
tropicswhenawarmlandbreezeblowsovercooleroceancurrents.Itis
especially noticeable on the longer range scales.
d(1.22h)=
Figure 1.8 - Sub-refraction.
Figure 1.9 - Super-refraction.
11
Extra Super-refraction or Ducting
Mostradaroperatorsareawarethatatcertaintimestheyareabletodetect
targetsatextremelylongranges,butatothertimestheycannotdetecttargets
withinvisualranges,eventhoughtheirradarsmaybeintopoperating
condition in both instances.
Thesephenomenaoccurduringextremecasesofsuper-refraction.Energy
radiatedatanglesof1˚orlessmaybetrappedinalayeroftheatmosphere
calledaSURFACERADIODUCT.Inthesurfaceradioductillustratedin
figure1.10,theradarraysarerefracteddownwardtothesurfaceofthesea,
reflectedupward,refracteddownwardagainwithintheduct,andsoon
continuously.
Theenergytrappedbytheductsufferslittleloss;thus,targetsmaybe
detectedatexceptionallylongranges.Surfacetargetshavebeendetectedat
rangesinexcessof1,400mileswithrelativelylow-poweredequipment.
Thereisagreatlossintheenergyoftheraysescapingtheduct,thus
reducing the chances for detection of targets above the duct.
Ductingsometimesreducestheeffectiveradarrange.Iftheantennais
belowaduct,itisimprobablethattargetsabovetheductwillbedetected.In
instancesofextremelylow-levelductswhentheantennaisabovetheduct,
surfacetargetslyingbelowtheductmaynotbedetected.Thelattersituation
does not occur very often.
Ducting Areas
Althoughductingconditionscanhappenanyplaceintheworld,the
climateandweatherinsomeareasmaketheiroccurrencemorelikely.In
somepartsoftheworld,particularlythosehavingamonsoonalclimate,
variationinthedegreeofductingismainlyseasonal,andgreatchangesfrom
daytodaymaynottakeplace.Inotherpartsoftheworld,especiallythosein
whichlowbarometricpressureareasrecuroften,theextentofnonstandard
propagation conditions varies considerably from day to day.
Figure1.11illustratesthedifferentplacesintheworldwhereknown
ductingoccursfrequently.Refertothemaptoseetheirlocationinrelationto
the climate that exists in each area during different seasons of the year.
AtlanticCoastoftheUnitedStates(Area1).Ductingiscommonin
summeralongthenorthernpartofthecoast,butintheFloridaregionthe
seasonal trend is the reverse, with a maximum in the winter season.
WesternEurope(Area2).Apronouncedmaximumofductingconditions
existsinthesummermonthsontheeasternsideoftheAtlanticaroundthe
British Isles and in the North Sea.
MediterraneanRegion(Area3).Availablereportsindicatethatthe
seasonalvariationintheMediterraneanregionisverymarked,withducting
moreorlesstheruleinsummer.Conditionsareapproximatelystandardin
winter.DuctinginthecentralMediterraneanareaiscausedbytheflowof
warm,dryairfromthesouth,whichmovesacrosstheseaandthusprovides
anexcellentopportunityfortheformationofducts.Inwinter,however,the
climateinthecentralMediterraneanismoreorlessthesameasAtlantic
conditions, therefore not favorable for duct formation.
ArabianSea(Area4).ThedominatingmeteorologicalfactorintheArabian
Searegionisthesouthwestmonsoon,whichblowsfromearlyJunetomid-
SeptemberandcoversthewholeArabianSeawithmoist-equatorialairupto
considerableheights.Whenthismeteorologicalsituationisdevelopedfully,no
occurrenceofductingistobeexpected.Duringthedryseason,ontheother
hand,conditionsaredifferent.Ductingthenistherule,nottheexception,andon
someoccasionsextremelylongranges(upto1,500miles)havebeenobserved
on fixed targets.
WhenthesouthwestmonsoonbeginsearlyinJune,ductingdisappearson
theIndiansideoftheArabianSea.Alongthewesterncoasts,however,
conditionsfavoringductingmaystilllinger.TheStraitofHormuz(Persian
Gulf)isparticularlyinterestingasthemonsoontherehastocontendwiththe
shamal(anorthwesterlywind)overIraqandthePersianGulffromthenorth.
Thestraititselfliesattheboundarybetweenthetwowindsystems;afrontis
formedwiththewarm,dryshamalontopandthecolder,humidmonsoon
underneath.Consequently,conditionsarefavorablefortheformationofan
extensiveduct,whichisofgreatimportancetoradaroperationintheStrait
of Hormuz.
BayofBengal(Area5).Theseasonaltrendofductingconditionsinthe
BayofBengalisthesameasintheArabianSea,withstandardconditions
duringthesummersouthwestmonsoon.Ductingisfoundduringthedry
season.
Figure 1.10 - Ducting.
12
Figure 1.11 - Ducting areas.
13
PacificOcean(Area6).Frequentoccurrencesofductingaround
Guadalcanal,theeastcoastofAustralia,andaroundNewGuineaandKorea
havebeenexperienced.ObservationsalongthePacificcoastoftheUnited
Statesindicatefrequentducting,butnoclearindicationofitsseasonaltrend
isavailable.MeteorologicalconditionsintheYellowSeaandSeaofJapan,
includingtheislandofHonshu,areapproximatelylikethoseofthe
northeasterncoastoftheUnitedStates.Therefore,ductinginthisarea
shouldbecommoninthesummer.ConditionsintheSouthChinaSea
approximatethoseoffthesoutheasterncoastoftheUnitedStatesonlyduring
thewintermonths,whenductingcanbeexpected.Duringtherestofthe
year,theAsiaticmonsoonmodifiestheclimateinthisarea,butno
informationisavailableontheprevalenceofductingduringthistime.Trade
windsinthePacificquitegenerallyleadtotheformationofratherlowducts
over the open ocean.
WEATHER FACTORS AFFECTING THE RADAR HORIZON
Theusualeffectsofweatheraretoreducetherangesatwhichtargetscan
bedetectedandtoproduceunwantedechoesontheradarscopewhichmay
obscurethereturnsfromimportanttargetsorfromtargetswhichmaybe
dangeroustoone’sship.Thereductionofintensityofthewaveexperienced
along its path is known asattenuation.
Attenuationiscausedbytheabsorptionandscatteringofenergybythe
variousformsofprecipitation.Theamountofattenuationcausedbyeachof
thevariousfactorsdependstoasubstantialdegreeontheradarwavelength.
Itcausesadecreaseinechostrength.Attenuationisgreateratthehigher
frequencies or shorter wavelengths.
Attenuation by rain, fog, clouds, hail, snow, and dust
Theamountofattenuationcausedbytheseweatherfactorsisdependent
upontheamountofwater,liquidorfrozen,presentinaunitvolumeofair
anduponthetemperature.Therefore,asonewouldexpect,theaffectscan
differwidely.Thefurthertheradarwaveandreturningechomusttravel
throughthismediumthenthegreaterwillbetheattenuationandsubsequent
decreaseindetectionrange.Thisisthecasewhetherthetargetisinor
outsidetheprecipitation.Acertainamountofattenuationtakesplaceeven
whenradarwavestravelthroughaclearatmosphere.Theaffectwillnotbe
noticeabletotheradarobserver.Theeffectofprecipitationstartstobecome
ofpracticalsignificanceatwavelengthsshorterthan10cm.Inanygivenset
ofprecipitationconditions,the(S-band)or10cmwillsufferlessattenuation
than the (X-band) or 3cm.
Rain
Inthecaseofraintheparticleswhichaffectthescatteringandattenuation
taketheformofwaterdroplets.Itispossibletorelatetheamountof
attenuationtotherateofprecipitation.Ifthesizeofthedropletisan
appreciableproportionofthe3cmwavelength,strongclutterechoeswillbe
producedandtherewillbeseriouslossofenergyduetoscatteringand
attenuation.Ifthetargetiswithintheareaofrainfall,anyechoesfrom
raindropswillfurtherdecreaseitsdetectionrange.Weakertargetresponses,
asfromsmallvesselsandbuoys,willbeundetectableiftheirechoesarenot
strongerthanthatoftherain.Averyheavyrainstorm,likethosesometimes
encountered in the tropics, can obliterate most of the (X-band) radar picture.
Continuousrainfalloveralargeareawillmakethecenterpartofthe
screenbrighterthantherestandtherainclutter,movingalongwiththeship,
lookssimilartoseaclutter.Itcanbeclearlyseenonlongrangescales.This
isduetoagradualdecreaseinreturningpowerasthepulsepenetratesfurther
into the rain area.
Fog
Inmostcasesfogdoesnotactuallyproduceechoesontheradardisplay,
butaverydensefogbankwhicharisesinpolarregionsmayproducea
significant reduction in detection range.
Avesselencounteringareasknownforindustrialpollutionintheformof
smog may find a somewhat higher degree of attenuation than sea fog.
14
Clouds
Thewaterdropletswhichformcloudsaretoosmalltoproducea
detectableresponseatthe3cmwavelength.Ifthereisprecipitationinthe
cloud then the operator can expect a detectable echo.
Hail
Withrespecttowater,hailwhichisessentiallyfrozenrainreflectsradar
energylesseffectivelythanwater.Therefore,ingeneraltheclutterand
attenuation from hail are likely to prove less detectable than that from rain.
Snow
Similartotheeffectsofhail,theoveralleffectofclutteronthepictureis
lessthanthatduetorain.Fallingsnowwillonlybeobservedonthedisplays
of3cmexceptduringheavysnowfallwhereattenuationcanbeobservedona
10cm set.
Thestrengthofechoesfromsnowdependsuponthesizeofthesnowflake
andtherateofprecipitation.Forpracticalpurposes,however,thesignificant
factoristherateofprecipitation,becausethewatercontentoftheheaviest
snowfall will very rarely equal that of even moderate rain.
Itisimportanttokeepinmindthatinareasreceivingandcollecting
snowfallandwherethesnowiscollectingonpossibledangertargetsitmay
renderthemlessdetectable.Accumulationofsnowproducesalimited
absorptioncharacteristicandreducesthedetectionrangeofanotherwise
strong target.
Dust
Thereisageneralreductioninradardetectioninthepresenceofdustand
sandstorms.Onthebasisofparticlesize,detectableresponsesareextremely
unlikely and the operator can expect a low level of attenuation.
Unusual Propagation Conditions
Similartolightwaves,radarwavesgoingthroughtheearth’satmosphere
aresubjecttorefractionandnormallybendslightlywiththecurvatureofthe
earth.Certainatmosphericconditionswillproduceamodificationofthis
normal refraction.
15
A BASIC RADAR SYSTEM
RADAR SYSTEM CONSTANTS
Beforedescribingthefunctionsofthecomponentsofamarineradar,thereare
certainconstantsassociatedwithanyradarsystemthatwillbediscussed.These
arecarrierfrequency,pulserepetitionfrequency,pulselength,andpower
relation.Thechoiceoftheseconstantsforaparticularsystemisdeterminedby
itsoperationaluse,theaccuracyrequired,therangetobecovered,thepractical
physical size, and the problems of generating and receiving the signals.
Carrier Frequency
Thecarrierfrequencyisthefrequencyatwhichtheradio-frequency
energyisgenerated.Theprincipalfactorsinfluencingtheselectionofthe
carrierfrequencyarethedesireddirectivityandthegenerationandreception
of the necessary microwave radio-frequency energy.
Forthedeterminationofdirectionandfortheconcentrationofthe
transmittedenergysothatagreaterportionofitisuseful,theantennashould
behighlydirective.Thehigherthecarrierfrequency,theshorterthe
wavelengthandhencethesmalleristheantennarequiredforagiven
sharpness of the pattern of radiated energy.
Theproblemofgeneratingandamplifyingreasonableamountsofradio-
frequencyenergyatextremelyhighfrequenciesiscomplicatedbythe
physicalconstructionofthetubestobeused.Thecommontubebecomes
impracticalforcertainfunctionsandmustbereplacedbytubesofspecial
design. Among these are theklystron andmagnetron.
Sinceitisverydifficulttoamplifytheradio-frequencyechoesofthe
carrierwave,radio-frequencyamplifiersarenotused.Instead,thefrequency
oftheincomingsignals(echoes)ismixed(heterodyned)withthatofalocal
oscillatorinacrystalmixertoproduceadifferencefrequencycalledthe
intermediatefrequency.Thisintermediatefrequencyislowenoughtobe
amplified in suitableintermediate frequency amplifier stages in the receiver.
Pulse Repetition Frequency
ThePulseRepetitionFrequency(PRF),sometimesreferredtoasPulse
RepetitionRate(PRR)isthenumberofpulsestransmittedpersecond.Some
characteristicvaluesmaybe600,1000,1500,2200and3000pulsesper
second.Themajorityofmodernmarineradarsoperatewithinarangeof400
to 4000 pulses per second.
Ifthedistancetoatargetistobedeterminedbymeasuringthetime
requiredforonepulsetotraveltothetargetandreturnasareflectedecho,it
isnecessarythatthiscyclebecompletedbeforethepulseimmediately
followingistransmitted.Thisisthereasonwhythetransmittedpulsesmust
beseparatedbyrelativelylongnontransmittingtimeperiods.Otherwise,
transmissionwouldoccurduringreceptionofthereflectedechoofthe
precedingpulse.Usingthesameantennaforbothtransmittingandreceiving,
therelativelyweakreflectedechowouldbeblockedbytherelativelystrong
transmitted pulse.
Sufficienttimemustbeallowedbetweeneachtransmittedpulseforan
echotoreturnfromanytargetlocatedwithinthemaximumworkablerange
ofthesystem.Otherwise,thereceptionoftheechoesfromthemoredistant
targetswouldbeblockedbysucceedingtransmittedpulses.Themaximum
measurablerangeofaradarsetdependsuponthepeakpowerinrelationto
thepulserepetitionrate.Assumingsufficientpowerisradiated,the
maximumrangeatwhichechoescanbereceivedmaybeincreasedthrough
loweringthepulserepetitionratetoprovidemoretimebetweentransmitted
pulses.ThePRRmustbehighenoughsothatsufficientpulseshitthetarget
andenougharereturnedtodetectthetarget.Themaximummeasurable
range,assumingthattheechoesarestrongenoughfordetection,canbe
determinedbydividing80,915(radarnauticalmilespersecond)bythePRR.
Withtheantennabeingrotated,thebeamofenergystrikesatargetfora
relativelyshorttime.Duringthistime,asufficientnumberofpulsesmustbe
transmittedinordertoreceivesufficientechoestoproducethenecessary
indicationontheradarscope.Withtheantennarotatingat15revolutionsper
minute,aradarsethavingaPRRof800pulsespersecondwillproduce
approximately9pulsesforeachdegreeofantennarotation.The
PERSISTENCEoftheradarscope,i.e.,ameasureofthetimeitretains
imagesofechoes,andtherotationalspeedoftheantenna,therefore,
determine the lowest PRR that can be used.
Pulse Length
Pulselengthisdefinedasthedurationofthetransmittedradarpulseandis
usually measured in microseconds.
Theminimumrangeatwhichatargetcanbedetectedisdetermined
largelybythepulselength.Ifatargetissoclosetothetransmitterthatthe
echoisreturnedtothereceiverbeforethetransmissionstops,thereception
16
oftheecho,obviously,willbemaskedbythetransmittedpulse.For
example,aradarsethavingapulselengthof1microsecondwillhavea
minimumrangeof164yards.Thismeansthattheechoofatargetwithinthis
rangewillnotbeseenontheradarscopebecauseofbeingmaskedbythe
transmitted pulse.
Sincetheradio-frequencyenergytravelsataspeedof161,829nautical
milespersecondor161,829nauticalmilesinonemillionmicroseconds,the
distancetheenergytravelsin1microsecondisapproximately0.162nautical
mileor328yards.Becausetheenergymustmakearoundtrip,thetarget
cannotbecloserthan164yardsifitsechoistobeseenontheradarscope
whileusingapulselengthof1microsecond.Consequently,relativelyshort
pulse lengths, about 0.1 microsecond, must be used for close-in ranging.
Manyradarsetsaredesignedforoperationwithbothshortandlongpulse
lengths.Manyoftheseradarsetsareshiftedautomaticallytotheshorter
pulselengthonselectingtheshorterrangescales.Ontheotherradarsets,the
operatormustselecttheradarpulselengthinaccordancewiththeoperating
conditions.Radarsetshavegreaterrangecapabilitieswhilefunctioningwith
thelongerpulselengthbecauseagreateramountofenergyistransmittedin
each pulse.
Whilemaximumdetectionrangecapabilityissacrificedwhenusingthe
shorterpulselength,betterrangeaccuracyandrangeresolutionareobtained.
Withtheshorterpulse,betterdefinitionofthetargetontheradar-scopeis
obtained;therefore,rangeaccuracyisbetter.RANGERESOLUTIONisa
measureofthecapabilityofaradarsettodetecttheseparationbetween
thosetargetsonthesamebearingbuthavingsmalldifferencesinrange.If
theleadingedgeofapulsestrikesatargetataslightlygreaterrangewhile
thetrailingpartofthepulseisstillstrikingaclosertarget,itisobviousthat
thereflectedechoesofthetwotargetswillappearasasingleelongated
image on the radarscope.
Power Relation
Theusefulpowerofthetransmitteristhatcontainedintheradiatedpulses
andiscalledthePEAKPOWERofthesystem.Powerisnormallymeasured
asanaveragevalueoverarelativelylongperiodoftime.Becausetheradar
transmitterisrestingforatimethatislongwithrespecttotheoperating
time,theaveragepowerdeliveredduringonecycleofoperationisrelatively
low compared with the peak power available during the pulse time.
Adefiniterelationshipexistsbetweentheaveragepowerdissipatedover
anextendedperiodoftimeandthepeakpowerdevelopedduringthepulse
time.
ThePULSEREPETITIONTIME,ortheoveralltimeofonecycleof
operation,isthereciprocalofthepulserepetitionrate(PRR).Otherfactors
remainingconstant,thelongerthepulselength,thehigherwillbethe
averagepower;thelongerthepulserepetitiontime,thelowerwillbethe
average power.
These general relationships are shown in figure 1.12.
Theoperatingcycleoftheradartransmittercanbedescribedintermsof
thefractionofthetotaltimethatradio-frequencyenergyisradiated.This
timerelationshipiscalledtheDUTYCYCLEandmayberepresentedas
follows:
Foraradarhavingapulselengthof2microsecondsandapulserepetition
rateof500cyclespersecond(pulserepetitiontime=2,000microseconds),
the
Figure 1.12 - Relationship of peak and average power.
averagepower
peakpower
-----------------------------------------
pulselength
pulserepetitiontime
-------------------------------------------------------------=
dutycycle
pulselength
pulserepetitiontime
-------------------------------------------------------------=
dutycycle
2µsec
2000µsec,
--------------------------0.001==
17
Likewise,theratiobetweentheaveragepowerandpeakpowermaybe
expressed in terms of the duty cycle.
Intheforegoingexampleassumethatthepeakpoweris200kilowatts.
Therefore,foraperiodof2microsecondsapeakpowerof200kilowattsis
suppliedtotheantenna,whilefortheremaining1998microsecondsthe
transmitteroutputiszero.Becauseaveragepowerisequaltopeakpowertimes
the duty cycle,
Highpeakpowerisdesirableinordertoproduceastrongechooverthe
maximumrangeoftheequipment.Lowaveragepowerenablesthe
transmittertubesandcircuitcomponentstobemadesmallerandmore
compact.Thus,itisadvantageoustohavealowdutycycle.Thepeakpower
thatcanbedevelopedisdependentupontheinterrelationbetweenpeakand
average power, pulse length, and pulse repetition time, or duty cycle.
COMPONENTS AND SUMMARY OF FUNCTIONS
Whilepulse-modulatedradarsystemsvarygreatlyindetail,theprinciples
ofoperationareessentiallythesameforallsystems.Thus,asinglebasic
radarsystemcanbevisualizedinwhichthefunctionalrequirementsare
essentially the same as for all specific equipments.
Thefunctionalbreakdownofabasicpulse-modulatedradarsystem
usuallyincludessixmajorcomponents,asshownintheblockdiagram,
figure1.13.Thefunctionsofthecomponentsmaybesummarizedas
follows:
ThepowersupplyfurnishesallACandDCvoltagesnecessaryforthe
operation of the system components.
Themodulatorproducesthesynchronizingsignalsthattriggerthe
transmittertherequirednumberoftimespersecond.Italsotriggersthe
indicator sweep and coordinates the other associated circuits.
Thetransmittergeneratestheradio-frequencyenergyintheformofshort
powerful pulses.
Theantennasystemtakestheradio-frequencyenergyfromthetransmitter,
radiatesitinahighlydirectionalbeam,receivesanyreturningechoes,and
passes these echoes to the receiver.
Thereceiveramplifiestheweakradio-frequencypulses(echoes)returned
by a target and reproduces them as video pulses passed to the indicator.
Theindicatorproducesavisualindicationoftheechopulsesinamanner
that furnishes the desired information.
dutycycle
averagepower
peakpower
-----------------------------------------=
averagepower200kwx0.0010.2kilowatt==
Figure 1.13 - Block diagram of a basic pulse-modulated radar system
18
FUNCTIONS OF COMPONENTS
Power Supply
Infigure1.13thepowersupplyisrepresentedasasingleblock.
Functionally,thisblockisrepresentative.However,itisunlikelythatanyone
supplysourcecouldmeetallthepowerrequirementsofaradarset.The
distributionofthephysicalcomponentsofasystemmaybesuchastomake
itimpracticaltogroupthepower-supplycircuitsintoasinglephysicalunit.
Differentsuppliesareneededtomeetthevaryingrequirementsofasystem
andmustbedesignedaccordingly.Thepowersupplyfunctionisperformed
byvarioustypesofpowersuppliesdistributedamongthecircuitcomponents
of a radar set.
Infigure1.14themodulator,transmitter,andreceiverarecontainedinthe
samechassis.Inthisarrangement,thegroupofcomponentsiscalleda
TRANSCEIVER.(Thetermtransceiverisanacronymcomposedfromthe
words TRANSmitter and reCEIVER.)
Modulator
Thefunctionofthemodulatoristoinsurethatallcircuitsconnectedwiththe
radarsystemoperateinadefinitetimerelationshipwitheachotherandthatthe
timeintervalbetweenpulsesisoftheproperlength.Themodulator
simultaneouslysendsasynchronizingsignaltotriggerthetransmitterandthe
indicatorsweep.Thisestablishesacontrolforthepulserepetitionrate(PRR)and
providesareferenceforthetimingofthetravelofatransmittedpulsetoatarget
and its return as an echo.
Transmitter
Thetransmitterisbasicallyanoscillatorwhichgeneratesradio-frequency
(r-f)energyintheformofshortpowerfulpulsesasaresultofbeingturned
onandoffbythetriggeringsignalsfromthemodulator.Becauseofthe
frequenciesandpoweroutputsrequired,thetransmitteroscillatorisaspecial
type known as a MAGNETRON.
Transmitting and Receiving Antenna System
Thefunctionoftheantennasystemistotakether-fenergyfromthe
transmitter,radiatethisenergyinahighlydirectionalbeam,receiveany
echoesorreflectionsoftransmittedpulsesfromtargets,andpassthese
echoes to the receiver.
Incarryingoutthisfunctionther-fpulsesgeneratedinthetransmitterare
conductedtoaFEEDHORNatthefocalpointofadirectionalreflector,from
whichtheenergyisradiatedinahighlydirectionalpattern.Thetransmitted
andreflectedenergy(returnedbythesamedualpurposereflector)are
conducted by a common path.
ThiscommonpathisanelectricalconductorknownasaWAVEGUIDE.
Awaveguideishollowcoppertubing,usuallyrectangularincrosssection,
havingdimensionsaccordingtothewavelengthorthecarrierfrequency,i.e.,
the frequency of the oscillations within the transmitted pulse or echo.
Becauseofthisuseofacommonwaveguide,anelectronicswitch,a
TRANSMIT-RECEIVE(TR)TUBEcapableofrapidlyswitchingfrom
transmittoreceivefunctions,andviceversa,mustbeutilizedtoprotectthe
receiverfromdamagebythepotentenergygeneratedbythetransmitter.The
TRtube,asshowninfigure1.14blocksthetransmitterpulsesfromthe
receiver.Duringtherelativelylongperiodswhenthetransmitterisinactive,
theTRtubepermitsthereturningechoestopasstothereceiver.Toprevent
anyoftheveryweakechoesfrombeingabsorbedbythetransmitter,another
deviceknownasanANTI-TR(A-TR)TUBEisusedtoblockthepassageof
these echoes to the transmitter.
19
Figure 1.14 - A basic radar system.
20
Thefeedhornattheupperextremityofthewaveguidedirectsthe
transmittedenergytowardsthereflector,whichfocusesthisenergyintoa
narrowbeam.Anyreturningechoesarefocusedbythereflectoranddirected
towardthefeedhorn.Theechoespassthroughboththefeedhornand
waveguideenroutetothereceiver.Theprinciplesofaparabolicreflectorare
illustrated in figure 1.15.
Sincether-fenergyistransmittedinanarrowbeam,particularlynarrow
initshorizontaldimension,provisionmustbemadefordirectingthisbeam
towardsatargetsothatitsrangeandbearingmaybemeasured.Normally,
thisisaccomplishedthroughcontinuousrotationoftheradarbeamatarate
ofabout10to20revolutionsperminutesothatitwillimpingeuponany
targetswhichmightbeinitspath.Therefore,inthisbasicradarsystemthe
upperportionofthewaveguide,includingthefeedhorn,andthereflectorare
constructedsothattheycanberotatedinthehorizontalplanebyadrive
motor.Thisrotatableantennaandreflectorassemblyiscalledthe
SCANNER.
Figure1.16illustratesaSLOTTEDWAVEGUIDEANTENNAandnotice
thatthereisnoreflectororfeedhorn.Thelastfewfeetofthewaveguideis
constructedsothatitcanberotatedinthehorizontalplane.Theforwardand
narrowerfaceoftherotatablewaveguidesectioncontainsaseriesofslots
fromwhichther-fenergyisemittedtoformanarrowradarbeam.Returning
echoesalsopassthroughtheseslotsandthenpassthroughthewaveguideto
the receiver.
Receiver
Thefunctionofthereceiveristoamplifyorincreasethestrengthofthe
veryweakr-fechoesandreproducethemasvideosignalstobepassedtothe
indicator.Thereceivercontainsacrystalmixerandintermediatefrequency
amplificationstagesrequiredforproducingvideosignalsusedbythe
indicator.
Figure 1.15 - Principles of a parabolic reflector.
Figure 1.16 - Slotted waveguide antenna.
21
Indicator
Theprimaryfunctionoftheindicatoristoprovideavisualdisplayofthe
rangesandbearingsofradartargetsfromwhichechoesarereceived.Inthis
basicradarsystem,thetypeofdisplayusedisthePLANPOSITION
INDICATOR(PPI),whichisessentiallyapolardiagram,withthe
transmittingship’spositionatthecenter.Imagesoftargetechoesare
receivedanddisplayedateithertheirrelativeortruebearings,andattheir
distancesfromthePPIcenter.Withacontinuousdisplayoftheimagesofthe
targets,themotionofthetargetrelativetothemotionofthetransmittingship
is also displayed.
Thesecondaryfunctionoftheindicatoristoprovidethemeansfor
operating various controls of the radar system.
TheCATHODE-RAYTUBE(CRT),illustratedinfigure1.17,istheheart
oftheindicator.TheCRTfaceorscreen,whichiscoatedwithafilmof
phosphorescentmaterial,isthePPI.TheELECTRONGUNattheopposite
endofthetube(seefigure1.18)emitsaverynarrowbeamofelectrons
whichimpingesuponthecenterofthePPIunlessdeflectedbyelectrostatic
orelectromagneticmeans.SincetheinsidefaceofthePPIiscoatedwith
phosphorescentmaterial,asmallbrightspotisformedatthecenterofthe
PPI.
Iftheelectronbeamisrapidlyandrepeatedlydeflectedradiallyfromthe
center,abrightlinecalledaTRACEisformedonthePPI.Shouldtheflowof
electronsbestopped,thistracewillcontinuetoglowforashortperiod
followingthestoppageoftheelectronbeambecauseofthephosphorescent
coating.TheslowdecayofthebrightnessisknownasPERSISTENCE;the
slower the decay the higher the persistence.
Attheinstantthemodulatortriggersthetransmitter,itsendsaTIMING
TRIGGERsignaltotheindicator.Thelattersignalactstodeflectthe
electronbeamradiallyfromthecenteroftheCRTscreen(PPI)toforma
traceoftheradialmovementoftheelectronbeam.Thisradialmovementof
theelectronbeamiscalledtheSWEEPorTIMEBASE.Whiletheterms
traceandsweeparefrequentlyusedinterchangeably,thetermtraceis
descriptive only of the visible evidence of the sweep movement.
SincetheelectronbeamisdeflectedfromthecenteroftheCRTscreen
witheachpulseofthetransmitter,thesweepmustberepeatedveryrapidly
evenwhenthelowerpulserepetitionratesareused.Withapulserepetition
rateof750pulsespersecond,thesweepmustberepeated750timesper
second.Thus,itshouldbequiteobviouswhythesweepappearsasasolid
luminouslineonthePPI.Thespeedofmovementofthepointof
impingementoftheelectronbeamisfarinexcessofthecapabilityof
detection by the human eye.
WhilethesweepmustberepeatedinaccordancewiththePRR,theactual
rateofradialmovementoftheelectronbeamisgovernedbythesizeofthe
CRTscreenandthedistancerepresentedbytheradiusofthisscreen
accordingtotherangescalebeingused.Ifthe20-milerangescaleis
selected,theelectronbeammustbedeflectedradiallyfromthecenterofthe
CRTscreenhavingaparticularradiusataratecorrespondingtothetime
requiredforradio-frequencyenergytotraveltwicethedistanceoftherange
scaleor40nauticalmiles.Whenusingthe20-milerangescale,theelectron
beammustmoveradiallyfromthecenteroftheCRTscreentoitsperiphery
in 247 microseconds.
Speed of radio frequency - frequency energy =
0.161829 nm per microsecond
Distance = Speed X Time
40 nm÷ 0.161829 nm per microsecond =
247 microseconds
Theobjectiveofregulatingtherateoftraveloftheelectronbeaminthis
manneristoestablishatimebaseonthePPIwhichmaybeusedfordirect
measurementsofdistancestotargetswithoutfurtherneedtotakeinto
Figure 1.17 - Electromagnetic cathode-ray tube.
22
Figure 1.18 - The sweep on the plan position indicator.
23
accountthefactthatthetransmittedpulseanditsreflectedechomakea
roundtriptoandfromthetarget.WiththeperipheryofthePPIrepresenting
adistanceof20milesfromthecenterofthePPIatthe20-milerangescale
setting,thetimerequiredfortheelectronbeamtomoveradiallyfromthe
centertotheperipheryisthesameasthetimerequiredforthetransmitted
pulsetotraveltoatargetat20milesandreturntotheantennaasareflected
echoorthetimetotravel40milesinthiscase.Itfollowsthatapointonthe
sweeportimebasehalfwaybetweenthecenterofthePPIanditsperiphery
representsadistanceof10milesfromthecenterofthePPI.Theforegoing
assumesthattherateoftraveloftheelectronbeamisconstant,whichisthe
usual case in the design of indicators for navigational radar.
Iftheantennaistrainedonatargetat10mileswhileusingthe20-mile
rangescale,thetimeforthe20-mileroundtripofthetransmittedpulseand
thereturningechois123.5microseconds.At123.5microseconds,following
theinstantoftriggeringthetransmitterandsendingthetimingtriggerpulse
totheindicatortodeflecttheelectronbeamradially,theelectronbeamwill
havemovedadistanceof10milesinitssweeporonthetimebase.On
receivingtheechoat123.5microsecondsaftertheinstantofthepulse,the
receiversendsavideosignaltotheindicatorwhichinturnactstointensify
orbrightentheelectronbeamatthepointinitssweepat123.5
microseconds,i.e.,at10milesonthetimebase.Thisbrighteningofthetrace
producedbythesweepatthepointcorrespondingtothedistancetothe
targetinconjunctionwiththepersistenceofthePPIproducesavisibleimage
ofthetarget.Becauseofthepulserepetitionrate,thispaintingofanimage
onthePPIisrepeatedmanytimesinashortperiod,resultinginasteady
glow of the target image if the target is a reasonably good reflector.
Innavigationalandcollisionavoidanceapplicationsofradar,theantenna
andthebeamofr-fenergyradiatedfromitarerotatedataconstantrate,
usuallyabout10to20revolutionsperminuteinordertodetecttargetsall
aroundtheobserver’sship.Intheprecedingdiscussionofhowatargetimage
ispaintedonthePPI,referenceismadeonlytoradialdeflectionofthe
electronbeamtoproducethesweeportimebase.Iftargetimagesaretobe
paintedattheirrelativebearingsaswellasdistancesfromthecenterofthe
PPI,thesweepmustberotatedinsynchronizationwiththerotationofthe
antenna.Justastheelectronbeammaybedeflectedradiallybyelectrostatic
orelectromagneticmeans,thesweepmayberotatedbythesamemeans.The
sweep is usually rotated electromagnetically in modern radars.
Astheantennaisrotatedpasttheship’sheading,thesweep,in
synchronizationwiththeantenna,isrotatedpastthe0˚graduationonthe
relativebearingdialofthePPI.Theimageofanytargetdetectedaheadis
paintedonthePPIatitsrelativebearinganddistancefromthecenterof
thePPI.Astargetsaredetectedinotherdirections,theirimagesare
paintedonthePPIattheirrelativebearingsanddistancesfromthecenter
of the PPI.
Uptothispointthediscussionofhowtargetinformationisdisplayedon
thePPIhasbeenlimitedtohowthetargetimagesarepainted,virtually
instantaneously,attheirdistancesandrelativebearingsfromthereference
shipatthecenterofthePPI.Itfollowsthatthroughcontinuousdisplay
(continuousbecauseofthepersistenceoftheCRTscreenandthepulse
repetitionrate)ofthepositionsoftargetsonthePPI,theirmotionsrelativeto
the motion of the reference ship are also displayed.
Insummary,theindicatorofthisbasicradarsystemprovidesthemeans
formeasuringanddisplaying,inausefulform,therelativebearingsand
distancestotargetsfromwhichreflectedechoesmaybereceived.In
displayingthepositionsofthetargetsrelativetothereferenceship
continuously,themotionsofthetargetsrelativetothemotionofthe
reference ship are evident.
24
FACTORS AFFECTING DETECTION, DISPLAY, AND MEASUREMENT OF RADAR TARGETS
FACTORS AFFECTING MAXIMUM RANGE
Frequency
Thehigherthefrequencyofaradar(radio)wave,thegreateristhe
attenuation(lossinpower),regardlessofweather.Lowerradarfrequencies
(longerwavelengths)have,therefore,beengenerallysuperiorforlonger
detection ranges.
Peak Power
Thepeakpowerofaradarisitsusefulpower.Rangecapabilitiesofthe
radarincreasewithpeakpower.Doublingthepeakpowerincreasesthe
range capabilities by about 25 percent.
Pulse Length
Thelongerthepulselength,thegreateristherangecapabilityoftheradar
because of the greater amount of energy transmitted.
Pulse Repetition Rate
Thepulserepetitionrate(PRR)determinesthemaximummeasurable
rangeoftheradar.Ampletimemustbeallowedbetweenpulsesforanecho
toreturnfromanytargetlocatedwithinthemaximumworkablerangeofthe
system.Otherwise,echoesreturningfromthemoredistanttargetsare
blockedbysucceedingtransmittedpulses.Thisnecessarytimeinterval
determines the highest PRR that can be used.
ThePRRmustbehighenough,however,thatsufficientpulseshitthe
targetandenoughechoesarereturnedtotheradar.Themaximum
measurablerangecanbedeterminedapproximatelybydividing81,000by
thePRR.
Beam Width
Themoreconcentratedthebeam,thegreateristhedetectionrangeofthe
radar.
Target Characteristics
Targetsthatarelargecanbeseenonthescopeatgreaterranges,provided
line-of-sightexistsbetweentheradarantennaandthetarget.Conducting
materials(aship’ssteelhull,forexample)returnrelativelystrongechoes
whilenonconductingmaterials(awoodhullofafishingboat,forexample)
return much weaker echoes.
Receiver Sensitivity
Themoresensitivereceiversprovidegreaterdetectionrangesbutaremore
subject to jamming.
Antenna Rotation Rate
Themoreslowlytheantennarotates,thegreateristhedetectionrangeof
the radar.
ForaradarsethavingaPRRof1,000pulsespersecond,ahorizontal
beamwidthof2.0˚,andanantennarotationrateof6RPM(1revolutionin
10secondsor36scanningdegreespersecond),thereis1pulsetransmitted
each0.036˚ofrotation.Thereare56pulsestransmittedduringthetime
required for the antenna to rotate through its beam width.
Withanantennarotationrateof15RPM(1revolutionin4secondsor90
scanningdegreespersecond),thereisonly1pulsetransmittedeach0.090˚
ofrotation.Thereareonly22pulsestransmittedduringthetimerequiredfor
the antenna to rotate through its beam width.
Fromtheforegoingitisapparentthatatthehigherantennarotationrates,
themaximumrangesatwhichtargets,particularlysmalltargets,maybe
detected are reduced.
BeamWidth
DegreesperPulse
-----------------------------------------------------
2.0°
0.036°
----------------56pulses==
BeamWidth
DegreesperPulse
-----------------------------------------------------
2.0°
0.090°
----------------22==pulses
25
FACTORS AFFECTING MINIMUM RANGE
Pulse Length
Theminimumrangecapabilityofaradarisdeterminedprimarilybythe
pulselength.Itisequaltohalfthepulselengthoftheradar(164yardsper
microsecondofpulselength).Electronicconsiderationssuchastherecovery
timeofthereceiverandtheduplexer(TRandanti-TRtubesassembly)
extendtheminimumrangeatwhichatargetcanbedetectedbeyondthe
range determined by the pulse length.
Sea Return
Seareturnorechoesreceivedfromwavesmaycluttertheindicatorwithin
andbeyondtheminimumrangeestablishedbythepulselengthandrecovery
time.
Side-Lobe Echoes
Targetsdetectedbytheside-lobesoftheantennabeampatternarecalled
side-lobeechoes.Whenoperatingnearlandorlargetargets,side-lobeechoes
maycluttertheindicatorandpreventdetectionofclosetargets,without
regard to the direction in which the antenna is trained.
Vertical Beam Width
Smallsurfacetargetsmayescapetheloweredgeoftheverticalbeam
when close.
FACTORS AFFECTING RANGE ACCURACY
Therangeaccuracyofradardependsupontheexactnesswithwhichthe
timeintervalbetweentheinstantsoftransmittingapulseandreceivingthe
echo can be measured.
Fixed Error
Afixedrangeerroriscausedbythestartingofthesweepontheindicator
beforether-fenergyleavestheantenna.Thezeroreferenceforallrange
measurementsmustbetheleadingedgeofthetransmittedpulseasitappearson
theindicator.Inasmuchaspartofthetransmittedpulseleaksdirectlyintothe
receiverwithoutgoingtotheantenna,afixederrorresultsfromthetimerequired
forr-fenergytogouptotheantennaandreturntothereceiver.Thiserrorcauses
the indicated ranges to be greater than their true values.
Adevicecalledatriggerdelaycircuitisusedtoeliminatethefixederror.
Bythismeansthetriggerpulsetotheindicatorcanbedelayedasmall
amount.Suchadelayresultsinthesweepstartingattheinstantanecho
wouldreturntotheindicatorfromaflatplaterightattheantennanotatthe
instant that the pulse is generated in the transmitter.
Line Voltage
Accuracyofrangemeasurementdependsontheconstancyoftheline
voltagesuppliedtotheradarequipment.Ifsupplyvoltagevariesfromits
nominalvalue,rangesindicatedontheradarmaybeunreliable.This
fluctuationusuallyhappensonlymomentarily,however,andafterashort
wait ranges normally are accurate.
Frequency Drift
Errorsinrangingalsocanbecausedbyslightvariationsinthefrequency
oftheoscillatorusedtodividethesweep(timebase)intoequalrange
intervals.Ifsuchafrequencyerrorexists,therangesreadfromtheradar
generally are in error by some small percentage of the range.
Toreducerangeerrorscausedbyfrequencydrift,precisionoscillatorsin
radarsusuallyareplacedinaconstanttemperatureoven.Theovenisalways
heated,sothereisnodriftofrangeaccuracywhiletherestofthesetis
warming up.
Calibration
TherangetoatargetcanbemeasuredmostaccuratelyonthePPIwhen
theleadingedgeofitspipjusttouchesafixedrangering.Theaccuracyof
thismeasurementisdependentuponthemaximumrangeofthescaleinuse.
Representativemaximumerrorinthecalibrationofthefixedrangeringsis
75yardsor1
1
/
2
percentofthemaximumrangeoftherangescaleinuse,
whicheverisgreater.Withtheindicatorsetonthe6-milerangescale,the
errorintherangeofapipjusttouchingarangeringmaybeabout180yards
orabout0.1nauticalmilebecauseofcalibrationerroralonewhentherange
calibration is within acceptable limits.
OnsomePPI’s,rangecanonlybeestimatedbyreferencetothefixed
rangerings.Whenthepipliesbetweentherangerings,theestimateis
usuallyinerrorby2to3percentofthemaximumrangeoftherangescale
setting plus any error in the calibration of the range rings.
Radarindicatorsusuallyhaveavariablerangemarker(VRM)or
26
adjustablerangeringwhichisthenormalmeansforrangemeasurements.
WiththeVRMcalibratedwithrespecttothefixedrangeringswithina
toleranceof1percentofthemaximumrangeofthescaleinuse,rangesas
measuredbytheVRMmaybeinerrorbyasmuchas2
1
/
2
percentofthe
maximumrangeofthescaleinuse.Withtheindicatorsetonthe8-mile
rangescale,theerrorinarangeasmeasuredbytheVRMmaybeinerrorby
as much as 0.2 nautical mile.
Pip and VRM Alignment
TheaccuracyofmeasuringrangeswiththeVRMisdependentuponthe
abilityoftheradarobservertoaligntheVRMwiththeleadingedgeofthepipon
thePPI.OnthelongerrangescalesitismoredifficulttoaligntheVRMwiththe
pipbecausesmallchangesinthereadingoftheVRMrangecounterdonotresult
in appreciable changes in the position of the VRM on the PPI.
Range Scale
Thehigherrangescalesettingsresultinreducedaccuracyoffixedrange
ringandVRMmeasurementsbecauseofgreatercalibrationerrorsandthe
greaterdifficultyofpipandVRMalignmentassociatedwiththehigher
settings.
PPI Curvature
BecauseofthecurvatureofthePPI,particularlyintheareanearitsperiphery,
rangemeasurementsofpipsneartheedgeareoflesseraccuracythanthe
measurements nearer the center of the PPI.
Radarscope Interpretation
Relativelylargerangeerrorscanresultfromincorrectinterpretationofa
landmassimageonthePPI.Thedifficultyofradarscopeinterpretationcan
be reduced through more extensive use of height contours on charts.
Forreliableinterpretationitisessentialthattheradaroperatingcontrols
beadjustedproperly.Ifthereceivergainistoolow,featuresatornearthe
shoreline,whichwouldreflectechoesatahighergainsetting,willnot
appearaspartofthelandmassimage.Ifthereceivergainistoohigh,the
landmassimageonthePPIwill“bloom”.Withbloomingtheshorelinewill
appear closer than it actually is.
Afinefocusadjustmentisnecessarytoobtainasharplandmassimageon
the PPI.
Becauseofthevariousfactorsintroducingerrorsinradarrange
measurements,oneshouldnotexpecttheaccuracyofnavigationalradarto
be better than + or - 50 yards under the best conditions.
FACTORS AFFECTING RANGE RESOLUTION
Rangeresolutionisameasureofthecapabilityofaradartodisplayas
separatepipstheechoesreceivedfromtwotargetswhichareonthesame
bearing and are close together.
Theprincipalfactorsthataffecttherangeresolutionofaradararethe
lengthofthetransmittedpulse,receivergain,CRTspotsize,andtherange
scale.Ahighdegreeofrangeresolutionrequiresashortpulse,lowreceiver
gain, and a short range scale.
Pulse Length
Twotargetsonthesamebearing,closetogether,cannotbeseenastwo
distinctpipsonthePPIunlesstheyareseparatedbyadistancegreaterthan
one-halfthepulselength(164yardspermicrosecondofpulselength).Ifa
radarhasapulselengthof1-microsecondduration,thetargetswouldhaveto
beseparatedbymorethan164yardsbeforetheywouldappearastwopips
on the PPI.
Radio-frequencyenergytravelsthroughspaceattherateofapproximately
328yardspermicrosecond.Thus,theendofa1-microsecondpulsetraveling
throughtheairis328yardsbehindtheleadingedge,orstart,ofthepulse.If
a1-microsecondpulseissenttowardtwoobjectsonthesamebearing,
separatedby164yards,theleadingedgeoftheechofromthedistanttarget
coincidesinspacewiththetrailingedgeoftheechofromtheneartarget.As
aresulttheechoesfromthetwoobjectsblendintoasinglepip,andrange
canbemeasuredonlytothenearestobject.Thereasonforthisblendingis
illustrated in figure 1.19.
InpartAoffigure1.19,thetransmittedpulseisjuststrikingthenear
target.PartBshowsenergybeingreflectedfromtheneartarget,whilethe
leadingedgeofthetransmittedpulsecontinuestowardthefartarget.Inpart
C,
1
/
2
microsecondlater,thetransmittedpulseisjuststrikingthefartarget;
theechofromtheneartargethastraveled164yardsbacktowardtheantenna.
Thereflectionprocessattheneartargetisonlyhalfcompleted.InpartD
echoesaretravelingbacktowardtheantennafrombothtargets.InpartE
reflectioniscompletedattheneartarget.Atthistimetheleadingedgeofthe
echofromthefartargetcoincideswiththetrailingedgeofthefirstecho.
Whentheechoesreachtheantenna,energyisdeliveredtothesetduringa
period of 2 microseconds so that a single pip appears on the PPI.
27
Figure 1.19 - Pulse length and range resolution.
28
Thedatabelowindicatestheminimumseparationinrangefortwotargets
to appear as separate echoes on the PPI for various pulse lengths.
Receiver Gain
Rangeresolutioncanbeimprovedbyproperadjustmentofthereceiver
gaincontrol.Asillustratedinfigure1.20,theechoesfromtwotargetsonthe
samebearingmayappearasasinglepiponthePPIifthereceivergain
settingistoohigh.Withreductioninthereceivergainsetting,theechoes
may appear as separate pips on the PPI.
CRT Spot Size
Therangeseparationrequiredforresolutionisincreasedbecausethespot
formedbytheelectronbeamonthescreenoftheCRTcannotbefocused
intoapointoflight.Theincreaseinechoimage(pip)lengthandwidthvaries
with the size of the CRT and the range scale in use.
Onthelongerrangescales,theincreaseinechosizebecauseofspotsize
is appreciable.
Range Scale
Thepipsoftwotargetsseparatedbyafewhundredyardsmaymergeonthe
PPIwhenoneofthelongerrangescalesisused.Theuseoftheshortestrange
scalepossibleandproperadjustmentofthereceivergainmayenabletheir
detectionasseparatetargets.Ifthedisplaycanbeoff-centered,thismaypermit
display of the targets on a shorter range scale than would be possible otherwise.
Pulse Length
(microseconds)
Range Resolution
(yards)
0.058
0.1016
0.2033
0.2541
0.582
1.2197
Figure 1.20 - Receiver gain and range resolution.
CRT Diameter
(Inches)
Range Scale
(nautical mi.)
Spot Length or Width
(yards)
NominalEffective
97.50.55
24220
1210.50.54
24185
1614.40.53
24134
29
FACTORS AFFECTING BEARING ACCURACY
Horizontal Beam Width
Bearingmeasurementscanbemademoreaccuratelywiththenarrower
horizontalbeamwidths.Thenarrowerbeamwidthsaffordbetterdefinition
ofthetargetand,thus,moreaccurateidentificationofthecenterofthetarget.
Severaltargetsclosetogethermayreturnechoeswhichproducepipsonthe
PPIwhichmerge,thuspreventingaccuratedeterminationofthebearingofa
single target within the group.
Theeffectivebeamwidthcanbereducedthroughloweringthereceiver
gainsetting.Inreducingthesensitivityofthereceiver,themaximum
detectionrangeisreduced,butthenarrowereffectivebeamwidthprovides
better bearing accuracy.
Target Size
Foraspecificbeamwidth,bearingmeasurementsofsmalltargetsare
moreaccuratethanlargetargetsbecausethecentersofthesmallerpipsof
the small targets can be identified more accurately.
Target Rate of Movement
Thebearingsofstationaryorslowlymovingtargetscanbemeasured
more accurately than the bearings of faster moving targets.
Stabilization of Display
StabilizedPPIdisplaysprovidehigherbearingaccuraciesthan
unstabilized displays because they are not affected by yawing of the ship.
Sweep Centering Error
IftheoriginofthesweepisnotaccuratelycenteredonthePPI,bearing
measurementswillbeinerror.Greaterbearingerrorsareincurredwhenthe
pipisnearthecenterofthePPIthanwhenthepipisneartheedgeofthePPI.
Sincethereisnormallysomecenteringerror,moreaccuratebearing
measurementscanbemadebychangingtherangescaletoshiftthepip
position away from the center of the PPI.
Parallax Error
Improperuseofthemechanicalbearingcursorwillintroducebearing
errors.Onsettingthecursortobisectthepip,thecursorshouldbeviewed
fromapositiondirectlyinfrontofit.Electronicbearingcursorsusedwith
somestabilizeddisplaysprovidemoreaccuratebearingmeasurementsthan
mechanicalbearingcursorsbecausemeasurementsmadewiththeelectronic
cursor are not affected by parallax or centering errors.
Heading Flash Alignment
Foraccuratebearingmeasurements,thealignmentoftheheadingflash
withthePPIdisplaymustbesuchthatradarbearingsareincloseagreement
withrelativelyaccuratevisualbearingsobservedfromneartheradar
antenna.
FACTORS AFFECTING BEARING RESOLUTION
Bearingresolutionisameasureofthecapabilityofaradartodisplayas
separatepipstheechoesreceivedfromtwotargetswhichareatthesame
range and are close together.
Theprincipalfactorsthataffectthebearingresolutionofaradarare
horizontal beam width, the range to the targets, and CRT spot size.
Horizontal Beam Width
Astheradarbeamisrotated,thepaintingofapiponthePPIbeginsas
soonastheleadingedgeoftheradarbeamstrikesthetarget.Thepaintingof
thepipiscontinueduntilthetrailingedgeofthebeamisrotatedbeyondthe
target.Therefore,thepipisdistortedangularlybyanamountequaltothe
effective horizontal beam width.
30
Asillustratedinfigure1.21,inwhichahorizontalbeamwidthof10˚is
usedforgraphicalclarityonly,theactualbearingofasmalltargethaving
goodreflectingpropertiesis090˚,butthepipaspaintedonthePPIextends
from095˚to085˚.Theleft5˚andtheright5˚arepaintedwhiletheantenna
isnotpointeddirectlytowardsthetarget.Thebearingmustbereadatthe
center of the pip.
Range of Targets
Assumingamorerepresentativehorizontalbeamwidthof2˚,thepipofa
ship400feetlongobservedbeamonatadistanceof10nauticalmilesona
bearingof090˚wouldbepaintedonthePPIbetween091.2˚and088.8˚,the
actualangularwidthofthetargetbeing0.4˚.Thepipofaship900feetlong
observedbeamonatthesamedistanceandbearingwouldbepaintedonthe
PPIbetween091.4˚and088.6˚,theangularwidthofthetargetbeing0.8˚.
Sincetheangularwidthsofthepipspaintedforthe400and900-foottargets
are1.4˚and1.8˚,respectively,anyattempttoestimatetargetsizebythe
angular width of the pip is not practical, generally.
SincethepipofasingletargetaspaintedonthePPIiselongated
angularlyanamountequaltobeamwidth,twotargetsatthesamerangemust
beseparatedbymorethanonebeamwidthtoappearasseparatepips.The
requireddistanceseparationdependsuponrange.Assuminga2˚beam
width,targetsat10milesmustbeseparatedbyover0.35nauticalmilesor
700yardstoappearasseparatepipsonthePPI.At5milesthetargetsmust
beseparatedbyover350yardstoappearasseparatepipsifthebeamwidth
is 2˚.
Figure1.22illustratesacaseinwhichechoesarebeingreceivedfromfour
targets,butonlythreepipsarepaintedonthePPI.TargetsAandBare
paintedasasinglepipbecausetheyarenotseparatedbymorethanonebeam
width;targetsCandDarepaintedasseparatepipsbecausetheyare
separated by more than one beam width.
Inasmuchasbearingresolutionisdeterminedprimarilybyhorizontal
beamwidth,aradarwithanarrowhorizontalbeamwidthprovidesbetter
bearing resolution than one with a wide beam.
Figure 1.21 - Angular distortion.
Figure 1.22 - Bearing resolution.
31
CRT Spot Size
Thebearingseparationrequiredforresolutionisincreasedbecausethe
spotformedbytheelectronbeamonthescreenoftheCRTcannotbe
focusedintoapointoflight.TheincreaseinthepipwidthbecauseofCRT
spot size varies with the size of the CRT and the range scale in use.
WAVELENGTH
Generally,radarstransmittingattheshorterwavelengthsaremoresubject
to the effects of weather than radars transmitting at the longer wavelengths.
Figure1.23illustratesthePPIdisplaysoftworadarsofdifferent
wavelengthsaboardashipsteaminginarainsquallandachoppysea.
Withoutuseofanti-rainandanti-seacluttercontrols,theclutterismore
massiveonthePPIoftheradarhavingtheshorterwavelength.Also,three
targets,whichcanbedetectedonthePPIoftheradarhavingthelonger
wavelength,cannotbedetectedonthePPIoftheradarhavingtheshorter
wavelength.Followinguseoftheanti-rainandanti-seacluttercontrols,the
threetargetsstillcannotbedetectedonthePPIoftheradarhavingthe
shorterwavelengthbecausetoomuchoftheenergyhasbeenabsorbedor
attenuated by the rain.
Similarly,figure1.24illustratesdetectionofclosetargetsbyaradar
havingarelativelylongwavelengthandnodetectionofthesetargetsbya
radar having a relatively short wavelength.
32
Twoidentical8milerangePPIpicturestakenonRaytheon3cm.and10cm.radarsinarainsquallandwithachoppysea.Threeshipsbearing
225˚, 294˚ and 330˚ shown on the 10 cm. radar right are not shown on the 3 cm. radar left.
Onbothradarstheanti-rainandanti-seaclutterdevicesareswitchedin.Thethreeshipsareclearlyvisibleonthe10cm.radarright.Thereareno
targets visible on the 3 cm. radar left as the echo power has been absorbed by rain.
Reproduced by Courtesy of the Raytheon Company.
Figure 1.23- Effects of rain and sea on PPI displays of radars having different wavelengths.
33
Twoidentical20milerangePPIpicturestakenonRaytheon3cm.and10cm.radarsshowingtheeffectsofseaclutter.Onthe10cm.radarright
targets inside the 5 mile range marker are clearly visible. On the 3 cm. radar left the close range targets are missing.
Onbothradarstheanti-seacluttercontrolhasbeencarefullyadjustedtoremoveseaclutter.Thecloserangetargetsareclearlyvisibleonthe10
cm. right, whereas they are missing on the 3 cm. radar left.
Reproduced by Courtesy of the Raytheon Company.
Figure 1.24 - Effects of sea on PPI displays of radars having different wavelengths.
34
TARGET CHARACTERISTICS
Thereareseveraltargetcharacteristicswhichwillenableonetargettobe
detectedatagreaterrangethananother,orforonetargettoproducea
stronger echo than another target of similar size.
Height
Sinceradarwavepropagationisalmostlineofsight,theheightofthe
targetisofprimeimportance.Ifthetargetdoesnotriseabovetheradar
horizon,theradarbeamcannotbereflectedfromthetarget.Becauseofthe
interference pattern, the target must rise somewhat above the radar horizon.
Size
Uptocertainlimits,targetshavinglargerreflectingareaswillreturn
strongerechoesthantargetshavingsmallerreflectingareas.Shouldatarget
bewiderthanthehorizontalbeamwidth,thestrengthoftheechoeswillnot
beincreasedonaccountofthegreaterwidthofthetargetbecausethearea
notexposedtotheradarbeamatanyinstantcannot,ofcourse,reflectan
echo.Sincetheverticaldimensionsofmosttargetsaresmallcomparedtothe
verticalbeamwidthofmarinenavigationalradars,thebeamwidthlimitation
isnotnormallyapplicabletotheverticaldimensions.However,thereisa
verticaldimensionlimitationinthecaseofslopingsurfacesorstepped
surfaces.Inthiscase,onlytheprojectedverticalarealyingwithinthe
distance equivalent of the pulse length can return echoes at any instant.
Aspect
Theaspectofatargetisitsorientationtotheaxisoftheradarbeam.With
changeinaspect,theeffectivereflectingareamaychange,dependingupon
theshapeofthetarget.Thenearertheanglebetweenthereflectingareaand
thebeamaxisisto90˚,thegreateristhestrengthoftheechoreturnedtothe
antenna.
Shape
Targetsofidenticalshapemaygiveechoesofvaryingstrength,depending
onaspect.Thusaflatsurfaceatrightanglestotheradarbeam,suchasthe
sideofasteelshiporasteepcliffalongtheshore,willreflectverystrong
echoes.Astheaspectchanges,thisflatsurfacewilltendtoreflectmoreof
theenergyofthebeamawayfromtheantenna,andmaygiveratherweak
echoes.Aconcavesurfacewilltendtofocustheradarbeambacktothe
antennawhileaconvexsurfacewilltendtoscattertheenergy.Asmooth
conicalsurfacewillnotreflectenergybacktotheantenna.However,echoes
may be reflected to the antenna if the conical surface is rough.
Texture
Thetextureofthetargetmaymodifytheeffectsofshapeandaspect.A
smoothtexturetendstoincreasethereflectionqualities,andwillincreasethe
strengthofthereflection,butunlesstheaspectandshapeofthetargetare
suchthatthereflectionisfocuseddirectlybacktotheantenna,thesmooth
surfacewillgiveapoorradarechobecausemostoftheenergyisreflectedin
anotherdirection.Ontheotherhand,aroughsurfacewilltendtobreakup
thereflection,andwillimprovethestrengthofechoesreturnedfromthose
targets whose shape and aspect normally give weak echoes.
Composition
Theabilityofvarioussubstancestoreflectradarpulsesdependsonthe
intrinsicelectricalpropertiesofthosesubstances.Thusmetalandwaterare
goodreflectors.Iceisafairreflector,dependingonaspect.Landareasvary
intheirreflectionqualitiesdependingontheamountandtypeofvegetation
andtherockandmineralcontent.Woodandfiberglassboatsarepoor
reflectors.Itmustberememberedthatallofthecharacteristicsinteractwith
eachothertodeterminethestrengthoftheradarecho,andnofactorcanbe
singled out without considering the effects of the others.
35
CHAPTER 2 — RADAR OPERATION
RELATIVE AND TRUE MOTION DISPLAYS
GENERAL
Therearetwobasicdisplaysusedtoportraytargetpositionandmotionon
thePPI’sofnavigationalradars.Therelativemotiondisplayportraysthe
motionofatargetrelativetothemotionoftheobservingship.Thetrue
motiondisplayportraystheactualortruemotionsofthetargetandthe
observing ship.
DependinguponthetypeofPPIdisplayused,navigationalradarsare
classifiedaseitherrelativemotionortruemotionradars.However,true
motionradarscanbeoperatedwitharelativemotiondisplay.Infact,radars
classifiedastruemotionradarsmustbeoperatedintheirrelativemotion
modeatthelongerrangescalesettings.Someradarsclassifiedasrelative
motionradarsarefittedwithspecialadaptersenablingoperationwithatrue
motiondisplay.Theseradarsdonothavecertainfeaturesnormally
associated with true motion radars, such as high persistence CRT screens.
RELATIVE MOTION RADAR
Throughcontinuousdisplayoftargetpipsattheirmeasuredrangesand
bearingsfromafixedpositionofownshiponthePPI,relativemotionradar
displaysthemotionofatargetrelativetothemotionoftheobserving(own)
ship.Withownshipandthetargetinmotion,thesuccessivepipsofthetarget
donotindicatetheactualortruemovementofthetarget.Agraphical
solutionisrequiredinordertodeterminetherateanddirectionoftheactual
movement of the target.
Ifownshipisinmotion,thepipsoffixedobjects,suchaslandmasses,
moveonthePPIatarateequaltoandinadirectionoppositetothemotionof
ownship.Ifownshipisstoppedormotionless,targetpipsmoveonthePPI
in accordance with their true motion.
36
Orientations of Relative Motion Display
Therearetwobasicorientationsusedforthedisplayofrelativemotionon
PPI’s.IntheHEADING-UPWARDdisplay,thetargetpipsarepaintedat
theirmeasureddistancesindirectionrelativetoownship’sheading.Inthe
NORTH-UPWARDdisplay,targetpipsarepaintedattheirmeasured
distancesintruedirectionsfromownship,northbeingupwardoratthetop
of the PPI.
Infigure2.1ownshiponaheadingof270˚detectsatargetbearing315˚
true.Thetargetpipispainted045˚relativetoship’sheadingonthis
Heading-Upwarddisplay.Infigure2.2thesametargetispaintedat315˚true
onaNorth-Upwarddisplay.Whilethetargetpipispainted045˚relativeto
theheadingflashoneachdisplay,theHeading-Upwarddisplayprovidesa
more immediate indication as to whether the target lies to port or starboard.
Stabilization
TheNorth-Upwarddisplayinwhichtheorientationofthedisplayisfixed
toanunchangingreference(north)iscalledaSTABILIZEDdisplay.The
Heading-Upwarddisplayinwhichtheorientationchangeswithchangesin
ownship’sheadingiscalledanUNSTABILIZEDdisplay.Someradar
indicatordesignshavedisplayswhicharebothstabilizedandHeading-
Upward.Inthesedisplays,thecathode-raytubesmustberotatedasownship
changesheadinginordertomaintainship’sheadingupwardoratthetopof
the PPI.
Figure 2.1 - Unstabilized Heading-Upward display.
Figure 2.2 - Stabilized North-Upward display.
37
TRUE MOTION RADAR
Truemotionradardisplaysownshipandmovingobjectsintheirtrue
motion.Unlikerelativemotionradar,ownship’spositionisnotfixedonthe
PPI.OwnshipandothermovingobjectsmoveonthePPIinaccordancewith
theirtruecoursesandspeeds.Alsounlikerelativemotionradar,fixedobjects
suchaslandmassesarestationary,ornearlyso,onthePPI.Thus,one
observes own ship and other ships moving with respect to landmasses.
Truemotionisdisplayedonmodernindicatorsthroughtheuseofa
microprocessorcomputingtargettruemotionratherthandependingonan
extremely long persistence phosphor to leave “trails”.
Stabilization
Usually,thetruemotionradardisplayisstabilizedwithNorth-Upward.
Withthisstabilization,thedisplayissimilartoaplotonthenavigational
chart.OnsomemodelsthedisplayorientationisHeading-Upward.Because
thetruemotiondisplaymustbestabilizedtoanunchangingreference,the
cathode-ray tube must be rotated to place the heading at the top or upward.
Radarscope Persistence and Echo Trails
Highpersistenceradarscopesareusedtoobtainmaximumbenefitfrom
thetruemotiondisplay.Astheradarimagesofthetargetsarepainted
successivelybytherotatingsweeponthehighpersistencescope,theimages
continuetoglowforarelativelylongerperiodthantheimagesonother
scopesoflesserpersistence.Dependingupontheratesofmovement,range
scale,anddegreeofpersistence,thisafterglowmayleaveavisibleechotrail
ortailindicatingthetruemotionofeachtarget.Iftheafterglowofthe
movingsweeporiginleavesavisibletrailindicatingthetruemotionofown
ship,estimatesofthetruespeedsoftheradartargetscanbemadeby
comparingthelengthsoftheirechotrailsortailswiththatofownship.
Becauseoftherequirementforresettingownship’spositiononthePPI,
there is a practical limit to the degree of persistence (see figure 2.3).
Reset Requirements and Methods
BecauseownshiptravelsacrossthePPI,thepositionofownshipmustbe
resetperiodically.Dependingupondesign,ownship’spositionmaybereset
manually,automatically,orbymanuallyoverridinganyautomaticmethod.
Usually,thedesignincludesasignal(buzzerorindicatorlight)towarnthe
observer when resetting is required.
AdesignmayincludeNorth-SouthandEast-Westresetcontrolstoenable
theobservertoplaceownship’spositionatthemostsuitableplaceonthe
PPI.Otherdesignsmaybemorelimitedastowhereownship’spositioncan
beresetonthePPI,beinglimitedtoapointfromwhichtheheadingflash
passes through the center of the PPI.
Theradarobservermustbealertwithrespecttoresetrequirements.To
avoideitheramanualorautomaticresetatthemostinopportunetime,the
radarobservershouldincludeinhisevaluationofthesituationa
determination of the best time to reset own ship’s position.
Figure 2.3 - True motion display.
38
RangesettingexamplesforRadiomarinetruemotionradarsetshaving
double stabilization are as follows:
Maximumviewingtimesbetweenautomaticresetsinthetruemotion
mode are as follows:
Theviewingtimeaheadcanbeextendedbymanuallyoverridingthe
automatic reset feature.
Modes of Operation
Truemotionradarscanbeoperatedwitheithertruemotionorrelative
motiondisplays,withtruemotionoperationbeinglimitedtotheshortand
intermediate range settings.
Intherelativemotionmode,thesweeporigincanbeoff-centeredto
extendtheviewahead.Withtheviewaheadextended,requirementsfor
changingtherangescalearereduced.Also,theoff-centerpositionofthe
fixedsweeporigincanpermitobservationofaradartargetonashorterrange
scalethanwouldbethecasewiththesweeporiginfixedatthecenterofthe
PPI.
Throughuseoftheshorterrangescale,therelativemotionoftheradar
target is more clearly indicated.
Types of True Motion Display
Whilefixedobjectssuchaslandmassesarestationary,ornearlyso,on
truemotiondisplays,fixedobjectswillbestationaryonthePPIonlyifthere
isnocurrentorifthesetanddriftarecompensatedforbycontrolsforthis
purpose.Dependentuponsetdesign,currentcompensationmaybeeffected
through set and drift controls or by speed and course-made-good controls.
Whenusingtruemotionradarprimarilyforcollisionavoidancepurposes,
thesea-stabilizeddisplayispreferredgenerally.Thelattertypeofdisplay
differsfromtheground-stabilizeddisplayonlyinthatthereisno
compensationforcurrent.Assumingthatownshipandaradarcontactare
affectedbythesamecurrent,thesea-stabilizeddisplayindicatestruecourses
andspeedsthroughthewater.Ifownshiphasleewayorisbeingaffectedby
current,theechoesofstationaryobjectswillmoveonthesea-stabilized
display.Smallechotrailswillbeformedinadirectionoppositetotheleeway
orset.Iftheechofromasmallrockappearstomoveduenorthat2knots,
thentheshipisbeingsetduesouthat2knots.Theusableafterglowofthe
CRTscreen,whichlastsfromabout1
1
/
2
to3minutes,determinesthe
minimumrateofmovementwhichcanbedetectedonthedisplay.The
minimumrateofmovementhasbeenfoundtobeabout1
1
/
2
knotsonthe6-
mile range scale and proportional on other scales.
Theground-stabilizeddisplayprovidesthemeansforstoppingthesmall
movementsoftheechoesfromstationaryobjects.Thisdisplaymaybeused
toobtainaclearerPPIpresentationortodetermineleewayortheeffectsof
current on own ship.
Intheground-stabilizeddisplayownshipmovesonthedisplayin
accordancewithitscourseandspeedovertheground.Thus,themovements
oftargetechoesonthedisplayindicatethetruecoursesandspeedsofthe
targets over the ground. Ground-stabilization is effected as follows:
(1)Thespeedcontrolisadjustedtoeliminateanymovementsofthe
echoesfromstationarytargetsdeadaheadordeadastern.Ifthe
echoesfromstationarytargetsdeadaheadaremovingtowardsown
ship,thespeedsettingisincreased;otherwisethespeedsettingis
decreased.
(2)Thecourse-made-goodcontrolisadjustedtoeliminateany
remainingmovementatrightanglestoownship’sheading.The
course-made-goodcontrolshouldbeadjustedinadirectioncounter
to the echo movement.
Therefore,bytrialanderrorprocedures,thedisplaycanbeground-
stabilizedrapidly.However,thedisplayshouldbeconsideredonlyasan
approximationofthecourseandspeedmadegoodovertheground.Among
otherfactors,theaccuracyoftheground-stabilizationisdependentuponthe
minimumamountofmovementwhichcanbedetectedonthedisplay.Small
errorsinspeedandcompasscourseinputsandothereffectsassociatedwith
anyradarsetmaycausesmallfalsemovementstoappearonthetruemotion
display.Theinformationdisplayedshouldbeinterpretedwithdueregardto
thesefactors.Duringaturnwhencompasserrorswillbegreaterandwhen
speedestimationismoredifficult,theradarobservershouldrecognizethat
the accuracy of the ground stabilization may be degraded appreciably.
Thevaryingeffectsofcurrent,wind,andotherfactorsmakeitunlikelythat
thedisplaywillremaingroundstabilizedforlongperiods.Consequently,the
displaymustbereadjustedperiodically.Suchreadjustmentsshouldbecarried
out only when they do not detract from the primary duties of the radar observer.
Whileinriversorestuaries,theonlydetectablemovementmaybethe
movementalongownship’sheading.Themovementsofechoesof
stationaryobjectsatrightanglestoownship’sheadingareusuallysmall
inthesecircumstances.Thus,inriversandestuariesadjustmentofthe
speedcontrolistheonlyadjustmentnormallyrequiredtoobtainground
stabilization of reasonable accuracy in these confined waters.
T
ype CRM-NID-75 (3.2cm) and T
ype CRM-N2D-30 (10cm)
Truemotionrangesettings1,2,6,
and 16 miles
Relativemotionrangesettings
1
/
2
, 1, 2, 6, 16, and 40 miles
Speed
(knots)
Range setting
(miles)
Initial view
ahead (miles)
Viewing time
(minutes)
20162666
1269.7541
823.2524
811.616
39
PLOTTING AND MEASUREMENTS ON PPI
THE REFLECTION PLOTTER
Thereflectionplotterisaradarscopeattachmentwhichenablesplottingof
positionandmotionofradartargetswithgreaterfacilityandaccuracyby
reductionoftheeffectofparallax(apparentdisplacementofanobjectdueto
observer’sposition).Thereflectionplotterisdesignedsothatanymarkmade
onitsplottingsurfaceisreflectedtoapointdirectlybelowonthePPI.
Hence,toplottheinstantaneouspositionofatarget,itisonlynecessaryto
makeagreasepencilmarksothatitsimagereflectedontothePPIjust
touches the inside edge of the pip.
Theplottershouldnotbemarkedwhenthedisplayisviewedataverylow
angle.Preferably,theobserver’seyepositionshouldbedirectlyoverthe
center of the PPI.
Basic Reflection Plotter Designs
Thereflectionplotteronamajorityofmarineradarsystemscurrently
offered use a flat plotting surface.
Thereflectionplottersillustratedinfigures2.4and2.5aredesignsthat
werepreviouslyusedaboardmanynavyandmerchantshipsandmaystillbe
inuse.Thecurvatureoftheplottingsurfaceasillustratedinfigure2.4
matches,butisoppositetothecurvatureofthescreenofthecathode-ray
tube,i.e.,theplottingsurfaceisconcavetotheobserver.Asemi-reflecting
mirrorisinstalledhalfwaybetweenthePPIandplottingsurface.The
plottingsurfaceisedge-lighted.Withoutthislightingthereflectionsofthe
grease pencil marks do not appear on the PPI.
Marking the Reflection Plotter
Themodernflatplottingsurfaceusesamirrorwhichmakesthemark
appearon,notabove,thesurfaceoftheoscilloscopeasdepictedinfigure
2.5.
Inmarkingtheolderflatplottershowninfigure2.5,thegreasepencil
isplacedoverthepipandthepointispressedagainsttheplottingsurface
withsufficientpressurethatthereflectedimageofthegreasepencilpoint
isseenonthePPIbelow.Thepointofthepencilisadjustedtofindthe
moreprecisepositionforthemarkorplot(atthecenterandleadingedge
oftheradarpip).Withthemoreprecisepositionfortheplotsofound,the
greasepencilpointispressedharderagainsttheplottingsurfacetoleave
a plot in the form of a small dot.
Inmarkingtheplottingsurfaceoftheconcaveglassplotters,thepoint
ofthegreasepencilisoffsetfromthepositionofthepip.Notingthe
positionofthereflectionofthegreasepencilpointonthePPI,alineis
drawnrapidlythroughthemiddleoftheleadingedgeoftheradarpip.A
secondsuchlineisdrawnrapidlytoforman“X”,whichistheplotted
positionoftheradartarget.Someskillisrequiredtoformthe
intersection at the desired point.
Cleanliness
Theplottingsurfaceofthereflectionplottershouldbecleaned
frequentlyandjudiciouslytoinsurethatpreviousmarkingsdonot
obscurenewradartargets,whichcouldappearundetectedbythe
observerotherwise.Acleaningagentwhichdoesnotleaveafilmresidue
shouldbeused.Anyoilyfilmwhichisleftbyanundesirablecleaning
agentorbythesmearofincompletelywipedgreasepencilmarkings
makestheplottingsurfacedifficulttomark.Aweaksolutionofammonia
andwaterisaneffectivecleaningagent.Duringplotting,aclean,softrag
should be used to wipe the plotting surface.
PLOTTING ON STABILIZED AND UNSTABILIZED
DISPLAYS
Stabilized North-Upward Display
Assumingthenormalconditioninwhichthestartofthesweepisat
thecenterofthePPI,thepipsofradartargetsarepaintedonthePPIat
theirtruebearingsatdistancesfromthePPIcentercorrespondingto
targetranges.BecauseofthepersistenceofthePPIandthenormally
continuousrotationoftheradarbeam,thepipsoftargetshaving
reasonablygoodreflectingpropertiesappearcontinuouslyonthePPI.As
targetsmoverelativetothemotionofownship,thepips,aspainted
successively,moveinthedirectionofthismotion.Withlapseoftime,the
pipspaintedearlierfadefromthePPI.Thus,itisnecessarytorecordthe
positionsofthepipsthroughplottingtopermitanalysisofthisradar
data.Failuretoplotthesuccessivepositionsofthepipsisconduciveto
the much publicizedRADAR ASSISTED COLLISION.
Throughperiodicallymarkingthepositionsofthepips,eitheronthe
glassplate(implosioncover)overtheCRTscreenorthereflection
plottermountedthereon,avisualindicationofthepastandpresent
positionsofthetargetsismadeavailablefortherequiredanalysis.This
analysisisaidedbytheHEADINGFLASH(HEADINGMARKER)
which is a luminous line of the PPI indicating ship’s heading.
40
Figure 2.4 - Reflection plotter having curved plotting surface.
41
Figure 2.5 - Reflection plotter having flat plotting surface.
42
Unstabilized Heading-Upward Display
PlottingontheunstabilizedHeading-Upwarddisplayissimilarto
plottingonthestabilizedNorth-Upwarddisplay.Sincethepipsare
paintedatbearingsrelativetotheheadingoftheobserver’sship,a
complicationariseswhentheheadingoftheobserver’sshipischanged.
Ifacontinuousgreasepencilplotistobemaintainedontheunstabilized
Heading-Upwardrelativemotiondisplayfollowingcoursechangesby
theobserver’sship,theplottingsurfaceofthereflectionplottermustbe
rotatedthesamenumberofdegreesasthecourseorheadingchangeina
directionoppositetothischange.Otherwise,theportionoftheplotmade
followingthecoursechangewillnotbecontinuouswiththeprevious
portionoftheplot.Alsotheunstabilizeddisplayisaffectedbyany
yawingoftheobserver’sship.Plotsmadewhiletheshipisoffthedesired
headingwillresultinanerraticplotoraplotoflesseraccuracythan
wouldbeaffordedbyastabilizeddisplay.Undersevereyawing
conditions,plottingontheunstabilizeddisplaymustbecoordinatedwith
theinstantsthattheshipisoncourseifanyreasonableaccuracyofthe
plot is to be obtained.
BecauseofthepersistenceoftheCRTscreenandtheilluminationof
thepipsattheirinstantaneousrelativebearings,astheobserver’sship
yaws or its course is changed the target pips on the PPI will smear.
Figure2.6illustratesanunstabilizedHeading-Upwardrelativemotion
displayforasituationinwhichaship’scourseandpresentheadingare
280˚,asindicatedbytheheadingflash.Theshipisyawingabouta
headingof280˚.Inthiscasethereisslightsmearingofthetargetpips.If
theship’scourseischangedtotherightto340˚asillustratedinfigure
2.7,thetargetpipssmeartotheleftthrough60˚,i.e.,anamountequal
andinadirectionoppositetothecoursechange.Thus,tomaintaina
continuousgreasepencilplotonthereflectionplotteritisnecessarythat
theplottingsurfaceofthisplotterberotatedinadirectionoppositeto
and equal to the course change.
Figure 2.6 - Effect of yawing on unstabilized display.
Figure 2.7 - Effect of course change on unstabilized display.
43
Figures2.8and2.9illustratethesamesituationappearingona
stabilizedNorth-Upwarddisplay.Thereisnopipsmearingbecauseof
yawing.Thereisnoshiftinginthepositionsofthetargetpipsbecauseof
thecoursechange.Anychangesinthepositionofthetargetpipsaredue
solelytochangesinthetruebearingsanddistancestothetargetsduring
thecoursechange.Theplotduringandfollowingthecoursechangeis
continuouswiththeplotprecedingthecoursechange.Thus,thereisno
needtorotatetheplottingsurfaceofthereflectionplotterwhenthe
display is stabilized.
Figure 2.8 - Stabilized display following course change.
Figure 2.9 - Stabilized display preceding course change.
44
RANGE AND BEARING MEASUREMENT
Mechanical Bearing Cursor
Themechanicalbearingcursorisaradiallineorcrosshairinscribed
onatransparentdiskwhichcanberotatedmanuallyaboutitsaxis
coincidentwiththecenterofthePPI.Thiscursorisusedforbearing
determination.Frequently,thediskisinscribedwithaseriesoflines
paralleltothelineinscribedthroughthecenterofthedisk,inwhich
casethebearingcursorisknownasaPARALLEL-LINECURSORor
PARALLELINDEX(seefigure2.10.)Toavoidparallaxwhenreading
the bearing, the lines are inscribed on each side of the disk.
WhenthesweeporiginisatthecenterofthePPI,theusualcasefor
relativemotiondisplays,thebearingofasmall,welldefinedtargetpipis
determinedbyplacingtheradiallineoroneoftheradiallinesofthecross
hairoverthecenterofthepip.Thetrueorrelativebearingofthepipcanbe
read from the respective bearing dial.
Variable Range Marker (Range Strobe)
Thevariablerangemarker(VRM)isusedprimarilytodeterminethe
rangestotargetpipsonthePPI.Amongitssecondaryusesisthatof
providingavisualindicationofalimitingrangeaboutthepositionofthe
observer’s ship, within which targets should not enter for reasons of safety.
TheVRMisactuallyasmallrotatingluminousspot.Thedistanceofthe
spotfromthesweeporigincorrespondstorange;ineffect,itisavariable
range ring.
Thedistancetoatargetpipismeasuredbyadjustingthecircledescribed
bytheVRMsothatitjusttouchestheleading(inside)edgeofthepip.The
VRMisadjustedbymeansofarangecrank.Thedistanceisreadonarange
counter.
Forbetterrangeaccuracy,theVRMshouldbejustbrightenoughtosee
and should be focused as sharply as possible.
Electronic Bearing Cursor
Thedesignsofsomeradarindicatorsmayincludeanelectronicbearing
cursorinadditiontothemechanicalbearingcursor.Thiselectroniccursoris
aluminouslineonthePPIusuallyoriginatingatthesweeporigin.Itis
particularlyusefulwhenthesweeporiginisnotatthecenterofthePPI(see
figure2.3).Bearingsaredeterminedbyplacingthecursorinapositionto
bisectthepip.Insettingtheelectroniccursorinthismanner,thereareno
parallaxproblemssuchasareencounteredintheuseofthemechanical
bearingcursor.Thebearingstothepipsortargetsarereadonanassociated
bearing indicator.
Theelectronicbearingcursormayhavethesameappearanceasthe
headingflash.Toavoidconfusionbetweenthesetwoluminouslines
originatingatthesweeporiginonthePPI,thedesignmaybesuchthatthe
electroniccursorappearsasadashedordottedluminousline.Another
designapproachusedtoavoidconfusionlimitsthepaintingofthecursorto
thatpartoftheradialbeyondthesettingoftheVRM.Withoutspecial
provisionfordifferentiatingbetweenthetwoluminouslines,theirbrightness
may be made different to serve as an aid in identification.
Inthesimplerdesignsofelectronicbearingcursors,thecursoris
independentoftheVRM,i.e.,thebearingisreadbycursorandrangeisread
bytherotatingVRM.Inmoreadvanceddesigns,theVRM(rangestrobe)
movesradiallyalongtheelectronicbearingastherangecrankisturned.This
serves to expedite the reading of the range and bearing to a pip.
Figure 2.10 - Measuring bearing with parallel-line cursor.
45
Interscan
ThetermINTERSCANisdescriptiveofvariousdesignsofelectronic
bearingcursors,thelengthsofwhichcanbevariedfordeterminingtherange
to a pip.
InterscansarepaintedcontinuouslyonthePPI;thepaintingsoftheother
electronicbearingcursorsarelimitedtoonepaintingforeachrotationofthe
antenna.Thus,theluminouslinesofthelattercursorstendtofadebetween
paintings.Thecontinuouslyluminouslineoftheinterscanservestoexpedite
measurements.
Insomedesignstheinterscanmaybepositionedatdesiredlocationson
thePPI;thelengthanddirectionoftheluminouslinemaybeadjustedto
servevariousrequirements,includingthedeterminationofthebearingand
distance between two pips.
Off-Center Display
Whilethedesignofmostrelativemotionradarindicatorsplacesthe
sweeporiginonlyatthecenterofthePPI,someindicatorsmayhavethe
capability for off-centering the sweep origin (see figure 2.11).
Theprimaryadvantageoftheoff-centerdisplayisthatforanyparticular
rangescalesetting,theviewaheadcanbeextended.Thislessensthe
requirementforchangingrangescalesettings.Theoff-centeringfeatureis
particularly advantageous in river navigation.
Withthesweeporiginoff-centered,thebearingdialsconcentricwiththe
PPIcannotbeuseddirectlyforbearingmeasurements.Iftheindicatordoes
nothaveanelectronicbearingcursor(interscan),theparallel-linecursormay
beusedforbearingmeasurements.Byplacingthecursorsothatoneofthe
parallellinespassesthroughboththeobserver’spositiononthePPI(sweep
origin)andthepip,thebearingtothepipcanbereadonthebearingdial.
Generally,theparallellinesinscribedonthediskaresospacedthatitwould
beimprobablethatoneoftheparallellinescouldbepositionedtopass
throughthesweeporiginandpip.Thisnecessitatesplacingthecursorsothat
theinscribedlinesareparalleltoalinepassingthroughthesweeporiginand
Figure 2.11 - Off-center display.
46
Expanded Center Display
Someradarindicatordesignshavethecapabilityforexpandingthecenter
ofthePPIontheshortestrangescale,1mileforinstance.Whileusingan
expandedcenterdisplay,zerorangeisatone-halfinch,forinstance,fromthe
centerofthePPIratherthanatitscenter.Withsweeprotationthecenterof
the PPI is dark out to the zero range circle.
Rangesmustbemeasuredfromthezerorangecircleratherthanthecenter
ofthePPI.Whilethedisplayisdistorted,thebearingsofpipsfromthecenter
ofthePPIarenotchanged.Throughshiftingclosetargetpipsradiallyaway
fromthePPIcenter,betterresolutionordiscriminationbetweenthepipsis
afforded.AlsobecauseofthenormalsmallcenteringerrorsofthePPI
display,theradialshiftingofthetargetpipspermitsmoreaccuratebearing
determinations.
Figure2.12illustratesanormaldisplayinwhichrangeismeasuredfrom
thecenterofthePPI.Figure2.13illustratesanexpandedcenterdisplayof
the same situation.
Figure 2.12 - Normal display.
Figure 2.13 - Expanded center display.
47
RADAR OPERATING CONTROLS
POWER CONTROLS
Indicator Power Switch
ThisswitchontheindicatorhasOFF,STANDBY,andOPERATE(ON)
positions.IftheswitchisturneddirectlyfromtheOFFtoOPERATE
positions,thereisawarm-upperiodofabout3minutesbeforetheradarset
isinfulloperation.Duringthewarm-upperiodthecathodesofthetubesare
heated,thisheatingbeingnecessarypriortoapplyinghighvoltages.Ifthe
switchisintheSTANDBYpositionforaperiodlongerthanthatrequiredfor
warm-up,theradarsetisplacedinfulloperationimmediatelyuponturning
theswitchtotheOPERATEposition.KeepingtheradarsetinSTANDBY
whennotinusetendstolessenmaintenanceproblems.Frequentswitching
fromOFF toOPERATE tends to cause tube failures.
Antenna (Scanner) Power Switch
Forreasonsforsafety,aradarsetshouldhaveaseparateswitchfor
startingandstoppingtherotationoftheantenna.Separateswitchingpermits
antennarotationfordeicingpurposeswhentheradarsetiseitherofforin
standbyoperation.Separateswitchingpermitsworkontheantennaplatform
whenpowerisappliedtoothercomponentswithoutthedangerattendanttoa
rotating antenna.
Special Switches
Evenwhentheradarsetisoff,provisionmaybemadeforapplyingpower
toheatersdesignedforkeepingthesetdry.Insuchcase,aspecialswitchis
provided for turning this power on and off.
Note:PriortoplacingtheindicatorpowerswitchintheOPERATEposition,
thebrilliancecontrol,thereceivergaincontrol,thesensitivitytimecontrol,
andthefasttimeconstantswitchshouldbeplacedattheirminimumoroff
positions.Thesettingofthebrilliancecontrolavoidsexcessivebrilliance
harmfultotheCRTonapplyingpower.Theothersettingsarerequiredprior
to making initial adjustments of the performance controls.
48
PERFORMANCE CONTROLS—INITIAL ADJUSTMENTS
Brilliance Control
AlsoreferredtoasIntensityorBrightnesscontrol.Thebrilliancecontrol,
whichdeterminestheoverallbrightnessofthePPIdisplay,isfirstadjustedto
makethetraceoftherotatingsweepvisiblebutnottoobright.Thenitis
adjustedsothatthetracejustfades.Thisadjustmentshouldbemadewiththe
receivergaincontrolatitsminimumsettingbecauseitisdifficulttojudge
therightdegreeofbrilliancewhenthereisaspeckledbackgroundonthe
PPI.Figures2.14,2.15,and2.16illustratetheeffectsofdifferentbrilliance
settings,thereceivergaincontrolbeingsetsothatthespeckledbackground
doesnotappearonthePPI.Withtoolittlebrilliance,thePPIdisplayis
difficult to see; with excessive brilliance, the display is unfocused.
Figure 2.14 - Too little brilliance.
Figure 2.15 - Normal brilliance.
Figure 2.16 - Excessive brilliance.
49
Receiver Gain Control
Thereceivergaincontrolisadjusteduntilaspeckledbackgroundjust
appearsonthePPI.Figures2.17,2.18,and2.19illustratetoolittlegain,
normalgain,andexcessivegain,respectively.Withtoolittlegain,weak
echoesmaynotbedetected;withexcessivegain,strongechoesmaynotbe
detectedbecauseofthepoorcontrastbetweenechoesandthebackgroundof
the PPI display.
Inadjustingthereceivergaincontroltoobtainthespeckledbackground,
theindicatorshouldbesetononeofthelongerrangescalesbecausethe
speckledbackgroundismoreapparentonthesescales.Onshiftingtoa
differentrangescale,thebrightnessmaychange.Generally,therequired
readjustmentmaybeeffectedthroughuseofthereceivergaincontrolalone
althoughthebrightnessofthePPIdisplayisdependentuponthesettingsof
thereceivergainandbrilliancecontrols.Insomeradarindicatordesigns,the
brilliancecontrolispresetatthefactory.Evenso,thebrilliancecontrolmay
havetobereadjustedattimesduringthelifeofthecathode-raytube.Also
thepresetbrilliancecontrolmayhavetobereadjustedbecauseoflarge
changes in ambient light levels.
Figure 2.17 - Too little gain.
Figure 2.18 - Normal gain.
Figure 2.19 - Excessive gain.
50
Tuning Control
Withoutshiporlandtargets,aperformancemonitor,oratuningindicator,
thereceivermaybetunedbyadjustingthemanualtuningcontrolfor
maximumseaclutter.Analternativetotheuseofnormalseaclutterwhichis
usuallypresentouttoafewhundredyardsevenwhentheseaiscalm,isthe
useofechoesfromtheship’swakeduringaturn.Whenseaclutterisused
formanualtuningadjustment,allanti-cluttercontrolsshouldbeeitheroffor
placedattheirminimumsettings.Also,oneoftheshorterrangescales
should be used.
PERFORMANCECONTROLS-ADJUSTMENTSACCORDINGTO
OPERATING CONDITIONS
Receiver Gain Control
Thiscontrolisadjustedinaccordancewiththerangescalebeingused.
Particularcautionmustbeexercisedsothatwhilevaryingitsadjustmentfor
betterdetectionofmoredistanttargets,theareanearthecenterofthePPIisnot
subjectedtoexcessivebrightnesswithinwhichclosetargetsmaynotbedetected.
Whendetectionatthemaximumpossiblerangeistheprimaryobjective,
thereceivergaincontrolshouldbeadjustedsothataspeckledbackgroundis
justvisibleonthePPI.However,atemporaryreductionofthegainsetting
may prove useful for detecting strong echoes from among weaker ones.
Fast Time Constant (FTC) Switch (Differentiator)
WiththeFTCswitchintheONposition,theFTCcircuitthrough
shorteningtheechoesonthedisplayreducesclutteronthePPIwhichmight
becausedbyrain,snow,orhail.Whenused,thiscircuithasaneffectover
theentirePPIandgenerallytendstoreducereceiversensitivityand,thus,the
strengths of the echoes as seen on the display.
Rain Clutter Control
Theraincluttercontrolprovidesavariablefasttimeconstant.Thus,it
providesgreaterflexibilityintheuseofFTCaccordingtotheoperating
conditions.WhethertheFTCisfixedorvariable,itprovidesthemeansfor
breakingupclutterwhichotherwisecouldobscuretheechoofatargetof
interest.Whennavigatinginconfinedwaters,theFTCfeatureprovidesbetter
definitionofthePPIdisplaythroughbetterrangeresolution.Also,theuseof
FTC provides lower minimum range capability.
Figure2.20illustratesclutteronthePPIcausedbyarainsquall.Figure
2.21illustratesthebreakupofthisclutterbymeansoftherainclutter
control.
Figure 2.20 - Clutter caused by a rain squall.
Figure 2.21 - Break up of clutter by means of rain clutter control.
51
Figure2.22illustratestheappearanceofaharboronthePPIwhenthe
FTCcircuitisnotbeingused.Figure2.23illustratestheharborwhenthe
FTCcircuitisbeingused.WithuseoftheFTCcircuit,thereisbetter
definition.
Figure 2.22 - FTC not in use.
Figure 2.23 - FTC in use.
52
Sensitivity Time Control (STC)
AlsocalledSEACLUTTERCONTROL,ANTI-CLUTTERCONTROL,
SWEPT GAIN, SUPPRESSOR.
Normally,theSTCshouldbeplacedattheminimumsettingincalmseas.
Thiscontrolisusedwithacircuitwhichisdesignedtosuppressseaclutteroutto
alimiteddistancefromtheship.Itspurposeistoenablethedetectionofclose
targetswhichotherwisemightbeobscuredbyseaclutter.Thiscontrolmustbe
usedjudiciouslyinconjunctionwiththereceivergaincontrol.Generally,one
shouldnotattempttoeliminateallseaclutterwiththiscontrol.Otherwise,
echoes from small close targets may be suppressed also.
Figures2.24,2.25,and2.26illustrateSTCsettingswhicharetoolow,
correct, and too high, respectively.
Figure 2.24 - STC setting too low.
Figure 2.25 - STC setting correct.
Figure 2.26 - STC setting too high.
53
Performance Monitor
Theperformancemonitorprovidesacheckoftheperformanceofthe
transmitterandreceiver.Beinglimitedtoacheckoftheoperationofthe
equipment,theperformancemonitordoesnotprovideanyindicationof
performanceasitmightbeaffectedbythepropagationoftheradarwaves
throughtheatmosphere.Thus,agoodcheckontheperformancemonitor
does not necessarily indicate that targets will be detected.
Whentheperformancemonitorisused,aplumeextendsfromthecenter
ofthePPI(seefigure2.27).Thelengthoftheplume,whichisdependent
uponthestrengthoftheechoreceivedfromtheechoboxinthevicinityof
theantenna,isanindicationoftheperformanceofthetransmitterandthe
receiver.Thelengthofthisplumeiscomparedwithitslengthwhentheradar
is known to be operating at high performance.
Anyreductionofover20percentoftherangetowhichtheplumeextends
whentheradarsetisoperatingatitshighestperformanceisindicativeofthe
needfortuningadjustment.Iftuningadjustmentdoesnotproduceaplume
lengthwithinspecifiedlimits,theneedforequipmentmaintenanceis
indicated.
Withmalfunctioningoftheperformancemonitor,theplumeappearsas
illustrated in figure 2.28.
Theeffectivenessoftheanti-cluttercontrolscanbecheckedbyinspecting
their effects on the plume produced by the echo from the echo box.
Figure 2.27 - Performance monitor plume.
Figure 2.28 - Appearance of plume when performance monitor is malfunctioning.
54
Pulse Lengths and Pulse Repetition Rate Controls
Onsomeradarsetsthepulselengthandpulserepetitionrate(PRR)are
changedautomaticallyinaccordancewiththerangescalesetting.Atthe
higherrangescalesettingstheradaroperationisshiftedtolongerpulse
lengthsandlowerpulserepetitionrates.Thegreaterenergyinthelonger
pulseisrequiredfordetectionatlongerranges.Thelowerpulserepetition
rateisrequiredinorderthatanechocanreturntothereceiverpriortothe
transmissionofthenextpulse.Attheshorterrangescalesettings,theshorter
pulselengthprovidesbetterrangeresolutionandshorterminimumranges,
thehigherpowerofthelongerpulsenotbeingrequired.Also,thehigher
pulserepetitionratesattheshorterrangescalesettingsprovidemore
frequentrepaintingofthepipsand,thus,sharperpipsonthePPIdesirable
for short range observation.
OnotherradarsetsthepulselengthandPRRmustbechangedbymanual
operationofcontrols.OnsomeofthesesetspulselengthandPRRcanbe
changedindependently.ThepulselengthsandPRR’sofradarsetsinstalled
aboardmerchantshipsusuallyarechangedautomaticallywiththerange
scale settings.
LIGHTING AND BRIGHTNESS CONTROLS
Reflection Plotter
Theilluminationlevelsofthereflectionplotterandthebearingdialsare
adjusted by a control, labeled PLOTTER DIMMER.
Thereflectionplotterlightingmustbeturnedoninordertoseereflected
imagesofthegreasepencilplotonthePPI.Withyellowish-green
fluorescence,yellowandorangegreasepencilmarkingsprovidetheclearest
imagesonthePPI;withorangefluorescence,blackgreasepencilmarkings
provide the clearest images.
Heading Flash
Thebrightnessoftheheadingflashisadjustedbyacontrol,labeled
FLASHERINTENSITYCONTROL.Thebrightnessshouldbekeptatalow
leveltoavoidmaskingasmallpiponthePPI.Theheadingflashshouldbe
turned off periodically for the same reason.
Electronic Bearing Cursor
Thebrightnessoftheelectronicbearingcursorisadjustedbyacontrolfor
thispurpose.Unlesstheelectronicbearingcursorappearsasadashedor
dottedline,thebrightnesslevelsoftheelectronicbearingcursorandthe
headingflashshouldbedifferenttoserveasanaidtotheiridentification.
Radarindicatorsarenowequippedwithaspring-loadedswitchto
temporarily disable the flash.
Fixed Range Markers
Thebrightnessofthefixedrangemarkersisadjustedbyacontrol,labeled
FIXEDRANGEMARKINTENSITYCONTROL.Thefixedrangemarkers
shouldbeturnedoffperiodicallytoavoidthepossibilityoftheirmaskinga
small pip on the PPI.
Variable Range Marker
Thebrightnessofthevariablerangemarkerisadjustedbythecontrol
labeledVARIABLERANGEMARKINTENSITYCONTROL.Thiscontrol
isadjustedsothattheringdescribedbytheVRMissharpandclearbutnot
too bright.
Panel Lighting
TheilluminationofthepanelisadjustedbythecontrollabeledPANEL
CONTROL.
MEASUREMENT AND ALIGNMENT CONTROLS
Range
Usually,rangesaremeasuredbymeansofthevariablerangemarker
(VRM).OnsomeradarstheVRMcanbeusedtomeasurerangesuptoonly
20milesalthoughthemaximumrangescalesettingis40miles.For
distances greater than 20 miles, the fixed range rings must be used.
Theradarindicatorsdesignedformerchantshipinstallationhave
rangecounterreadingsinmilesandtenthsofmiles.Accordingtothe
rangecalibration,thereadingsmaybeeitherstatuteornauticalmiles.
Therangecounterhasthreedigits,thelastorthirddigitindicatingthe
rangeintenthsofamile.AstheVRMsettingisadjusted,therangeis
readinstepsoftenthsofamile.TheVRMcontrolmayhavecoarseand
55
finesettings.Thecoarsesettingpermitsrapidchangesintherange
settingoftheVRM.Thefinesettingpermitstheoperatortomakesmall
adjustmentsoftheVRMmorereadily.Foraccuraterangemeasurements,
thecircledescribedbytheVRMshouldbeadjustedsothatitjusttouches
the inside edge of the pip.
Bearing
Onmostradarindicatorsbearingsaremeasuredbysettingthemechanical
bearingcursortobisectthetargetpipandreadingthebearingonthebearing
dial.
WithunstabilizedHeading-Upwarddisplays,truebearingsarereadonthe
outer,rotatabledialwhichisseteithermanuallyorautomaticallytoship’s
true heading.
WithstabilizedNorth-Upwarddisplays,truebearingsarereadonthe
fixeddial.Withlossofcompassinputtotheindicator,thebearingsasread
onthelatterdialarerelative.Someradarindicatorsdesignedforstabilized
North-Upwarddisplayshaverotatablerelativebearingdials,thezero
graduationsofwhichcanbesettotheheadingflashforreadingrelative
bearings.
Someradarindicators,especiallythosehavingtruemotiondisplays,may
haveanelectronicbearingcursorandassociatedbearingindicator.The
electronic cursor is particularly useful when the display is off-centered.
Sweep Centering
Foraccuratebearingmeasurementbythemechanicalbearingcursor,the
sweeporiginmustbeplacedatthecenterofthePPI.Someradarindicators
havepanelcontrolswhichcanbeusedforhorizontalandverticalshiftingof
thesweeporigintoplaceitatthecenterofthePPIand,thus,atthepivot
pointofthemechanicalbearingcursor.Onotherradarindicatorsnothaving
panelcontrolsforcenteringthesweeporigin,thesweepmustbecenteredby
makingthoseadjustmentsinsidetheindicatorcabinetasareprescribedin
the manufacturer’s instruction manual.
Center Expansion
SomeradarindicatorshaveaCENTEREXPANDSWITCHwhichisused
todisplacezerorangefromthecenterofthePPIontheshortestrangescale
setting.WiththeswitchintheONposition,thereisdistortioninrangebutno
distortioninthebearingsofthepipsdisplayedbecausetheexpansionis
radial.Usingcenterexpansion,thereisgreaterseparationbetweenpipsnear
thecenterofthePPIand,thus,betterbearingresolution.Also,bearing
accuracyisimprovedbecausecenteringerrorshavelessereffectonaccuracy
withgreaterdisplacementofpipsfromthePPIcenter.Whencenter
expansionisused,thefixedrangeringsexpandwiththecenter.However,the
rangemustbemeasuredfromtheinnercircleasopposedtothecenterofthe
PPI.
Theuseofthecenterexpansioncanbehelpfulinanti-clutter
adjustment.
Heading Flash Alignment
Foraccuratebearingmeasurements,thealignmentoftheheadingflash
withthePPIdisplaymustbesuchthatradarbearingsareincloseagreement
withrelativelyaccuratevisualbearingsobservedfromneartheradar
antenna.
Onsomeradarindicators,theheadingflashmustbesetbyaPICTURE-
ROTATECONTROLaccordingtothetypeofdisplaydesired.Shouldthere
beanyappreciabledifferencebetweenradarandvisualbearings,adjustment
oftheheadingflashcontactsisindicated.Thelatteradjustmentshouldbe
madeinaccordancewiththeprocedureprescribedinthemanufacturer’s
instructionmanual.However,thefollowingproceduresshouldprovehelpful
in obtaining an accurate adjustment:
(1)Adjustthecenteringcontrolstoplacethesweeporiginatthecenterof
the PPI as accurately as is possible.
(2)Inselectinganobjectforsimultaneousvisualandradarbearing
measurements, select an object having a small and distinct pip on the PPI.
(3)Selectanobjectwhichliesnearthemaximumrangeofthescalein
use. This object should be not less than 2 nautical miles away.
(4)Observethevisualbearingsfromapositionasclosetotheradar
antenna as is possible.
(5)Useasthebearingerrortheaverageofthedifferencesofseveral
simultaneous radar and visual observations.
(6)Afteranyheadingflashadjustment,checktheaccuracyofthe
adjustment by simultaneous radar and visual observations.
Range Calibration
Therangecalibrationoftheindicatorshouldbecheckedatleastonceeach
watch,beforeanyeventrequiringhighaccuracy,andmoreoftenifthereis
anyreasontodoubttheaccuracyofthecalibration.Acalibrationcheck
madewithinafewminutesafteraradarsethasbeenturnedonshouldbe
checked again 30 minutes later, or after the set has warmed up thoroughly.
ThecalibrationcheckissimplythecomparisonofVRMandfixedrange
ringrangesatvariousrangescalesettings.Inthischecktheassumptionsare
56
thatthecalibrationofthefixedrangeringsismoreaccuratethanthatofthe
VRM,andthatthecalibrationofthefixedrangeringsisrelativelystable.
Oneindicationoftheaccuracyoftherangeringcalibrationisthelinearityof
thesweeportimebase.Sincerangeringsareproducedbybrighteningthe
electronbeamatregularintervalsduringtheradialsweepofthisbeam,equal
spacing of the range rings is indicative of the linearity of the time base.
Representativemaximumerrorsincalibratedfixedrangeringsare75yardsor
1.5percentofthemaximumrangeoftherangescaleinuse,whicheverisgreater.
Thus,ona6-milerangescalesettingtheerrorintherangeofapipjusttouching
arangeringmaybeabout180yardsorabout0.1nauticalmile.Sincefixedrange
ringsarethemostaccuratemeansgenerallyavailablefordeterminingrange
whentheleadingedgeofthetargetpipisattherangering,itfollowsthatranging
byradarislessaccuratethanmanymayassume.Oneshouldnotexpectthe
accuracyofnavigationalradartobebetterthanplusorminus50yardsunderthe
best conditions.
EachrangecalibrationcheckismadebysettingtheVRMtotheleadingedge
ofafixedrangeringandcomparingtheVRMrangecounterreadingwiththe
rangerepresentedbythefixedrangering.TheVRMreadingshouldnotdiffer
fromthefixedrangeringvaluebymorethan1percentofthemaximumrangeof
thescaleinuse.Forexample,withtheradarindicatorsetonthe40-milerange
scaleandtheVRMsetatthe20-milerangering,theVRMrangecounterreading
should be between 19.6 and 20.4 miles.
TRUE MOTION CONTROLS
Thefollowingcontrolsarerepresentativeofthoseadditionalcontrolsused
inthetruemotionmodeofoperation.Ifthetruemotionradarsetdesign
includesprovisionforgroundstabilizationofthedisplay,thisstabilization
maybeeffectedthroughuseofeithersetanddriftorspeedandcourse-
made-good controls.
Operating Mode
Sincetruemotionradarsaredesignedforoperationintruemotionand
relativemotionmodes,thereisacontrolontheindicatorpanelforselecting
the desired mode.
Normal Reset Control
SinceownshipisnotfixedatthecenterofthePPIinthetruemotion
mode,ownship’spositionmustberesetperiodicallyonthePPI.Ownship’s
positionmayberesetmanuallyorautomatically.Automaticresetis
performedatdefinitedistancesfromthePPIcenter,accordingtotheradar
setdesign.Withthenormalresetcontrolactuated,resetmaybeperformed
automaticallywhenownshiphasreachedapositionbeyondthePPIcenter
abouttwothirdstheradiusofthePPI.Whetherownship’spositionisreset
automaticallyormanually,ownship’spositionisresettoanoff-center
positiononthePPI,usuallyatapositionfromwhichtheheadingflashpasses
throughthecenterofthePPI.Thisoff-centerpositionprovidesmoretime
beforeresettingisrequiredthanwouldbethecaseifownship’sposition
were reset to the center of the PPI.
Delayed Reset Control
Withthedelayedresetcontrolactuated,resetisperformedautomatically
whenownshiphasreachedapositionclosertotheedgeofthePPIthanwith
normalreset.Witheitherthenormalordelayedresetcontrolactuated,thereisan
alarm signal which gives about 10 seconds forewarning of automatic resetting.
Manual Reset Control
Themanualresetcontrolpermitstheresettingofownship’spositionat
any desired time.
Manual Override Control
Themanualoverridecontrolwhenactuatedpreventsautomaticresetting
ofownship’sposition.Thiscontrolisparticularlyusefulifacritical
situationshoulddevelopjustpriortothetimeofautomaticresetting.Shifting
fromnormaltodelayedresetcanalsoprovidemoretimeforevaluatinga
situation before resetting occurs.
Ship’s Speed Input Selector Control
Ownship’sspeedandcoursebeingnecessaryinputstothetruemotion
radarcomputer,theship’sspeedinputselectorcontrolpermitseithermanual
inputofship’sspeedorautomaticinputofspeedfromaspeedlog.Withthe
controlinthemanualposition,ship’sspeedinknotsandtenthsofknotscan
be set in steps of tenths of knots.
Set and Drift Controls
Setanddriftcontrols,ortheirequivalent,providemeansforground
stabilizationofthetruemotiondisplay.Whenthereisaccuratecompensation
57
forsetanddrift,thereisnomovementofstationaryobjectsonthePPI.
Withoutsuchcompensation,slightmovementsofstationaryobjectsmaybe
detectedonthePPI.ThesetcontrolmaybelabeledDRIFTDIRECTION;
the drift control may be labeledDRIFT SPEED.
Speed and Course Made Good Controls
Theradarsetdesignmayincludespeedandcoursemadegoodcontrolsin
lieuofsetanddriftcontrolstoeffectgroundstabilizationofthetruemotion
display.Thecoursemadegoodcontrolpermitstheinputofacorrection,
withinlimitsofabout25˚tothecourseinputtotheradarset.Thespeed
controlpermitstheinputofacorrectiontothespeedinputfromthe
underwater speed log or from an artificial (dummy) log.
Zero Speed Control
IntheZEROposition,thezerospeedcontrolstopsthemovementofown
shiponthePPI;intheTRUEpositionownshipmovesonthePPIatarate
set by the speed input.
59
CHAPTER 3 — COLLISION AVOIDANCE
RELATIVE MOTION
IntheUniversethereisnosuchconditionasabsoluterestorabsolute
motion.Anobjectisonlyatrestorinmotionrelativetosomereference.A
mountainontheearthmaybeatrestrelativetotheearth,butitisinmotion
relativetothesun.Althoughallmotionisrelative,asusedhereactualortrue
motionismovementwithrespecttotheearth;relativemotionismotionwith
respecttoanarbitrarilyselectedobject,whichmayormaynothaveactualor
true motion.
Theactualortruemotionofanobjectusuallyisdefinedintermsofits
directionandrateofmovementrelativetotheearth.Iftheobjectisaship,
thismotionisdefinedintermsofthetruecourseandspeed.Themotionof
anobjectalsomaybedefinedintermsofitsdirectionandrateofmovement
relativetoanotherobjectalsoinmotion.Therelativemotionofaship,orthe
motionofoneshiprelativetothemotionofanothership,isdefinedinterms
oftheDirectionofRelativeMovement(DRM)andtheSpeedofRelative
Movement(SRM).Eachformofmotionmaybedepictedbyavelocity
vector,alinesegmentrepresentingdirectionandrateofmovement.Before
furtherdiscussionofvelocityvectorsandtheirapplication,asituation
involving relative motion between two ships will be examined.
Infigure3.1,shipA,atgeographicpositionA1,ontruecourse000˚at15
knotsinitiallyobservesshipBonthePPIbearing180˚at4miles.The
bearinganddistancetoshipBchangesasshipAproceedsfromgeographic
positionA1toA3.ThechangesinthepositionsofshipBrelativetoshipA
areillustratedinthesuccessivePPIpresentationscorrespondingtothe
geographicpositionofshipsAandB.LikewiseshipB,atgeographic
positionB1,ontruecourse026˚at22knotsinitiallyobservesshipAonthe
PPIbearing000˚at4miles.ThebearinganddistancetoshipAchangesas
shipBproceedsfromgeographicpositionB1toB3.Thechangesinthe
positionsofshipArelativetoshipBareillustratedinthesuccessivePPI
presentations corresponding to the geographic positions of ships A and B.
Figure 3.1 - Relative motion between two ships.
60
IftheradarobserveraboardshipAplotsthesuccessivepositionsofship
BrelativetohispositionfixedatthecenterofthePPI,hewillobtainaplot
calledtheRELATIVEPLOTorRELATIVEMOTIONPLOTasillustrated
in figure 3.2.
IftheradarobserveraboardshipBplotsthesuccessivepositionsofship
ArelativetohispositionfixedatthecenterofthePPI,hewillobtaina
relativeplotillustratedinfigure3.3.TheradarobserveraboardshipAwill
determinethattheDirectionofRelativeMovement(DRM)ofshipBis064˚
whereastheradarobserveraboardshipBwilldeterminethattheDRMof
ship A is 244˚.
Figure 3.2 - Motion of ship B relative to ship A.
Figure 3.3 - Motion of ship A relative to ship B.
61
Ofprimarysignificanceatthispointisthefactthatthemotiondepictedby
therelativeplotoneachPPIisnotrepresentativeofthetruemotionortrue
courseandspeedoftheothership.Figure3.4illustratestheactualheading
ofshipBsuperimposedupontherelativeplotobtainedbyshipA.Relative
motiondisplaysdonotindicatetheaspectsofshiptargets.Foreitherradar
observertodeterminethetruecourseandspeedoftheothership,additional
graphical constructions employing relative and true vectors are required.
Figure3.5illustratesthetimedmovementsoftwoships,RandM,with
respecttotheearth.Thisplot,similartotheplotmadeinordinarychart
navigationwork,iscalledageographical(navigational)plot.ShipR
proceedingoncourse045˚,ataconstantspeedpassesthroughsuccessive
positionsR
1
,R
2,
R
3
,R
4
...equallyspacedatequaltimeintervals.Therefore,
thelinesegmentsconnectingsuccessivepositionsrepresentdirectionand
rateofmovementwithrespecttotheearth.Thustheyaretruevelocity
vectors.Likewise,forshipMoncourse325˚thelinesegmentsconnecting
theequallyspacedplotsforequaltimeintervalsrepresenttruevelocity
vectorsofshipM.AlthoughthemovementofRrelativetoMorMrelative
toRmaybeobtainedbyadditionalgraphicalconstructionorbyvisualizing
thechangesinbearingsanddistancesbetweenplotscoordinatedintime,the
geographicalplotdoesnotprovideadirectpresentationoftherelative
movement.
Figure3.6illustratesamodificationoffigure3.5inwhichthetruebearing
linesandrangesofothershipMfromownshipRareshownatequaltime
intervals.OnplottingtheserangesandbearingsfromafixedpointR,the
movementofMrelativetoownshipRisdirectlyillustrated.Thelines
betweentheequallyspacedplotsatequaltimeintervalsprovidedirection
andrateofmovementofMrelativetoRandthusarerelativevelocity
vectors.
Figure 3.4 - The actual heading of ship B.
Figure 3.5 - True velocity vectors.
Figure 3.6 - Relative velocity vectors.
62
Thetruevelocityvectordepictingownship’struemotioniscalledown
ship’strue(course-speed)vector;thetruevelocityvectordepictingtheother
ship’struemotioniscalledothership’strue(course-speed)vector;the
relativevelocityvectordepictingtherelativemotionbetweenownshipand
the other ship is called therelative (DRM-SRM)vector.
Intheforegoingdiscussionandillustrationoftrueandrelativevelocity
vectors,themagnitudesofeachvectorweredeterminedbythetimeinterval
between successive plots.
Actuallyanyconvenienttimeintervalcanbeusedaslongasitisthesame
foreachvector.Thuswithplotsequallyspacedintime,ownship’strue
(course-speed)vectormagnitudemaybetakenasthelinesegmentbetween
R
1
andR
3
,R
1
andR
4
,R
2
andR
4
,etc.,aslongasthemagnitudesoftheother
two vectors are determined by the same time intervals.
AplotofthesuccessivepositionsofothershipMinthesamesituationon
arelativemotiondisplayonthePPIoftheradarsetaboardownshipR
wouldappearasinfigure3.7.WithaRelativeMovementLine(RML)drawn
throughtheplot,theindividualsegmentsoftheplotcorrespondingto
relativedistancestraveledperelapsedtimearerelative(DRM-SRM)vectors,
althoughthearrowheadsarenotshown.Theplot,calledtheRELATIVE
PLOTorRELATIVEMOTIONPLOT,istheplotofthetruebearingsand
distancesofshipMfromownshipR.Iftheplotswerenottimed,vector
magnitudewouldnotbeindicated.Insuchcasestherelativeplotwouldbe
related to the (DRM-SRM) vector in direction only.
Figure3.8illustratesthesamesituationasfigure3.7plottedona
ManeuveringBoard.ThecenteroftheManeuveringBoardcorrespondsto
thecenterofthePPI.AswiththePPIplot,allrangesandtruebearingsare
plotted from a fixed point at the center, point R.
Figure3.8illustratesthattherelativeplotprovidesanalmostdirect
indicationoftheCLOSESTPOINTOFAPPROACH(CPA).TheCPAisthe
true bearing and distance of the closest approach of one ship to another.
Figure 3.7 - Relative Plot.
Figure 3.8 - Relative Plot on the Maneuvering Board.
63
THE VECTOR TRIANGLE
Intheforegoingdiscussion,therelativemotionofothershipMwith
respecttoownshipRwasdevelopedgraphicallyfromthetruemotionsof
shipMandshipR.Theusualproblemistodeterminethetruemotion(true
courseandspeed)oftheothershipM,knowingownship’struemotion(true
courseandspeed)and,throughplotting,determiningthemotionofshipM
relative to own ship R.
Thevectortriangleisagraphicalmeansofaddingorsubtractingtwo
velocityvectorstoobtainaresultantvelocityvector.Todeterminethetrue
(course-speed)vectorofothershipM,thetrue(course-speed)vectorofown
shipRisaddedtotherelative(DRM-SRM)vectorderivedfromtherelative
plot, or the timed motion of other ship M relative to own ship R.
Intheadditionofvectors,thevectorsarelaidendtoend,takingcarethat
eachvectormaintainsitsdirectionandmagnitude,thetwoessentialelements
ofavector.Justasthereisnodifferencewhether5isaddedto3or3isadded
to5,thereisnodifferenceintheresultantvectorwhethertherelative(DRM-
SRM)vectorislaidattheendofownship’strue(course-speed)vectoror
ownship’strue(course-speed)vectorislaidattheendoftherelative(DRM-
SRM)vector.Becauseofthenotationsusedinthismanual,therelative
(DRM-SRM)vectorislaidattheendofownship’strue(course-speed)
vector, unless otherwise specified.
Theresultantvector,thetrue(course-speed)vectorofothershipM,is
foundbydrawingavectorfromtheoriginofthetwoconnectedvectorsto
theirendpoint.Unlessthetwovectorsaddedhavethesameoropposite
directions,atrianglecalledthevectortriangleisformedondrawingthe
resultant vector.
Insightintothevalidityofthisproceduremaybeobtainedthroughthe
mariner’s experience with the effect of a ship’s motion on the wind.
Ifashipissteamingduenorthat15knotswhilethetruewindis10knots
fromduenorth,themarinerexperiencesarelativewindof25knotsfromdue
north.Assumingthatthemarinerdoesnotknowthetruewind,itmaybe
foundbylayingownship’strue(course-speed)vectorandtherelativewind
(DRM-SRM) vector end to end as in figure 3.9.
Infigure3.9,ownship’strue(course-speed)vectorislaiddowninadue
northdirection,usingavectormagnitudescaledfor15knots.Attheendof
thelattervector,therelativewind(DRM-SRM)vectorislaiddowninadue
southdirection,usingavectormagnitudescaledfor25knots.Ondrawing
theresultantvectorfromtheoriginofthetwoconnectedvectorstotheirend
point, a true wind vector of 10 knots in a due south direction is found.
Ifownshipmaintainsaduenorthcourseat15knotsasthewinddirection
shifts,therelativewind(DRM-SRM)vectorchanges.Inthiscaseavector
triangleisformedonaddingtherelativewind(DRM-SRM)vectortoown
ship’s true (course-speed) vector (see figure 3.10).
Figure 3.9 - Relative and true wind vectors.
Figure 3.10 - Wind vector triangle.
64
Returningnowtotheproblemofrelativemotionbetweenshipsandusing
thesamesituationasinfigure3.7,atimedplotofthemotionofothershipM
relative to own ship R is made on the PPI as illustrated in figure 3.11.
Assumingthatthetrue(course-speed)vectorofothershipMisunknown,
itmaybedeterminedbyaddingtherelative(DRM-SRM)vectortoown
ship’s true (course-speed) vector.
Thevectorsarelaidendtoend,whilemaintainingtheirrespective
directionsandmagnitudes.Theresultantvector,thetrue(course-speed)
vectorofothership,isfoundbydrawingavectorfromtheoriginofthetwo
connected (added) vectors to their end point.
VECTOR EQUATIONS
Where:
em is other ship’s true (course-speed) vector.
er is own ship’s true (course-speed) vector.
rm is relative (DRM-SRM) vector.
em = er + rm
er = em - rm
rm = em - er
(See figure 3.12)
Figure 3.11 - Vector triangle on PPI.
Figure 3.12 - True and relative vectors.
65
Todeterminevectoremfromvectorserandrm,vectorserandrmare
addedbylayingthemendtoendanddrawingaresultantvector,em,fromthe
origin of the two connected vectors to their end point (see figure 3.13).
Todeterminevectorerfromvectorsemandrm,vectorrmissubtracted
fromvectorembylayingvectorrm,withitsdirectionreversed,attheendof
vectoremanddrawingaresultantvector,er,fromtheoriginofthetwo
connected vectors to their end point (see figure 3.14).
Todeterminevectorrmfromvectorsemander,vectorerissubtracted
fromvectorembylayingvectorer,withitsdirectionreversed,attheendof
vectoremanddrawingaresultantvectorfromtheoriginofthetwo
connected vectors to their end point (see figure 3.15).
Figure 3.13 - Addition of own ship’s true (course-speed) vector and the relative (DRM-SRM)
vector to find the true (course-speed) vector of the other ship.
Figure 3.14 - Subtraction of the relative (DRM-SRM) vector from other ship’s true (course-
speed) vector to find own ship’s true (course-speed) vector.
Figure 3.15 - Subtraction of own ship’s true (course-speed) vector from other ship’s true
(course-speed) vector to find the relative (DRM-SRM) vector.
66
MANEUVERING BOARD
MANEUVERING BOARD FORMAT
TheManeuveringBoardisadiagramwhichcanbeusedinthesolutionof
relativemotionproblems.Printedingreenonwhite,itisissuedintwosizes,
10 inches and 20 inches, charts 5090 and 5091, respectively.
Chart5090,illustratedinfigure3.16,consistsprimarilyofapolar
diagramhavingequallyspacedradialsandconcentriccircles.Theradialsare
printedasdottedlinesat10˚intervals.The10concentriccirclesarealso
dottedexceptfortheinnercircleandtheoutercompletecircle,whichhasa
10-inchdiameter.Dottedradialsandarcsofconcentriccirclesarealso
printedintheareaofthecornersofthe10-inchsquareframingthepolar
diagram.
The10-inchcircleisgraduatedfrom0˚atthetop,through360˚withthe
graduations at each 10˚ coinciding with the radials.
Theradialsbetweenconcentriccirclesaresubdividedinto10equalparts
bythedotsandsmallcrossesfromwhichtheyareformed.Exceptforthe
innercircle,thearcsoftheconcentriccirclesbetweenradialsaresubdivided
into10equalpartsbythedotsandsmallcrossesfromwhichtheyare
formed. The inner circle is graduated at 5˚ intervals.
Thus,exceptfortheinnercircle,allconcentriccirclesandthearcsof
concentriccirclesbeyondtheoutercompletecirclearegraduatedatone-
degree intervals.
Inthelabelingoftheoutercompletecircleat10˚intervals,thereciprocal
valuesareprintedinsidethiscircle.Forexample,theradiallabeledas0˚is
also labeled as 180˚.
Intheleft-handmargintherearetwoverticalscales(2:1and3:1);inthe
right-hand margin there are two vertical scales (4:1 and 5:1).
Alogarithmictime-speed-distancescaleandinstructionsforitsuseare
printed at the bottom.
Chart 5090 is identical to chart 5091 except for size.
PLOTTING ON MANEUVERING BOARD
Ifradartargetstobeplottedliewithin10milesofownshipandthe
distancestothesetargetsaremeasuredinmiles,andtenthsofmiles,the
ManeuveringBoardformatisparticularlyadvantageousforrelativelyrapid
transferplotting,i.e.,plottingtarget(radarcontact)informationtransferred
from the radarscope.
Theextensionofthedottedradialsandarcsofconcentriccirclesintothe
cornersoftheManeuveringBoardpermitsplottingwiththesamefacility
whenthedistancestothetargetsarejustbeyond10milesandtheirbearings
correspond to these regions.
InplottingtherangesandbearingsofradartargetsontheManeuvering
Board,theradarobservergenerallymustselectanoptimumdistancescale.
Forradartargetsatdistancesbetween10and20miles,the2:1scaleisthe
bestselection,unlessthetargetscanbeplottedwithinthecornersofthe
ManeuveringBoardusingthe1:1scale.Theobjectiveistoprovideasmuch
separationbetweenindividualplotsasispossibleforbothclarityand
accuracy of plotting.
Whilegenerallyeitherthe1:1or2:1scaleissuitableforplottingthe
relativepositionsoftheradarcontactsincollisionavoidanceapplications
whentherangesaremeasuredinmiles,theradarobserveralsomustselecta
suitablescaleforthegraphicalconstructionofthevectortriangleswhenthe
sides of these triangles are scaled in knots.
Toavoidconfusionbetweenscalesbeingusedfordistanceandspeedin
knots,theradarobservershouldmakeanotationontheManeuveringBoard
astowhichscaleisbeingusedfordistanceandwhichscaleisbeingusedfor
speedinknots.However,rapidradarplottingtechniques,withinthescopeof
usingaselectedportionoftherelativeplotdirectlyastherelative(course-
speed) vector, may be employed with the Maneuvering Board.
Asillustratedinfigure3.18,theplottingofrelativepositionsonthe
ManeuveringBoardrequirestheuseofastraightedgeandapairofdividers.
Thedistancescaleisselectedinaccordancewiththeradarrangesetting.To
avoid mistakes, the distance scale used should be circled.
Asillustratedinfigure3.19,theconstructionofownshipstrue(course-
speed)vectorscaledinknotsandoriginatingfromthecenterofthe
ManeuveringBoardalsorequirestheuseofastraightedgeandpairof
dividers.
Intheuseofaseparaterelativeplotandvectortrianglescaledinknots,the
directionoftherelative(DRM-SRM)vectormustbetransferredfromthe
relative plot by parallel rules or by sliding one triangle against another.
67
Figure 3.16 - Maneuvering Board.
68
Figure 3.17 - Speed triangle and relative plot on the Maneuvering Board.
69
Figure 3.18 - Plotting relative positions on the Maneuvering Board.
70
Figure 3.19 - Constructing a true vector on the Maneuvering Board.
71
Relative Movement Problems
Relative movement problems may be divided into two general categories:
(1)Tracking:fromobservedrelativemovementdata,determiningthe
actual motion of the ship or ships being observed.
(2)Maneuvering:knowing,orhavingpreviouslydeterminedtheactual
motionoftheshipsinvolvedintheproblem,ascertainingthe
necessarychangestoactualmotiontoobtainadesiredrelative
movement.
Threeseparateanddistinctplotsareavailableforthesolutionofrelative
movement problems:
(1)Geographical or navigational plot.
(2)Relative plot.
(3)Vector diagram (Speed Triangle).
Eachoftheseplotsprovidesamethodeitherforcompletesolutionsorfor
obtainingadditionaldatarequiredinthesolutionofmorecomplexproblems.
Intheforegoingtreatmentofthegeographicalandrelativeplots,thetrue
andrelativevectornatureofthoseplotswasillustrated.Butintheuseof
vectorsitisusuallymoreconvenienttoscalethemagnitudesofthevectors
inknotswhileatthesametimeutilizingoptimumdistanceandspeedscales
forplottingaccuracy.Therefore,ifthegeographicalandrelativeplotsare
usedonlyforobtainingpartoftherequireddata,othermeansmustbe
employedincompletingthesolution.Thisothermeansisthevectordiagram
which is a graphical means of adding or subtracting vectors.
Whenthevectordiagramisscaledinknotsitiscommonlycalledthe
Speed Triangle. Figure 3.20 illustrates the construction of a speed triangle in
whichthetruevectors,scaledinknots,aredrawnfromacommonpointe
(forearth)atthecenterofthepolardiagram.Thetruevectorofthereference
shipiser;thetruevectorofshipM,commonlycalledthemaneuveringship,
isem,andtherelativevectorisrm.Thevectordirectionsareshownbythe
arrowheads.
Thedirectionoftherelativevectorrminthespeedtriangleisthesameas
theDRMintherelativeplot.TheDRMistheconnectinglinkbetweenthe
twodiagrams.Also,themagnitude(SRM)oftherelativevectorinthespeed
triangleisdeterminedbytherateofmotionofshipMalongtheRMLofthe
relative plot.
Ifinfigure3.20thetruevectorofthereferenceshipwereknownandthe
relativevectorwerederivedfromtherateanddirectionoftherelativeplot,
thevectorscouldbeaddedtoobtainthetruevectorofthemaneuveringship
().Intheadditionofvectors,thevectorsareconstructedend
toendwhilemaintainingvectormagnitudeanddirection.Thesumisthe
magnitudeanddirectionofthelinejoiningtheinitialandterminalpointsof
the vectors.
Ifinfigure3.20thetruevectorofthemaneuveringshipwereknownas
wellasthatofthereferenceship,therelativevectorcouldbeobtainedby
subtractingthetruevectorofthereferenceshipfromthetruevectorofthe
maneuvering ship ().
Inthisvectorsubtraction,thetruevectorsareconstructedendtoendas
before, but the direction of the reference ship true vector is reversed.
Ifinfigure3.20thetruevectorofthemaneuveringshipwereknownas
wellastherelativevector,thetruevectorofthereferenceshipcouldbe
obtainedbysubtractingtherelativevectorfromthetruevectorofthe
maneuvering ship ().
Butinthepracticalapplicationofconstructingtwooftheknownvectors,
emerrm+=
Figure 3.20 - Speed triangle and relative plot.
rmemer–=
eremrm–=
72
thethirdvectormaybefoundbycompletingthetriangle.Theformulasas
suchmaybeignoredaslongascareisexercisedtoinsurethatthevectorsare
constructedintherightdirection.Particularcaremustbeexercisedtoinsure
thattheDRMisnotreversed.Therelativevectorrmisalwaysinthe
directionoftherelativemovementasshownontherelativeplotandalways
join the heads of the true vectors at pointsr andm.
Fundamentaltothisconstructionofthespeedtriangle(vectordiagram)
withtheoriginofthetruevectorsatthecenterofthepolardiagramisthe
factthatthelocationswheretheactualmovementistakingplacedonot
affecttheresultsofvectoradditionorsubtraction.Or,forgiventruecourses
andspeedsofthereferenceandmaneuveringships,thevectordiagramis
independentoftherelativepositionsoftheships.Inturn,theplaceof
constructionofthevectordiagramisindependentofthepositionofthe
relative plot.
Infigure3.20thevectordiagramwasconstructedwiththeoriginsofthe
truevectorsatthecenterofthepolardiagraminordertomakemosteffective
useofthecompassroseanddistancecirclesinconstructingtruevectors.But
inthisapplicationofthevectordiagraminwhichthevectormagnitudesare
scaledinknots,todeterminethetruevectorofthemaneuveringshipan
intermediatecalculationisrequiredtoconverttherateofrelativemovement
torelativespeedinknotsbeforetherelativevectormaybeconstructedwith
itsoriginattheheadofthetruevectorofthereferenceship.This
intermediatecalculationaswellasthetransferoftheDRMtothevector
diagrammaybeavoidedthroughdirectuseoftherelativeplotastherelative
vector.Inthisapplicationthevectordiagramisconstructedwiththetrue
vectorssettothesamemagnitudescaleastherelativevector.Thisscaleis
the distance traveled per the time interval of the relative plot.
Therearetwobasictechniquesusedintheconstructionofthistypeof
vectordiagram.Figures3.21and3.22(a)illustratetheconstructioninwhich
thereferenceship’struevectorisdrawntoterminateattheinitialplotofthe
segmentoftherelativeplotuseddirectlyastherelativevector.Thevector
diagramiscompletedbyconstructingthetruevectorofthemaneuvering
shipfromtheoriginofthereferenceship’struevector,terminatingattheend
oftherelativevector.Figure3.22(b)illustratestheconstructioninwhichthe
referenceship’struevectorisdrawntooriginateatthefinalplotofthe
segmentoftherelativeplotuseddirectlyastherelativevector.Thevector
diagramiscompletedbyconstructingthetruevectorofthemaneuvering
shipfromtheoriginoftherelativevector,terminatingattheheadofthe
referenceship’struevector.Inthelattermethodtheadvantagesofthe
conventionalvectornotationarelost.Eithermethodisfacilitatedthroughthe
useofconvenienttimelapses(selectedplottingintervals)suchas3or6
minutes,orothermultiplesthereof,withwhichwellknownrulesofthumb
may be used in determining the vector lengths.
Figure 3.21 - Vector diagram.
Figure 3.22 - Vector diagrams.
73
Figure3.23illustratesthateventhoughthevectordiagrammaybe
constructedinitiallyinaccordancewithaparticularselectedplotting
interval,thevectordiagramsubsequentlymaybesubdividedorexpandedin
geometricallysimilartrianglesastheactualtimelapseoftheplotdiffers
fromthatpreviouslyselected.Ifownship’struevectorerisdrawninitially
foratimelapseof6minutesandtheactualplotisof8minutesduration,
vectorerisincreasedinmagnitudebyonethirdpriortocompletingthe
vector diagram.
Figure 3.23 - Vector diagram.
74
THE LOGARITHMIC TIME-SPEED-DISTANCE NOMOGRAM
AtthebottomoftheManeuveringBoardanomogramconsistingofthree
equallyspacedlogarithmicscalesisprintedforrapidsolutionoftime,speed,
and distance problems.
Thenomogramhasalogarithmicscaleforeachofthetermsofthebasic
equation:
Distance = Speed x Time
Theupperscaleisgraduatedlogarithmicallyinminutesoftime;the
middlescaleisgraduatedlogarithmicallyinbothmilesandyards;andthe
lowerscaleisgraduatedlogarithmicallyinknots.Bymarkingthevaluesof
twoknowntermsontheirrespectivescalesandconnectingsuchmarksbya
straightline,thevalueofthethirdtermisfoundattheintersectionofthis
line with the remaining scale.
Figure3.24illustratesasolutionforspeedwhenadistanceof4milesis
traveledin11minutes.Onlyoneofthethreescalesisrequiredtosolvefor
time,speed,ordistanceifanytwoofthethreevaluesareknown.Anyoneof
thethreelogarithmicscalesmaybeusedinthesamemannerasasliderule
fortheadditionorsubtractionoflogarithmsofnumbers.Becausetheupper
scaleislarger,itsuseforthispurposeispreferredforobtaininggreater
accuracy.
Figure 3.24 - Logarithmic time-speed-distance nomogram.
75
Whenusingasinglelogarithmicscaleforthesolutionofthebasic
equationwithspeedunitsinknotsanddistanceunitsinmilesorthousandsof
yards,either60or30hastobeincorporatedinthebasicequationforproper
cancellation of units.
Figure3.24illustratestheuseoftheupperscaleforfindingthespeedin
knotswhenthetimeinminutesandthedistanceinmilesareknown.Inthis
problemthetimeis11minutesandthedistanceis4miles.Onepointofa
pairofdividersissetatthetimeinminutes,11,andthesecondpointatthe
distanceinmiles,4.Withoutchangingthespreadofthedividersortheright-
leftrelationship,setthefirstpointat60.Thesecondpointwillthenindicate
thespeedinknots,21.8.Ifthespeedandtimeareknown,placeonepointat
60andthesecondpointatthespeedinknots,21.8.Withoutchangingthe
spreadofthedividersortheright-leftrelationship,placethefirstpointatthe
timeinminutes,11.Thesecondpointthenwillindicatethedistancein
miles, 4.
Inthemethoddescribed,therewasnorealrequirementtomaintainthe
right-leftrelationshipofthepointsofthepairofdividersexcepttoinsure
thatforspeedsoflessthan60knotsthedistanceinmilesislessthanthetime
inminutes.Ifthespeedisinexcessof60knots,thedistanceinmileswill
always be greater than the time in minutes.
Ifthedistanceisknowninthousandsofyardsorifthedistanceistobe
foundinsuchunits,adividerpointissetat30ratherthanthe60usedwith
miles.Ifthespeedislessthan30knotsinthisapplication,thedistancein
thousandsofyardswillalwaysbelessthanthetimeinminutes.Ifthespeed
isinexcessof30knots,thedistanceinthousandsofyardswillalwaysbe
greater than the time in minutes.
Forspeedsoflessthan60knotsandwhenusingalogarithmicscalewhich
increasesfromlefttoright,thedistancegraduationalwaysliestotheleftof
thetimeinminutesgraduation;thespeedinknotsgraduationalwaysliesto
the left of the 60 graduation.
Theuseofthesinglelogarithmicscaleisbaseduponthefundamental
propertyoflogarithmicscalesthatequallengthsalongthescalerepresent
equalvaluesofratios.Forexample,ifonehastheratio1/2andwiththe
dividersmeasuresthelengthbetween1and2,hefindsthesamelength
between2and4,5.5and11.0,oranyothertwovaluesoneofwhichishalf
theother.Inusingthesinglelogarithmicscaleforthesolutionofaspecific
probleminwhichashiptravels10nauticalmilesin20minutes,thebasic
formula is rearranged as follows:
Onsubstitutingknownnumericalvaluesandcancelingunits,theformula
is rearranged further as:
Theratio10/20hasthesamenumericalvalueastheratioSpeed(knots)/
60.Sinceeachratiohasthesamenumericalvalue,thelengthasmeasuredon
thelogarithmicscalebetweenthedistanceinnauticalmiles(10)andthetime
inminutes(20)willbethesameasthelengthbetween60andthespeedin
knots.Thus,onmeasuringthelengthbetween10and20andmeasuringthe
same length from 60 the speed is found to be 30 knots.
Speed
Discenauticalmiles()tan
Timeminutes()
--------------------------------------------------------------------
times
60min.
1hr.
----------------------
=
Speedknots()
60
----------------------------------------
10
20
------=
76
NAUTICAL SLIDE RULES
Severalslideruleshavebeendesignedforthesolutionoftime,speed,and
distanceproblems.Thecircularslideruleillustratedinfigure3.25has
distancegraduationsinbothnauticalmilesandyards.Onenauticalmileis
assumedtobeequalto2,000yards.Onsettingtwoknownvaluestotheir
respectivearrowheads,thevaluesoughtisfoundatthethirdarrowhead.
Thus,thereisrelativelylittlechanceforerrorintheuseofthissliderule.
Whilethenauticalmilesandyardsgraduationsaredifferentiatedclearlyby
theirnumbering,thenauticalmilesgraduationsaregreenandtheyards
graduationsareblack.Thereisanotationonthebaseofthesliderulewith
respect to this color code.
Therearestraightsliderulesdesignedspecificallyforthesolutionoftime,
speed,anddistanceproblems.Thefixedandslidingscalesarelabeledsoas
to avoid blunders in their use.
GRAPHICAL RELATIVE MOTION SOLUTIONS
Thissectionprovidesexamplesolutionsoftypicalrelativemotion
problemsencounteredwhileavoidingcollisionatsea.Thesolutionstothese
problemsmaybederivedfromradarplotsmadeonthePPI,areflection
plottermountedonthePPI,orfromradarplotinformationtransferredtoa
separate polar plotting diagram such as the Maneuvering Board.
Untilrecently,transferplottingtechniquesorthetransferofradarplot
informationtoaseparatepolarplottingdiagramweregivenprimary
emphasisinthetrainingofradarobservers.Studiesoftheincreasing
numbersofcollisionsamongradar-equippedshipshavedirectedattentionto
thefactthattoomanymariners,usuallytrainedonlyintransferplotting
techniques,werenotmakingeffectiveuseoftheirradarsbecauseofa
number of factors, including:
(1)Theirperformanceofmultipledutiesaboardmerchantshipswithlittle
if any assistance.
(2)Theproblemsinherenttotransferplotting,suchasthetimelagin
measuringtherangesandbearingsandtransferringthisdatatoaseparate
plot, and the possibility of error in transferring the data.
(3)Theirattentionbeingdirectedawayfromtheradarindicatorandthe
subsequentmovementsofthetargetsandtheappearanceofnewtargetson
thePPIwhilerecording,plotting,andconstructinggraphicalsolutionsona
separate plotting diagram.
(4)Inamultipleradarcontactsituation,theconfusionandgreater
probabilityforblundersassociatedwiththeconstructionofoverlapping
vectortriangles,thevectorsofwhichmustberelatedtoseparaterelative
plots.
Figure 3.25 - Nautical slide rule.
77
(5)Thegenerallackofcapabilityofcompetentradarobserversto
determineexpeditiouslyinitialrelativemotionsolutionsformorethanabout
twoorthreeradarcontactsimposingpossibledangeratonetimewhileusing
conventionaltransferplottingtechniques.Thelattercapabilitygenerally
requirestheuseofatleasttwocompetentradarobservers.Evasiveactionby
oneormoreoftheradarcontactsmayresultinanextremelyconfusing
situation,thetimelysolutionofwhichmaynotbepracticablebymeansof
transfer plotting techniques.
RAPID RADAR PLOTTING
TheexpressionRAPIDRADARPLOTTINGisdescriptiveoftechniques
usedtoobtainsolutionstorelativemotionproblemsbymakingtherequired
graphicalconstructionsonthePPIorreflectionplotterasopposedtotheuse
ofaseparateplottingdiagramfortheseconstructions.Thesetechniques
makedirectuseofthetimedrelativemotionplotonthePPIastherelative
(DRM-SRM)vector.Theothertwovectorsofthevectortrianglearescaled
inaccordancewiththescaleoftherelative(DRM-SRM)vector.Thus,the
magnitudesofallvectorsaregovernedbythesameintervaloftime,the
distancescaleoftheradarrangesetting,andtherespectiveratesof
movement.
Thedirectuseofthetimedrelativemotionplotastherelative(DRM-
SRM)vectoreliminatesthenecessityformakingmeasurementsofthe
bearings and ranges of the radar targets for plotting on a separate diagram.
ThisinformationisobtainedsimplybymarkingthetargetpipsonthePPI
bygreasepencil.Thus,rapidradarplottingtechniques,whenfeasible,
permittheradarobservertoemploysimplerprocedureswhilebeingableto
devote more time to radar observation.
TRANSFER PLOTTING
Relativemotionsolutionsderivedfromradardatatransferredtoaplotting
diagramcanbedeterminedthroughthedirectuseofatimedsegmentofthe
relativeplotastherelative(DRM-SRM)vectorofthevectortriangleasin
rapidradarplotting.Usually,however,thevectortriangleisscaledinknots
withtheoriginofeachtruevectoratthecenteroftheplottingdiagram.In
thistransferplottingtechnique,theseparaterelativeplotandvectortriangle
arerelatedinthattherelative(DRM-SRM)vectorofthevectortriangle
scaled in knots is derived from the relative plot.
Asillustratedinfigure3.26,ownship’strue(course-speed)vectoreris
constructedfromthecenteroftheManeuveringBoardinthedirectionof
ownship’struecourse(090˚)withitsmagnitudescaledinknots.The2:1
scaleintheleftmarginisusedforscalingthevectorsofthevectortriangle
(speedtriangle)inknots.Usingapairofdividers,ownship’sspeedof12
knots is picked off the 2:1 scale to determine the length of vectorer.
Usingthedistancescaleonwhichtherelativeplotisbased,i.e.,the2:1
scale(circledasanaidinavoidingthesubsequentuseofthewrongdistance
scale),therelativedistancebetweentimedplotsM
1
/0720andM
2
/29is
measuredas3.3miles.WithothershipMhavingmoved3.3milesin9
minutesrelativetoownshipR,thespeedofrelativemovement(SRM)is22
knots.
Sincethedirectionoftherelative(DRM-SRM)vectoristhatofthe
directionofrelativemovement(DRM),i.e.,thedirectionalongtherelative
movementline(RML)fromM1toM2,allinformationneededfor
constructing the relative (DRM-SRM) vector is available.
TransferringtheDRMfromtherelativeplotbyparallelrulersorother
means,alineisdrawnfromtheextremityofownship’strue(course-speed)
vectorerinthesamedirectionastheDRM.Thelengthoftherelativevector
rmistakenfromthe2:1scaleusedinconstructingownship’struevectorer.
Thetrue(course-speed)vectorofothershipM,vectorem,isfoundby
completingthetriangle.ThespeedofothershipMinknotsisfoundby
setting the length of the vectorem to the 2:1 scale.
SELECTION OF PLOTTING TECHNIQUES
Theprimaryadvantageoftransferplottingisthehigheraccuracyafforded
bythelargevectortrianglesscaledinknots.Also,theplottingdiagramsused
provideapermanentrecord.Foraspecificsituation,theselectionofthe
basictechniquetobeusedshouldbebasedupontherelativeadvantagesand
disadvantagesofeachtechniqueastheypertaintothatsituation.Whilethe
individual’sskillintheuseofaparticulartechniqueisalegitimatefactorin
techniqueselection,thecompetentradarobservershouldbeskilledinthe
use of both basic techniques, i.e., transfer plotting and rapid radar plotting.
DuringdaylightwhenthehoodmustbemountedoverthePPI,therapid
radarplottingtechniquegenerallyisnotpractical.Evenwithhandaccess
holesinthehood,directplottinggenerallyistooawkwardtobefeasiblefor
reasonablyaccuratesolutions.However,theuseofablackoutcurtaininstead
ofahoodenablestheuseoftherapidradarplottingtechniqueduring
daylightaslongasthecurtainadequatelyshieldsthePPIfromambientlight.
Sincemosthooddesignsdonotpermitmorethanoneobservertoviewthe
radarscopeatonetime,blackoutcurtainarrangementswhichpermitmore
thanoneobservertoviewtheradarscopeatonetimeshouldenablesafer
radarobservationthanhooddesignswhichlimitobservationtooneobserver.
78
Figure 3.26 - Determining the true course and speed of the other ship by transfer plotting.
79
Rapidradarplottingtechniquesareparticularlyvaluablewhenrapid,
approximatesolutionshavehigherprioritythanmoreaccuratesolutions
derivedfromtimeconsumingmeasurementofradarinformationandtransfer
ofthisinformationtoseparateplottingsheetsforgraphicalconstructions
thereon.Thefeasibilityoftherapidradarplottingtechniquesisenhanced
whenusedwithreflectionplottersmountedonthelargersizesofPPI’s.The
feasibilityisenhancedfurtheratthelowerradarrangescalesettings.With
thelargerPPI’sandatthelowerrangescalesettings,largervectortriangles
areformedforaparticularplottinginterval.Theselargertrianglesprovide
moreaccuratesolutions.Plottingandgraphicalconstructionerrors
associatedwiththeuseofthegreasepencilhavelessereffectsonthe
accuracyofthesolutionwhenthedisplayissuchthatlargervectortriangles
are formed.
Inmanysituationsitispreferabletoobtainanapproximatesolution
rapidlyonwhichtobaseearlyandsubstantialevasiveactionratherthanwait
foramoreaccuratesolution.Intheuseofrapidlyobtainedapproximate
solutions,theradarobservershould,ofcourse,incorporateinhissolutiona
largersafetyfactorthanwouldbethecasewithmoretediousandaccurate
solutions.Shouldtheradarobserveremploymoretimeconsumingand
accuratetechniques,thereisalwaysthepossibilitythatevasiveactionbythe
othershipwillnullifyhissolution.Thesameistrueforearlyand
approximatesolutions,butsuchwouldhavetheadvantageofbeingacted
uponwhiletheshipsareatgreaterdistancesfromoneanother.Itisfarbetter
thatanymisunderstandingsastotheintentionsandactionsoftheshipbe
incurred while the ships are farther apart.
Figure3.27illustratesatransferplottingsolutionforonlytwocontacts
initiallyimposingdanger.Fromthisillustrationitshouldbereadilyapparent
thatacompetentradarobserverhavingmultipleresponsibilitiesonthe
navigationbridgewithlittle,ifany,assistancewouldhavetodirecthis
attentionprimarilytothetransferplottingtask.Particularlyiftherewere
threeradarcontactsinitiallyimposingdanger,theprobabilityforsolution
mistakesgenerallywouldbesignificantlygreaterbecauseofthegreater
possibilityofconfusionassociatedwiththeoverlappingvectors.Ifoneor
moreofthecontactsshouldchangecourseorspeedduringthesolution,
evaluation of the situation could become quite difficult.
Figure 3.27 - Multiple-contact solution by transfer plotting.
80
Theuseofrapidradarplottingtechniquesinamultipleradarcontact
situationshouldtendtoreducesolutionmistakesorblundersbecauseofthe
usualseparationofthevectortriangles.Throughconstructingthevector
trianglesdirectlyonthePPIorreflectionplotter,theprobabilityoftimely
detectionofnewcontactsandanymaneuversofcontactsbeingplotted
shouldbegreaterwhileusingrapidradarplottingtechniquesthanwhile
using transfer plotting.
Shouldtheradarobserverchoosetouseaseparateplottingsheetforeach
ofthecontactsinamultipleradarcontactsituationtoavoidanyoverlapping
ofvectortrianglesintransferplotting,thismultipleusageofplottingsheets
canintroducesomedifficultyinrelatingeachgraphicalsolutiontothePPI
display.ThroughconstructingthevectortrianglesdirectlyonthePPI
display,thegraphicalsolutionscanberelatedmorereadilytothePPI
display.Also,thedirectplottingiscompatiblewithatechniquewhichcanbe
usedtoevaluatetheeffectofanyplannedevasiveactionontherelative
movementsofradarcontactsforwhichtruecourseandspeedsolutionshave
not been obtained.
Theforegoingdiscussionofthecomparativeadvantagesofrapidradar
plottingovertransferplottinginamultipleradarcontactsituationdoesnot
meantoimplythatrapidradarplottingtechniquesalwaysshouldbeused
wheneverfeasible.Eachbasictechniquehasitsindividualmerits.Insome
situations,themoreaccuratesolutionsaffordedbytransferplottingmay
justifythegreatertimerequiredforproblemsolution.However,theradar
observershouldrecognizethatthesmallobservationalandplottingerrors
normallyincurredcanintroducesignificanterrorinanapparentlyaccurate
transferplottingsolution.Atransferplottingsolutionmayindicatethata
contactonacoursenearlyoppositetothatofownshipwillpasstostarboard
whiletheactualsituationisthateachshipwillpassporttoportifnoevasive
actionistaken.Ifinthissituationownship’scourseischangedtotheleftto
increasetheCPAtostarboard,thecourseoftheothershipmaybechanged
toitsrighttoincreasetheCPAofacorrectlyevaluatedportpassing.Such
action taken by own ship could result in a collision.
81
RADAR PLOTTING SYMBOLS
(See Alternative Radar Plotting Symbols)
RELATIVE PLOTVECTOR TRIANGLE
SymbolMeaningSymbolMeaning
ROwn Ship.eTheoriginofanyship’strue(course-speed)vector;
fixed with respect to the earth.
MOther Ship.
M
1
First plotted position of other ship.rTheendofownship’strue(course-speed)vector,er;
the origin of the relative (DRM-SRM) vector,rm.
M
2
, M
3
Later positions of other ship.
M
x
PositionofothershiponRMLatplannedtimeof
evasive action; point of execution.
r
1
,r
2
Theendsofalternativetrue(course-speed)vectorsfor
own ship.
NRMLNew relative movement line.erOwn ship’s true (course-speed) vector.
RMLRelative movement line.mTheendofothership’strue(course-speed)vector,em;
the end of the relative (DRM-SRM) vector,rm.
DRM
Directionofrelativemovement;alwaysinthe
direction ofM
1
→
M
2
→
M
3
........
emOther ship’s true (course-speed) vector.
SRMSpeed of relative movement.rmTherelative(DRM-SRM)vector;alwaysinthe
direction of M
1
→
M
2
→
M
3
........
MRMMilesofrelativemovement;relativedistancetraveled.
CPAClosed point of approach.
82
Figure 3.28 - Examples of use of radar plotting symbols.
83
GRAPHICAL SOLUTIONS ON THE REFLECTION PLOTTER
RAPID RADAR PLOTTING
CLOSEST POINT OF APPROACH
Todeterminetheclosestpointofapproach(CPA)ofacontactby
graphicalsolutiononthereflectionplotter,followtheproceduregiven
below.
(1)Plotatleastthreerelativepositionsofthecontact.Iftherelative
positionslieinastraightornearlystraightline,fairalinethroughthe
relativepositions.Extendthisrelativemovementline(RML)pastthe
center of the PPI.
(2)Crankoutthevariablerangemarker(VRM)untiltheringdescribed
by it is tangent to the RML as shown in figure 3.29. The point of
tangency is the CPA.
(3)TherangeatCPAisthereadingoftheVRMcounter;thebearingat
CPAisdeterminedbymeansofthemechanicalbearingcursor,
parallel-linecursor,orothermeansforbearingmeasurementfromthe
center of the PPI.
Note:TheRMLshouldbereconstructedifthecontactdoesnotcontinueto
plot on the RML as originally constructed.
TRUE COURSE AND SPEED OF CONTACT
Todeterminethetruecourseandspeedofacontactbygraphicalsolution
on the reflection plotter, follow the procedure given below.
(1)AssoonaspossibleafteracontactappearsonthePPI,plotitsrelative
positiononthereflectionplotter.Labelthepositionwiththetimeof
the observation as shown in figure 3.29. When there is no doubt with
respecttothehouroftheplot,itisonlynecessarytoshowthelasttwo
digits,i.e.,theminutesafterthehour.Inthoseinstanceswherean
undulylongwaitwouldnotberequireditmightbeadvantageousto
delaystartingthetimedplotuntilthetimeissometenthofanhour...,
6minutes,12minutes,18minutes,etc.,afterthehour.Thistiming
couldsimplifytheuseofthe6-minuteplottingintervalnormallyused
with the rapid radar plotting technique.
(2)Examinetherelativeplottodeterminewhetherthecontactisona
steadycourseatconstantspeed.Ifso,therelativepositionsplotina
straightornearlystraightline;therelativepositionsareequally
Figure 3.29 - Closest point of approach.
84
(3)Withthecontactonasteadycourseatconstantspeed,selectasuitable
relativepositionastheoriginoftherelativespeed(DRM-SRM)
vector; label this plot r as shown in figure 3.30.
(4)Cranktheparallel-linecursoruntilitslinesareparalleltotheheading
flash.Asshowninfigure3.30,placetheappropriateplasticruleso
thatonenotchisatranditsstraightedgeisparalleltothelinesofthe
cursorandtheheadingflash.Theruleisscaledfora6-minuterun
between notches.
(5)Selectthetimeintervalforthesolution,12minutesforexample.
Accordingly,theorigineofownship’strue(course-speed)vectorer
isatthesecondnotchfromr;m,theheadofthecontact’strue(course-
speed)vector,isattheplot12minutesbeyondrinthedirectionof
relative movement.
(6)Construct the contact’s true (course-speed) vectorem.
(7)Cranktheparallel-linecursorsothatitslinesareparalleltovectorem
asshowninfigure3.31.Thecontact’struecourseisreadonthetrue
bearingdialusingtheradiallineoftheparallel-linecursor;the
contact’struespeedisestimatedbyvisualcomparisonwithown
ship’struevectorer.Forexampleifemisabouttwo-thirdsthelength
ofer,thecontact’sspeedisabouttwo-thirdsownship’sspeed.Or,the
notchedrulecanbeusedtodeterminethespeedcorrespondingtothe
length ofem.
Figure 3.30 - Use of the notched plastic rule.
Figure 3.31 - Use of parallel-line cursor to find true course of contact.
85
COURSE TO PASS AT SPECIFIED CPA
Theprocedurefordeterminingownship’snewcourseand/orspeedto
reduce the risk of collision is given below.
(1)Continuingwiththeplotusedinfindingthetruecourseand
speed of the contact, mark the point of execution (Mx) on the RML as
showninfigure3.32.MxisthepositionofthecontactontheRMLat
theplannedtimeofevasiveaction.Thisactionmaybetakenata
specificclocktimeorwhentherangetothecontacthasdecreasedtoa
specified value.
(2)CranktheVRMtothedesireddistanceatCPA.Thisisnormallythe
distancespecifiedforthedangerorbufferzone.Ifthefixedrangeringsare
displayedandonerangeringisequaltothisdistance,itwillnotbenecessary
to use the VRM.
(3)FromMxdrawthenewRMLtangenttotheVRMcircle.Twolines
can be drawn tangent to the circle, butthe line drawn in figure 3.32 fulfills
the requirement that the contact pass ahead of own ship. If the new RML
crosses the heading flash, the contact will pass ahead.
Toavoidparallax,theappropriatesectoroftheVRMmaybemarkedon
thereflectionplotterandthenewRMLdrawntoitratherthanattemptingto
draw the new RML tangent to the VRM directly.
(4)Usingtheparallel-linecursor,drawalineparalleltothenewRML
throughmorthefinalplot(relativeposition)usedindeterminingthecourse
andspeedofthecontact.Thislineisdrawnfromminadirectionoppositeto
thenewDRMbecausethenewrelativespeed(DRM-SRM)vectorwillbe
paralleltothenewRMLandthehead(m)ofthenewvector(r'm)willliein
the new DRM away from the origin, r'.
(5)Avoidingbycoursechangeonly,themagnitudeofown’strue(course-
speed)vectorremainsconstant.Therefore,thesamenumberofnotcheson
theplasticruleusedforownship’struevectorforthecontact’scourseand
speedsolutionareusedforownship’snewtruevectorer'.Withonenotch
setate,therulerisadjustedsothatthethirdnotchawayintersectstheline
drawn parallel to the new RML. As shown in figure 3.32, the intersection at
r'istheheadoftherequirednewtruevectorforownship(er');itisthe
origin of the new relative speed vector,r'm.
Thepreviouslydescribeduseoftheplasticruler,ineffect,rotatesvector
eraboutitsorigin;theheadofthevectordescribesanarcwhichintersects
the line drawn parallel to the new RLM atr'.
Ifthespeedofthecontactweregreaterthanownship’sspeed,there
wouldbetwointersectionsand,thus,twocoursesavailabletoproducethe
desireddistanceatCPA.Generally,thepreferredcourseisthatwhichresults
inthehigherrelativespeed(thelongerrelativespeedvector)inorderto
expedite safe passing.
Figure 3.32 - Evasive action.
86
SPECIAL CASES
Insituationswherecontactsareoncoursesoppositetoownship’scourse
orareonthesamecourseasownshipbutatslowerorhigherspeeds,the
relativemovementlinesareparalleltoownship’scourseline.Ifacontact
hasthesamecourseandspeedasownship,thereisnorelativemovement
line;allrelativepositionslieatonepointataconstanttruebearingand
distancefromownship.Ifacontactisstationaryordeadinthewater,the
relativevectorrmandownship’struevectorerareequalandopposite,and
coincident. Withe andm coincident, there is no vectorem.
Thesolutionsofthesespecialcasescanbeeffectedinthesamemanneras
thosecasesresultingintheconventionalvectortriangle.However,novector
triangle is formed; the vectors lie in a straight line and are coincident.
Infigure3.33contactsA,B,C,andDareplottedfora12-minuteinterval;
ownship’struevectorerisscaledinaccordancewiththistime.Inspectionof
theplotforcontactArevealsthattheDRMisoppositetoownship’scourse;
therelativespeedisequaltoownship’sspeedplusthecontact’sspeed.The
contactisonacourseoppositetoownship’scourseataboutthesamespeed.
InspectionoftheplotforcontactBrevealsthattheDRMisoppositeto
ownship’scourse;therelativespeedisequaltoownship’sspeedminusthe
contact’sspeed.Thecontactisonthesamecourseasownshipataboutone-
half own ship’s speed.
InspectionoftheplotforcontactCrevealsthattheDRMisoppositeto
ownship’scourse;therelativespeedisequaltoownship’sspeedplusthe
contact’sspeed.Thecontactisonacourseoppositetoownship’scourseat
about the same speed.
InspectionoftheplotforcontactDrevealsthattheDRMisthesameas
ownship’scourse;therelativespeedisequaltothecontact’sspeedminus
ownship’sspeed.Thecontactisonthesamecourseasownshipatabout
twice own ship’s speed.
87
Figure 3.33 - Special cases.
88
CONSTRUCTING THE PLASTIC RULE USED WITH RAPID RADAR PLOTTING
Whenplottingbytherapidradarplottingtechnique,acolored6to8-inch
flexibleplasticstraightedgeisnormallyusedtoconstructthevectorsand
otherlinesegmentsonthereflectionplotter.Thefollowingprocedurecanbe
usedtoconstructthedesiredscaleforvectormagnitudesonthestraightedge.
(1)Switchtheradarindicatortoanappropriateplottingrange,24miles
for example.
(2)Crankoutthevariablerangemarker(VRM)toanintegralvalueof
range,5milesforexample.Markthereflectionplotteratthe
intersectionoftheVRMandtheheadingflashasshowninfigure
3.34. This point will represent zero on the scale to be constructed for
subsequent transfer to the plastic strip.
(3)Computethedistanceownshipwilltravelin6minutesataspeed
expectedtobeusedincollisionavoidance.Ataspeedof21knots,
own ship will travel 2.1 miles in 6 minutes.
(4)Sincethezeromarkisat5milesonthePPI,crankouttheVRMto7.1
milesandmarkthereflectionplotterattheintersectionoftheVRM
andtheheadingflashtoobtainthescalespacingfor2.1miles.Repeat
thisprocedurewiththeVRMsetat9.2,11.3,and13.4milestoobtain
otherscalegraduations2.1milesapart.Thelengthbetweenscale
marksat5.0and7.1milesprovidesthemagnitudeof6-minute
vectorsat21knots;thelengthbetweenscalemarksat5.0and9.2
provides the magnitudes of 12-minute vectors at 21 knots, etc.
(5) As shown in figure 3.35, lay the plastic strip adjacent to the
graduationmarksonthereflectionplotterandparalleltotheheading
flash.Extendthegreasepencilmarksontotheplasticstrip.Withthe
scaletransferredtotheplasticstrip,apermanentruleismadeby
notchingthescaleontheplasticstrip.Thenotchesintheruleshown
in figure 3.35 have been drawn large and angular for illustration
purposesonly.Theyshouldbeaboutthesizeandshapeofthecross-
section of the lead used in the grease pencil.
(6)Severalrulesarenormallyused,eachgraduatedforaparticularrange
scalesettingandownshipspeed.Therangeandspeedshouldbe
prominently marked on each rule.
Figure 3.34 - Constructing the scale.
Figure 3.35 - Graduating the rule.
89
EXAMPLES
e-r-m TRIANGLE
EXAMPLE 1
.
DETERMINATION OF CLOSEST POINT OF APPROACH (CPA)
EXAMPLE 2
.
COURSE AND SPEED OF A RADAR CONTACT
EXAMPLE 3
.
COURSE AND SPEED OF RADAR CONTACT BY THE LADDER METHOD
EXAMPLE 4
.
COURSE TO PASS A SHIP AT A SPECIFIED CPA
Own Ship’s Speed is Greater Than That of Other Ship
EXAMPLE 5
.
COURSE TO PASS A SHIP AT A SPECIFIED CPA
Own Ship’s Speed is Less Than That of Other Ship
EX
AMPLE 6
.
VERIFICATION OF FIXED OBJECTS OR RADAR CONTACTS DEAD IN THE WATER
EXAMPLE 7
.
AVOIDANCE OF MULTIPLE CONTACTS WITHOUT FIRST DETERMINING TRUE COURSES AND SPEEDS
OF THE CONTACTS
EXAMPLE 8
.
DETERMINING THE CLOSEST POINT OF APPROACH FROM THE GEOGRAPHICAL PLOT
90
EXAMPLE 1
DETERMINATION OF CLOSEST POINT OF APPROACH (CPA)
Situation:
Withownshiponcourse070˚andtheradarsetonthe12-milerange
scale, other ship M is observed as follows:
Required:
(1) Direction of relative movement (DRM).
(2) Speed of relative movement (SRM).
(3) Bearing and range at closest point of approach (CPA).
(4) Estimated time of arrival at CPA.
Solution:
(1)Plotandlabeltherelativepositions,M
1
,M
2
,andM
3
,usingthe1:1
scale;fairalinethroughtherelativepositions;extendthisline,therelative
movement line (RML), beyond the center of the Maneuvering Board.
(2)ThedirectionoftheRMLfromtheinitialplotM
1
,isthedirectionof
relative movement (DRM): 236˚.
(3)Measuretherelativedistance(MRM)betweenanytwotimedplots
ontheRML,preferablybetweenthetwobestplotswiththegreatesttime
separation.Inthisinstance,measurethedistancebetweenM
1
andM
3
:
3.0miles.Usingthecorrespondingtimeinterval(1000-1012=12m),
obtainthespeedofrelativemovement(SRM)fromtheLogarithmic
Time-Speed-DistanceScaleatthebottomoftheManeuveringBoard:15
knots.
(4)Fromthecenteroftheradarplottingsheet,R,drawaline
perpendiculartotheRML;labeltheintersectionCPA.Thedirectionofthe
CPAfromthecenteroftheplottingsheet,i.e.,ownship’sposition,isthe
bearingoftheCPA:326˚;thedistancefromthecenterorownshipisthe
range at CPA: 0.9 mile.
(5)MeasurethedistancefromM
3
toCPA:6.0miles.Usingthisdistance
andthespeedofrelativemovement(SRM):15knots,obtainthetime
intervalfrom1012(thetimeofplotM
3
)bymeansoftheTime-Speed-
DistanceScale:24
m
.TheestimatedtimeofarrivalatCPAis1012+24
m
=
1036.
Answers:
(1)DRM236˚;(2)SRM15knots;(3)CPA326˚,0.9mile;(4)ETAat
CPA 1036.
TimeBearingRange (miles)Rel. position
1000050˚9.0M1
1006049˚7.5M2
1012047˚6.0M3
91
EXAMPLE 1
Notes:
1.Thereshouldbesufficientplotsto
insureaccurateconstructionoftheRML
fairedthroughtheplots.Shouldonlytwo
plotsbemade,therewouldbenomeans
ofdetectingcourseorspeedchangesby
theothership.Thesolutionisvalidonly
iftheothershipmaintainscourseand
speedconstant.Preferably,thetimed
plotsshouldbemadeatequaltime
intervals.Equalspacingoftheplots
timedatregularintervalsandthe
successiveplottingoftherelative
positionsinastraightlineindicatethat
theothershipismaintainingconstant
course and speed.
2.Thistransferplottingsolution
requiredindividualmeasurementsand
recordingoftherangesandbearingsof
therelativepositionofshipMatintervals
oftime.Italsoentailedthenormal
requirementofplottingtherelative
positionsonthePPIorreflectionplotter.
Visualizingtheconcentriccirclesofthe
ManeuveringBoardasthefixedrange
ringsofthePPI,afastersolutionmaybe
obtainedbyfairingalinethroughthe
greasepencilplotonthePPIand
adjustingtheVRMsothatthecircle
describedistangenttoorjusttouchesthe
RML.TherangeatCPAisthesettingof
theVRM;thebearingatCPAandthe
DRMmaybefoundbyuseofthe
parallel-linecursor(parallelindex).The
timeoftheCPAcanbedeterminedwith
reasonableaccuracythroughvisual
inspection,i.e.,thelengthalongtheRML
fromM
3
toCPAbyquickvisual
inspectionisabouttwicethelength
betweenM
1
andM
3
representingabout
24 minutes.
92
EXAMPLE 2
COURSE AND SPEED OF A RADAR CONTACT
Situation:
OwnshipRisoncourse340˚,speed15knots.Theradarissetonthe12-
milerangescale.Aradarcontact,shipM,isobservedtobechangingcourse,
andpossiblyspeed,betweentimes0953and1000.Whilekeepingaclose
watchoftherelativemovement,therelativepositionsofMaremarkedat
frequent intervals on the reflection plotter by grease pencil.
Required:
(1)CourseandspeedofshipMwhenMhassteadiedoncourseandspeed.
Solution:
(1)Withthedecisionmadethatthesolutionwillbeobtainedbyrapid
radarplotting,thesolutionisstartedwhileMisstillmaneuveringthrough
determining:(a)thedistanceownshipwilltravelthroughthewaterduringa
timelapseof6minutesand(b)thelengthofsuchdistanceonthePPIatthe
range setting in use.
(i)Thedistancetraveledbyownshipin6minutesisone-tenthofthe
speed in knots, or 1.5 nautical miles.
(ii)Thelengthof1.5nauticalmilesonthePPImaybefoundthroughuse
ofthevariablerangemarker(VRM).CranktheVRMouttoaconvenient
starting point, 6 miles for instance.
MarktheintersectionoftheVRMandtheheadingflash.CranktheVRM
outto7.5milesandmarktheintersectionoftheVRMandtheheadingflash.
Thelengthbetweenthetwomarks(1.5mi.)istransferredtoashortplastic
rule.
(2)ObservationofthePPIrevealsthatbetween1000and1006,Misona
steadycourseatconstantspeed(successiveplotsformastraightlineonthe
scope;plotsforequaltimeintervalsareequallyspaced).Drawtherelative
movementline(RML)fromthe1000plot(M
1
)throughthe1006plot(M
3
),
extending beyond the center of the PPI.
(3)Setcenterlineofparallel-linecursortoheadingflash.Atthe1000plot
(M
1
)placetheplasticrule,markedforthe6-minuterunofownship,parallel
tothecursorlines.Inthedirectionofownship’scourse,drawalineof1.5
mileslengthwhichendsatthe1000plot.Twosidesofthevectortriangle
havebeenformed(erandrm).Thesolutionisobtainedbycompletingthe
triangle to form true (course-speed) vectorem.
(4)Oncompletingthetriangle,thethirdside,vectorem,representsthe
truecourseandrateofmovementofM.Thetruecoursemaybereadby
adjustingtheparallel-linecursorparalleltothethirdside,truevectorem.
ThespeedofMinknotsmaybeestimatedbycomparingthelengthofem
withthelengthofer,thetrue(course-speed)vectorofownshipR,thespeed
of which in knots is known.
Answers:
(1) Course 252˚, speed 25 knots.
93
EXAMPLE 2
Heading-Upward
Unstabilized PPI Display
with Stabilized True
Bearing Dial
Scale: 12-mile range setting
Note:
Insomecasesitmaybe
desirabletoconstructownship’s
truevectororiginatingattheend
ofthesegmentoftherelativeplot
useddirectlyastherelative
vectorrm.Ifappliedtothiscase,
the6-minuterunofownship
wouldbedrawnfromthe1006
plotinthedirectionofownship’s
course.Oncompletingthe
triangle,thethirdsidewould
representthetruecourseandrate
of movement of M.
94
EXAMPLE 3
COURSE AND SPEED OF RADAR CONTACT BY THE LADDER METHOD
Situation:
OwnshipRisoncourse120˚,speed15knots.Theradarissetonthe6-
milerangescalebecausesmallwoodenvesselsareexpectedtobe
encountered.Therangescalesettingisbeingshiftedperiodicallytolonger
rangesforpossibledetectionofdistanttargets.Aradarcontactisbeing
plottedonthereflectionplotter.Inspectionoftheplotrevealsthatthecontact
is on steady course at constant speed (see solution step (2) of example 2).
Required:
(1) Course and speed of the radar contact.
Solution:
(1)Withthedecisionmadethatthesolutionswillbeobtainedbyrapid
radarplotting,theradarobserverfurtherelectstousetheLadderMethodin
ordertobeabletorefinethesolutionastherelativeplotforthecontact
develops with time.
(2)Fora6-minuteintervaloftime,ownshipat15knotsruns1.5nautical
miles through the water; the run for 12 minutes is 3.0 nautical miles.
(3)Drawownship’strue(course-speed)vectorerinthedirectionofown
ship’struecourse,withtheheadofthevectoratthe0506plot;thelengthof
thisvectorisdrawninmultiplesof6-minuterunsofownshipand
subsequentlysubdividedbyeyetoformaladder.Sincethetimedplotonthe
relativemovementlinestartsat0506,thestartingpointofthe6-minuterun
ofownshipislabeled12;thestartingpointofthe12-minuterunislabeled
18.
(4)Thefirstsolutionisobtainedattime0512bydrawingalinefromthe
12-graduationorrungontheladdertothe0512plotontheRML.Thisline,
whichcompletesthevectortrianglefora6-minuterun,representsthetrue
courseandrateofmovementofthecontact.Thetruecourseandspeedofthe
contact is obtained as in solution step (4) of Example 2.
(5)Thesecondsolutionisobtainedattime0515bydrawingalinefrom
the15-graduationorrungontheladdertothe0515plotontheRML.This
line,whichcompletesthevectortrianglefora9-minuterun,representsthe
true course and rate of movement of the contact.
Answers:
(1) Course 072˚, Speed 17 knots.
95
EXAMPLE 3
Heading-Upward
Unstabilized PPI Display
with Stabilized True
Bearing Dial
Scale: 6-mile range setting
Notes:
1.Usingtheladdermethod,the
radarobserverisabletoobtainan
approximatesolutionquicklyand
thenrefinethesolutionastheplot
develops.
2.Thissolutionwassimplified
bystartingthetimedplotatsome
tenth of an hour after the hour.
96
EXAMPLE 4
COURSE TO PASS A SHIP AT A SPECIFIED CPA
(Own ship’s speed is greater than that of other ship)
Situation:
OwnshipRisoncourse188˚,speed18knots.Theradarissetonthe12-
milerangescale.OthershipM,havingbeenobservedandplottedbetween
times1730and1736,isoncourse258˚at12knots.ShipsMandRareon
collision courses. Visibility is 2.0 nautical miles.
Required:
(1)CourseofownshipRat18knotstopassaheadofothershipMwitha
CPAof3.0nauticalmilesifcourseischangedtotherightwhentherangeis
6.5 nautical miles.
Solution:
(1)ContinuingwiththeplotonthePPIusedinfindingthetruecourseand
speedofothershipM,plotM
x
bearing153˚,6.5nauticalmilesfromR.
AdjusttheVRMto3.0nauticalmiles,thedesireddistanceatCPA.FromM
x
drawalinetangenttotheVRMcircleatM
3
.FromM
x
twolinescanbe
drawntangenttothecircle,butthepointoftangencyatM
3
fulfillsthe
requirementthatownshippassaheadoftheothershiporthatothershipM
pass astern of own ship R.
(2)Fromtheoriginofthetruevectorsofthevectortriangleusedin
findingthetruecourseandspeedofshipM,pointe,describeanarcofradius
1.8nauticalmiles.SinceownshipRwillnotchangespeedinthemaneuver,
thedistanceandcorrespondingPPIlengthofownship’struevector(1.8
nauticalmilesfora6-minuterunofownshipat18knots)isusedasthe
radius of the arc.
(3)Usingtheparallel-linecursor,drawalinethroughM
2
paralleltothe
new RML (M
x
M
3
) to intersect the arc drawn in (2).
(4)TheintersectionofthearcwiththelinethroughM
2
paralleltothenew
RMLestablishestheheadoftheownship’snewtrue(course-speed)vector
drawnfrompointe.Therefore,ownship’snewcoursewhenothershipM
reachesrelativepositionM
x
isrepresentedbythetruevectordrawnfrom
point e to the intersection atr
1
.
Answers:
(1) Course 218˚.
Notes:
1.ActuallythearcintersectingthelinedrawnM
2
inadirectionopposite
tothenewDRMwouldalsointersectthesamelineifextendedinthenew
DRM.Butanewcourseofownshipbaseduponthisintersectionwould
reversethenewDRMorreversethedirectiontheothershipwouldploton
the new RML.
2.IfthespeedofothershipMweregreaterthanownshipR,therewould
betwocoursesavailableat18knotstoproducethedesireddistanceatCPA.
Generally,thepreferredcourseisthatwhichresultsinthehighestrelative
speed in order to expedite the safe passing.
97
EXAMPLE 4
North-Upward
Stabilized PPI Display
Scale: 12-mile range setting
Notes: (Continued)
3.Afterownship’scoursehas
beenchanged,othershipR
shouldplotapproximatelyalong
thenewRML,asdrawnandin
thedesireddirectionofrelative
movement.Thiscontinuityofthe
plotfollowingacoursechangeby
ownshipisoneoftheprimary
advantagesofastabilized
display.Immediatelyfollowing
anyevasiveaction,oneshould
inspectthePPItodetermine
whetherthetarget’sbearingis
changingsufficientlyandinthe
desireddirection.Withthe
stabilizeddisplay,theansweris
before the radar observer’s eyes.
98
EXAMPLE 5
COURSE TO PASS SHIP AT A SPECIFIED CPA
(Own ship’s speed is less than that of other ship)
Situation:
OwnshipRisoncourse340˚,speed15knots.Theradarissetonthe12-
milerangescale.OthershipM,havingbeenobservedandplottedbetween
times0300and0306,isoncourse249˚at25knots.SincetheCPAwillbe
1.5nauticalmilesat310˚ifbothshipsmaintaintheircoursesandspeeds
untiltheyhavepassed,thedistanceatCPAisconsideredtooshortfor
adequate safety.
Required:
(1)CourseofownshipRat15knotstopassasternofothershipMwitha
CPAof3.0nauticalmilesifcourseischangedtotherightwhentherangeto
ship M is 6.0 nautical miles.
Solution:
(1)ContinuingwiththeplotonthePPIusedinfindingthetruecourse,
speed,andCPAofshipM,plotM
x
ontheRML6.0nauticalmilesfromown
shipR.SettheVRMto3.0nauticalmiles,thedesireddistanceatCPA(in
thiscasetheVRMsettingiscoincidentwiththefirstfixedrangering).From
M
x
twolinescanbedrawntangenttotheVRMcircle,butthepointof
tangencyatM
3
fulfillstherequirementthatownshippassasternofother
ship M.
(2)Fromtheoriginofthetruevectorsofthevectortriangleusedin
findingthetruecourseandspeedofshipM,pointe,describeanarcofradius
1.5nauticalmiles.Sinceownshipwillnotchangespeedinthemaneuver,
thedistanceandcorrespondingPPIlengthofownship’struevector(1.5
nauticalmilesfora6-minuterunofownshipat15knots)isusedasthe
radius of the arc.
(3)Usingtheparallel-linecursor,drawalinethroughM
2
paralleltothe
new RML (M
x
M
3
) to intersect the arc drawn in (2).
(4)SincethespeedofothershipMisgreaterthanthatofownshipR,the
arcintersectsthelinethroughM
2
attwopoints.Eachintersectionestablishes
aheadofapossiblenewownship’struevector.Ofthetwopossiblevectors
oneprovidesahigherspeedofrelativemovementthantheother.Generally,
thetruevectorwhichprovidesthehigherSRMorlongerrelativevectoris
chosentoexpeditethepassing.However,inthisexampleacoursechangeto
therightisspecified.Thisrequirestheuseofvectorer
1
,whichprovidesthe
higher SRM.
(5)Withthisunstabilized,Heading-UpwardPPIdisplay,thereisa
complicationarisingfromtheplotshiftingequalandoppositetotheamount
anddirectionofthecoursechange.Somereflectionplotterdesignshave
provisionsforeithermanualorautomaticshiftingoftheirplottingsurfaces
tocompensatefortheshiftingoftheplot.Withoutthiscapability,thereisno
continuityinthegreasepencilplotfollowingcoursechangesbyownship.
Consequently,itisnecessarytoerasetheplotandreplottheothership’s
relativepositionwhenownshipsteadiesoncourse.WiththeVRMsetto3.0
miles,thenewRMLmustbedrawntangenttothecircledescribedbythe
VRM.Theothershipmustbewatchedcloselytoinsurethatitsrelative
movement conforms with the new RML.
Answers:
(1) Course 030˚.
99
EXAMPLE 5
Heading–Upward
Unstabilized PPI Display
with Stabilized True
Bearing Dial
Scale: 12-mile range setting
Note:
Examinationoftheplotreveals
thatifownshipRmaintainsits
originaltruecourse(340˚),the
intersectionoftheoriginaltrue
vectorer of own ship with the line
drawnthroughM
2
paralleltothe
newRMLprovidestheheadofthe
vectorer
2
requiredtoeffectthe
desiredCPAwithoutcourse
change.Sincethelengthofvector
er
2
isapproximatelyhalfthatof
theoriginalvectorer,an
instantaneouschangeto
approximatelyhalftheoriginal
speedwouldproducethedesired
results.Alesserchangeofcourse
totherightinconjunctionwitha
speedreductioncouldbeusedto
compensate for deceleration.
100
EXAMPLE 6
VERIFICATION OF FIXED OBJECTS OR RADAR CONTACTS DEAD IN THE WATER
Situation:
OwnshipRisoncourse340˚,speed20knots.Theradarissetatthe24-
mile range scale. Radar observations are made as follows:
The RML is parallel to and the DRM is opposite to own ship’s course, 340˚.
Required:
CourseandspeedofMinordertoverifywhetherMisdeadinthewater
or a terrestrial object.
Solution:
(1)OnthePPI,preferablyareflectionplottermountedthereon,plotM
1
,
M
2
,M
3
.Drawtherelativemovementline(RML)throughtherelative
positions, M
1
, M
2
, M
3
.
(2)Usingthesamedistancescaleastheradarrangesetting,determinethe
lengthofthetrue(course-speed)vectorerofownshipRforatimeinterval
of 36 minutes: 12 miles.
(3)Drawtruevectorerinthedirectionofownship’scoursewithitshead
atrelativepositionM
1
.If,aftersuchgraphicalconstruction,thevectororigin
eliesoverrelativepositionM
3
,thelengthoftheemvectorwouldbezero.
Thus,thetruespeedoftheobservedcontactwouldbezero.Evenifthe
observedtargetisdeadinthewaterorafixedobject,smallobservationaland
plottingerrorswillfrequentlyindicateasmallvalueoftruespeedforthe
contact.
TimeBearingRange (miles)Rel. position
1200017˚22.8M
1
1218029˚17.4M
2
1236046˚14.4M
3
101
EXAMPLE 6
Heading-Upward
Unstabilized PPI Display
with Stabilized True
Bearing Dial
Scale: 24-mile range setting
102
EXAMPLE 7
AVOIDANCE OF MULTIPLE CONTACTS WITHOUT FIRST DETERMINING THE TRUE
COURSES AND SPEEDS OF THE CONTACTS
Situation:
OwnshipRisoncourse000˚,speed20knots.Withthestabilizedrelative
motiondisplayradarsetatthe12-milerangesetting,radarcontactsA,B,
andCareobservedandplotteddirectlyonthePPIorreflectionplotter.The
plots at time 1000 are considered as the initial plots in the solution.
Required:
(1)DeterminethenewrelativemovementlinesforcontactsA,B,andC
whichwouldresultfromownshipchangingcourseto065˚andspeedto15
knots at time 1006.
(2)Determinewhethersuchcourseandspeedchangewillresultin
desirable or acceptable CPA’s for all contacts.
Solution:
(1)WiththecenterofthePPIastheirorigin,drawownship’struevectors
erander'forthecourseandspeedineffectortobeputineffectattimes
1000and1006,respectively.Usingthedistancescaleoftheradar
presentation,draweachvectoroflengthequaltothedistanceownshipR
willtravelthroughthewaterduringthetimeintervaloftherelativeplot
(relativevector),6minutes.Vectorer,havingaspeedof20knots,isdrawn
2.0milesinlengthintruedirection000˚;vectorer',havingaspeedof15
knots, is drawn 1.5 miles in length in true direction 065˚.
(2) Draw a dashed line betweenr andr'.
(3)ForcontactsA,B,andC,offsettheinitialplots(A
1
,B
1
,andC
1
)inthe
samedirectionanddistanceasthedashedliner-r';labeleachsuchoffsetplot
r'.
(4)Ineachrelativeplot,drawastraightlinefromtheoffsetinitialplot,r',
throughthefinalplot(A
2
orB
2
orC
2
).Thelinesr'A
2
,r'B
2
,andr'C
2
representthenewRML'swhichwouldresultfromacoursechangeto065˚
and speed change to 15 knots at time 1006.
Answers:
(1)New RML of contact A-DRM 280˚
New RML of contact B-DRM 051˚
New RML of contact C-DRM 028˚
(2)Inspectionofthenewrelativemovementlinesforallcontacts
indicatesthatifallcontactsmaintaincourseandspeed,allcontactswillplot
alongtheirrespectiverelativemovementlinesatasafedistancesfromown
ship R on course 065˚, speed 15 knots.
Explanation:
Thesolutionisbasedupontheuseoftherelativeplotastherelative
vector.Witheachcontactmaintainingtruecourseandspeed,theemvector
foreachcontactremainsstaticwhileownship’ser'vectorisrotatedaboute
tothenewcourseandchangedinmagnitudecorrespondingtothenew
speed.
103
EXAMPLE 7
North-Upward
Stabilized PPI Display
Scale: 12-mile range setting
104
EXAMPLE 8
DETERMINING THE CLOSEST POINT OF APPROACH FROM THE GEOGRAPHICAL PLOT
Situation:
OwnshipRisoncourse000˚,speed10knots.Thetruebearingsand
rangesofanothershipareplottedfromownship’ssuccessivepositionsto
form a geographical (navigational) plot:
Required:
(1) Determine the closest point of approach.
Solution:
(1)Sincethesuccessivetimedpositionsofeachshipofthegeographical
plotindicaterateofmovementandtruedirectionoftravelforeachship,each
linesegmentbetweensuccessiveplotsrepresentsatruevelocityvector.
Equalspacingoftheplotstimedatregularintervalsandthesuccessive
plottingofthetruepositionsinastraightlineindicatethattheothershipis
maintaining constant course and speed.
(2)Thesolutionisessentiallyareversaloftheprocedureinrelative
motionsolutionsinwhich,fromtherelativeplotandownship’struevector,
thetruevectoroftheothershipisdetermined.Accordingly,thetruevectors
fromthetwotrueplotsforthesametimeinterval,0206-0212forexample,
are subtracted to obtain the relative vector (rm = em - er).
(3)Therelative(DRM-SRM)vectorrmisextendedbeyondownship’s
0212 position to form the relative movement line (RML).
(4)Theclosestpointofapproach(CPA)isfoundbydrawingalinefrom
own ship’s 0212 plot perpendicular to the relative movement line.
Answers:
(1) CPA 001˚, 2.2 miles.
TimeBearingRange (miles)Rel. position
0200074˚7.3T
1
0206071˚6.3T
2
0212067˚5.3T
3
105
EXAMPLE 8
Note:
Eitherthetime0200,0206,or
0212plotsoftheothershipcan
beusedastheoriginofthetrue
vectorsofthevectordiagram.
Usingthetime0200plotasthe
originandatimeintervalof6
minutesforvectormagnitude,the
lineperpendiculartothe
extendedrelativemovementline
wouldbedrawnfromthetime
0206 plot of own ship.
WhiletheManeuveringBoard
hasbeenusedinillustratingthe
solution,thetechniqueis
applicabletosolutionsforCPA
ontruemotiondisplays.See
PRACTICALSOLUTIONFOR
CPAINTRUEMOTION
MODE.
106
ALTERNATIVE RADAR PLOTTING SYMBOLS
Thealternativeradarplottingsymbolsdescribedinthissectionwere
derivedfromthoseusedinRealTimeMethodofRadarPlottingbyMaxH.
CarpenterandCaptainWayneM.WaldooftheMaritimeInstituteof
TechnologyandGraduateStudies,LinthicumHeights,Maryland.Theabove
manualshouldbereferredtoforamorecompleteexplanationofthesymbols
and their use in radar plotting.
Theexplanationofthealternativesymbolsasgivenherefollowsan
approachdifferentfromthatusedbyCarpenterandWaldo.Thetwo
approaches should be helpful to the student.
Thealternativesymbolsaredeemedtoprovidesimplerandmore
representationalsymbologyforRapidRadarPlottingthandoesthe
ManeuveringBoardsymbology,whichhasvalueforrelativemotion
solutionsofgreatervarietythanthosenormallyassociatedwithcollision
avoidance.Greatersimplicityisaffordedbyusingthesamesymbolsforthe
relativemotionplotandthecorrespondingsideofthevectordiagram
(triangle).Thesymbolsaredeemedtobemorerepresentationalinthatthe
symbols suggest their meaning.
Asshowninfigure3.36,therelativemotionplotislabeledR-M;thetrue
motionplotislabeledT-M.Intherelativemotioncase,thefirstplotisatR;
thesecondplot(ortheplotforthetimeintervaltobeusedinthesolution)is
labeledM.Thus,R-Misdescriptiveoftherelativemotionplotted.Likewise
withthefirstplotbeinglabeledTinthetruemotioncase,T-Misdescriptive
of the true motion plotted.
Asisalsoshowninfigure3.36,theplotsareannotatedwithtimeintwo
digits(forminutesoftime).Preferablythefirstplotisforzerotimerather
thanclocktime.Suchpracticeisenhancedwiththeuseofasuitabletimer
whichcanbereadilyresetasrequired.Suchpractice,whichisfollowedhere,
facilitatesplottingatdesiredintervalsandalsoenablesmoreaccuratetiming
of the plot.
Whenusingthissymbologyintextualreferences,timeintervalfromzero
timeisindicatedasasubscriptofasymbolwhenappropriate.Forexample,
therelativeplot(orrelativevector)forplottinginterval3minutesmaybe
shown as
R
00
—M
03
Figure 3.36 - Relative and true motion plots.
107
Inactualplottingonthereflectionplotter,theplacementofthetime
annotation is affected by practical considerations, including clutter.
WithconsiderationatthispointthatRapidRadarPlottingmakesdirect
useoftherelativeplotastherelativevectorofthevectordiagram(triangle),
thesymbolsfortheothertwovectorsorsidesofthetrianglearenow
described.
Sincetheothertwovectorsaretruevectors,thesymbolTisusedto
indicatetheoriginofbothvectorsatacommonpoint.Oneofthetruevectors
mustendatR,theotheratM.ThetruevectorT-Risownship’s(reference
shipRintheothersymbology)true(course-speed)vector;theothertrue
vectorT-Mistheothership’s(othershipMintheothersymbology)true
(course-speed) vector.
Ownship’struevectorT-RbeingsuggestiveoftheabbreviationTRfor
track,inturnsuggeststruecourseandspeed.Or,usingacombinationof
symbologies,thesymbolT-RsuggeststruevectorforreferenceshipR(own
ship).
Theothership’struevectorT-Missuggestiveoftruemotion(oftheother
ship,orofothershipM,usingacombinationofsymbologies).Seefigure
3.37 for the R-T-M triangle.
Nowthinkingintermsoftruemotionratherthantruecourseandspeedof
theothership,theabbreviationsDTMandSTMareusedtoindicate
direction of true motion and speed of true motion, respectively.
In brief the vectors are comprised of the following elements:
R-M:DRM & SRM
T-R:Course & Speed (of own ship)
T-M:DTM & STM (of other ship)
AbbreviationscommontobothsymbologiesareCPA(ClosestPoint
Approach),DRM(DirectionofRelativeMovement),SRM(Speedof
RelativeMovement)andNRML(NewRelativeMovementLine).In
additiontoDTM(DirectionofContact’sTrueMotion)andSTM(Speedof
Contact’sTrueMotion),thealternativesymbologyusesMCPAforminutes
toCPA.ThesymbolRisusedtoindicatetheheadofownship’struevector
followingachangeofcourseorspeedorbothtoobtainanewRML.The
symbol M is also used to indicate the point of execution.
Figure 3.37 -R-T-M triangle.
108
ThefollowingisanalternativepresentationoftheR-T-Mtrianglewhich
does not use vector terminology.
Byexaminingthecombinationgeographic(true)andrelativeplotin
figure3.38,itcanbeseenthatT-Mofthetriangleisthepathactually
followedbytheothershipattherateofitsactualspeed.Atthetimeofthe
firstobservationfromT',theothershipwasactuallyatT,notR.Alsoown
shipwasatT',notR'.However,attheendoftheplottinginterval,theother
shipwasactuallyatMandownshipwasactuallyatR'.Butallobservations
oftheothershipwereactuallyplottedfromR'.Thus,thefirstobservation
placedtheothershipatR;successiveobservationsplacetheothershipat
points alongR-M untilM was reached at the end of the plotting interval.
Intheabovepresentationthetruemotionoftheothershipisgiven.Butin
thenormalcourseofradarobservationforcollisionavoidancepurposes,this
motionmustbedetermined.WithR-Mderivedbyplotting,itcanbeseenby
inspectionthatTofthetrianglecanbelocatedbyconstructingT-Rinthe
directionofownship’scourseandscaledaccordingtothedistanceownship
travelsduringtheplottinginterval.Aftersuchconstruction,thetriangleis
completed to findT-M (DTM & STM).
STANDARD PLOTTING PERIOD
Astandardplottingperiod,whichvariesinasimple,easilyremembered
relationshipwiththerangescalesetting,canbeusedtofacilitatescalingT-
RordeterminingSTMfromT-M.Theuseofstandardplottingperiodis
enhancedwhenthePPIhassixfixedrangeringsandtherangescalesare
1
1
/
2
, 3, 6, 12, 24, and 48 miles.
Thestandardplottingperiodenablesthedirectuseoftherangering
separationasthespeedscaleasshownbelow.Onagivenproperlyadjusted
(forlinearity)PPIwithsixrangerings,theringseparationis5centimeters.
Onthe6-milescale,thisseparation(5centimeters)represents1nautical
mile.Onthe12-milescale,thesameseparationbetweenrings(5
centimeters)represents2nauticalmiles;andonthe24-milescale,4nautical
miles,etc.Withdistanceinmilestraveledin6minutesbeingnumerically
equaltoone-tenthofthespeedinknots,at20knotsavesseltravels2miles
in6minutes.Thus,onthefrequentlyused12-milescale,avesselsteaming
at20knots(relativeortrue)travelsadistance(relativeortrue)equaltothe
rangeringseparation(5centimetersor2nauticalmiles)inthenumberof
minutes(6)equaltoonehalfoftherangescaleinmiles(12).Withtherange
scalechangedto6miles,avesselat20knots(relativeortrue)stilltravelsa
distance(relativeortrue)equaltotherangeringseparation(5centimeters
nowcorrespondingto1nauticalmile)duringthenumberofminutes(3)
equal to one half of the range scale in miles (6).
Whateverthespeedofownshiporoftheothershipmaybe,forthesix-
ringPPIhavingthescalesasdescribedabove,thestandardplottingperiod
remains:aperiodinminutesequaltoonehalfoftherangescaleinmiles.
Forexampleonthe12-milescaleandusingtheassociated6-minutestandard
plottingperiod,avesselat20knotstravelsoneringseparation(5
centimeters)duringtheplottingperiod;at10knotsthevesseltravelsonehalf
oftheringseparationduringthesameperiod.Thusasinglespeedscalecan
becalibratedlinearlyforusewithdifferentrangescales.Buttheassociated
standard plotting period must be used with each range scale.
Figure 3.38 - Combination geographic (true) and relative plot.
109
Insummary,thestandardplottingperiodmakesonerangeringseparation
equalto20knotswhatevertherangescalesettingmaybe.Multiplesand
sub-multiplesofthisonerangeringseparationfor20knotsestablishother
speeds as shown in figure 3.39.
Thestandardplottingintervalsbaseduponthesix-ringPPIandrange
scalesdescribedaboveandupononerangeringseparationcorrespondingto
20 knots are summarized as follows:
IfthePPIhasfourfixedrangerings,standardplottingperiodscanbe
establishedinlikemannerforonerangeringseparationequalto20knots.
Aswiththesix-ringPPI,thestandardplottingperioddoublesastherange
scaledoubles.Theonlydifferenceisthatthestandardplottingperiodis
three-fourths of the range scale setting, instead of one-half.
Range Scale (miles)Standard Plotting Period
126 min.
63 min.
390 sec.
1.545 sec.
Figure 3.39 - Standard plotting period scale. Under “black light” illumination a plastic scale of chartreuse color has been found to be most useful.
110
SUMMARY OF ALTERNATIVE PLOTTING SYMBOLS
R-T-M TRIANGLE
RELATIVE PLOTVECTOR TRIANGLE
SymbolMeaningSymbolMeaning
R
00
Firstplottedpositionofothership;plottedpositionof
other ship at time 00.
T
03
Theoriginofanyship’strue(course-speed)vector;fixed
withrespecttotheearth.Thesubscriptistheplotting
period used to construct the triangle.
M
03
, M
06
Plottedpositionsofothershipattimes03and06,
respectively.
R
00
The head of own ship’s true (course-speed) vector,
T
03
-R
00
;theoriginoftherelative(DRM-SRM)vector,
R
00
-M
03
.
M
x
PositionofothershiponRMLatplannedtimeofevasive
action; point of execution.
RMLRelative movement line.T
03
-R
00
Own ship’s true (course-speed) vector.
NRMLNew relative movement line.T
03
-M
03
Othership’strue(course-speed)vector.Thesubscriptis
the plotting period used to construct the triangle.
DRMDirectionofrelativemovement;alwaysinthedirectionof
R
00
→ M
03
→ M
06
........
DTMDirection of other ship’s true motion.
SRMSpeed of relative movement.STMSpeed of other ships true motion.
CPAClosest point of approach.R
00
-M
03
Therelative(DRM-SRM)vector;alwaysinthedirection
ofR
00
→ M
03
→ M
06
........
MCPAMinutes to CPA.
TCPATime to CPA.R
c
Theheadofownship’strue(course-speed)vector
followingcourseorspeedchangeorbothtoobtainanew
RML.
R
c
-M
03
Therelative(DRM-SRM)vector;alwaysinthedirection
of the new RML (M
x
M
x+3
M
x+6
...).
T
03
-R
c
Ownship’strue(course-speed)vectorrequiredtoobtain
new RML.
111
Figure 3.40 - Alternative plotting symbols.
112
ALTERNATIVE GRAPHICAL SOLUTIONS ON THE REFLECTION PLOTTER
R-T-M TRIANGLE
CLOSEST POINT OF APPROACH
Todeterminetheclosestpointofapproach(CPA)ofacontactby
graphicalsolutiononthereflectionplotter,followtheproceduregiven
below.
(1)Plotatleastthreerelativepositionsofthecontact.Iftherelative
positionslieinastraightornearlystraightline,fairalinethroughthe
relativepositions.Extendthisrelativemovementline(RML)pastthe
center of the PPI.
(2)Crankoutthevariablerangemarker(VRM)untiltheringdescribed
byitistangenttotheRMLasshowninfigure3.41.Thepointof
tangency is the CPA.
(3)TherangeatCPAisthereadingoftheVRMcounter;thebearingat
CPAisdeterminedbymeansofthemechanicalbearingcursor,
parallel-linecursor,orothermeansforbearingmeasurementfromthe
center of the PPI.
Note:TheRMLshouldbereconstructedifthecontactdoesnotcontinueto
plot on the RML as originally constructed.
TRUE COURSE AND SPEED OF CONTACT
Todeterminethetruecourseandspeedofacontactbygraphicalsolution
on the reflection plotter, follow the procedure given below.
(1)AssoonaspossibleafteracontactappearsonthePPI,plotitsrelative
positiononthereflectionplotter.Labelthepositionwiththetimeof
theobservationasshowninfigure3.41.Asrecommendedin
AlternativePlottingSymbols,thefirstplotislabeledastimezero.
Subsequentrelativepositionsareplottedandlabeledat3-minute
intervals,preferablyusingasuitabletimingdevicewhichcanbereset
to zero time when desired.
(2)Examinetherelativeplottodeterminewhetherthecontactisona
steadycourseatconstantspeed.Ifso,therelativepositionsplotina
straightornearlystraightline;therelativepositionsareequally
spaced for equal time intervals as shown in figure 3.41.
(3)Withthecontactonasteadycourseatconstantspeed,R
00
,theplotfor
zerotime,istheoriginoftherelative(DRM-SRM)vector.Atplottime
03,thisvectorisR
00
-M
03
;atplottime06,thisvectorisR
00
-M
06
.Note
thattherelativemotionandrelativevectorarealwaysinthedirectionof
R
00
M
03
M
06
.
Figure 3.41 - Closest point of approach.
113
(4)Cranktheparallel-linecursoruntilitslinesareparalleltotheheading
flash.Asshowninfigure3.42,placethestandardplottingperiodscale
sothatitsstraightedgeisparalleltothelinesofthecursorandthe
heading flash and the zero speed graduation is atR
00
.
(5)Giventhatownshipisoncourse000˚at30knotsandtherangescale
settingis12miles,thestandardplottingperiodis6minutes;the30-
knotgraduationonthescalecorrespondstoT
06
.Theheadoftheother
ship’strue(course-speed)vectorisatM
06
beyondR
00
inthedirection
of relative movement (DRM).
(6)Construct the other ship’s true (course-speed) vectorT
06
-M
06
.
(7)Cranktheparallel-linecursorsothatitslinesareparalleltovector
T
06
-M
06
asshowninfigure3.43.Theothership’sdirectionoftrue
motion(DTM)isreadonthetruebearingdialusingtheradiallineof
theparallel-linecursor;theothership’sspeedoftruemotion(STM)is
measuredbythestandardplottingperiodscaleorestimatedbyvisual
comparisonwithownship’struevectorT
06
-R
00
.Forexample,ifT
00
-
M
06
isabouttwo-thirdsthelengthofT
06
-R
00
,theothership’sspeed
of true motion is about two-thirds own ship’s speed.
Figure 3.42 - Use of the standard plotting period scale.
Figure 3.43 - Use of parallel-line cursor to find true course of contact.
114
COURSE TO PASS AT SPECIFIED CPA
Theprocedurefordeterminingownship’snewcourseand/orspeedto
reduce the risk of collision is given below.
(1)Continuingwiththeplotusedinfindingthetruecourseandspeedof
theothership,markthepointofexecution(M
x
)ontheRMLasshownin
figure3.44.M
x
isthepositionofthecontactontheRMLattheplannedtime
ofevasiveaction.Thisactionmaybetakenataspecificclocktimeorwhen
the range to the other ship has decreased to a specified value.
(2)CranktheVRMtothedesireddistanceatCPA.Thisisnormallythe
distancespecifiedforthedangerorbufferzone.Ifthefixedrangeringsare
displayedandonerangeringisequaltothisdistance,itwillnotbenecessary
to use the VRM.
(3)FromM
x
drawthenewRMLtangenttotheVRMcircle.Twolinescan
be drawn tangent to the circle, but the line drawn in figure 3.44 fulfills the
requirementthattheothershippassaheadofownship.IfthenewRML
crosses the heading flash, the other ship will pass ahead.
(4)Usingtheparallel-linecursor,drawalineparalleltothenewRML
throughM
06
orthefinalplot(relativeposition)usedindeterminingthe
courseandspeedofthecontact.ThislineisdrawnfromM
06
inadirection
oppositetothenewDRMbecausethenewrelativespeed(DRM-SRM)
vectorwillbeparalleltothenewRMLandthehead(M
06
)ofthenewvector
(R
c
M
06
) will lie in the new DRM away from the origin,R
c
.
(5)Avoidingbycoursechangeonly,themagnitudeofownship’strue
(course-speed)vectorremainsconstant.Therefore,thesamespeed
graduationonthestandardplottingintervalscaleusedtoconstructT
06
-R
00
is
setatT
06
.Thescaleisthenadjustedsothatitszerograduationintersectsthe
line drawn parallel to the new RML. As shown in figure 3.44, the
intersectionatR
c
istheheadoftherequirednewtrue(course-speed)vector
for own ship,T
06
-R
c
.
Thepreviouslydescribeduseoftheplasticruler,ineffect,rotatesvector
T
06
-R
c
aboutitsorigin;theheadofthevectordescribesanarcwhich
intersects the line drawn parallel to the new RML atR
c
.
Ifthespeedofthecontactweregreaterthanownship’sspeed,there
wouldbetwointersectionsand,thus,twocoursesavailabletoproducethe
desireddistanceatCPA.Generally,thepreferredcourseisthatwhichresults
inthehigherrelativespeed(thelongerrelativespeedvector)inorderto
expedite safe passing.
Figure 3.44 - Evasive action.
115
SPECIAL CASES
Insituationswherecontactsareoncoursesoppositetoownship’scourse
orareonthesamecourseasownshipbutatslowerorhigherspeeds,the
relativemovementlinesareparalleltoownship’scourseline.Ifacontact
hasthesamecourseandspeedasownship,thereisnorelativemovement
line;allrelativepositionslieatonepointataconstanttruebearingand
distancefromownship.Ifacontactisstationaryordeadinthewater,the
relativevectorR-Mandownship’struevectorT-Rareequalandopposite,
and coincident. WithT andM coincident, there is no vectorT-M.
Thesolutionsofthesespecialcasescanbeeffectedinthesamemanneras
thosecasesresultingintheconventionalvectortriangle.However,novector
triangle is formed; the vectors lie in a straight line and are coincident.
In figure 3.45 contacts A, B, C, and D are plotted for a 12-minute interval;
ownship’struevectorT
12
-R
00
isscaledinaccordancewiththistime.
InspectionoftheplotforcontactArevealsthattheDRMisoppositetoown
ship’scourse;therelativespeedisequaltoownship’sspeedplusthe
contact’sspeed.Thecontactisonacourseoppositetoownship’scourseat
about the same speed.
InspectionoftheplotforcontactBrevealsthattheDRMisoppositeto
ownship’scourse;therelativespeedisequaltoownship’sspeedminusthe
contact’sspeed.Thecontactisonthesamecourseasownshipataboutone-
half own ship’s speed.
InspectionoftheplotforcontactCrevealsthattheDRMisoppositeto
ownship’scourse;therelativespeedisequaltoownship’sspeedplusthe
contact’sspeed.Thecontactisonacourseoppositetoownship’scourseat
about the same speed.
InspectionoftheplotforcontactDrevealsthattheDRMisthesameas
ownship’scourse;therelativespeedisequaltothecontact’sspeedminus
ownship’sspeed.Thecontactisonthesamecourseasownshipatabout
twice own ship’s speed.
BLACK LIGHT ILLUMINATION
“Blacklight”illuminationofthereflectionplotterpermitstheuseofthe
standardplottingperiodscalewithouttheuseofnotchesinthescalethat
wouldotherwiseberequired.However,whenthistypeofilluminationis
usedtofacilitatescalingbymeansofagraduatedscale,suchillumination
shouldbeusedonlywhilescalingbecauseittendstomakethevideoonthe
PPIlessvisible.Therefore,meansshouldbereadilyavailabletoextinguish
this illumination when it is not required.
Theshaftofthegreasepencilaswellasthestandardplottingperiodscale
should be fluorescent.
116
Figure 3.45 - Special cases.
117
EXAMPLES
R-T-M TRIANGLE
EXAMPLE 9
.
DETERMINATION OF CLOSEST POINT OF APPROACH (CPA)
EXAMPLE 10
.
COURSE AND SPEED OF A RADAR CONTACT
EXAMPLE 11
.
COURSE AND SPEED OF RADAR CONTACT BY THE LADDER METHOD
EXAMPLE 12
.
COURSE TO PASS A SHIP AT A SPECIFIED CPA
Own ship’s Speed is Greater Than That of Other Ship
EXAMPLE 13
.
COURSE TO PASS A SHIP AT A SPECIFIED CPA
Own ship’s Speed is Less Than That of Other Ship
EXAMPLE 14
.
VERIFICATION OF FIXED OBJECTS OR RADAR CONTACTS DEAD IN THE WATER
EXAMPLE 15
.
AVOIDANCE OF MULTIPLE CONTACTS WITHOUT FIRST DETERMINING TRUE COURSES AND
SPEEDS OF THE CONTACTS
118
EXAMPLE 9
DETERMINATION OF CLOSEST POINT OF APPROACH (CPA)
Situation:
Withownshiponcourse070˚andtheradarsetonthe12-milerange
scale, the other ship is observed as follows:
Required:
(1) Direction of relative movement. (DRM)
(2) Speed of relative movement. (SRM)
(3) Bearing and range at closest point of approach. (CPA)
(4) Estimated time of arrival at CPA.
Solution:
(1)Plotandlabeltherelativepositions,R
00
,M
06
,andM
12
,usingthe1:1
scale;fairalinethroughtherelativepositions;extendthisline,therelative
movement line (RML), beyond the center of the Maneuvering Board.
(2)ThedirectionoftheRMLfromtheinitialplotR
00
isthedirectionof
relative movement (DRM): 236˚.
(3)Measuretherelativedistancebetweenanytwotimedplotsonthe
RML,preferablybetweenthetwobestplotswiththegreatesttime
separation.Inthisinstance,measurethedistancebetweenR
00
andM
12
:3.0
miles.Usingthecorrespondingtimeinterval(1000-1012=12
m
),obtainthe
speedofrelativemovement(SRM)fromtheLogarithmicTime-Speed-
Distance Scale at the bottom of the Maneuvering Board: 15 knots.
(4)FromthecenteroftheManeuveringBoard,drawalineperpendicular
totheRML;labeltheintersectionCPA.ThedirectionoftheCPAfromthe
centeroftheplottingsheet,i.e.,ownship’sposition,isthebearingofthe
CPA:326˚;thedistancefromthecenterorownshipistherangeatCPA:0.9
mile.
(5)MeasurethedistancefromM
12
toCPA:6.0miles.Usingthisdistance
andthespeedofrelativemovement(SRM):15knots,obtaintheminutesto
CPA(MCPA)from1012(thetimeofplotM
12
)bymeansoftheTime-
Speed-DistanceScale:24m.TheestimatedtimeofarrivalatCPAis1012+
24
m
= 1036.
Answers:
(1)DRM236˚(2)SRM15knots;(3)CPA326˚,0.9mile;(4)ETAatCPA
1036.
TimeBearingRange (miles)Rel. position
1000050˚9.0R00
1006049˚7.5M06
1012047˚6.0M12
119
EXAMPLE 9
Notes:
1.Thereshouldbesufficientplotsto
insureaccurateconstructionoftheRML
fairedthroughtheplots.Shouldonlytwo
plotsbemade,therewouldbenomeans
ofdetectingcourseorspeedchangesby
theothership.Thesolutionisvalidonly
iftheothershipmaintainscourseand
speedconstant.Preferably,thetimed
plotsshouldbemadeatequaltime
intervals.Equalspacingoftheplots
timedatregularintervalsandthe
successiveplottingoftherelative
positionsinastraightlineindicatethat
theothershipismaintainingconstant
course and speed.
2.Thistransferplottingsolution
requiredindividualmeasurementsand
recordingoftherangesandbearingsof
therelativepositionofshipMatintervals
oftime.Italsoentailedthenormal
requirementofplottingtherelative
positionsonthePPIorreflectionplotter.
Visualizingtheconcentriccirclesofthe
ManeuveringBoardasthefixedrange
ringsofthePPI,afastersolutionmaybe
obtainedbyfairingalinethroughthe
greasepencilplotonthePPIand
adjustingtheVRMsothatthecircle
describedistangenttoorjusttouchesthe
RML.TherangeatCPAisthesettingof
theVRM;thebearingatCPAandthe
DRMmaybefoundbyuseofthe
parallel-linecursor(parallelindex).The
timeoftheCPAcanbedeterminedwith
reasonableaccuracythroughvisual
inspection,i.e.,thelengthalongtheRML
fromM
12
toCPAbyquickvisual
inspectionisabouttwicethelength
betweenR
00
andM
12
,representingabout
24 minutes.
120
EXAMPLE 10
COURSE AND SPEED OF A RADAR CONTACT
Situation:
Ownshipisoncourse340˚,speed15knots.Theradarissetonthe12-
milerangescale.Aradarcontactisobservedtobechangingcourse,and
possiblyspeed,betweentimes0953and1000.Whilekeepingaclosewatch
oftherelativemovement,therelativepositionsofthecontactaremarkedat
frequent intervals on the reflection plotter by grease pencil.
Required:
(1)Courseandspeedofthecontactwhenithassteadiedoncourseand
speed.
Solution:
(1)Thesolutionisstartedbeforethecontactsteadiesoncourseandspeed
through planning:
(a)Sincethecontactisbeingobservedonthe12-milerangescale,the
standardplottingperiodforusewiththesixfixedrangeringsis6minutes.
(b)Theobserveranticipatesthatafterthecontacthasbeenobservedtobe
onasteadycourseatconstantspeedfor6minuteshewillbeabletoobtain
arapidsolutionbyusingthespacingbetweenrangeringsasaspeedscale.
(2)ObservationofthePPIrevealsthatbetween1000and1006,the
contactisonasteadycourseatconstantspeed(successiveplotsforma
straightlineonthescope;plotsforequaltimeintervalsareequallyspaced).
Drawtherelativemovementline(RML)fromthe1000plot(R
00
)through
the 1006 plot (M
06
), extending beyond the center of the PPI.
(3)Setcenterlineofparallel-linecursortoheadingflash.Placethe
standardplottingperiodscaleparalleltothelinesonthecursorandwithits
zerograduationatR
00
.The15-knotgraduationonthescalecorrespondsto
T
06
.Twosidesofthevectordiagram(triangle)havebeenformed:T
06
-R
00
andR
00
-M
06
.Thesolutionisobtainedbycompletingthetriangletoformthe
contact’s true (course-speed) vectorT
06
-M
06
.
(4)Thedirectionofthecontact’struemotion(DMT)canbereadby
adjustingtheparallel-linecursorparalleltoT
06
-M
06
.Aftersuchadjustment,
theradiallineofthecursorindicatestheDTMortruecourseofthecontact.
Thespeedofthecontact’struemotion(STM)canbemeasuredbythe
standardplottingperiodscale,oritcanbeestimatedbycomparingthelength
ofT
06
-M
06
withT
06
-R
00
, the speed of which in knots is known.
Answers:
(1) Course 252˚, speed 25 knots.
121
EXAMPLE 10
Heading-Upward
Unstabilized PPI Display
with Stabilized True
Bearing Dial
Scale: 12-mile range setting
Notes:
1.Inthisexamplewiththe
contactobservedtobechanging
course,andpossiblyspeed,
betweentimes0953and1000,it
wasnecessarytodelay
constructionofownship’strue
vector(T
06
-R
00
)untilafter1000.
However,whenitisnotknown
thatthecontactisonotherthana
steadycourseatconstantspeed,
thesolutioncanoftenbe
expeditedbyconstructingT
06
-
R
00
soonaftertheinitial
observationandthendetermining
whetherthecontactisonasteady
courseatconstantspeed.Ifsuch
isthecase,thetriangleis
completed at time 06.
2.Withthedisplayofthefixed
rangerings,apracticalsolution
canbeobtainedwithouttheuse
ofthestandardplottingperiod
scalebyvisualizingthevector
diagram(triangle)usingthe
spacingbetweenrangeringsas
the speed scale.
122
EXAMPLE 11
COURSE AND SPEED OF RADAR CONTACT BY THE LADDER METHOD
Situation:
Ownshipisoncourse120˚,speed20knots.Theradarissetonthe6-mile
rangescalebecausesmallwoodenvesselsareexpectedtobeencountered.
Therangescalesettingisbeingshiftedperiodicallytolongerrangesfor
possibledetectionofdistanttargets.Aradarcontactisbeingplottedonthe
reflectionplotter.Inspectionoftheplotrevealsthatthecontactisonsteady
course at constant speed (see solution step (2) of example 10).
Required:
(1) Course and speed of the radar contact.
Solution:
(1)Withthedecisionmadethatthesolutionswillbeobtainedbyrapid
radarplotting,theradarobserverfurtherelectstousetheLadderMethodin
ordertobeabletorefinethesolutionastherelativeplotforthecontact
develops with time.
(2)Sincethecontactisbeingobservedonthe6-milerangescale,the
standard plotting period for use with the six fixed range rings is 3 minutes.
(3)Setthecenterlineoftheparallel-linecursortoheadingflash.Placethe
standardplottingperiodscaleparalleltothelinesofthecursorandwithits
zerograduationatR
00
.The20-knotgraduationonthescalecorrespondsto
T
03
.Theladderisdrawninmultiplesandsub-multiplestoT
03
-R
00
:The40-
knotgraduationcorrespondstoT
06
;the30-knotgraduationcorrespondsto
T
4.5
; and the 10-knot graduation corresponds toT
1.5
.
(4)Withtheassumptionthatthecontactisonasteadycourseatconstant
speed,thefirstsolutionisobtainedattime1.5(90seconds)byconstructing
vectorT
1.5
-M
1.5
.Attime03itisseenthatthecontactisonasteadycourseat
constantspeed.Thesolutionobtainedattime03bycompletingvectorT
03
-
M
03
isarefinementoftheearliersolution.Assumingthatthecontact
maintainscourseandspeed,solutionsobtainedatlatertimesshouldbeof
increasing accuracy.
(5)Thedirectionofthecontact’struemotion(DTM)attime06canbe
readbyadjustingtheparallel-linecursorparalleltoT
06
-M
06
.Aftersuch
adjustment,theradiallineofthecursorindicatestheDTMortruecourseof
thecontact.Thespeedofthecontact’struemotion(STM)canbemeasured
bythestandardplottingperiodscale,oritcanbeestimatedbycomparingthe
lengthofT
06
-M
06
withT
06
-R
00
,thespeedofwhichinknots(20)isknown.
Notethatalthoughthe40-knotgraduationonthestandardplottingperiod
scalecorrespondstotime06,vectorsT
1.5
-R
00
,T
03
-R
00
,T
4.5
-R
00
,andT
06
-
R
00
are all 20-knot vectors.
Answers:
(1) Course 072˚, Speed 22 knots.
123
EXAMPLE 11
Heading-Upward
Unstabilized PPI Display
with Stabilized True
Bearing Dial
Scale: 6-mile range setting
Notes:
1.Usingtheladdermethod,the
radarobserverisabletoobtainan
approximatesolutionquicklyand
thenrefinethesolutionastheplot
develops.
2.Thissolutionwassimplified
bystartingthetimedplotatsome
tenth of an hour after the hour.
124
EXAMPLE 12
COURSE TO PASS A SHIP AT A SPECIFIED CPA
(Own ship’s speed is greater than that of other ship)
Situation:
Ownshipisoncourse188˚,speed18knots.Theradarissetonthe12-
milerangescale.Betweentimes1730and1736ashiphasbeenobservedto
beonacollisioncoursewithownship.Byrapidradarplotting,itisfoundto
be on course 258˚ at 12 knots. The visibility is 2.0 nautical miles.
Required:
(1)Courseofownshipat18knotstopassaheadoftheothershipwitha
CPAof3.0nauticalmilesifcourseischangedtotherightwhentherangeis
6.5 nautical miles.
Solution:
(1)ContinuingwiththeplotonthePPIusedinfindingthetruecourseand
speedoftheothership,plotM
x
ontheRML6.5nauticalmilesfromown
ship.AdjusttheVRMto3.0nauticalmiles,thedesireddistanceatCPA.
FromM
x
drawalinetangenttotheVRMcircle.FromM
x
twolinescanbe
drawntangenttothecircle,butthelineasdrawnfulfillstherequirementthat
ownshippassaheadoftheothershiporthattheothershippassasternof
own ship.
(2)Fromtheoriginofthetruevectorsofthevectortriangleusedin
findingtheDTMandSTMoftheothership,T
06
,describeanarcofradius
equal to the length ofT
06
-R
00
.
(3)Withtheaidoftheparallel-linecursor,drawalinethroughM
06
parallel to the new RML to intersect the arc drawn in (2).
(4)TheintersectionofthearcwiththelinethroughM
06
paralleltothe
newRMLestablishestheheadofvectorT
06
-R
c
,ownship’strue(course-
speed) vector required to obtain new RML.
Answers:
(1) Course 218˚.
Notes:
1.ActuallythearcintersectingthelinedrawnfromM
06
inadirection
oppositetothenewDRMwouldalsointersectthesamelineifextendedin
thenewDRM.Butanewcourseofownshipbaseduponthisintersection
wouldreversethenewDRMorreversethedirectiontheothershipwould
plot on the new RML.
2.Ifthespeedoftheothershipweregreaterthanthatofownship,there
wouldbetwocoursesavailableat18knotstoproducethedesireddistanceat
CPA.
125
EXAMPLE 12
North-Upward
Stabilized PPI Display
Scale: 12-mile range setting
Notes: (continued)
Generally,thepreferredcourse
isthatwhichresultsinthehighest
relativespeedinordertoexpedite
the safe passing.
3.Afterownship’scoursehas
beenchanged,theothership
shouldplotapproximatelyalong
thenewRML,asdrawnandin
thedesireddirectionofrelative
movement.Thiscontinuityofthe
plotfollowingacoursechangeby
ownshipisoneoftheprimary
advantagesofastabilized
display.Immediatelyfollowing
anyevasiveaction,oneshould
inspectthePPItodetermine
whetherthetarget’sbearingis
changingsufficientlyandinthe
desireddirection.Withthe
stabilizeddisplay,theansweris
before the radar observer’s eyes.
126
EXAMPLE 13
COURSE TO PASS SHIP AT A SPECIFIED CPA
(Own ship’s speed is less than that of other ship)
Situation:
Ownshipisoncourse340˚,speed15knots.Theradarissetonthe12-
milerangescale.Betweentimes0300and0306,ashiphasbeenobservedto
beonacollisioncoursewithownship.Byrapidradarplotting,itisfoundto
be on course 249˚ at 25 knots. The visibility is 2.0 nautical miles.
Required:
(1)Courseofownshipat15knotstopassasternoftheothershipwith
CPAof3.0nauticalmilesifcourseischangedtotherightwhentherangeis
6.0 nautical miles.
Solution:
(1)ContinuingwiththeplotonthePPIusedinfindingthetruecourse,speed,
andCPAoftheothership,plotM
x
ontheRML6.0nauticalmilesfromown
ship.AdjusttheVRMto3.0nauticalmiles,thedesireddistanceatCPA.From
M
x
twolinescanbedrawntangenttotheVRMcircle,butthelineasdrawn
fulfills the requirement that own ship pass astern of the other ship.
(2)Fromtheoriginofthetruevectorsofthevectortriangleusedin
findingtheDTMandSTMoftheothership,T
06
,describeanarcofradius
equal toT
06
-R
00
.
(3)Withtheaidoftheparallel-linecursor,drawalinethroughM
06
parallel to the new RML to intersect the arc drawn in (2).
(4)Sincethespeedoftheothershipisgreaterthanthatofownship,the
arcintersectsthelinethroughM
06
attwopoints.Eachintersection
establishesaheadofapossiblenewownship’struevector.Ofthetwo
possiblevectorsoneprovidesahigherspeedofrelativemovementthanthe
other.Generally,truevectorwhichprovidesthehigherSRMorlonger
relativevectorischosentoexpeditethepassing.However,inthisexamplea
coursechangetotherightisspecified.ThisrequirestheuseofvectorT
06
-
R
c1
, which provides the higher SRM.
(5)Withthisunstabilized,Heading-UpwardPPIdisplay,thereisa
complicationarisingfromtheplotshiftingequalandoppositetotheamount
anddirectionofthecoursechange.Somereflectionplotterdesignshave
provisionsforeithermanualorautomaticshiftingoftheirplottingsurfaces
tocompensatefortheshiftingoftheplot.Withoutthiscapability,thereisno
continuityinthegreasepencilplotfollowingcoursechangesofownship.
Consequently,itisnecessarytoerasetheplotandreplottheothership’s
relativepositionwhenownshipsteadiesoncourse.WiththeVRMsetto3.0
miles,thenewRMLmustbedrawntangenttothecircledescribedbythe
VRM.Theothershipmustbewatchedcloselytoinsurethatitsmovement
conforms with the new RML.
Answers:
(1) Course 030˚.
127
EXAMPLE 13
Heading-Upward
Unstabilized PPI Display
with Stabilized True
Bearing Dial
Scale: 12-mile range setting
Note:
Examinationoftheplotreveals
thatifownshipmaintainsits
originaltruecourse(340˚),the
intersectionoftheoriginaltrue
vectorT
06
-R
00
ofownshipwith
thelinedrawnthroughM
06
paralleltothenewRMLprovides
theheadofthevectorT
06
-R
C2
requiredtoeffectthedesiredCPA
withoutcoursechange.Sincethe
lengthofvectorT
06
-R
C2
is
approximatelyhalfthatofthe
originalvectorT
06
-R
00
,an
instantaneouschangeto
approximatelyhalftheoriginal
speedwouldproducethedesired
results.Alesserchangeofcourse
totherightinconjunctionwitha
speedreductioncouldbeusedto
compensate for deceleration.
128
EXAMPLE 14
VERIFICATION OF FIXED OBJECTS OR RADAR CONTACTS DEAD IN THE WATER
Situation:
Ownshipisoncourse340˚,speed20knots.Theradarissetatthe24-mile
range scale. Radar observations are made as follows:
TheRMLisparalleltoandtheDRMisoppositetoownship’scourse,
340˚.
Required:
Courseandspeedofcontactinordertoverifywhetheritisdeadinthe
water or a terrestrial object.
Solution:
(1)OnthePPI,preferablyonewithareflectionplottermountedthereon,
plotR
00
,M
18
,M
36
.Drawtherelativemovementline(RML)throughthese
relative positions.
(2)Usingthesamedistancescaleastheradarrangesetting,determinethe
lengthofthetrue(course-speed)vectorT-Rofownshipforatimeintervalof
36 minutes: 12 miles.
(3)DrawtruevectorT
36
-R
00
inthedirectionofownship’scoursewithits
headatrelativepositionR
00
.Ifaftersuchgraphicalconstruction,thevector
originliesoverrelativepositionM
36
,thelengthoftheT
36
-M
36
vectorwould
bezero.Thus,thetruespeedoftheobservedcontactwouldbezero.Evenif
theobservedtargetisdeadinthewaterorafixedobject,smallobservational
andplottingerrorswillfrequentlyindicateasmallvalueoftruespeedforthe
contact.
TimeBearingRange (miles)Rel. position
1200017˚22.8R00
1218029˚17.4M18
1236046˚14.4M36
129
EXAMPLE 14
Heading-Upward
Unstabilized PPI Display
with Stabilized True
Bearing Dial
Scale: 24-mile range setting
130
EXAMPLE 15
AVOIDANCE OF MULTIPLE CONTACTS WITHOUT FIRST DETERMINING THE TRUE COURSES
AND SPEEDS OF THE CONTACTS
Situation:
Ownshipisoncourse000˚,speed20knots.Withthestabilizedrelative
motiondisplayradarsetatthe12-milerangesetting,radarcontactsA,B,
andCareobservedandplotteddirectlyonthePPIorreflectionplotter.The
plots at time 1000 are considered as the initial plots in the solution.
Required:
(1)DeterminethenewrelativemovementlinesforcontactsA,B,andC
whichwouldresultfromownshipchangingcourseto065˚andspeedto15
knots at time 1006.
(2)Determinewhethersuchcourseandspeedchangewillresultin
desirable or acceptable CPA’s for all contacts.
Solution:
(1)WiththecenterofthePPIastheirorigin,drawownship’struevectors
T-RandT-R
c
forthecourseandspeedineffectortobeputineffectattimes
1000and1006,respectively.Usingthedistancescaleoftheradar
presentation,draweachvectoroflengthequaltothedistanceownshipwill
travelthroughthewaterduringthetimeintervaloftherelativeplot(relative
vector),6minutes.VectorT-R,havingaspeedof20knots,isdrawn2.0
milesinlengthintruedirection000˚;vectorT-R
c
,havingaspeedof15
knots, is drawn 1.5 miles in length in true direction 065˚.
(2) Draw a broken line betweenR andR
c
.
(3)ForcontactsA,B,andC,offsettheinitialplots(A
1
,B
1
,andC
1
)inthe
samedirectionanddistanceasthebrokenlineR-R
c
;labeleachsuchoffset
plot R
c
.
(4)Ineachrelativeplot,drawastraightlinefromtheoffsetinitialplotR
c
,
throughthefinalplot(A
2
orB
2
orC
2
).ThelinesR
c
A
2
,R
c
B
2
,andR
c
C
2
representthenewRML’swhichwouldresultfromacoursechangeto065˚
and speed change to 15 knots at time 1006.
Answers:
(1)New RML of contact A—DRM 280˚
New RML of contact B—DRM 051˚
New RML of contact C—DRM 028˚
(2)Inspectionofthenewrelativemovementlinesforallcontacts
indicatesthatifallcontactsmaintaincourseandspeed,allcontactswillplot
alongtheirrespectiverelativemovementlinesatsafedistancesfromown
ship on course 065˚, speed 15 knots.
Explanation:
Thesolutionisbasedupontheuseoftherelativeplotastherelative
vector.Witheachcontactmaintainingtruecourseandspeed,thetruevector
foreachcontactremainsstaticwhileownship’struevectorisrotatedabout
itsoriginTtothenewcourseandchangedinmagnitudecorrespondingtothe
new speed.
131
EXAMPLE 15
North-Upward
Stabilized PPI Display
Scale: 12-mile range setting
132
PRACTICAL SOLUTION FOR CPA IN TRUE MOTION MODE
ApracticalsolutionforCPAinthetruemotionmodeisdependentupona
featurenormallyprovidedwithatruemotionradar:someformofelectronic
bearingline(EBL)thatcanholdtherangeandbearingtowhichset.Withthe
EBLoriginatingatownshipmovingintruemotiononthePPI,itfollows
thatiftheEBLisheldataninitialsetting,theendoftheEBLmovesatthe
samespeedasownshipalongaparallelpath.OrtheendoftheEBLfollows
own ship in true motion.
Thetruemotionsofownshipandofacontactareshowninfigure3.46
afterobservationforabout3minutes.Withownship(atthecenterofthe
rangerings)oncourse000˚at20knots,itstailhasalengthaboutequalto
the1-milerangeringinterval,1milebeingthedistanceownshiptravelsin3
minutesat20knots.Thetailofthecontactbearing045˚at4milesindicates
thatthecontactisontruecourse280˚at30knots.Atthispointitshouldbe
notedthattheaccuracyofthetruemotiondisplayedisdependentuponthe
accuraciesofownshipcourseandspeedinputs,particularlythespeedinput,
andothererrorsassociatedwithdeadreckoning,suchasthosedueto
currents.Therefore,truemotionsolutionsshouldbeconsideredmore
approximate than those derived from stabilized relative motion displays.
Duetothefactthatunlikerelativemotion,thetruemotionisnotactually
observedbutisdeducedfromobservedrelativemotionandestimatedown
shipcourseandspeedovergroundinputs,thetruemotiondisplayedonthe
PPI is better calleddeduced true motion.
Figure3.47showstheEBLsetatthecontactattheinitialposition(time
00),whichislabeledT
00
.Ownship’spositionatthistimeisalsolabeled00.
Ifownshipisdeadreckonedtothetime03positionasshowninfigure3.48,
withtheEBLholdingtherangeandbearingtowhichsetattime00,theend
oftheEBL,movinginparallelmotionatthesamerateasthetruemotionof
ship,arrivesatR
03
atthesametimeasownshipreachesthetime03dead
reckoningposition.Duringthistimethecontactmovesindeducedtrue
motionfromitsinitialposition,T
00
toM
03
asshowninfigure3.48.Withthe
motionsofownshipandofthecontactproducingthetwotruevectorsofthe
R-T-Mtriangle,thetriangleiscompletedtoprovidetherelativevectorR
03
-
M
03
,theextensionofwhichprovidestheRML,bymeansofwhichtheCPA
is determined. See figure 3.49.
WiththeEBLholdingtheinitialrangeandbearing,itfollowsthatthe
motionsofthecontactandoftheendoftheEBLfromtheinitialposition
continuouslygeneratetheR-T-Mtriangle.ThereforetheR-T-Mtrianglecan
becompletedatanytimebetweentimes00and03byconstructingthe
relativevectorfromtheendoftheEBLtothepositionthecontactoccupies
atthesametime.Figure3.50showsthecompletionoftheR-T-Mtriangleat
times01,02,and03.However,asindicatedabove,thetrianglecanbe
completedatanytime.TherelativevectorandtheRMLcanbeobtained
withoutanydirectconsiderationofplottime.Thisfactenhancesthe
practicalityofthesolution.Itenablesreal-timevisualizationoftheRML
throughobservationofthecurrentpositionofthecontactinrelationtothe
endofthemovingEBL.This,inturn,enablestheobservertodeterminethe
CPA very quickly.
ShouldtheCPAbelessthandesired,aproceduresimilartoobtaininga
desiredCPAonarelativemotiondisplay(seeexamples12and13)canbe
used. As shown in figure 3.51, the CPA is increased by course change only.
TheCPAismeasuredfromthepositionownshipoccupiesonthePPIat
plot time 03.
ThispracticalsolutionforCPAinthetruemotionmodewasdevisedby
CaptainWayneM.Waldo,Head,All-weatherNavigationDepartment,
MaritimeInstituteofTechnologyandGraduateStudies,LinthicumHeights,
Maryland.
133
Own ship’s course 000˚
speed 20 knots
Contact’s course 280˚
speed 30 knots
Range-ring interval: 1 mile
Figure 3.46- True motion display
134
Own ship’s course 000˚
speed 20 knots
Contact’s course 280˚
speed 30 knots
Range-ring interval: 1 mile
Figure 3.47- Electronic bearing line set at initial time position of contact moving in true motion.
135
Own ship’s course 000˚
speed 20 knots
Contact’s course 280˚
speed 30 knots
Range-ring interval: 1 mile
Figure 3.48- True motion display with electronic bearing line holding the bearing and range at which initially set.
136
Own ship’s course 000˚
speed 20 knots
Contact’s course 280˚
speed 30 knots
Range-ring interval: 1 mile
Figure 3.49- Solution for CPA on true motion display.
137
Own ship’s course 000˚
speed 20 knots
Contact’s course 280˚
speed 30 knots
Figure 3.50- Construction ofR-T-M triangle at any time.
138
Own ship’s course 000˚
speed 20 knots
Contact’s course 280˚
speed 30 knots
Range-ring interval: 1 mile
Desired CPA: 1.5 miles
Figure 3.51- Solution for desired CPA.
139
SITUATION RECOGNITION
INTRODUCTION
TherulesforSituationRecognitionweredevelopedbyMr.MaxH.
CarpenterandCaptainWayneM.Waldo,formermembersofthefacultyfor
theMaritimeInstituteofTechnologyandGraduateStudies,Linthicum
Heights,Maryland.ThefollowinginformationisprintedfromSectionVIIof
theReal Time Method of Radar Plotting.
AsyourRTMplottingskillsincreasesowillyourabilitytoinstantly
recognizedangeroussituationswithoutaplot.Thisskillcanbedescribedas
SituationRecognition,andmakesuseofeverythingyouhavelearnedand
practiced thus far.
Thisabilitytorecognizeasituationasyouviewitonradarwillmarkyou
as an exceptionally competent mariner.
Inariskofcollisionsituation,thetrueorcompassdirectionofrelative
movementmustbechanged.Simplerulesforrapidpredictionofthechange
inthecompassdirectionofrelativemovement(DRM)ofaradarcontact
resultingfromacourseorspeedchangebyownshipcanbeinvaluable,
particularly in confusing multiple-contact situations.
Therulescanbeusedonlywhenusingastabilizedrelativemotion
display.Attemptingtoapplytheserulesusinganunstabilizedradardisplay
couldbeverydangeroussinceahighdegreeofcompassorientationis
requiredtodiscoverandavoidtheriskofcollision.Preferably,the
radarscope should have high persistence.
SituationRecognitioncanbethoughtasatwo-stepprocedure.Thefirstis
toascertaintheriskofcollisionasrequiredbytheRulesoftheroad.The
secondistorecognizethoseactionsyoucantakewhichwillreducetherisk
of collision, i.e. increase the passing distance
Stepone;isrelativelysimpleprovidedyouobeytheinstructiongivenin
theSteeringandsailingRulesandascertaintheriskofcollision,by
“carefullywatchingthe
compassbearingofanapproachingvessel.
Therefore,yourradarmustgiveyouthecompassreferenceyouneedto
recognizeriskofcollision.Thismeansthatthesituationataglancerequires
agyrostabilizeddisplay.Unlessyourradarissoequippedthatyoucan,ata
glance,observethecompassbearingchangeofallapproachingvesselsyou
areseriouslyhandicapped.Thereisnowayyoucan,ataglance,determine
theriskofcollisionbyobservingtherelativebearingsofapproaching
vessels.Torepeat:thereisonlyonemethodthatis100%reliablein
determiningriskofcollisioneithervisuallyorbyradar,andthatistheone
givenintheSteeringandSailingRules.Inthisgameofcollisionavoidanceif
youcannotsatisfactorilyanswertherequirementsofstepone,itis
impossible to evaluate the actions required in step two.
Steptwo;consistsofdecidingwhichofthefourbasiccollisionavoidance
maneuverswillbestincreasethepassingdistance(turnleft,turnright,speed
up,slowdown).Thisisrelativelyeasyforyouhavebeenmakingthesesame
decisionsallyourlife.Ifwhileyouaremovingyouvisuallyobservean
objectcomingtowardsyou,youcanveryquicklydecidehowbesttoavoida
collisionbyeitherturningrightorleft,speedinguporslowingdown.Youdo
exactlythesamethingusingaradartoobservecontactscomingtowardsthe
center of the scope.
140
RULES FOR SPEED CHANGE
Thefollowingrulesprovidepredictionsofhowacontact’srelativemotion
changeswithaspeedchangebyownship.Thepredictionsarevalid
irrespective of the position of the contact in range and bearing.
Reduced Speed
Therelativeplotmovesup-the-scopewhenownshipreducesspeedor
stops.
Increased Speed
The relative plot moves down-the-scope when own ship increases speed.
Speed of Relative Motion (SRM)
Theeffectivenessofaturningmaneuverdepends,inpart,upontheSRM
oftheradartarget.AtargetwhoseSRMishighwillshowlesschangein
relative motion than a similarly located contact with a low SRM.
Assumetwocontactsoncollisioncoursesapproachingtheobserver’s
vesselatthesamespeed,withonecontact40˚ontheobserver’sportbow
andtheother40˚tostarboard.Arightturnwillresultinasmallchangein
theDRMofthecontacttostarboardandamuchlargerchangeintheoneto
port.Thedifferenceisexplainedbythefactthattheturntowardthe
starboardcontactraiseditsSRM,makingitmoredifficulttochange.The
portcontact’sSRMwasreduced.Asaresult,theamountofDRMchange
was greater.
Thus,theeffectivenessofaturntoavoidacontactisenhancedbyturning
away from the contact. This is illustrated in Figure 3.52.
SITUATION DISPLAYS
Theseriesofillustrationswhichfollow,showsvariousstepsinevaluating
theresultsofownship’smaneuversusingonlythedirectionofrelative
motionaspresented,anddemonstratestheimmediatereadabilityof
informationsufficienttomakeriskofcollisionassessmentandmaneuver.
Thesephotographsweretakenofa16inchstabilizednorthuprelative
motionradar,therangesettingis6miles.ViewsAandBshowthesituation
uptothedecisiontimeof3minutes.ViewsCthruJshowtheresultsoffour
simulator runs demonstrating each basic maneuver.
Theseillustrationsshowthatitispossibleforthemaneuveringofficerto
haveinstantaneous,readilyavailable,at-a-glanceinformationwhichwill
“hangin”whenthegoinggetsroughandwhenorientationseemstobethe
mostthreatened.Thisisimportant,foritisdifficulttoassessamaneuverby
readingalistofnumbersconcerningthethreatandthenmentallytryingto
associate those numbers with what own ship is doing.
APPLICATION
Figures3.53to3.56illustratetheuseoftherulesinevaluatingtheeffects
of evasive action by own ship.
Whenthecontactisfasterthanownship,theeffectofownship’s
evasiveactiononthecompassdirectionofrelativemovementis
generallylessthanitwouldbeifownshipwerethefastership.Notethat
thecontactisalwaysfasterthanownshipintheup-the-scopeandacross-
the-scope cases.
InmakingmaneuveringdecisionsusingtheDRMtechnique,speed
informationonaratiobasisisadequate.Theobserverneedonlyknow
whetherthecontact’sspeedisaboutone-half,three-fourths,ortwiceown
ship’s speed for example.
Figure 3.52 - Effects of a course change against targets
with different speeds of relative motion.
141
View AUponswitchingfromstandbytoon,wediscover3contacts.Noriskof
collision is available therefore no maneuver decision can be made.
View BAftertheendof3minutesthedirectionofrelativemotionrevealsthatriskof
collisionexistswithcontactsonthestarboardbowandbeam.Inotherwordsthe
compass bearing is not changing on these two contacts.
View CAttheendof5minutesadecisiontoturnright60°hasresultedinachangein
DRMofallcontacts.ThecontactasternhaschangedhisDRMfromupto
across category.
View DApproximately10minutesfromthestarttheMastercanbegincomingbacktobase
course expecting to achieve 1.5 mile CPA on all targets.
Reproduced by Courtesy of Maritime Institute of Technology and Graduate Studies, Linthicum Heights, Maryland.
Figure 3.53 - Predicting effects of evasive action.
142
View ESame situation as Fig. 3 at five minutes, but with a 35 deg. left turn.
Note “down” contact has moved to his left, “up” contact to his right.
View FThedecisionnineminutesfromfirstobservationfor35deg.leftprojectsa1.5mile
CPA.Noticethebeamcontacthaslostmostofitsrelativemotion,thusrevealinghis
course and speed to be about the same as own ship’s at this instant.
View GThisistheoriginalsituationplusfiveminutes.TheMasterinthisinstance
decided to stop. Note that all DRM is swinging forward.
View HAfter 11 minutes, the action to stop has resulted in a close quarters situation.
Reproduced by Courtesy of Maritime Institute of Technology and Graduate Studies, Linthicum Heights, Maryland.
Figure 3.54 - Predicting effects of evasive action.
143
View IAtfiveminutesthedecisiontoincreasespeedfromhalftofullaheadresultsin
aswingofallDRMaft.ItisapparentthatvesselwhoseDRMis195deg.will
pass close but clear.
View JAfter10minutesitisobviousthatallcontactswillpassclear,butcontactwhose
DRM is 195˚ will clear by only one-half mile.
View KA high density situation.View LTryingfora1-mileCPAinthehighdensitysituationillustratedinViewKthe
conningofficercomestocourse060˚.After2minuteshenotesthatthecontact
bearing 125˚ will pass too close. Therefore, he starts to come to course 125˚.
Reproduced by Courtesy of Maritime Institute of Technology and Graduate Studies, Linthicum Heights, Maryland.
Figure 3.55 - Predicting effects of evasive action.
View M The relative plots of all contacts are changing according to the rules.View NAfter 6 minutes the conning officer can resume his original course.
Reproduced by Courtesy of Maritime Institute of Technology and Graduate Studies, Linthicum Heights, Maryland.
Figure 3.56- Predicting effects of evasive action.
145
RULES FOR MANEUVERING
Tomaneuverusingtheinformationfrom“situationrecognition”requires
atechniquewhoseeffectivenesshasbeendemonstratedintheradar
laboratoryandiscurrentlybeingusedatsea.Thistechniquemakesuseof
the“natural”abilityweallhaveinavoidingcollisionwithmovingobjectsin
dailylife.Thisabilityis,anunderstandingofrelativemotion.Inthis
techniqueweusetheDirectionofRelativeMotion(DRM)asthekeytothe
whole thing.
Inconsideringthiskey,let’srememberthatanycollisionavoidance
systemrequires,asaminimum,astabilizedradarwhichhasthehigh
persistencephosphorC.R.T.Withthiswehaveadisplayfromwhichwecan
obtaintheinformationontheDRMalmostataglance.Withafewsimple
rulesconcerningthisdirectionofrelativemotion,andaDeckOfficerwith
maneuveringexperience,wenowhaveacompetentmarinecollision
avoidance system.
Inviewinganyradarscope,thedirectioninwhichtheship’sheading
flasherispointingcanbedescribedas“upthescope”.Thereciprocalofitis
adirectionoppositetotheheadingflasher,or“downthescope”.Acontact
movingatrightanglestotheheadingflasheranywhereonthescopewould
be described as “across the scope”.
TherulesweusetoshowthatDRMisthe“key”arebasedsolelyonthe
relationshipofDRMwithreferencetoownship’sheadingflasher.These
rulesalertthedeckofficertotheexpectedeffectonDRMasaresultofany
collisionavoidanceaction,suchasanycourseorspeedchange.Wehave
threespecificrulesconcerningcoursechange,twospecificrulesconcerning
speedchange,andtwosubordinateruleswhichapplytothetechnique
described therein.
Rulenumberone:Anycontactappearingonthescope,regardlessof
positioninrangeandbearingwhosedirectionofrelativemotionisup-the-
scope,fromafewdegreesup,toparalleltotheheadingflasher,whenown
shipturnsright,thedirectionofrelativemotionoftheobservedthreatwill
turn to its left.
Rulenumbertwo:Anycontactwhosedirectionofrelativemotionis
down-the-scope,thatis,anywherefromafewdegreesdown,toparallelto
theheadingflasherbutintheoppositedirection,whenownshipturnsright,
thedirectionofrelativemotionwillturntoitsright.(ViewsA-D)Thisrule
also applies in the case of a left turn as shown in (Views E and F).
Rulenumberthree:AnycontactwhoseDRMisacross-the-scopeisin
“limbo”.Changingofownship’scourseleftorrightwillhaveverylittle
effectonthecrossingcontactsDRMuntilit’scategoryischangedtoeithera
“downcontact”or“upcontact”,andthenthecontactwillfollowrulesOne
or Two as stated previously (View F).
Rulenumberfour:Ifownshipreducesspeedorstops,allrelative
motionobservedonyourscopewillswingforwardor“up-the-scope”,no
matter where they are. (View G).
Rulenumberfive:Conversely,ifownshipincreasesspeed,allrelative
motion will swing aft, or down the scope. (View I).
Theexperiencedmarinerofcourseknowsthatanycontactwhoserelative
motionisup-the-scopeisafastership.thisfactalsoappliestocontacts
whosedirectionofrelativemotionisatrightanglestotheheadingflasheras
in rule three contacts.
ThoughspecificspeedisnotavailableinusingtheDRMtechnique,the
speedinformationisadequateformakingdecisionsinmaneuvering.The
experiencedofficerusuallyhandlesspeedonthebasisofaratio.Isthe
threat’s relative speed faster or slower than own ship’s speed?
Rulenumbersix:Ifcontact’srelativespeedishigh,theeffectofown
ship’s avoiding action is low.
Rulenumberseven:Ifcontact’srelativespeedislow,theeffectofown
ship’s avoiding action is high.
TostateRules6and7inanotherway,ifthecontactisfasterthanown
ship,itislikelytobehardertomaneuveragainst.Ifitisslower,thenown
ship essentially is in command of the situation.
147
CHAPTER 4 — RADAR NAVIGATION
RADARSCOPE INTERPRETATION
Initspositionfindingornavigationalapplication,radarmayservethe
navigatorasaveryvaluabletoolifitscharacteristicsandlimitationsare
understood.Whiledeterminingpositionthroughobservationoftherange
andbearingofacharted,isolated,andwelldefinedobjecthavinggood
reflectingpropertiesisrelativelysimple,thistaskstillrequiresthatthe
navigatorhaveanunderstandingofthecharacteristicsandlimitationsofhis
radar.Themoregeneraltaskofusingradarinobservingashorelinewhere
theradartargetsarenotsoobviousorwelldefinedrequiresconsiderable
expertisewhichmaybegainedonlythroughanadequateunderstandingof
the characteristics and limitations of the radar being used.
Whiletheplanpositionindicatordoesprovideachartlikepresentation
whenalandmassisbeingscanned,theimagepaintedbythesweepisnota
truerepresentationoftheshoreline.Thewidthoftheradarbeamandthe
lengthofthetransmittedpulsearefactorswhichacttodistorttheimage
paintedonthescope.Briefly,thewidthoftheradarbeamactstodistortthe
shorelinefeaturesinbearing;thepulselengthmayacttocauseoffshore
features to appear as part of the landmass.
Themajorproblemisthatofdeterminingwhichfeaturesinthevicinityof
theshorelineareactuallyreflectingtheechoespaintedonthescope.
Particularlyincaseswherealowlyingshoreisbeingscanned,theremaybe
considerable uncertainty.
Anassociatedproblemisthefactthatcertainfeaturesontheshorewill
notreturnechoes,eveniftheyhavegoodreflectingproperties,simply
becausetheyareblockedfromtheradarbeambyotherphysicalfeaturesor
obstructions.Thisfactorinturncausesthechartlikeimagepaintedonthe
scope to differ from the chart of the area.
Ifthenavigatoristobeabletointerpretthechartlikepresentationon
hisradarscope,hemusthaveatleastanelementaryunderstandingofthe
characteristicsofradarpropagation,thecharacteristicsofhisradarset,
thereflectingpropertiesofdifferenttypesofradartargets,andtheability
toanalyzehischarttomakeanestimateofjustwhichchartedfeatures
aremostlikelytoreflectthetransmittedpulsesortobeblockedfromthe
radarbeam.Whilecontourlinesonthecharttopographyaidthe
navigatormateriallyinthelattertask,experiencegainedduringclear
weathercomparisonofthevisualcross-bearingplotandtheradarscope
presentation is invaluable.
LAND TARGETS
Onrelativeandtruemotiondisplays,landmassesarereadilyrecognizable
becauseofthegenerallysteadybrillianceoftherelativelylargeareaspainted
onthePPI.Alsolandshouldbeatpositionsexpectedfromknowledgeofthe
ship’snavigationalposition.Onrelativemotiondisplays,landmassesmove
indirectionsandatratesoppositeandequaltotheactualmotionofthe
observer’sship.Individualpipsdonotmoverelativetooneanother.Ontrue
motiondisplays,landmassesdonotmoveonthePPIifthereisaccurate
compensationforsetanddrift.Withoutsuchcompensation,i.e.,whenthe
truemotiondisplayissea-stabilized,onlyslightmovementsoflandmasses
may be detected on the PPI.
Whilelandmassesarereadilyrecognizable,theprimaryproblemisthe
identificationofspecificfeaturessothatsuchfeaturescanbeusedforfixing
thepositionoftheobserver’sship.Identificationofspecificfeaturescanbe
quitedifficultbecauseofvariousfactors,includingdistortionresultingfrom
beamwidthandpulselengthanduncertaintyastojustwhichcharted
featuresarereflectingtheechoes.Thefollowinghintsmaybeusedasanaid
in identification:
(a)Sandspitsandsmooth,clearbeachesnormallydonotappearonthe
PPIatrangesbeyond1or2milesbecausethesetargetshavealmostnoarea
thatcanreflectenergybacktotheradar.Rangesdeterminedfromthese
targetsarenotreliable.Ifwavesarebreakingoverasandbar,echoesmaybe
returnedfromthesurf.Wavesmay,however,breakwelloutfromtheactual
shoreline,sothatrangingonthesurfmaybemisleadingwhenaradar
position is being determined relative to shoreline.
(b)Mudflatsandmarshesnormallyreflectradarpulsesonlyalittlebetter
thanasandspit.Theweakechoesreceivedatlowtidedisappearathightide.
Mangrovesandotherthickgrowthmayproduceastrongecho.Areasthatare
indicatedasswampsonachart,therefore,mayreturneitherstrongorweak
echoes,dependingonthedensityandsizeofthevegetationgrowinginthe
area.
(c)Whensanddunesarecoveredwithvegetationandarewellbackfrom
alow,smoothbeach,theapparentshorelinedeterminedbyradarappearsas
thelineofthedunesratherthanthetrueshoreline.Undersomeconditions,
sanddunesmayreturnstrongechosignalsbecausethecombinationofthe
148
verticalsurfaceofthevegetationandthehorizontalbeachmayformasortof
corner reflector.
(d)LagoonsandinlandlakesusuallyappearasblankareasonaPPI
becausethesmoothwatersurfacereturnsnoenergytotheradarantenna.In
someinstances,thesandbarorreefsurroundingthelagoonmaynotappear
on the PPI because it lies too low in the water.
(e)Coralatollsandlongchainsofislandsmayproducelonglinesof
echoeswhentheradarbeamisdirectedperpendiculartothelineofthe
islands.Thisindicationisespeciallytruewhentheislandsareclosely
spaced.Thereasonisthatthespreadingresultingfromthewidthoftheradar
beamcausestheechoestoblendintocontinuouslines.Whenthechainof
islandsisviewedlengthwise,orobliquely,however,eachislandmay
produceaseparatepip.Surfbreakingonareefaroundanatollproducesa
ragged, variable line of echoes.
(f)Submergedobjectsdonotproduceradarechoes.Oneortworocks
projectingabovethesurfaceofthewater,orwavesbreakingoverareef,may
appearonthePPI.Whenanobjectissubmergedentirelyandtheseais
smooth over it, no indication is seen on the PPI.
(g)Ifthelandrisesinagradual,regularmannerfromtheshoreline,
nopartoftheterrainproducesanechothatisstrongerthantheecho
fromanyotherpart.Asaresult,ageneralhazeofechoesappearson
thePPI,anditisdifficulttoascertaintherangetoanyparticularpartof
the land.
Landcanberecognizedbyplottingthecontact.Caremustbeexercised
whenplottingbecause,asashipapproachesorgoesawayfromashore
behindwhichthelandrisesgradually,aplotoftherangesandbearingstothe
landmayshowan“apparentcourseandspeed.Thisphenomenonis
demonstratedinfigure4.1.InviewAtheshipis50milesfromtheland,but
becausetheradarbeamstrikesatpoint1,wellupontheslope,theindicated
rangeis60miles.InviewBwheretheshipis10milesclosertoland,the
indicatedrangeis46milesbecausetheradarechoisnowreturnedfrom
point2.InviewCwheretheshipisanother10milescloser,theradarbeam
strikesatpoint3,evenlowerontheslope,sothattheindicatedrangeis32
miles.Iftheserangesareplotted,thelandwillappeartobemovingtoward
the ship.
Infigure4.1,asmooth,gradualslopeisassumed,sothataconsistentplot
isobtained.Inpractice,however,theslopeofthegroundusuallyisirregular
andtheploterratic,makingithardtoassignadefinitespeedtotheland
contact.Thesteepertheslopeoftheland,thelessisitsapparentspeed.
Furthermore,becausetheslopeofthelanddoesnotalwaysfalloffinthe
directionfromwhichtheshipapproaches,theapparentcourseofthecontact
neednotalwaysbetheoppositeofthecourseoftheship,asassumedinthis
simple demonstration.
(h)Blotchysignalsarereturnedfromhillygroundbecausethecrestof
eachhillreturnsagoodechoalthoughthevalleybeyondisinashadow.If
highreceivergainisused,thepatternmaybecomesolidexceptforthevery
deep shadows.
(i)Lowislandsordinarilyproducesmallechoes.Whenthickpalmtreesor
otherfoliagegrowontheisland,strongechoesoftenareproducedbecause
thehorizontalsurfaceofthewateraroundtheislandformsasortofcorner
reflectorwiththeverticalsurfacesofthetrees.Asaresult,woodedislands
givegoodechoesandcanbedetectedatamuchgreaterrangethanbarren
islands.
Figure 4.1 - Apparent course and speed of land target.
149
SHIP TARGETS
WiththeappearanceofasmallpiponthePPI,itsidentificationasaship
canbeaidedbyaprocessofelimination.Acheckofthenavigational
positioncanoverrulethepossibilityofland.Thesizeofthepipcanbeused
tooverrulethepossibilityoflandorprecipitation,bothusuallyhavinga
massiveappearanceonthePPI.TherateofmovementofthepiponthePPI
can overrule the possibility of aircraft.
Havingeliminatedtheforegoingpossibilities,theappearanceofthepipat
amediumrangeasabright,steady,andclearlydefinedimageonthePPI
indicates a high probability that the target is a steel ship.
Thepipofashiptargetmaybrightenattimesandthenslowlydecreasein
brightness.Normally,thepipofashiptargetfadesfromthePPIonlywhen
the range becomes too great.
RADAR SHADOW
WhilePPIdisplaysareapproximatelychartlikewhenlandmassesare
beingscannedbytheradarbeam,theremaybesizableareasmissing
fromthedisplaybecauseofcertainfeaturesbeingblockedfromthe
radarbeambyotherfeatures.Ashorelinewhichiscontinuousonthe
PPIdisplaywhentheshipisatonepositionmaynotbecontinuous
whentheshipisatanotherpositionandscanningthesameshoreline.
Theradarbeammaybeblockedfromasegmentofthisshorelinebyan
obstructionsuchasapromontory.Anindentationintheshoreline,such
asacoveorbay,appearingonthePPIwhentheshipisatoneposition
maynotappearwhentheshipisatanotherpositionnearby.Thus,radar
shadowalonecancauseconsiderabledifferencesbetweenthePPI
displayandthechartpresentation.Thiseffectinconjunctionwiththe
beamwidthandpulselengthdistortionofthePPIdisplaycancause
even greater differences.
BEAM WIDTH AND PULSE LENGTH DISTORTION
Thepipsofships,rocks,andothertargetsclosetoshoremaymergewith
theshorelineimageonthePPI.Thismergingisduetothedistortioneffects
ofhorizontalbeamwidthandpulselength.TargetimagesonthePPIalways
aredistortedangularlybyanamountequaltotheeffectivehorizontalbeam
width.Also,thetargetimagesalwaysaredistortedradiallybyanamountat
leastequaltoone-halfthepulselength(164yardspermicrosecondofpulse
length).
Figure4.2illustratestheeffectsofship’sposition,beamwidth,andpulse
lengthontheradarshoreline.Becauseofbeamwidthdistortion,astraight,
ornearlystraight,shorelineoftenappearscrescent-shapedonthePPI.This
effectisgreaterwiththewiderbeamwidths.Notethatthisdistortion
increases as the angle between the beam axis and the shoreline decreases.
150
Figure 4.2 - Effects of ship’s position, beam width, and pulse length on radar shoreline.
151
SUMMARY OF DISTORTIONS
Figure4.3illustratesthedistortioneffectsofradarshadow,beamwidth,
andpulselength.ViewAshowstheactualshapeoftheshorelineandthe
landbehindit.Notethesteeltoweronthelowsandbeachandthetwoships
atanchorclosetoshore.TheheavylineinviewBrepresentstheshorelineon
thePPI.Thedottedlinesrepresenttheactualpositionandshapeofall
targets. Note in particular:
(a) The low sand beach is not detected by the radar.
(b)Thetoweronthelowbeachisdetected,butitlookslikeashipina
cove.Atcloserrangethelandwouldbedetectedandthecove-shapedarea
wouldbegintofillin;thenthetowercouldnotbeseenwithoutreducingthe
receiver gain.
(c)Theradarshadowbehindbothmountains.Distortionowingtoradar
shadowsisresponsibleformoreconfusionthananyothercause.Thesmall
island does not appear because it is in the radar shadow.
(d)Thespreadingofthelandinbearingcausedbybeamwidthdistortion.
Lookattheuppershoreofthepeninsula.Theshorelinedistortionisgreater
tothewestbecausetheanglebetweentheradarbeamandtheshoreis
smaller as the beam seeks out the more westerly shore.
(e)ShipNo.1appearsasasmallpeninsula.Herpiphasmergedwiththe
land because of the beam width distortion.
(f)ShipNo.2alsomergeswiththeshorelineandformsabump.This
bumpiscausedbypulselengthandbeamwidthdistortion.Reducing
receivergainmightcausetheshiptoseparatefromland,providedtheshipis
nottooclosetotheshore.TheFTCcouldalsobeusedtoattempttoseparate
the ship from land.
Figure 4.3 - Distortion effects of radar shadow, beam width, and pulse length.
152
RECOGNITION OF UNWANTED ECHOES AND EFFECTS
Thenavigatormustbeabletorecognizevariousabnormalechoesand
effects on the radarscope so as not to be confused by their presence.
Indirect (False) Echoes
Indirectorfalseechoesarecausedbyreflectionofthemainlobeofthe
radarbeamoffship’sstructuressuchasstacksandkingposts.Whensuch
reflectiondoesoccur,theechowillreturnfromalegitimateradarcontactto
theantennabythesameindirectpath.Consequently,theechowillappearon
thePPIatthebearingofthereflectingsurface.Thisindirectechowillappear
onthePPIatthesamerangeasthedirectechoreceived,assumingthatthe
additional distance by the indirect path is negligible (see figure 4.4).
Characteristicsbywhichindirectechoesmayberecognizedare
summarized as follows:
(1) The indirect echoes will usually occur in shadow sectors.
(2)Theyarereceivedonsubstantiallyconstantbearingsalthoughthetrue
bearing of the radar contact may change appreciably.
(3) They appear at the same ranges as the corresponding direct echoes.
(4) When plotted, their movements are usually abnormal.
(5) Their shapes may indicate that they are not direct echoes.
Figure4.5illustratesamassiveindirectechosuchasmaybereflectedbya
landmass.
Figure 4.4 - Indirect echo.
Figure 4.5 - Indirect echo reflected by a landmass.
153
Side-lobe Effects
Side-lobeeffectsarereadilyrecognizedinthattheyproduceaseriesof
echoesoneachsideofthemainlobeechoatthesamerangeasthelatter.
Semi-circlesorevencompletecirclesmaybeproduced.Becauseofthelow
energyoftheside-lobes,theseeffectswillnormallyoccuronlyattheshorter
ranges.Theeffectsmaybeminimizedoreliminatedthroughuseofthegain
andanticluttercontrols.Slottedwaveguideantennashavelargelyeliminated
the side-lobe problem (see figure 4.6).
Multiple Echoes
Multipleechoesmayoccurwhenastrongechoisreceivedfromanother
shipatcloserange.Asecondorthirdormoreechoesmaybeobservedon
theradarscopeatdouble,triple,orothermultiplesoftheactualrangeofthe
radar contact (see figure 4.7).
Second-Trace (Multiple-Trace) Echoes
Second-traceechoes(multiple-traceechoes)areechoesreceivedfroma
contactatanactualrangegreaterthantheradarrangesetting.Ifanechofroma
distanttargetisreceivedafterthefollowingpulsehasbeentransmitted,theecho
willappearontheradarscopeatthecorrectbearingbutnotatthetruerange.
Second-traceechoesareunusualexceptunderabnormalatmosphericconditions,
orconditionsunderwhichsuper-refractionispresent.Second-traceechoesmay
berecognizedthroughchangesintheirpositionsontheradarscopeonchanging
thepulserepetitionrate(PRR);theirhazy,streaky,ordistortedshape;andtheir
erratic movements on plotting.
Asillustratedinfigure4.8,atargetpipisdetectedonatruebearingof
090˚atadistanceof7.5miles.OnchangingthePRRfrom2000to1800
pulsespersecond,thesametargetisdetectedonabearingof090˚ata
distanceof3miles(seefigure4.9).Thechangeinthepositionofthepip
indicatesthatthepipisasecond-traceecho.Theactualdistanceofthetarget
isthedistanceasindicatedonthePPIplushalfthedistancetheradarwave
travels between pulses.
154
Figure 4.6 - Side-lobe effects.Figure 4.7 - Multiple echoes.
Figure 4.8 - Second-trace echo on 12-mile range scale.Figure 4.9 - Position of second-trace echo on 12-mile range scale after changing PRR.
155
Figure4.10illustratesnormal,indirect,multiple,andsideechoesonaPPI
with an accompanying annotated sketch.
Electronic Interference Effects
Electronicinterferenceeffects,suchasmayoccurwheninthevicinityof
anotherradaroperatinginthesamefrequencybandasthatoftheobserver’s
ship,isusuallyseenonthePPIasalargenumberofbrightdotseither
scatteredatrandomorintheformofdottedlinesextendingfromthecenter
to the edge of the PPI.
Interferenceeffectsaregreateratthelongerradarrangescalesettings.The
interferenceeffectscanbedistinguishedeasilyfromnormalechoesbecausethey
do not appear in the same places on successive rotations of the antenna.
Blind and Shadow Sectors
Stacks,masts,samsonposts,andotherstructuresmaycauseareductionin
theintensityoftheradarbeambeyondtheseobstructions,especiallyifthey
areclosetotheradarantenna.Iftheangleattheantennasubtendedbythe
obstructionismorethanafewdegrees,thereductionoftheintensityofthe
radarbeambeyondtheobstructionmaybesuchthatablindsectoris
produced.Withlesserreductionintheintensityofthebeambeyondthe
obstructions,shadowsectors,asillustratedinfigure4.11,canbeproduced.
Withintheseshadowsectors,smalltargetsatcloserangemaynotbe
detected while larger targets at much greater ranges may be detected.
From the Use of Radar at Sea, 4th Ed. Copyright 1968, The Institute of Navigation, London. Used by permission.
Figure 4.10 - Normal, indirect, multiple, and side echoes.
156
Spoking
SpokingappearsonthePPIasanumberofspokesorradiallines.Spoking
iseasilydistinguishedfrominterferenceeffectsbecausethelinesarestraight
on all range-scale settings and are lines rather than a series of dots.
ThespokesmayappearallaroundthePPI,ortheymaybeconfinedtoa
sector.Shouldthespokingbeconfinedtoanarrowsector,theeffectcanbe
distinguishedfromaramarksignalofsimilarappearancethrough
observationofthesteadyrelativebearingofthespokeinasituationwhere
thebearingoftheramarksignalshouldchange.Theappearanceofspoking
is indicative of need for equipment maintenance.
Sectoring
ThePPIdisplaymayappearasalternatelynormalanddarksectors.This
phenomenonisusuallyduetotheautomaticfrequencycontrolbeingoutof
adjustment.
Serrated Range Rings
Theappearanceofserratedrangeringsisindicativeofneedforequipment
maintenance.
PPI Display Distortion
Aftertheradarsethasbeenturnedon,thedisplaymaynotspread
immediatelytothewholeofthePPIbecauseofstaticelectricityinsidethe
CRT.Usually,thisstaticelectricityeffect,whichproducesadistortedPPI
display, lasts no longer than a few minutes.
Hour-Glass Effect
Hour-glasseffectappearsaseitheraconstrictionorexpansionofthe
displaynearthecenterofthePPI.Theexpansioneffectissimilarin
appearancetotheexpandedcenterdisplay.Thiseffect,whichcanbecaused
byanonlineartimebaseorthesweepnotstartingontheindicatoratthe
sameinstantasthetransmissionofthepulse,ismostapparentwhenin
narrow rivers or close to shore.
Overhead Cable Effect
TheechofromanoverheadpowercableappearsonthePPIasasingleecho
alwaysatrightanglestothelineofthecable.Ifthisphenomenonisnot
recognized,theechocanbewronglyidentifiedastheechofromashipona
steadybearing.Avoidingactionresultsintheechoremainingonaconstant
bearingandmovingtothesamesideofthechannelastheshipalteringcourse.
Thisphenomenonisparticularlyapparentforthepowercablespanningthe
Straits of Messina. See figure 4.12 for display of overhead cable effect.
Figure 4.11 - Shadow sectors.
157
Figure 4.12 - Overhead cable effect.
158
AIDS TO RADAR NAVIGATION
Variousaidstoradarnavigationhavebeendevelopedtoaidthenavigator
inidentifyingradartargetsandforincreasingthestrengthoftheechoes
received from objects which otherwise are poor radar targets.
RADAR REFLECTORS
Buoysandsmallboats,particularlythoseboatsconstructedofwood,are
poorradartargets.Weakfluctuatingechoesreceivedfromthesetargetsare
easilylostintheseaclutterontheradarscope.Toaidinthedetectionofthese
targets,radarreflectors,ofthecornerreflectortype,maybeused.Thecorner
reflectorsmaybemountedonthetopsofbuoysorthebodyofthebuoymay
be shaped as a corner reflector, as illustrated in figure 4.13.
Eachcornerreflectorillustratedinfigure4.14consistsofthreemutually
perpendicular flat metal surfaces.
Aradarwaveonstrikinganyofthemetalsurfacesorplateswillbe
reflectedbackinthedirectionofitssource,i.e.,theradarantenna.Maximum
energywillbereflectedbacktotheantennaiftheaxisoftheradarbeam
makesequalangleswithallthemetalsurfaces.Frequentlycornerreflectors
are assembled in clusters to insure receiving strong echoes at the antenna.
RADAR BEACONS
Whileradarreflectorsareusedtoobtainstrongerechoesfromradar
targets,othermeansarerequiredformorepositiveidentificationofradar
targets.Radarbeaconsaretransmittersoperatinginthemarineradar
frequencybandwhichproducedistinctiveindicationsontheradarscopesof
shipswithinrangeofthesebeacons.Therearetwogeneralclassesofthese
beacons:raconwhichprovidesbothbearingandrangeinformationtothe
targetandramarkwhichprovidesbearinginformationonly.However,ifthe
ramarkinstallationisdetectedasanechoontheradarscope,therangewill
be available also.
Figure 4.13 - Radar reflector buoy.
Figure 4.14 - Corner reflectors.
159
Racon
Raconisaradartransponderwhichemitsacharacteristicsignalwhen
triggeredbyaship’sradar.Thesignalmaybeemittedonthesamefrequency
asthatofthetriggeringradar,inwhichcaseitisautomaticallysuperimposed
ontheship’sradardisplay.Thesignalmaybeemittedonaseparate
frequency,inwhichcasetoreceivethesignaltheship’sradarreceivermust
becapableofbeingtunedtothebeaconfrequencyoraspecialreceivermust
beused.Ineithercase,thePPIwillbeblankexceptforthebeaconsignal.
“Frequencyagile”raconsarenowinwidespreaduse.Theyrespondtoboth3
and 10 centimeter radars.
TheraconsignalappearsonthePPIasaradiallineoriginatingatapoint
justbeyondthepositionoftheradarbeaconorasaMorsecodesignal
displayed radially from just beyond the beacon (see figures 4.15 and 4.16).
Raconsarebeingusedasrangesorleadinglines.Therangeisformedby
tworaconssetupbehindeachotherwithaseparationintheorderof2to4
nauticalmiles.OnthePPIscopethe“paint”receivedfromthefrontandrear
racons form the range.
Somebridgesarenowequippedwithraconswhicharesuspendedunder
the bridge to provide guidance for safe passage.
The maximum range for racon reception is limited by line of sight.
Figure 4.15 - Racon signal.
Figure 4.16 - Coded racon signal.
160
Ramark
Ramarkisaradarbeaconwhichtransmitseithercontinuouslyoratintervals.
ThelattermethodoftransmissionisusedsothatthePPIcanbeinspected
withoutanyclutterintroducedbytheramarksignalonthescope.Theramark
signalasitappearsonthePPIisaradiallinefromthecenter.Theradiallinemay
beacontinuousnarrowline,aseriesofdashes,aseriesofdots,oraseriesofdots
and dashes (see figures 4.17 and 4.18).
Figure 4.17 - Ramark signal appearing as a dotted line.
Figure 4.18 - Ramark signal appearing as a dashed line.
161
RADAR FIXING METHODS
RANGE AND BEARING TO A SINGLE OBJECT
Preferably,radarfixesobtainedthroughmeasuringtherangeandbearing
toasingleobjectshouldbelimitedtosmall,isolatedfixedobjectswhichcan
beidentifiedwithreasonablecertainty.Inmanysituations,thismethodmay
betheonlyreliablemethodwhichcanbeemployed.Ifpossible,thefix
shouldbebaseduponaradarrangeandvisualgyrobearingbecauseradar
bearingsarelessaccuratethanvisualgyrobearings.Aprimaryadvantageof
themethodistherapiditywithwhichafixcanbeobtained.Adisadvantage
isthatthefixisbasedupononlytwointersectingpositionlines,abearing
lineandarangearc,obtainedfromobservationsofthesameobject.
Identification mistakes can lead to disaster.
TWO OR MORE BEARINGS
Generally,fixesobtainedfromradarbearingsarelessaccuratethanthose
obtainedfromintersectingrangearcs.Theaccuracyoffixingbythismethod
isgreaterwhenthecenterbearingsofsmall,isolated,radar-conspicuous
objects can be observed.
Becauseoftherapidityofthemethod,themethodaffordsameansfor
initiallydetermininganapproximatepositionforsubsequentuseinmore
reliable identification of objects for fixing by means of two or more ranges.
TANGENT BEARINGS
Fixingbytangentbearingsisoneoftheleastaccuratemethods.Theuse
oftangentbearingswitharangemeasurementcanprovideafixof
reasonably good accuracy.
Asillustratedinfigure4.19,thetangentbearinglinesintersectatarange
fromtheislandobservedlessthantherangeasmeasuredbecauseofbeam
widthdistortion.Righttangentbearingsshouldbedecreasedbyanestimate
ofhalfthehorizontalbeamwidth.Lefttangentbearingsshouldbeincreased
bythesameamount.Thefixistakenasthatpointontherangearcmidway
between the bearing lines.
Itisfrequentlyquitedifficulttocorrelatetheleftandrightextremitiesofthe
islandaschartedwiththeislandimageonthePPI.Therefore,evenwith
compensationforhalfofthebeamwidth,thebearinglinesusuallywillnot
intersect at the range arc.
TWO OR MORE RANGES
Inmanysituations,themoreaccurateradarfixesaredeterminedfrom
nearlysimultaneousmeasurementsoftherangestotwoormorefixed
objects.Preferably,atleastthreerangesshouldbeusedforthefix.The
numberofrangeswhichitisfeasibletouseinaparticularsituationis
dependentuponthetimerequiredforidentificationandrangemeasurements.
Inmanysituations,theuseofmorethanthreerangearcsforthefixmay
introduce excessive error because of the time lag between measurements.
Ifthemostrapidlychangingrangeismeasuredfirst,theplotwillindicate
lessprogressalongtheintendedtrackthanifitweremeasuredlast.Thus,
lesslagintheradarplotfromtheship’sactualpositionisobtainedthrough
measuring the most rapidly changing ranges last.
Similartoavisualcross-bearingfix,theaccuracyoftheradarfixis
dependentupontheanglesofcutoftheintersectingpositionlines(range
arcs).Forgreateraccuracy,theobjectsselectedshouldproviderangearcs
withanglesofcutascloseto90˚asispossible.Incaseswheretwo
identifiableobjectslieinoppositeornearlyoppositedirections,theirrange
Figure 4.19 - Fixing by tangent bearings and radar range.
162
arcs,eventhoughtheymayintersectatasmallangleofcutormaynot
actuallyintersect,incombinationwithanotherrangearcintersectingthemat
anangleapproaching90˚,mayprovideafixofhighaccuracy(seefigure
4.20).Theneartangencyofthetworangearcsindicatesaccurate
measurementsandgoodreliabilityofthefixwithrespecttothedistanceoff
the land to port and starboard.
Small,isolated,radar-conspicuousfixedobjectsaffordthemostreliable
andaccuratemeansforradarfixingwhentheyaresosituatedthattheir
associated range arcs intersect at angles approaching 90˚.
Figure4.21illustratesafixobtainedbymeasuringtherangestothreewell
situatedradar-conspicuousobjects.Thefixisbasedsolelyuponrange
measurementsinthatradarrangesaremoreaccuratethanradarbearingseven
whensmallobjectsareobserved.Notethatinthisratheridealsituation,apoint
fixwasnotobtained.Becauseofinherentradarerrors,anypointfixshouldbe
treated as an accident dependent upon plotting errors, the scale of the chart, etc.
Whileobservedradarbearingswerenotusedinestablishingthefixassuch,
the bearings were useful in the identification of the radar-conspicuous objects.
Astheshiptravelsalongitstrack,thethreeradar-conspicuousobjectsstill
affordgoodfixingcapabilityuntilsuchtimeastheanglesofcutoftherange
arcshavedegradedappreciably.Atsuchtime,otherradar-conspicuous
objectsshouldbeselectedtoprovidebetteranglesofcut.Preferably,thefirst
newobjectshouldbeselectedandobservedbeforetheanglesofcuthave
degradedappreciably.Incorporatingtherangearcofthenewobjectwith
rangearcsofobjectswhichhaveprovidedreliablefixesaffordsmore
positive identification of the new object.
MIXED METHODS
Whilefixingbymeansofintersectingrangearcs,theusualcaseisthat
twoormoresmall,isolated,andconspicuousobjects,whicharewellsituated
toprovidegoodanglesofcut,arenotavailable.Thenavigatormustexercise
considerableskillinradarscopeinterpretationtoestimatewhichcharted
featuresareactuallydisplayed.Ifinitiallytherearenowelldefinedfeatures
displayedandthereisconsiderableuncertaintyastotheship’sposition,the
navigatormayobservetheradarbearingsoffeaturestentativelyidentifiedas
Figure 4.20 - Radar fix.
Figure 4.21 - Fix by small, isolated radar-conspicuous objects.
163
asteptowardstheirmorepositiveidentification.Ifthecross-bearingfixdoes
indicatethatthefeatureshavebeenidentifiedwithsomedegreeofaccuracy,
theestimateoftheship’spositionobtainedfromthecross-bearingfixcanbe
usedasanaidinsubsequentinterpretationoftheradardisplay.Withbetter
knowledgeoftheship’sposition,thefactorsaffectingthedistortionofthe
radardisplaycanbeusedmoreintelligentlyinthecourseofmoreaccurate
interpretation of the radar display.
Frequentlythereisatleastoneobjectavailablewhich,ifcorrectly
identified,canenablefixingbytherangeandbearingtoasingleobject
method.Afixsoobtainedcanbeusedasanaidinradarscopeinterpretation
for fixing by two or more intersecting range arcs.
Thedifficultieswhichmaybeencounteredinradarscopeinterpretation
duringatransitmaybesogreatthataccuratefixingbymeansofrangearcsis
notobtainable.Insuchcircumstances,rangearcshavingsomedegreeof
accuracycanbeusedtoaidintheidentificationofobjectsusedwiththe
range and bearing method.
Withcorrectidentificationoftheobjectobserved,theaccuracyofthefix
obtainedbytherangeandbearingtoasingleobjectmethodusuallycanbe
improvedthroughtheuseofavisualgyrobearinginsteadoftheradar
bearing.Particularlyduringperiodsoflowvisibility,thenavigatorshouldbe
alert for visual bearings of opportunity.
Whilethebestmethodorcombinationofmethodsforaparticular
situationmustbelefttothegoodjudgmentoftheexperiencednavigator,
factors affecting method selection include:
(1)Thegeneralneedforredundancy—butnottosuchextentthattoo
much is attempted with too little aid or means in too little time.
(2) The characteristics of the radar set.
(3) Individual skills.
(4) The navigational situation, including the shipping situation.
(5) The difficulties associated with radarscope interpretation.
(6) Angles of cut of the position lines.
PRECONSTRUCTION OF RANGE ARCS
Small,isolated,radar-conspicuousobjectspermitpreconstructionofrange
arcsonthecharttoexpediteradarfixing.Thispreconstructionispossible
becausetherangecanbemeasuredtothesamepointoneachobject,ornearly
so,astheaspectchangesduringthetransit.Withfixedradartargetsoflesser
conspicuous,thenavigator,generally,mustcontinuallychangethecentersofthe
range arcs in accordance with his interpretation of the radarscope.
Toexpediteplottingfurther,thenavigatormayalsopreconstructaseries
ofbearinglinestotheradar-conspicuousobjects.Thedegreeof
preconstructionofrangearcsandbearinglinesisdependentuponacceptable
chartclutterresultingfromthearcsandlinesaddedtothechart.Usually,
preconstructionislimitedtoacriticalpartofapassageortotheapproachto
an anchorage.
164
CONTOUR METHOD
Thecontourmethodofradarnavigationconsistsofconstructingaland
contouronatransparenttemplateaccordingtoaseriesofradarrangesand
bearingsandthenfittingthetemplatetothechart.Thepointoforiginofthe
ranges and bearings defines the fix.
Thismethodmayprovidemeansforfixingwhenitisdifficultto
correlatethelandmassimageonthePPIwiththechartbecauseofalack
offeaturesalongtheshorelinewhichcanbeidentifiedindividually.The
accuracyofthemethodisdependentuponthenavigator’sabilityto
estimatethecontoursofthelandmostlikelytobereflectingtheechoes
formingthelandmassimageonthePPI.Evenwithconsiderableskillin
radarscopeinterpretation,thenavigatorcanusuallyobtainonlyan
approximatefitofthetemplatecontourwiththeestimatedlandcontour.
Theremayberelativelylargegapsinthefitcausedbyradarshadow
effects.Thus,theremaybeconsiderableuncertaintywithrespecttothe
accuracyofthepointfix.Thecontourmethodismostfeasiblewhenthe
landrisessteeplyatorneartheshoreline,thusenablingamoreaccurate
estimate of the reflecting surfaces.
Figure4.22illustratesarectangulartemplateonthebottomsideofwhich
radialsaredrawnat5-degreeintervals.Theradialsaredrawnfromasmall
hole,whichisthepositionoftheradarfixwhenthetemplateisfittedtothe
chart.
Inmakingpreparationforuseofthetemplate,thetemplateistackedtothe
range(distance)scaleofthechart.Astherangesandbearingstoshoreare
measuredat5or10-degreeintervals,thetemplateisrotatedaboutthezero-
distancegraduationandmarkedaccordingly.Acontourlineisfairedthrough
the marks on each radial.
Oninitiallyfittingthecontourtemplatetothechart,thetemplateshould
beorientedtotruenorth.Becauseofnormalbearingerrorsinradar
observations,thetemplatewillnotnecessarilybealignedwithtruenorth
when the best fit is obtained subsequently.
Figure 4.22 - Transparent template used with contour method.
165
IDENTIFYING A RADAR-INCONSPICUOUS OBJECT
Situation:
ThereisdoubtthatapiponthePPIrepresentstheechofromabuoy,a
radar-inconspicuousobject.Onthechartthereisaradar-conspicuousobject,
arock,inthevicinityofthebuoy.Thepipoftherockisidentifiedreadilyon
the PPI.
Required:
Identify the pip which is in doubt.
Solution:
(1)Measurethebearinganddistanceofthebuoyfromtherockonthe
chart.
(2)DeterminethelengthofthisdistanceonthePPIaccordingtothe
range scale setting.
(3)Rotatetheparallel-linecursortothebearingofthebuoyfromtherock
(see figure 4.23).
(4)Withrubber-tippeddividerssettotheappropriatePPIlength,setone
pointoverthepipoftherock;usingtheparallellinesofthecursoras
aguide,setthesecondpointinthedirectionofthebearingofthe
buoy from the rock.
(5)Withthedividerssoset,thesecondpointliesovertheunidentified
pip.Subjecttotheaccuracylimitationsofthemeasurementsand
normalprudence,thepipmaybeevaluatedastheechoreceivedfrom
the buoy.
NOTE:Duringlowvisibilityaradar-conspicuousobjectcanbeused
similarly to determine whether another ship is fouling an anchorage berth.
Figure 4.23 - Use of parallel-line cursor to identify radar-inconspicuous object.
166
FINDING COURSE AND SPEED MADE GOOD BY PARALLEL-LINE CURSOR
Situation:
Ashipsteaminginfogdetectsaprominentrockbyradar.Becauseofthe
unknowneffectsofcurrentandotherfactors,thenavigatorisuncertainofthe
course and speed being made good.
Required:
To determine the course and speed being made good.
Solution:
(1)Make a timed plot of the rock on the reflection plotter.
(2)Aligntheparallel-linecursorwiththeplottodeterminethecourse
beingmadegood,whichisinadirectionoppositetotherelative
movement (see figure 4.24).
(3)Measurethedistancebetweenthefirstandlastplotsandusingthe
timeinterval,determinethespeedofrelativemovement.Sincethe
rock is stationary, the relative speed is equal to that of the ship.
NOTE:Thisbasictechniqueisusefulfordeterminingwhethertheship
isbeingsetofftheintendedtrackinpilotwaters.Observingaradar-
conspicuousobjectandusingtheparallel-linecursor,alineisdrawn
throughtheradar-conspicuousobjectinadirectionoppositetoown
ship’s course.
Byobservingthesuccessivepositionsoftheradar-conspicuousobject
relativetothisline,thenavigatorcandeterminewhethertheshipisbeingset
to the left or right of the intended track.
Figure 4.24 - Use of parallel-line cursor to find course and speed made good.
167
USE OF PARALLEL-LINE CURSOR FOR ANCHORING
Situation:
Ashipismakinganapproachtoananchorageoncourse290˚.The
directionoftheintendedtracktotheanchorageis290˚.Allowingforthe
radiusofthelettinggocircle,theanchorwillbeletgowhenaradar-
conspicuousisletis1.0mileaheadoftheshipontheintendedtrack.A
decisionismadetouseaparallel-linecursortechniquetokeeptheshipon
theintendedtrackduringthelastmileoftheapproachtotheanchorageand
todeterminethetimeforlettinggo.Beforethelatterdecisionwasmade,the
navigator’sinterpretationofthestabilizedrelativemotiondisplayrevealed
that,evenwithchangeinaspect,theradarimageofajettytostarboardcould
be used to keep the ship on the intended track.
Required:
Maketheapproachtotheanchorageontheintendedtrackandletthe
anchor go when the islet is 1.0 mile ahead along the intended track.
Solution:
(1)Fromthechartdeterminethedistanceatwhichtheheadofthejetty
willbepassedabeamwhentheshipisoncourseandontheintended
track.
(2)Aligntheparallel-linecursorwiththedirectionoftheintendedtrack,
290˚ (see figure 4.25).
(3)Usingtheparallellinesofthecursorasaguide,draw,atadistance
fromthecenterofthePPIasdeterminedinstep(2),therelative
movementlinefortheheadofthejettyinadirectionoppositetothe
direction of the intended track.
(4)Makeamarkat290˚and1.0milefromthecenterofthePPI;label
this mark “LG” for letting go.
(5)Makeanothermarkat290˚and1.0milebeyondtheLGmark;label
this mark “1”.
(6)Subdividetheradialbetweenthemarksmadeinsteps(4)and(5).
Thissubdivisionmaybelimitedto0.1mileincrementsfromtheLG
mark to the 0.5 mile graduation.
(7)Iftheshipisontheintendedtrack,theRMLshouldextendfromthe
radarimageoftheheadofthejetty.Iftheshipkeepsontheintended
track,theimageofthejettywillmovealongtheRML.Iftheship
deviatesfromtheintendedtrack,theimageofthejettywillmove
awayfromtheRML.Correctiveactionistakentokeeptheimageof
the jetty on the RML.
(8)Withtheshipbeingkeptontheintendedtrackbykeepingtheimage
ofthejettyontheRML,thegraduationsoftheradialinthedirection
oftheintendedtrackprovidedistancestogo.Whenthemarklabeled
“1”justtouchestheleadingedgeofthepipoftheisletahead,thereis
1miletogo.Whenthemarklabel“.5”justtouchestheleadingedge
ofthelatterpip,thereis0.5miletogo,etc.Theanchorshouldbelet
gowhenthemarklabeled“LG”justtouchestheleadingedgeofthe
pip of the islet.
168
Figure 4.25 - Use of parallel-line cursor for anchoring.
169
PARALLEL INDEXING
ParallelIndexinghasbeenusedformanyyears.ItwasdefinedbyWilliam
BurgerintheRadarObserversHandbook(1957,page.98)asequidistantly
spacedparallellinesengravedonatransparentscreenwhichfitsonthePPI
andcanberotated.Thisconceptofusingparallellinestoassistinnavigation
hasbeenextensivelyusedinEuropetoassistinmaintainingaspecified
track,alteringcourseandanchoring.Itisbestsuitedforusewithastabilized
radar.Whenusinganunstabilizedradar,itcanposesomedangertoan
individual that is unaware of problems inherent in this type of display.
WiththeadventofARPAwithmovableEBLs(ElectronicBearingLines)
andNavigationLines,parallelindexingonscreencanbeaccomplishedwith
greateraccuracy.Indexlinesthatareatexactbearingsanddistancesoffcan
bedisplayedwithgreaterease.Anumberofdiagramsareincludedonthe
pagesthatfollowtoexplaintheuseofparallelindexingtechniquesaswellas
its misuse.
Cross Index Range (“C”)
Thedistanceofanobjectwhenabeamifthevesselwastopassthe
navigationmark.Aparallellineisdrawnthroughthismark.The
perpendiculardistancefromthecenterofthedisplaytothisparallellineis
theCross Index Range (1964, Admiralty Manual of Navigation).
Dead Range (“D”)
Thedistanceatwhichanobjecttrackingonaparallellinewouldbeona
new track line (ahead of or behind the beam bearing of the object).
Wheel Over Point (“W”)
Thepointatwhichtheactualmaneuverismadetoinsurethattheobject
being“indexed”isonthenewtracklinetakingintoaccounttheadvanceand
transfer of the vessel.
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182
THE FRANKLIN CONTINUOUS RADAR PLOT TECHNIQUE
TheFranklinContinuousRadarPlottechniqueprovidesmeansfor
continuouscorrelationofasmallfixed,radar-conspicuousobjectwithown
ship’spositionandmovementrelativetoaplannedtrack.Thetechnique,as
developedbyMasterChiefQuartermasterByronE.Franklin,U.S.Navy,
whileservingaboardUSSINTREPID(CVS-11),isarefinementofthe
parallel-cursor(parallel-index)techniquesusedasameansforkeepingown
ship on a planned track or for avoiding navigational hazards.
Rangesandbearingsoftheconspicuousobjectfromvariouspoints,
includingturningpoints,ontheplannedtrackaretransferredfromthechart
tothereflectionplottermountedonastabilizedrelativemotionindicator.On
plottingtherangesandbearingsandconnectingthemwithlinesegments,the
navigatorhasavisualdisplayofthepositionoftheconspicuousobject
relative to the path it should follow on the PPI (see figure 4.26).
Ifthepipoftheconspicuousobjectispaintedsuccessivelyonthe
constructedpath(plannedrelativemovementlineorseriesofsuchlines),the
navigatorknowsthat,withinthelimitsofaccuracyoftheplotandtheradar
display,hisshipisontheplannedtrack.Withtheplotlabeledwithrespectto
time,heknowswhetherheisaheadorbehindhisplannedschedule.Ifthe
pipsarepaintedtotheleftorrightoftheRML,actionrequiredtoreturnto
theplannedtrackisreadilyapparent.However,eitherofthefollowingrules
ofthumbmaybeused:(1)UsingtheDRMasthereferencedirectionforany
offsetsofthepips,theshipistotheleftoftheplannedtrackifthepipsare
paintedtotheleftoftheplannedRML;theshipistotherightoftheplanned
trackifthepipsarepaintedtotherightoftheplannedRML.(2)While
facinginthedirectionoftraveloftheconspicuousobjectonthePPI,theship
istotheleftorrightoftheplannedtrackifthepipsarepaintedleftorrightof
the planned RML, respectively.
Throughtakingsuchcorrectiveactionasisnecessarytokeepthe
conspicuousobjectpipontheRMLinaccordancewiththeplannedtime
schedule,continuousradarfixingis,ineffect,accomplished.Thisfixinghas
thelimitationofbeingbasedupontherangeandbearingmethod,more
subjecttoidentificationmistakesthanthemethodusingthreeormore
intersecting range arcs.
Exceptforthelimitationsofbeingrestrictedwithrespecttotherange
scalesettingandsomePPIclutterproducedbytheconstructionofthe
plannedRML,thetechniquedoesnotinterferewiththeuseofthePPIfor
fixingbyothermeans.Preferably,thetechniqueshouldbeusedin
conjunctionwitheithervisualfixingorfixingbymeansofthreeormore
intersectingrangearcs.Fixingbyeithermeansshouldestablishwhetherthe
radar-conspicuousobjecthasbeenidentifiedcorrectly.Withverificationthat
theradar-conspicuousobjecthasbeenidentifiedcorrectly,requirementsfor
frequent visual fixes or fixes by range measurements are less critical.
Becauseofthenormaltimelaginthelatestradarfixplottedonthechart,
inspectionofthepositionofthepipoftheradar-conspicuousobjectrelative
totheplannedRMLshouldprovideamoretimelyindicationastowhether
theshipistotheleftorrightoftheplannedtrackorwhethertheshiphas
turned too early or too late according to plan.
Oncetheradar-conspicuousobjecthasbeenidentifiedcorrectly,the
plannedRMLenablesrapidre-identificationinthosesituationswherethe
radarscopecannotbeobservedcontinuously.Also,thisidentificationofthe
conspicuousobjectwithrespecttoitsmovementalongtheplannedRML
provides means for more certain identification of other radar targets.
WhiletheplannedRMLcanbeconstructedthroughuseofthebearing
cursorandthevariablerangemarker(rangestrobe),theuseofplastic
templatesprovidesgreaterflexibilityintheuseofthetechnique,particularly
whentherearerequirementsforuseofmorethanonerangescalesettingora
needforshiftingtoadifferentradar-conspicuousobjectduringapassage
throughrestrictedwaters.WithaplannedRMLforaspecificradar-
conspicuousobjectcutinaplastictemplateforaspecificrangescalesetting
available,theplannedRMLcanbetracedrapidlyonthePPI.With
availabilityofothertemplatespreparedfordifferentrangescalesettingsor
differentobjectsandassociatedrangescalesettings,theplannedRMLas
neededcanbetracedrapidlyonthePPI.Othertemplatescanbepreparedfor
alternative planned tracks.
Iftherangescalesettingiscontinuouslyadjustableor“rubberizeditmay
bepossibletoconstructthetemplatebytracingtheplannedtrackonachart
havingascalewhichcanbeduplicatedonthePPI.Becausetheplanned
RMLisoppositetotheplannedtrack,thetrackcutinthetemplatemustbe
rotated 180˚ prior to tracing the planned RML on the PPI.
183
Figure 4.26 - The Franklin continuous radar plot technique.
184
TRUE MOTION RADAR RESET IN RESTRICTED WATERS
Whenusingtruemotiondisplays,thenavigatorshouldexercisecarein
decidingwhenandwheretoresetownship’spositiononthePPI.While
navigatinginrestrictedwaters,hemustinsurethathehasadequatewarning
ahead;throughsoundplanning,hemustavoidanyneedforresettingthe
display at critical times.
Thefollowingisanexampleofresettingatruemotiondisplayforaship
enteringtheRiverTyne.Thespeedmadegoodis6knots.Thenavigator
desires to maintain a warning ahead of at least 1 mile (see figure 4.27).
At 1000
Ownshipisresettothesouthonthe3-milerangescaletodisplayarea
AsothatTynemouthisjustshowingandsufficientwarningtothenorth
is obtained for the turn at about 1030.
At 1024
Ownshipisresettothesoutheastonthe1.5-milerangescaletodisplay
area B before the turn at 1030.
At 1040
OwnshipisresettotheeasttodisplayareaC.Theresethasbeencarried
outearlytoavoidaresetintheentranceandtoshowalltrafficuptoSouth
Shields.
At 1055
OwnshipisresettothenortheasttodisplayareaD.Theresethasbeen
carriedoutearlybeforethebendoftheriveratSouthShieldsandtoplace
the bend at Tyne Dock near the center of the display.
At 1117
Own ship is reset to the east to display area E.
At 1133
OwnshipisresettothenortheasttodisplayareaF.Theresethasbeen
carriedoutbeforethebendatHebburnanduptothenortheastbecausethe
ship is making good a southwest direction.
At 1200
Own ship is reset to the southeast to display area G.
185
Figure 4.27 - Resetting a true motion display.
186
RADAR DETECTION OF ICE
Radarcanbeaninvaluableaidinthedetectionoficeifusedwiselybythe
radarobserverhavingknowledgeofthecharacteristicsofradarpropagation
andthecapabilitiesofhisradarset.Theradarobservermusthavegood
appreciationofthefactthaticecapableofcausingdamagetoashipmaynot
bedetectedevenwhentheobserverismaintainingacontinuouswatchofthe
radarscope and is using operating controls expertly.
Whennavigatinginthevicinityoficeduringlowvisibility,acontinuous
watchoftheradarscopeisanecessity.Forreasonablyearlywarningofthe
presenceofice,rangescalesettingsofabout6or12milesareprobably
thosemostsuitable.Suchsettingsshouldprovideampletimeforevasive
actionafterdetection.Becauseanyicedetectedbyradarmaybelost
subsequentlyinseaclutter,itmaybeadvisabletomaintainageographical
plot.Thelatterplotcanaidindifferentiatingbetweeniceagroundordrifting
andshiptargets.Ifanicecontactisevaluatedasaniceberg,itshouldbe
givenawideberthbecauseoftheprobabilityofgrowlersinitsvicinity.Ifice
contactsareevaluatedasbergybitsorgrowlers,theradarobservershouldbe
alertforthepresenceofaniceberg.Becausethesmallericemayhavecalved
recentlyfromaniceberg,theradarobservershouldmaintainaparticularly
close watch to windward of the smaller ice.
ICEBERGS
Whilelargeicebergsmaybedetectedinitiallyatrangesof15to20miles
inacalmsea,thestrengthsofechoesreturnedfromicebergsareonlyabout
1
/
60
ofthestrengthsofechoeswhichwouldbereturnedfromasteelshipof
equivalent size.
Becauseoftheshapeoftheiceberg,thestrengthsofechoesreturned
mayhavewidevariationwithchangeinaspect.Also,becauseofshape
andaspect,theicebergmayappearontheradarscopeasseparateechoes.
Tabularicebergs,havingflattopsandnearlyverticalsideswhichmay
riseasmuchas100feetabovetheseasurface,arecomparativelygood
radar targets.
Generally,icebergswillbedetectedatrangesnotlessthan3miles
because of irregularities in the sloping faces.
BERGY BITS
Bergybits,extendingatmostabout15feetabovetheseasurface,usually
cannotbedetectedbyradaratrangesgreaterthan3miles.However,they
maybedetectedatrangesasgreatas6miles.Becausetheirechoesare
generallyweakandmaybelostinseaclutter,bergybitsweighingseveral
hundred or a few thousand tons can impose considerable hazard to a ship.
GROWLERS
Growlers,extendingatmostabout6feetabovetheseasurface,are
extremelypoorradartargets.Beingsmoothandroundbecauseofwave
action,aswellassmall,growlersarerecognizedasthemostdangeroustype
of ice that can be encountered.
Inaroughseaandwithseaclutterextendingbeyond1mile,growlers
largeenoughtocausedamagetoashipmaynotbedetectedbyradar.Even
withexpertuseofreceivergain,pulselength,andanti-cluttercontrols,
dangerous growlers in waves over 4 feet in height may not be detected.
Inacalmseagrowlersarenotlikelytobedetectedatarangeexceeding2
miles.
187
RADAR SETTINGS FOR RADARSCOPE PHOTOGRAPHY
Radarsettingsareanimportantfactorinpreparinggoodquality
radarscopephotography.Anaturaltendencyistoadjusttheradarimageso
thatitpresentsasuitablevisualdisplay,butthis,almostinvariably,produces
poorphotographicresults.Usuallytheresultingphotographisbadly
overexposedandlackingindetail.Anothertendencyistotrytorecordtoo
muchinformationononephotographsuchthattheclutterofbackground
returnsactuallyobscuresthetargetimages.Inbothcases,thebasicproblem
isacombinationofgainandintensitycontrol.Abasicruleofthumbisif
imagerylooksrighttovisualinspection,itwillprobablyoverexposethe
recordingfilm.Asaruleofthumb,iftheimageintensityisadjustedsothat
weakreturnsarejustvisible,thenaonesweepexposureshouldproducea
reasonably good photograph.
Thefollowinglistofeffectsassociatedwithvariousradarsettingscanbe
used as an aid in avoiding improper settings for radarscope photography:
(1)Excessivebrightnessproducesanoverallmilkyorintenselybright
appearanceoftheimages.Individualreturnswillbloomexcessively
andappearunfocused.Itbecomesdifficulttodistinguishthedivision
between land and water, and ground and cultural returns.
(2)Impropercontrastresultsinalackofbalanceinthegreytonal
gradations on the scope, greatly degrading the interpretive quality.
(3)Highgainresultsin“blooming”ofallbrightreturnsadversely
affectingtheimageresolution.Highgainalsocausestheformationof
a “hot spot” at the sweep origin.
(4)Lowgainresultsinalossofweaktomediumreturns.Theresultwill
bepoorinterpretivequalitywheretherearefewbrighttargets
illuminated due to absence of definitive target patterns on the scope.
(5)Excessivelybrightbearingcursors,headingflashes,andrangemarkers
resultinwidecursors,flashes,andmarkerswhichmayobscure
significant images.
(6)Improperradarscopeorcamerafocuswillresultinextremelyfuzzyor
blurred imagery.
188
NAVIGATIONAL PLANNING
Beforetransitinghazardouswaters,theprudentnavigatorshoulddevelop
afeasibleplanforderivingmaximumbenefitfromavailablenavigational
means.Indevelopinghisplan,thenavigatorshouldstudythecapabilities
andlimitationsofeachmeansaccordingtothenavigationalsituation.He
shoulddeterminehowonemeans,suchascross-bearingfixing,canbestbe
supported by another means, such as fixing by radar-range measurements.
Thenavigatormustbepreparedfortheunexpected,includingthe
possibilitythatatsomepointduringthetransititmaybenecessarytodirect
themovementsofthevesselprimarilybymeansofradarobservations
becauseofasuddenobscurityofchartedfeatures.Withoutadequate
planningfortheuseofradarastheprimarymeansforinsuringthesafetyof
thevessel,considerabledifficultyanddelaymaybeincurredbeforethe
navigatorisabletoobtainreliablefixesbymeansofradarfollowinga
sudden loss of visibility.
Anintendedtrackwhichmaybeidealforvisualobservationsmayimpose
severelimitationsonradarobservations.Insomecasesamodificationofthis
intendedtrackcanaffordincreasedcapabilityforreliableradarobservations
withoutundulydegradingthereliabilityofthevisualobservationsor
increasingthelengthofthetransitbyasignificantamount.Inthatthe
navigatorofaradar-equippedvesselalwaysmustbepreparedtouseradaras
theprimarymeansofnavigatinghisvesselwhileinpilotwaters,the
navigatorshouldeffectareasonablecompromisebetweentherequirements
forvisualandradarfixingwhiledeterminingtheintendedtrackforthe
transit.
Thevalueofradarfornavigationinpilotwatersislargelylostwhenitis
notmannedcontinuouslybyacompetentobserver.Withoutcontinuous
manningtheproblemsassociatedwithreliableradarscopeinterpretationare
toogreat,usually,forpromptandeffectiveuseoftheradarastheprimary
meansofinsuringthesafetyofthevessel.Thecontinuousmanningofthe
radarisalsorequiredforobtainingthebestradarscopepresentationthrough
properadjustmentsoftheoperatingcontrolsasthenavigationalsituation
changesorasthereisaneedtomakeadjustmentstoidentifyspecific
features.
Withradarbeingusedtosupportvisualfixingduringatransitof
hazardouswaters,visualobservationscanbeusedasanaidinthe
identificationofradarobservations.Throughcomparingtheradarplotwith
thevisualplot,thenavigatorcanevaluatetheaccuraciesoftheradar
observations.Withradaractuallybeingusedtosupportvisualfixing,the
transitiontotheuseofradarastheprimarymeanscanbeeffectedwithlesser
difficultyandwithgreatersafetythanwouldbethecaseiftheradarwerenot
continuously manned and used to support visual fixing.
Whilethenavigationalplanmustbepreparedinaccordancewiththe
manninglevelandindividualskillsaswellasthenavigationalsituation,
characteristicsofnavigationalaidsorequipment,characteristicsofradar
propagation,etc.,thenavigatorshouldrecognizethenavigationallimitations
imposedbylackofprovisionforcontinuousmanningoftheradar.Atransit,
whichmaybeeffectedwithareasonablemarginofsafetyiftheradaris
mannedcontinuouslybyacompetentobserver,mayimposetoomuchriskif
provision is not made for the continuous manning of the radar.
Theprovisionforcontinuousmanningoftheradarbyadesignatedand
competentobserverdoesnotnecessarilymeanthatotherresponsible
navigationalpersonnelshouldnotobservetheradarscopefromtimetotime.
Infacttheobservationsbyothernavigationalpersonnelarehighlydesirable.
Accordingtothenavigationalplan,thedesignatedobservermayberelieved
byamoreexperiencedandproficientobserverintheeventthatradarmustbe
usedastheprimarymeansofinsuringthesafetyofthevesselatsomepoint
duringthetransit.Insucheventtheobserverwhohasbeenmanningthe
radarshouldbeabletobriefhisreliefrapidlyandreliablywithrespecttothe
radarsituation.Assumingthatthepreviousobserverhasmadeoptimum
rangesettingsaccordingtoplanatvariouspointsonthetrack,thenew
observershouldbeabletomakeeffectiveuseoftheradaralmost
immediately.Ifthismoreproficientobserverhasbeenmakingfrequent
observationsoftheradarscope,aidedbycommentoftheobserver
continuouslymanningtheradar,anybriefingrequirementsonactually
relieving the other observer should be minimal.
Ifradaristobeusedeffectivelyinhazardouswaters,itisessentialthat
provisionsbemadefortheradarobserverandotherresponsiblenavigational
personneltobeabletoinspectthechartintheimmediatevicinityofthe
radarindicator.Thepracticeofleavingaradarindicatorinstalledinthe
wheelhousetoinspectthechartinthechartroomishighlyunsatisfactoryin
situationsrequiringpromptandreliableradarscopeinterpretation.Theradar
observermustbeabletomakefrequentinspectionsofthechartwithout
unduedelaysbetweensuchinspectionsandsubsequentradarobservations.
AcontinuouscorrelationofthechartandthePPIdisplayisrequiredfor
reliable radarscope interpretation.
Ifthenavigationalplotismaintainedonachartotherthanthatusedbythe
radarobserverforradarscopeinterpretation,theobserver’schartshould
includethebasicplanningdata,suchastheintendedtrack,turningbearings,
danger bearings, turning ranges, etc.
189
Inplanningfortheeffectiveuseofradar,itisadvisabletohaveadefinite
procedureandstandardizedterminologyformakingverbalreportsofradarand
visualobservations.Atpointsonthetrackwheresimultaneousvisualandradar
observationsaretobemade,thelackofanadequatereportingprocedurewill
maketherequiredcoordinationundulydifficult.Reportsofradarobservations
canbesimplifiedthroughtheuseofappropriateannotationsonthechartand
PPI.Forexample,achartedrockwhichisidentifiedonthePPIcanbedesignated
as“A”;anotherradar-conspicuousobjectcanbedesignatedas“B,”etc.Withthe
chartsimilarlyannotated,thevariousobjectscanbereportedinaccordancewith
their letter designations.
SPECIAL TECHNIQUES
Inthatthenavigatorofaradar-equippedvesselalwaysmustbepreparedto
useradarashisprimarymeansofnavigationinpilotwaters,duringtheplanning
foratransitofthesewatersitbehooveshimtostudythenavigationalsituation
withrespecttoanyspecialtechniqueswhichcanbeemployedtoenhancetheuse
ofradar.Theeffectivenessofsuchtechniquesusuallyisdependentupon
adequatepreparationfortheiruse,includingspecialconstructionsonthechartor
the preparation of transparent chart overlays.
ThecorrelationofthechartandthePPIdisplayduringatransitof
confinedwatersfrequentlycanbeaidedthroughtheuseofatransparent
chartoverlayonwhichproperlyscaledconcentriccirclesareinscribedasa
meansofsimulatingthefixedrangeringsonthePPI.Byplacingthecenter
oftheconcentriccirclesatappropriatepositionsonthechart,thenavigatoris
abletodeterminebyrapidinspection,andwithcloseapproximation,just
wherethepipsofcertainchartedfeaturesshouldappearwithrespecttothe
fixedrangeringsonthePPIwhenthevesselisatthosepositions.This
techniquecompensatesforthedifficultyimposedbyviewingthePPIatone
scaleandthechartatanotherscale.Throughstudyofthepositionsofvarious
chartedfeatureswithrespecttothesimulatedfixedrangeringsonthe
transparencyasthecenterofthesimulatedringsismovedalongtheintended
track, certain possibilities for unique observations may be revealed.
Identifying Echoes
Byplacingthecenteroftheproperlyscaledsimulatedrangering
transparencyovertheobserver’smostprobablepositiononthechart,the
identificationofechoesisaided.Thepositionsoftherangeringsrelativeto
themoreconspicuousobjectsaidinestablishingthemostprobableposition.
Withbetterpositioningofthecenterofthesimulatedrings,morereliable
identification is obtained.
Fixing
Byplacingthesimulatedrangeringtransparencyoverthechartsothatthe
simulatedringshavethesamerelationshiptochartedobjectsastheactual
rangeringshavetothecorrespondingechoes,theobserver’spositionis
found at the center of the simulated range rings.
Undersomeconditions,theremaybenotbeenoughsuitableobjectsand
correspondingechoestocorrelatewiththerangeringstoobtainthedesired
accuracy.
Thismethodoffixingshouldbeparticularlyusefulaboardsmallcraft
withlimitednavigationalpersonnel,equipment,andplottingfacilities.This
methodshouldservetoovercomedifficultiesassociatedwithunstabilized
displays and lack of a variable range marker.
191
CHAPTER 5 —AUTOMATIC RADAR PLOTTING AIDS (ARPA)
INTRODUCTION
Theavailabilityoflowcostmicroprocessorsandthedevelopmentof
advancedcomputertechnologyduringthe1970sand1980shavemadeit
possibletoapplycomputertechniquestoimprovecommercialmarineradar
systems.RadarmanufacturesusedthistechnologytocreatetheAutomatic
RadarPlottingAids(ARPA).ARPAsarecomputerassistedradardata
processingsystemswhichgeneratepredictivevectorsandothership
movement information.
TheInternationalMaritimeOrganization(IMO)hassetoutcertain
standardsamendingtheInternationalConventionofSafetyofLifeatSea
requirementsregardingthecarryingofsuitableautomatedradarplottingaids
(ARPA).TheprimaryfunctionofARPAscanbesummarizedinthe
statementfoundundertheIMOPerformanceStandards.Itstatesa
requirementofARPAs....“inordertoimprovethestandardofcollision
avoidanceatsea:Reducetheworkloadofobserversbyenablingthemto
automaticallyobtaininformationsothattheycanperformaswellwith
multipletargetsastheycanbymanuallyplottingasingletarget”.Aswecan
seefromthisstatementtheprincipaladvantagesofARPAareareductionin
theworkloadofbridgepersonnelandfullerandquickerinformationon
selected targets.
AtypicalARPAgivesapresentationofthecurrentsituationanduses
computertechnologytopredictfuturesituations.AnARPAassessestherisk
ofcollision,andenablesoperatortoseeproposedmaneuversbyownship.
WhilemanydifferentmodelsofARPAsareavailableonthemarket,the
following functions are usually provided:
1.True or relative motion radar presentation.
2.Automatic acquisition of targets plus manual acquisition.
3.Digitalread-outofacquiredtargetswhichprovidescourse,speed,range,
bearing, closest point of approach (CPA, and time to CPA (TCPA).
4.Theabilitytodisplaycollisionassessmentinformationdirectlyonthe
PPI,usingvectors(trueorrelative)oragraphicalPredictedAreaof
Danger (PAD) display.
5.Theabilitytoperformtrialmaneuvers,includingcoursechanges,speed
changes, and combined course/speed changes.
6.Automatic ground stabilization for navigation purposes.
ARPAprocessesradarinformationmuchmorerapidlythanconventional
radarbutisstillsubjecttothesamelimitations.ARPAdataisonlyas
accurate as the data that comes from inputs such as the gyro and speed log.
STAND-ALONE AND INTEGRAL ARPA’s
Overthepast10years,themostsignificantchangestotheARPAsystems
hasbeenintheirdesign.ThemajorityofARPAsmanufacturedtoday
integrate the ARPA features with the radar display.
TheinitialdevelopmentanddesignofARPAswereStand-aloneunits.
Thatistheyweredesignedtobeanadditiontotheconventionalradarunit.
AlloftheARPAfunctionswereinstalledonboardasaseparateunitbut
neededtointerfacedwithexistingequipmenttogetthebasicradardata.The
primarybenefitswerecostandtimesavings.Thisofcoursewasnotthemost
idealsituationandeventuallyitwastheintegralARPAthatgradually
replaced the stand-alone unit.
ThemodernintegralARPAcombinestheconventionalradardatawiththe
computerdataprocessingsystemsintooneunit.Themainoperational
advantage is that both the radar and ARPA data are readily comparable.
192
ARPA DISPLAY
Fromthetimeradarwasfirstintroducedtothepresentdaytheradar
picturehasbeenpresentedonthescreenofacathoderaytube.Althoughthe
cathoderaytubehasretaineditsfunctionovertheyears,thewayinwhich
thepictureispresentedhaschangedconsiderably.Fromaboutthemid-1980s
thefirstraster-scandisplaysappeared.Theradial-scanPPIwasreplacedbya
raster-scanPPIgeneratedonatelevisiontypeofdisplay.TheintegralARPA
andconventionalradarunitswitharaster-scandisplaywillgraduallyreplace
the radial-scan radar sets.
Thedevelopmentofcommercialmarineradarenteredanewphaseinthe
1980swhenraster-scandisplaysthatwerecompliantwiththeIMO
Performance Standards were introduced.
Theradarpictureofaraster-scansyntheticdisplayisproducedona
televisionscreenandismadeupofalargenumberofhorizontallineswhich
formapatternknownasaraster.Thistypeofdisplayismuchmorecomplex
thantheradial-scansyntheticdisplayandrequiresalargeamountof
memory.thereareanumberofadvantagesfortheoperatorofaraster-scan
displayandconcurrentlytherearesomedeficienciestoo.Themostobvious
advantageofaraster-scandisplayisthebrightnessofthepicture.This
allowstheobservertoviewthescreeninalmostallconditionsofambient
light.Outofallthebenefitsofferedbyaraster-scanradaritisthisability
whichhasassureditssuccess.Anotherdifferencebetweentheradial-scan
andraster-scandisplaysisthatthelatterhasarectangularscreen.Thescreen
sizeisspecifiedbythelengthofthediagonalandthewidthandheightofthe
screenwithanapproximateratioof4:3.Theraster-scantelevisiontubes
haveamuchlongerlifethanatraditionalradarCRT.Althoughthetubesare
cheaperovertheircounterpart,thecomplexityofthesignalprocessing
makes it more expensive overall.
Raster-scan PPI
TheIMOPerformanceStandardsforradartoprovideaplandisplaywith
aneffectivedisplaydiameterof180mm,250mm,or340mmdependingupon
thegrosstonageofthevessel.Withthediameterparametersalreadychosen,
themanufacturerhasthentodecidehowtoarrangetheplacementofthe
digitalnumericaldataandcontrolstatusindicators.Theraster-scandisplay
makes it easier for design engineers in the way auxiliary data can be written.
Monochrome and Color CRT
Amonochromedisplayisonewhichdisplaysonecolorandblack.The
generalmonochrometelevisionuseswhiteasthecolor.Thishoweverisnot
anappropriatecolorfortheconditionsunderwhichacommercialmarine
radarisviewed.Unlikeatelevisionscreen,marineradardisplaystendtobe
viewedfromtheshorterdistanceandtheobserverhasagreater
concentrationonthedetailsofthescreenandthereforeissubjectto
eyestrain.Forthisreasonthecolormostcommontomonochromeraster-
scanapplicationswasgreen.Thegreenphosphorprovidescomfortable
viewing by reducing eye strain and stress.
ThecolortubeCRTdiffersfromitsmonochromecounterpartinthatithas
three electron guns, which are designated as red, green, and blue.
193
FEATURES AND OPERATING INSTRUCTIONS FOR A MODERN RASTER SCAN RADAR AND ARPA
INTRODUCTION
Thefollowingparagraphsdescribethefeaturesandoperatinginstructions
oftheFurunoHeavy-DutyHighPerformanceRasterScanRadarandARPA
ModelFR/FAR-28x5series.OnlyselectedportionsoftheFurunooperating
instructionsarepresentedinthismanual.Forthecompleteoperating
instructions you should contact a Furuno dealer or representative.
The purpose of this section is to provide a sample of the technical
instructions that should be available to the officer. As a radar observer you
should thoroughly read and understand the operating instructions for the
radarunitsthatyouwillbeusing.Operatinginstructingwillofcoursediffer
not only between different radar manufactures’ but also with different
models for the same manufacturer.
Aswithallequipment,theoperatorshouldbecompletelyfamiliarwith
thesafetyinstructionspriortoturningontheradar.Thereareanumberof
dangers,warningsandcautionsthatshouldbefollowedbythoseoperating
theseradars.Failuretofollowtheappropriatesafetyinstructionscouldresult
in serious injury or death.
FEATURES
TheFR-2805andFAR-2805seriesofRadarandARPAsaredesignedto
fullymeettheexactingrulesoftheInternationalMaritimeOrganization
(IMO) for installations on all classes of vessels.
Thedisplayunitemploysa28inchdiagonalmulticoloredCRT.It
providesaneffectiveradarpictureof360mmdiameterleavingsufficient
space for on screen alpha-numeric data.
Targetdetectionisenhancedbythesophisticatedsignalprocessing
techniquesuchasmulti-levelquantization(MLQ),echostretch,echo
average,andabuilt-inradarinterferencerejector.Audibleandvisualguard
zonealarmsareprovidedasstandard.Othership’smovementisassessedby
trailsoftargetechoesorbyelectronicplotting.TheFAR-2805seriesARPA
furtherprovidestargetassessmentbyhistoricalplots,vectorsandtargetdata
table.
OnscreendatareadoutsincludeCPA,TCPA,range,bearing,speed/course
onupto3targetsatatime.TheARPAfunctionsincludeautomatic
acquisitionofupto20targets,ormanualacquisitionof40targets.In
addition,theARPAfeaturesdisplayofatrafficlane,buoys,dangerous
points, and other important reference points.
Figure 5.1 - FR-2805 Series Radar Display Unit Overview
194
GENERAL FEATURES
•Daylight-bright high-resolution display
•28inchdiagonalCRTpresentsradarpictureof360mmeffectivediameter
with alphanumeric data area around it
•Userfriendlyoperationbycombinationoftactilebacklittouchpads,a
trackball and rotary controls
•Audio-visual alert for targets in guard zone
•Echo trail to assess targets’ speed and course by simulated afterglow
•Electronicplottingofupto10targetsindifferentsymbols(Thisfunction
is disabled when ARPA is activated)
•Electronic parallel index lines
•Interswitch (optional) built in radar or ARPA display unit
•EnhancedvisualtargetdetectionbyEchoAverage,EchoStretch,
Interference Rejector, and multi-level quantization
•Stylish display
•Choiceof10,25or50KWoutputforX-band;30KWoutputforS-band,
either in the transceiver aloft (gearbox) or RF down (transceiver in bridge)
•Exclusive FURUNO MIC low noise receiver
ARPA FEATURES
•Acquires up to 20 targets automatically
•Movementoftrackedtargetsshownbytrueorrelativevectors(Vector
length 1 to 99 min. selected in 1 min steps)
•Settingofnavlines,buoymarksandothersymbolstoenhancenavigation
safety
•On-screendigitalreadoutsofrange,bearing,course,speed,CPA,TCPA,
BCR(BowCrossingRange)andBCT(BowCrossingTime)oftwotargets
out of all tracked targets.
•Audibleandvisualalarmsagainstthreateningtargetscominginto
operator-selectedCPA/TCPAlimits,losttargets,twoguardrings,visual
alarm against system failure and target full situation
•Electronicplottingofupto10targetsindifferentsymbols(Thisfunction
is disabled when ARPA is activated)
•Electronic parallel index lines
•Interswitching (optional) built in radar or ARPA display unit
•EnhancedvisualtargetdetectionbyEchoAverage,EchoStretch,
Interference Rejector, and multi-level quantization
•Stylish display
•Choiceof10,25or50kWoutputforX-band;30kwoutputforS-band,
either in the transceiver aloft (gearbox) or RF down (transceiver in bridge)
•Exclusive FURUNO MIC low noise receiver
195
Figure 5.2 - Main Control Panel
196
DISPLAY CONTROLS - MODE PANEL
HM OFF
Temporarily erases the heading marker.
ECHO TRAILS
Shows trails of target echoes in the form of simulated afterglow.
MODE
Selects presentation modes: Head-up, Head-up/TB, North-up, Course-up,
and True Motion.
GUARD ALARM
Used for setting the guard alarm.
EBL OFFSET
Activates and deactivates off-centering of the sweep origin.
BKGR COLOR
Selects the background color.
INDEX LINES
Alternately shows and erases parallel index lines.
X2 ZOOM
enlarges a user selected portion of picture twice as large as normal. (R-type
only)
CU, TM RESET
Resets the heading line to 000 in course-up mode; moves own ship position
50% radius in stern direction in the true motion mode.
INT REJECT
Reduces mutual radar interference
RANGE RINGS
Adjusts the brightness of range rings.
Figure 5.3 - Mode Panel
197
DISPLAY CONTROLS - PLOTTING KEYPAD
ORIGIN MARK
Show and erases the origin mark (a reference point).
VECTOR TRUE/REL
Selects true or relative vector.
VECTOR TIME
Sets vector length in time.
RADAR MENU
Opens and closes RADAR menus.
E-PLOT, AUTO PLOT MENU
Opens and closes E-plot and AUTO PLT menus.
NAV MENU
Opens and closes NAV menu.
KEYS 0-9
Select plot symbols. Also used for entering numeric data.
CANCEL
Terminates plotting of a specified target or all tracked targets.
ENTER
Used to save settings on menu screen.
TARGET DATA
Displays the acquired target data.
TARGET BASED DATA
Own ship’s speed is measured relative to a fixed target.
AUTO PLOT
Activates and deactivates the Auto Plotter.
TRIAL
Initiates a trial maneuver.
LOST TARGET
Silences the lost target audible alarm and erases the lost target symbol.
HISTORY
Shows and erases past positions of tracked targets.
MARK
Enter/erase mark.
CHART ALIGN
Used to align chart data.
VIDEO PLOT
Turns the video plotter on/off.
Figure 5.4 - Plotting keypad and tuning compartment
198
OPERATION
TURNING ON POWER
ThePOWERswitchislocatedatthelowerrightcornerofthedisplay.
Pushittoswitchontheradarset.Toturnofftheradar,pushitagain;the
switchwillextend.Thescreenshowsthebearingscaleanddigitaltimer
approximately15secondsafterpower-on.Thetimercountsdownthree
minutesofwarm-uptime.Duringthisperiodthemagnetron,orthe
transmittertube,iswarmedfortransmission.Whenthetimerhasreached
0:00,thelegendSTBYappearsindicatingthattheradarisnowreadyto
transmit pulses.
Inwarm-upandstandbycondition,youwillseethemessageBRGSIG
MISSING.Thisisnormalbecauseabearingsignalisnotyetgeneratedwhen
theantennaisnotrotating.ONTIMEandTXTIMEvaluesshownatthe
bottomofthescreenarethetimecountsinhoursandtenthsofhourwhenthe
radar has been powered on and transmitted.
TRANSMITTER ON
WhentheSTANDBYstatusisdisplayedonthescreen,presstheTransmit
switch labeled ST-BY/TX on the control panel of the display unit.
Theradarisinitiallysettopreviouslyusedrangeandpulsewidth.Other
settingssuchasbrilliancelevels,VRMs,ELBsandmenuoptionselections
are also set to previous settings.
TheTransmitswitchtogglestheradarbetweenSTANDBYand
TRANSMITstatus.TheantennastopsinSTANDBYstatusandrotatesin
TRANSMIT status.
Notes:
1.IftheantennadoesnotrotateinTRANSMITstatus,checkwhetherthe
antenna switch in the tuning compartment is in the OFF position.
2.Themagnetronageswithtimeresultinginareductionofoutputpower.It
ishighlyrecommendedthattheradarbesettoSTANDBYstatuswhennot
used for an extended period of time.
CRT BRILLIANCE
OperatetheBRILLcontrolonthecontrolpanelofthedisplayunitto
adjusttheentirescreenbrightness.Notethattheoptimumpointof
adjustmentvarieswithambientlightconditions,especiallybetweendaytime
and nighttime.
Note:TheCRTbrillianceshouldbeadjustedbeforeadjustingrelative
brilliance levels on the BRILLIANCE menu to be explained later.
199
TUNING THE RECEIVER
Auto Tune
Theradarreceiveristunedautomaticallyeachtimethepoweristurned
on,thusthereisnofrontpanelcontrolfortuningpurpose.Thetuning
indicatorandthelabelAUTOTUNEatthetoprightcornerofthedisplay
unitshowthetuningcircuitisworking.IfthelabelAUTOTUNEisnot
displayed,checkthattheTUNEselectorintuningcompartmentistheAUTO
position.
Manual Tune
Ifyouarenotsatisfiedwiththecurrentautotunesetting,followthese
steps to fine-tune the receiver:
1.Push the tune control so that it pops up.
2.SettheTUNEselectorinthetuningcompartmenttoMANformanual
tuning.
3.Whileobservingthepictureonthe48milescale,slowlyadjustTUNE
control and find the best tuning point.
4.SotheTUNEselectortoAUTOandwaitforabout10secondsorfour
scanner rotations.
5.Makesurethattheradarhasbeensettothebesttuningpoint.This
conditioniswherethetuningindicatorlightstoabout80%ofitstotal
length.
6.Push the TUNE control into the retracted position.
Video Lockup Recovery
Videolockup,orpicturefreeze,canoccurunexpectedlyondigital
rasterscanradars.Thisismainlycausedbyheavyspikenoiseinthepower
lineandcanbenoticedbycarefullywatchingthenearlyinvisiblesweepline.
Ifyoususpectthatthepictureisnotupdatedeveryscanoftheantennaorno
keyentryisacceptednotwithstandingtheapparentlynormalpicture,do
Quick Start to restore normal operation:
1.Turn off the power switch and turn it on again within five seconds.
2.Push the ST-BY switch in the tuning compartment.
3.Push the Transmit switch labeled ST-BY/TX for Transmit status.
200
ON-SCREEN LEGENDS AND MARKERS
Figure 5.5
201
Figure 5.6 - Data display
202
DEGAUSSING THE CRT SCREEN
Eachtimetheradaristurnedon,thedegaussingcircuitautomatically
demagnetizestheCRTscreentoeliminatecolorcontaminationcausedby
earth’s magnetism or magnetized ship structure.
Thescreenisalsodegaussedautomaticallywhenownshiphasmadea
significantcoursechange.Whilebeingdegaussed,thescreenmaybe
disturbedmomentarilywithverticallines.Ifyouwishtodegaussbymanual
operationatanarbitrarytime,openandpresstheDegaussswitchinthe
tuning compartment.
INITIALIZING THE GYRO READOUT
Providedthatyourradarisinterfacedwithagyrocompass,ship’sheading
isdisplayedatthetopofthescreen.Uponturningontheradar,aligntheon-
screenGYROreadoutwiththegyrocompassreadingbytheprocedure
shownbelow.Onceyouhavesettheinitialheadingcorrectly,resettingisnot
usuallyrequired.However,iftheGYROreadoutgoeswrongforsome
reason, repeat the procedure to correct it.
1.OpenthetuningcompartmentandpresstheHOLDbutton.TheGyro
LED lights.
2.PresstheUPorDOWNbuttontoduplicatethegyrocompassreadingat
theonscreenGYROreadout.Eachpressofthesebuttonschangesthe
readoutby0.1-degreesteps.Tochangethereadoutquickly,holdtheUP
or DOWN button for over two seconds.
3.PresstheHOLDswitchwhentheonscreenGYROreadouthasmatched
the gyrocompass reading. The Gyro LED goes out.
Note:TheHOLDbuttonisusedtodisengagethebuilt-ingyrointerfacefrom
thegyrocompassinputintheeventthatyouhavedifficultyinfine-adjusting
theGYROreadoutduetoship’syawing,forexample.Wheninitializingthe
GYROreadoutataberth(wherethegyrocompassreadingisusuallystable),
you may omit steps 1 and 3 above.
PRESENTATION MODES
This radar has the following presentation modes:
Relative Motion (RM)
Head-up:Unstabilized
Head-up TB:Head-upwithcompass-stabilizedbearingscale(True
Bearing)
Course-up:Compass-stabilized relative to ship’s intended course
North-up:Compass-stabilized with reference to north)
True Motion (TM)
North-up:Ground or sea stabilized with compass and speed inputs
SELECTING PRESENTATION MODE
PresstheMODEkeyonthemodepanel.EachtimetheMODEkeyis
pressed,thepresentationmodeandmodeindicationattheupper-leftcorner
of the screen change cyclically.
LossofGyroSignal:Whenthegyrosignalislost,thepresentationmode
automaticallybecomeshead-upandtheGYROreadoutatthescreentop
showsasterisks(***.*).ThemessageSETHDGappearsattheupperofthe
screen.Thiswarningstaysonwhenthegyrosignalisrestored,towarnthe
operatorthatthereadoutmaybeunreadable.PresstheMODEkeytoselect
anotherpresentationmode(theasterisksareerasedatthispoint).Then,align
theGYROreadoutwiththegyrocompassreadingandpresstheCANCEL
key to erase the message SET HDG.
203
Head-up Mode (Figure 5.7)
Adisplaywithoutazimuthstabilizationinwhichthelineconnectingthe
center with the top of the display indicates own ship’s heading.
Thetargetpipsarepaintedattheirmeasureddistancesandintheir
directions relative to own ship’s heading.
Ashortlineonthebearingscaleisthenorthmarkerindicatingcompass
north.Afailureofthegyroinputwillcausethenorthmarkertodisappear
andtheGYROreadouttoshowasterisks(***.*)andthemessageSETHDG
appears on the screen.
Course-up Mode (Figure 5.8)
Anazimuthstabilizeddisplayinwhichalineconnectingthecenterwith
thetopofthedisplayindicatesownship’sintendedcourse(namely,own
ship’spreviousheadingjustbeforethismodehasbeenselected).Targetpips
arepaintedattheirmeasureddistancesandintheirdirectionsrelativetothe
intendedcoursewhichismaintainedatthe0°positionwhiletheheading
markermovesinaccordancewithship’syawingandcoursechanges.This
modeisusefultoavoidsmearingofpictureduringcoursechange.Aftera
coursechange,pressthe(CU,TMRESET)keytoresetthepicture
orientation if you wish to continue using the course up mode.
Figure 5.7 - Head-up Mode
Figure 5.8 - Course-up Mode
204
Head-up TB (True Bearing) Mode (Figure 5.9)
Radar echoes are shown in the same way as in the head-up mode. The
difference from normal head-up presentation lies in the orientation of the
bearing scale. The bearing scale is compass stabilized, that is, it rotates in
accordance with the compass signal, enabling you to know own ship’s
heading at a glance.
Thismodeisavailableonlywhentheradarininterfacedwitha
gyrocompass.
North-up Mode (Figure 5.10)
In the north-up mode, target pips are painted at their measured distances
andintheirtrue(compass)directionsfromownship,northbeingmaintained
UP of the screen. The heading marker changes its direction according to the
ship’s heading.
Ifthegyrocompassfails,thepresentationmodechangestohead-upand
thenorthmarkerdisappears.Also,theGYROreadoutshowsasterisks
(***.*) and the message SET HDG appears on the screen.
Figure 5.9 - Head-up TB (True Bearing) Mode
Figure 5.10 - North-up Mode
205
True Motion Mode (Figure 5.11)
Ownshipandothermovingobjectsmoveinaccordancewiththeirtrue
coursesandspeeds.Allfixedtargets,suchaslandmasses,appearas
stationary echoes.
Whenownshipreachesapointcorrespondingto75%oftheradiusofthe
display,theownshipisautomaticallyresettoapointof50%radiusopposite
to the extension of the heading marker passing through the display center.
Resetting can be made at any moment before the ship reaches the limit by
pressing the (CU, TM RESET) key. Automatic resetting is preceded by a
beep sound.
Ifthegyrocompassfails,thepresentationmodeischangedtothehead-up
modeandthenorthmarkerdisappears.TheGYROreadoutatthetopofthe
screenshowsasterisks(***.*)andthemessageSETHDGappearsonthe
screen.
Figure 5.11 - True Motion Mode
206
SELECTING THE RANGE SCALE
Thedisplayrangescaleischangedin13stepsontheR-type(11stepson
theIMO-type)bypressingthe(+)and(-)keys.Theselectedrangescaleand
range ring interval are shown at the upper left corner on the screen.
Thedisplayrangecanbeexpandedby75%(100%inR-type)inany
direction by using the off-centering control.
SELECTING THE PULSEWIDTH
Thepulsewidthinuseisdisplayedattheupper-leftpositionofthescreen
using the abbreviations shown in the table above.
Appropriatepulsewidthsarepresenttoindividualrangescalesand
functionkeys.Therefore,youarenotusuallyrequiredtoselectthem.Ifyou
arenotsatisfiedwiththecurrentpulsewidthsettings,however,itispossible
to change them by the radar menu operation shown below.
Youcanchoosethepulsewidth1or2onthescales0.5to24nmrangeson
X-band models and 0.75 to 24 nm ranges on S-band models.
Thedisplayrangecanbeexpandedby75%(100%inR-type)inany
direction by using the off-centering control.
Selecting Pulsewidth 1 or 2
1.PresstheRADARMENUkeyontheplottingkeypadtoshowthe
FUNCTION menu.
2.Pressthe(1)keytoselect(orhighlight)PLUSEWIDTH1or2as
appropriate.
3.Press the (1) key to select menu item 1 PULSEWIDTH.
4.PresstheENTERkeytoconcludeyourselectionfollowedbytheRADAR
MENU key to close the FUNCTION menu.
Presetting Pulsewidths 1 and 2
Pulsewidth1and2canbepresetonthePulsewidth1and2menus.Shown
below are examples of the pulsewidth setup procedure:
1.ToenableselectionofS1(0.07microseconds)andS2(0.15
microseconds)pulsewidthonthe0.5nmrangeonanX-bandmodel,
selectS1at0.5nmonthePULSEWIDTH1menuandM1at3nmonthe
PULSEWIDTH 2 menu.
2.ToenableselectionofS2(0.15microseconds)andM1(0.3microseconds)
pulsewidthonthe3nmrangeonanX-bandmodel,selectS2at3nmin
thePULSEWIDTH1menuandM1at3nminthePULSEWIDTH2
menu.
Alongerpulseprovidesanincreaseddetectionrange,butwithreduced
discrimination.Ifyouneeddiscriminationinpreferencetodetection,choose
a shorter pulse.
Example:ToselectS1(0.07us)asPulsewidth1forthe0.5nmrange,display
thePULSEWIDTH1menufollowingthestepsshownaboveandhitthe(2)
keytochoose“20.5NM>”Furtherhitthe(2)keyuntilthemenuoption
“S1” is highlighted to the right of “2 0.5” NM.”
ADJUSTING THE SENSITIVITY
TheGAINcontrol(seeFigure5.14)isusedtoadjustthesensitivityofthe
receiver,andthustheintensityofechoesastheyappearonthescreen.It
shouldbeadjustedsothatspeckledbackgroundnoiseisjustvisibleonthe
screen.
TobecomeacquaintedwiththewaytheGAINcontrolworks,tryrotating
it between fully counterclockwise and clockwise positions while observing
the radar picture. You will notice that clockwise rotation increases the echo
intensity level. A low gain setting results in the loss of weak echoes and a
reduced detection range. If you turn the GAIN control too far clockwise for
an excessive gain setting, desired echoes will be masked in the strong
background noise.
207
SUPPRESSING SEA CLUTTER
Inroughweatherconditionsreturnsfromtheseaservicearereceivedover
severalmilesaroundownshipandmaskclosetargets.Thissituationcanbe
improvedbyproperlyadjustingtheA/CSEA(Anti-cluttersea)control(see
Figure 5.15).
Automatic Anti-clutter Control
Theeasiestwaytosuppresstheserviceclutteristousetheautomatic
control.PresstheA/CAUTOkey(seeFigure5.15)nexttotheEBLrotary
controlattheleftcorneronthecontrolpanel.Useofafunctionkeyisalsoa
goodmethodforreducingseaclutter.Forthispurpose,presettingisrequired.
Consult a Furuno representative.
Manual Anti-clutter Control
Fromthefullycounterclockwiseposition,slowlyturntheA/CSEA
controlclockwise.Foroptimumtargetdetection,youshouldleavespeckles
of the surface return slightly visible.
Theant-clutterseacontrolisoftenreferredtoasSTC(SensitivityTime
Control)whichdecreasestheamplificationofthereceiverimmediatelyafter
aradarpulseidtransmitted,andprogressivelyincreasesthesensitivityasthe
range increases.
AcommonmistakeistooveradjusttheA/CSEAcontrolsothatthe
surfaceclutteriscompletelyremoved.Byrotatingthecontrolfully
clockwise,youseehowdangerousthiscanbe;adarkzoneiscreatednear
thecenterofthescreenandclose-intargetscanbelost.Thisdarkzoneis
evenmoredangerousifthegainhasnotbeenproperlyadjusted.Always
leavealittlesurfacecluttervisibleonthescreen.Ifnosurfaceclutteris
observed(onverycalmwater),setthecontrolatthefullycounterclockwise
position.
SUPPRESSING PRECIPITATION CLUTTER
Inadverseweatherconditions,clouds,rain,orsnowproducealotof
spray-likespuriousechoesandimpairstargetdetectionoveralongdistance.
Thissituationcanbeimprovedbyusingafunctionkeyprovidedthatitis
programmed.Ifthefunctionkeyfailstoofferafavorablesuppressionofthe
rainclutter,adjusttheA/CRAINcontrol(seeFigure5.16)onthefront
control panel.
TheA/CRAINcontroladjuststhereceiversensitivityastheA/CSEA
controldoesbutratherinalongertimeperiod(longerrange).Clockwise
rotation of this control increases the anti-clutter effect.
INTERFERENCE REJECTOR
Mutualradarinterferencemayoccurinthevicinityofanothershipborneradar
operatinginthesamefrequencyband(9GHzforX-band,3GHzforS-band).It
isseenonthescreenasanumberofbrightspikeseitherinirregularpatternsorin
theformofusuallycurvedspoke-likedottedlinesextendingfromthecenterto
theedgeofthepicture.Thetypeofinterferencecanbereducedbyactivatingthe
interference rejector circuit.
Theinterferencerejectorisakindofsignalcorrelationcircuit.It
comparesthereceivedsignalsoversuccessivetransmissionsandsuppresses
randomlyoccurringsignals.Therearethreelevelsofinterferencerejection
dependingonthenumberoftransmissionsthatarecorrelated.Theseare
indicatedbythelegendslR1,lR2andlR3attheupperleftpositionofthe
screen.
PresstheINTREJECTkeytoactivatetheinterferencerejectorcircuit.
Successivepressesofthekeyincreasetheeffectofinterferencerejection,up
tolevel3.Afourthpressdeactivatestheinterferencerejector.Switchoffthe
interferencerejectorwhennointerferenceexists;otherwiseweaktargets
may be lost.
Note:Forstablereceptionofcertaintypesofradarbeacons(racons)or
SART(SearchandRescueRadarTransponder)asrequiredbySOLAS1974
asamended1988(GMDSS),itisrecommendedtoturntheinterference
rejector off.
208
MEASURING THE RANGE (Figure 5.12)
Usethefixedrangeringstoobtainaroughestimateoftherangetothe
target.Theyareconcentricsolidcirclesaboutownship,orthesweeporigin.
Thenumberofringsisautomaticallydeterminedbytheselectedrangescale
andtheirintervalisdisplayedattheupperleftpositionofthescreen.Press
theRINGSkeyonthemodepaneltoshowthefixedrangeringsiftheyare
notdisplayed.SuccessivepressesoftheRINGSkeygraduallyincreasetheir
brightness in 4 steps and fifth press erases the range rings.
Use the Variable Range Markers (VRM) for more accurate measurement
of the range of the target. There are two VRMs, No.1 and No.2, which
appear as dashed rings so that you can discriminate them from the fixed
range rings. The two VRMs can be distinguished from each other by
different lengths of dashes.
PresstheVRMONkeytodisplayeitheroftheVRMs.Successivepresses
oftheVRMONkeytoggletheactiveVRMbetweenNo.1andNo.2andthe
currently active VRM readout is circumscribed by>.....<.
AligntheactiveVRMwiththeinneredgeofthetargetofinterestandread
itsdistanceatthelowerrightcornerofthescreen.EachVRMremainsatthe
samegeographicaldistancewhenyouoperatetheRANGE+orRANGE-
key.ThismeansthattheapparentradiusoftheVRMringchangesin
proportiontotheselectedrangescale.PresstheVRMOFFkeytoeraseeach
VRM.
MEASURING THE BEARING (Figure 5.13)
UsetheElectronicBearingLines(EBL)totakebearingsofatarget.There
aretwoEBLs,No.1andNo.2whicharetoggledbysuccessivepressesofthe
EBLONkey.EachEBLisastraightdashedlineextendingoutfromtheown
shippositionuptothecircumferenceoftheradarpicture.Thefinedashed
line is the No.1 EBL and the course dashed one is the No.2 EBL.
PresstheELBONkeytodisplayeitheroftheEBLs.Successivepressesof
theEBLONkeytoggletheactiveELBbetweenNo.1andNo.2andthecurrently
active EBL readout is circumscribed by >... <.
Rotate the EBL rotary control clockwise or counterclockwise until the
activeEBLbisectsthetargetofinterest,andreaditsbearingatthelowerleft
corner of the screen. The EBL readout is affixed by “R” (relative) if it is
relative to own ship’s heading, T (true) if it is referenced to the north, as
determined by RADAR 2 menu settings.
EachEBLcarriesarangemarker,orashortlinecrossingtheEBLatright
anglesanditsdistancefromtheEBLoriginisindicatedattheVRMreadout
whether or not the corresponding VRM is displayed. The range marker
changes its position along the EBL with the rotation of the VRM control.
Press the EBL OFF key to erase each EBL.
Figure 5.12 - Measuring the range
Figure 5.13 - Measuring the bearing
209
COLLISION ASSESSMENT BY OFFSET EBL
TheoriginoftheEBLcanbeplacedanywherewiththetrackballtoenable
measurementofrangeandbearingbetweenanytargets.Thisfunctionisalso
usefulforassessmentofthepotentialriskofcollision.Toassesspossibility
of collision:
1.Press the EBL ON key to display or activate an EBL (No.1 or 2).
2.Placethecursor(+)onatargetofinterest(Aintheillustratedexample)by
operating the trackball.
3.PresstheEBLOFFSETkeyonthemodepanel,andtheoriginofthe
activeEBLshiftstothecursorposition.PresstheEBLOFFSETkeyagain
to anchor the EBL origin.
4.After waiting for a few minutes (at least 3 minutes), operate the EBL
controluntiltheEBLbisectsthetargetatthenewposition(A’).TheEBL
readout shows the target ship’s course, which may be true or relative
depending on the settings on the RADAR 2 menu.
Ifrelativemotionisselected,itisalsopossibletoreadCPAbyusinga
VRMasshowninfigure5.14.IftheEBLpassesthroughthesweeporigin
(ownship)asillustratedinfigure5.15,thetargetshipisonacollision
course.
5.ToreturntheEBLorigintotheownship’sposition,presstheEBL
OFFSET key again.
Figure 5.14 - Evaluating target ship’s course and CPA in relative motion mode
Figure 5.15 - Target ship on collision course
210
MEASURING RANGE AND BEARING BETWEEN TWO TARGETS
PresstheEBLOFFSETkey,andplacetheoriginofNo.1EBL,for
example,onatargetofinterest(target1infigure5.16)byoperatingthe
trackball.
TurntheEBLcontroluntiltheEBLpassesthroughanothertargetof
interest (target 2).
TurntheVRMcontroluntiltherangemarkeralignswithtarget2.The
activeVRMreadoutatthelowerrightcornerofthescreenindicatesthe
distance between the two targets.
Youcanrepeatthesameprocedureonthirdandfourthtargets(targets3
and 4) by using No.2 EBL and No. 2 VRM.
Bearingisshownrelativetoownshipwithsuffix“R”orasatruebearing
withsuffix“T”dependingonEBLrelative/truesettingsontheRADAR2
menu.ToreturntheEBLorigintotheownshipposition,presstheEBL
OFFSET key again.
SETTING A GUARD ZONE (GUARD ALARM)
Theguardzone(guardalarm)featureshouldneverberelieduponasthe
solemeansfordetectingtheriskofpotentialcollision.Theoperatorofaship
isnotrelievedoftheresponsibilitytokeepvisuallookoutforavoiding
collisions, whether or not the radar is in use.
Aguardzone(guardalarm)maybesettoalertthenavigatortotargets
(ships,landmasses,etc.)enteringacertainareawithvisualandaudible
alarms.
Theguardzone(guardalarm)hasafixedwidthof0.5nmintheradial
directionandisadjustableonlywithin3.0to6.0nmfromownship.The
guardzone(guardalarm)canbesettoanysectoranglebetween0°and360°
in any direction.
To set the guard zone (guard alarm):
1.Placethecursor(+)atpoint“A”usingthetrackballandpresstheGUARD
ALARMkeyonthemodepanel(leftkeygroup).ThemessageSET
GUARD appears at the bottom right corner of the screen.
2.Movethecursor(+)topoint“B”andpresstheGUARDALARMkey.
Then,aguardzone(guardalarm)asillustratediscreatedandthelabel
GUARDappearsinsteadofSETGUARDatthelowerrightcornerofthe
screen.
Note:Ifyouwishtocreateaguardzone(guardalarm)havinga360°
coveragearoundownship,setpoint“B”inalmostthesamedirection
(approx. +/-3°)as point “A” and press the GUARD ALARM key.
SILENCING AUDIBLE ALARM, REACTIVATING GUARD
ALARM
Atargetenteringtheguardzoneproducesbothvisual(flashing)and
audible(beeping)alarms.Tosilencetheaudiblealarm,presstheGUARD
ALARM key, and the label GUARD ACK replaces GUARD on the display.
Thiswilldeactivatetheaudiblealarmbutwillnotstoptheflashingofthe
targetintheguardzone.Toreactivatetheaudiblealarm,presstheGUARD
ALARM key again.
Figure 5.16 - Measuring range and bearing between two targets
211
DISABLING GUARD ZONE (GUARD ALARM)
Hold the GUARD ALARM key depressed for at least 3 seconds.
Note:Theguardalarmisgiventotargetshavingacertainlevelofecho
strength.Thisleveldoesnotalwaysimplyalandmass,reef,shipsorother
surfaceobjectsbutcanmeanreturnsfromtheseasurfaceorprecipitation.
ProperlyadjusttheGAIN,A/CSEA,andA/CRAINcontrolstoreduce
noise to avoid generation of guard alarm against false target detection.
INWARD AND OUTWARD GUARD ALARMS
OntheR-type,aninwardoroutwardguardalarmcanbeselectedonthe
RADAR2menu.OntheIMOtype,onlytheinwardguardalarmisavailable.
Theinwardguardalarmgeneratesvisualandaudiblewarningswhenatarget
enterstheguardzonefromanydirection.Theoutwardguardalarmis
produced when a target leaves the guard zone.
OFF-CENTERING
Ownshipposition,orsweeporigin,canbedisplacedtoexpandtheview
fieldwithoutswitchingtoalargerrangescale.OntheR-type,thesweep
origincanbeoffcenteredtoapointspecifiedbythecursor,upto100%of
therangeinuseinanydirection.OntheIMOtype,thesweeporigincanbe
offcenteredtothecursorposition,butnotmorethan75%oftherangein
use;ifthecursorissetbeyond75%oftherangescale,thesweeporiginwill
beoffcenteredtothepointof75%ofthelimit.Thisfeatureisnotavailable
on the longest range scale.
To off center the radar picture:
1.Placethecursoratapositionwhereyouwishtomovethesweeporiginby
operating the trackball.
2.Press the OFF CENTER key. Then, the sweep origin is off centered to the
cursor position.
3.To cancel off centering, press the OFF CENTER key again.
The picture cannot be off centered in the true motion mode.
ECHO STRETCH
Onlongrangestargetechoestendtoshrinkinthebearingdirection,
makingthemdifficulttosee.Onshortandmediumrangessuchas1.5,3and
6nauticalmilescales,thesamesizetargetsgetsmalleronthescreenasthey
approachtheownship.Theseareduetoinherentpropertyoftheradiation
patternproducedbytheantenna.Toenhancetargetvideo,usetheecho
stretchfunction.Therearetwotypes:echostretch1forlongrangedetection
and echo stretch 2 on 1.5-6 nautical mile scales.
To activate the echo stretch:
1.PresstheRADARMENUkeyontheplottingkeypadtoshowthe
FUNCTIONS menu.
2.Press the (2) key to select 2 ECHO STRETCH.
3.Press (2) until Echo Stretch option 1, 2 or OFF as desired is highlighted.
4.PresstheENTERkeytoconcludeyourselectionfollowedbytheRADAR
MENU key to close the FUNCTIONS menu.
Notes:
1.Ifthe1.5nmrangeispresetforpulsewidthofS1(0.08microseconds)or
S2(0.2microseconds),andthe3nmscaleforS2(0.2),theechostretch
function is not available on these range scales.
2.Theechostretchfunctionmagnifiesnotonlysmalltargetpipsbutalso
returnsfromseasurface,rainandradarinterference.Forthisreasonmake
surethesetypesofinterferencehavebeensufficientlysuppressedbefore
activating this function.
ECHO AVERAGING
Theechoaveragefeatureeffectivelysuppressesseaclutter.Echoes
receivedfromstabletargetssuchasshipsappearonthescreenatalmostthe
samepositioneveryrotationoftheantenna.Ontheotherhand,unstable
echoes such as sea clutter appear at random positions.
Todistinguishrealtargetechoesfromseaclutter,thisradarperformsscan-
to-scancorrelation.Correlationismadebystoringandaveragingecho
signalsoversuccessivepictureframes.Ifanechoissolidandstable,itis
presentedinitsnormalintensity.Seaclutterisaveragedoversuccessive
212
scansresultinginthereducedbrilliance,makingiteasiertodiscriminatereal
targets from sea clutter.
Toproperlyusetheechoaveragefunction,itisrecommendedtofirst
suppress sea clutter with the A/C SEA control and then to do the following:
1.PresstheRADARMENUkeyontheplottingkeypadtoshowthe
functions menu.
2.Press the (3) key to select 3 ECHO STRETCH.
3.Press (3) until echo average option 1, 2 or OFF as desired is highlighted.
OFF: No averaging effect
•Helpsdistinguishtargetsfromseaclutterandsuppressesbrillianceof
unstable echoes
•Distinguishes small stationary targets such as navigation buoys
•Stably displays distant targets
4.PresstheENTERkeytoconcludeyourselectionfollowedbytheRADAR
MENU key to close the FUNCTIONS menu.
Echoaveragingusesscantoscansignalcorrelationtechniquebasedon
thetruemotionoverthegroundofeachtarget.Thus,smallstationarytargets
suchasbuoyswillbeshownwhilesuppressingrandomechoessuchassea
clutter.Trueechoaverageisnothowevereffectiveforpickingupsmall
targets running at high speeds over the ground.
Echoaverageisinoperablewhenagyrocompasssignalisnotavailable.If
youwishtousethisfeaturewithoutagyrocompasssignal,consultaFuruno
representative.
Manualspeedentryisdoneatmenuitem6SHIP’SSPEEDonthe
FUNCTIONS menu which is accessed by pressing the RADAR MENU key.
CAUTION:DonotusetheEchoAveragefeatureunderheavypitchingand
rolling; loss of true targets can result.
ELECTRONIC PLOTTING AID (E-PLOT)
Amaximumof10operatorselectedtargetscanbeplottedelectronically
(manually)toassesstheirmotiontrend.Fivepastpositionscanbedisplayed
foreachoftheplottedtargets.Ifyouentera6thplotonacertaintarget,the
oldest plot (past position) will be erased.
Avectorappearswhenyouenterasecondplotforthetargetandis
updatedeachtimeanewplotisentered.Thevectorshowsthetargetmotion
trend based on its latest two plots.
Alphanumericreadoutsattheupperrighthandcornerofthescreenshow
range, bearing, course, speed, CPA, and TCPA of the last plotted target.
Itshouldbenotedthatthevectorandalphanumericdataarenotupdated
in real time, but only when you enter a new plot.
Note:EPArequiresownspeedinput(automaticormanual)andacompass
signal.Thevectoranddataareupdatedonrealtimebetweenplotentries,but
donotneglecttoplotanewpositionoveralongperiodoftime.Otherwise,
theaccuracywillbereduced.Notethattheplotswillbelostwhenthe
compass fails; start the plotting exercise again.
Plotting a Target
To perform electronic plotting:
1.Place the cursor (+) on a target of interest by operating the trackball.
2.Selectadesiredplotsymbolbypressingoneoftheplotsymbolkeyson
the plotting keypad.
3.PresstheACQkeyontheoperatorcontrolpanel,andtheselectedplot
symbol is marked at the cursor position.
4.WatchingtheEPAtime(TIMxx:xx)shownattheupperrightmarginof
thescreen,waitforatleast30seconds.Placethecursor(+)onthetarget
atitsnewlocation,selectthesameplotsymbolforthetargetandpressthe
ACQkey.Theplotsymbolmovestothenewtargetpositionandprevious
position is marked by a small dot.
5.Toacquireothertargets,repeattheabovestepsselectingdifferentplot
symbols.
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Note:Ifatargetonceplottedisnotplottedagainwithin10minutes,the
warming“UPDATEPLOTNO”willappearontheupperrightmarginofthe
screenandtheplotsymbolofthetargetflashes.Ifyouwanttocontinue
plottingthistarget,reacquireitwithinfiveminutes.Otherwise,thetarget
willberegardedasa“losttarget”anditsplotsymbolandtargetdatawillbe
erased.Thelargertheplottinginterval,thelessaccuratetheplottedtarget
data.Plottingofeachtargetshouldnormallybemadeevery3or6minutesas
far as possible.
Whenatargethasbeenplottedmorethanonce,theradarcalculatesits
motion rend and automatically displays a vector on the target.
True or Relative Vector
Truevectorscanbedisplayedrelativetoownship’sheading(Relative)or
withreferencetothenorth(True).PresstheVECTORTRUE/RELkeyto
selecttheproperindication.Thisfeatureisavailableinallpresentation
modes(gyrocompassmustbeworkingcorrectly).Thecurrentvectormodeis
indicated at the upper right corner of the screen.
Vector Time
Vectortime(orthelengthofvectors)canbesetto30sec,1,2,3,6,12,15
or30minutesandtheselectedvectortimeisindicatedattheupperright
cornerofthescreen.PresstheVECTORTIMEkeyuntilthedesiredvector
timeisreached.Thevectortipshowsanestimatedpositionofthetargetafter
theselectedvectortimeelapses.Itcanbevaluabletoextendthevector
length to evaluate the risk of collision with any target.
Target Data
Theradarcalculatesmotiontrends(range,bearing,course,speed,CPA,
and TCPA) of all plotted targets.
Inheadupandheaduptruebearingmodes,targetbearing,courseand
speedshownintheupperrighttargetdatafieldbecometrue(suffix“T”)or
relative(suffix“R”)toownshipinaccordancewithtrue/relativevector
setting.Innorthup,courseup,andtruemotionmodes,thetargetdatafield
alwaysdisplaystruebearing,truecourseandspeedoverthegroundor
through the water.
Reading the Target Data
Pressthecorrespondingplotsymbolkey,andthefollowingtargetdatais
displayed.
RNG/BRG:(Range/Bearing):Rangeandbearingfromownshiptolast
plotted target with suffix “T” or “R” plot symbol.
CSE/SPD:(Course/Speed):Courseandspeedaredisplayedforthelast
plotted target with suffix “T” or “R” plot symbol.
CPA/TCPA:CPAisaclosestrangethetargetwillapproachtoownship.
TCPAisthetimetoCPA.Bothareautomaticallycalculated.TCPAis
countedupto99.9minutesandbeyondthis.,itisindicatedasTCPA>*99.9
MIN.
BCR/BCT:BCR(BowCrossRange)istherangeatwhichtargetwillcross
ownship’sbow.BCT(BowCrossTime)istheestimatedtimeatwhichtarget
will cross own
Terminating Target Plotting
WithE-plotyoucanplotupto10targets.Youmaywishtoterminate
plotting of less important targets to newly plot other threatening targets.
BySymbol:Toterminateplottingofacertaintarget,pressthecorresponding
plot symbol key. Then press the CANCEL key.
WithTrackball:Placethecursor(+)onatargetwhichyoudonotwanttobe
tracked any longer by operating the trackball and press the CANCEL key.
AllTargets:Toterminateplottingofalltargetsatonce,pressandholdthe
CANCELkeyuntilallplotsymbolsandmarksdisappearinabout3seconds.
Entering Own Ship’s Speed
EPArequiresanownshipspeedinputandcompasssignal.Thespeedcan
beenteredfromaspeedlog(automatic)orthroughtheplottingkeypad
(manual).
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Automatic Speed Input
1.PresstheRADARMENUkeyontheplottingkeypadtoshowthe
functions menu,
2.Press the (6) key to select menu item 6 SHIP’S SPEED.
3.Press the (6) key to select (or Highlight) LOG option.
4.PresstheENTERkeytoconfirmyourselectionfollowedbytheRADAR
MENUkeytoclosetheFUNCTIONSmenu.Theship’sspeedreadoutat
thescreentopshowsownship’sspeedfedfromthespeedlogprecededby
the label “LOG”.
Notes:
1.IMOResolutionA.823(19)forARPArecommendsthataspeedlogtobe
interfacedwithanARPAshouldbecapableofprovidingthrough-the-water
speed data.
2.BesurnottoselectLOGwhenaspeedlogisnotconnected.Ifthelog
signalisnotprovided,theship’sspeedreadoutatthescreentopwillbe
blank.
Manual Speed Input
Iftheradarisnotinterfacedwithaspeedlog,orthespeedlogdoesnot
feed correct speed enter the ship’s speed as follows:
1.PresstheRADARMENUkeyonplottingkeypadtoshowthe
FUNCTIONS menu.
2.Press the (6) key to select menu 6 SHIP’S SPEED.
3.Press the (6) key to select menu 6 SHIP’s SPEED.
4.PresstheENTERkeytoconfirmselection.Atthispoint,“MAN+XX.KT”
appears at the bottom of the FUNCTIONS menu.
5.Entertheshipspeedbyhittingcorrespondingnumerickeysfollowedby
theENTERwithoutomittingleadingzeros,ifany.Asanexample,ifthe
ship speed is 8 knots, punch (0) (8) (ENTER).
6.PresstheRADARMENUkeytocloseFUNCTIONSmenu.Theship
speeddisplayedatthescreentopshowsownshipspeedenteredbythe
label “MAN”.
215
TARGET TRAILS (ECHO TRAILS)
Echotrailsaresimulatedafterglowoftargetechoesthatrepresenttheir
movementsrelativetoownshiportruemovementswithrespecttotruenorth
inasingletoneorgradualshadingdependingonthesettingsontheRADAR
1 menu.
True or Relative Trails
Youmaydisplayechotrailsintrueorrelativemotion.Relativetrailsshow
relativemovementsbetweentargetsandownship.Truemotiontrailsrequire
agyrocompasssignalandownshipspeedinputtocanceloutownship’s
movementandpresenttruetargetmovementsinaccordancewiththeirover
the ground speeds and courses.
Refertotheautomaticandmanualspeedinputproceduresforentering
own ship’s speed information.
Note:WhentruetrailisselectedontheRMmode,thelegendTRUETRAIL
appearsinred.NotruerelativeselectiononTM,itisonlyTRUETRAILon
TM mode.
To select true or relative echo trail presentation:
1.PresstheRADARMENUkeyontheplottingkeypadtoshowthe
FUNCTIONS menu.
2.Press the (0) key to show the SYSTEM SETTING 1 menu.
3.Press the (2) key to show the RADAR 1 menu.
4.Press the (6) key to select menu item 6 TRAIL REF.
5.Press the (6) key to select (or highlight) REL (Relative) or TRUE option.
6.PresstheENTERkeytoconfirmyourselectionfollowedbytheRADAR
MENU key to close the menu.
Trail Gradation
Echotrailsmaybeshowninmonotoneorgradualshading.Gradual
shadingpaintsthetrailsgettingthinnerwithtimejustliketheafterglowon
an analog PPI radar.
Selectionofmonochromeorgradualshadingrequiresalmostthesame
operationasfortrueorrelativetrailssetupproceduredescribedaboveexcept
that you should:
•Pressthe(7)keytoselectmenuitem7TRAILGRAD(graduation)in
step 4, and
•Pressthe(7)keytoselect(orhighlight)GGL(singletone)orMULT
(multiple shading) option in step 5.
Displaying and Erasing Echo Trails
PresstheECHOTRAILSkeytoactivateordeactivatetheechotrails
feature.
EachpressoftheECHOTRAILSkeywithin5secondscyclically
changesechotraillength(time)to30seconds,1,3,6,15,and30minutes,
continuousechotrailingandOFF.Thecurrentechotrailsettingisdisplayed
at the lower right corner of the screen.
Supposethat“3MIN”hasjustbeenselected.IftheECHOTRAILSkeyis
hitmorethan5secondslater,echotrailsareremovedfromthedisplay
(memory)stillalivewithechotrailtimercountgoingon).Nexthittingofthe
keycallsouttheechotrailsonthescreen.Toproceedtolongerplotintervals,
successivelypushtheECHOTRAILSkeywithahitandreleaseaction.The
larger the echo trail length, the larger the larger the echo trail plot interval.
Note:HoldingtheECHOTRAILSkeydepressedforabout3secondswill
cause a loss of echo trail data so far stored in an in memory.
Resetting Echo Trails
Toreset(orclear)theechotrailmemory,holdtheECHOTRAILSkey
depressedforabout3seconds.Echotrailsareclearedandthetrailing
processrestartsfromtimecountzeroatcurrentechotrailplotinterval.When
memoryassignedtoechotrailingbecomestheechotrailtimeratthelower
rightcornerofthescreenfreezesandtheoldesttrailsareerasedtoshowthe
latest trails.
216
PARALLEL INDEX LINES
Parallelindexlinesareusefulforkeepingaconstantdistancebetween
ownshipandcoastlineorapartnershipwhennavigating.Indexlinesare
drawninparallelwiththeNo.2EBL(no.2EBLmustbeactive).The
orientationoftheindexlinesiscontrolledwiththeEBLcontrolandthe
intervalsbetweenthelinesadjustedwiththeVRMrotarycontrol(provided
that No. 2 VRM is active).
MaximumnumberoftheindexlinecanbesettheinitialSettingmenu:2,
3, or 6.
Displaying and Erasing the Index Lines
1.Press the INDEX LINES key if the index lines are not already shown.
2.MakesurethattheNo.2EBLisactiveandorienttheindexlinesina
desired direction with the EBL rotary control.
3.To erase the index lines, press the INDEX LINES key again.
Adjusting Index Line Intervals
1.PresstheRADARMENUkeyontheplottingkeypadtoshowthe
FUNCTIONS menu.
2.Press the (7) key to select menu item 7 INDEX LINES.
3.Pressthe(7)keytoselector(highlight)No.2VRMorMAN(manual)
option.
4.Press the ENTER key to conclude your selection.
5.IfyouhaveselectedMANinstep3above,“MAN=XX.XXNM”appears
atthebottomofthefunctionsmenu.Enteradesiredlineintervalby
hittingnumerickeysfollowedbytheENTERkeywithoutomitting
leadingzeroes,ifany.Therearesixindexlinesbutthenumberoflines
visibleonthescreenmaybelessthansixdependingonthelinesetting
interval.
6.IfyouhaveselectedNO.2VRMinstep3above,makesurethattheNo.2
VRMisactiveandadjustthespacingbetweentheindexlinesby
operating the VRM control.
7.Press the RADAR MENU key to close the functions menu.
ANCHOR WATCH
Theanchorwatchfeaturehelpsyoumonitorwhetherownshipisdragged
bywindand/ortidewhileatanchor.Thisfeaturerequiresshippositiondata
fromasuitableradionavigationalaid.Providedthatownship’sphysicaldata
hasbeenentered,anownshipmarkcanbedisplayedwhentheanchorwatch
featureisactivated.Themessage“ANCHORWATCHERR”appearsinred
when position data is not inputted.
Notes:
1.TheownshipmarkisavailableontheR-typeradaronly;unavailableon
the IMO type.
2.Theownshipmarkiscreatedwithdataonownship’slength,width,radar
antennalocation,etc.Todisplayanownshipmark,askyournearestFuruno
representative.
Activating Anchor Watch
To set up the anchor watch feature:
1.OntheANCHORWATCHmenu,pressthe(2)keytoselectmenuitem2
ANCHOR WATCH OFF/ON.
2.Furtherpressthe(2)keytoselect(orhighlight)ON,followedbythe
ENTERkeytoconcludeyourselection.ThelabelWATCHappearsatthe
lower left corner of the screen.
3.Pressthe(3)keytoselectmenuitem3ALARMOFF/ON.Furtherpress
the(3)keytoselect(orhighlight)ONorOFF,followedbytheENTER
keytoconcludeyourselection.(Thisoperationdetermineswhetherto
activate the anchor watch audible alarm).
Alarm range setting
Pressthe(4)keytoselectmenuitem4ALARMRANGEonthe
ANCHORWATCHmenu.Enteradesiredalarmrangebetween0.1and
9.999nauticalmileswithnumerickeysandpresstheENTERkeyto
conclude your key input.
Ananchorwatchalarmcirclethusestablishedshowsupasaredcircleon
thescreen.Whenownshipisdraggedoutofthisalarmcircle,anaudible
alarm is generated and the on screen label ANCHOR WATCH turns red.
Tosilencetheaudiblealarm,presstheAUDIOOFFkeyonthecontrol
panel.
217
Showing Drag Line
Pressthe(5)keytoselectmenuitem5HISTORYontheANCHOR
WATCHmenu.Furtherpressthe(5)keytoselectON,followedbythe
ENTER key to conclude your selection.
Adragline,oraseriesofdotsalongwhichownshipwascarriedbywind
andwatercurrent,appearsasillustratedbelow.Duringthefirst50minute
period,dotsorownship’spastpositionsareplottedeveryminute.When50
dotshavebeenplottedin50minutes,theplotintervalbecomes2minutes
andupto25dotsareplottedduringthesucceeding50minuteperiod.Next,
thedotintervalbecomes4minutesandthemaximumnumberofdotswillbe
12.
Anchor Watch in Standby or Transmit Status
OntheR-typetheanchorwatchfeatureisavailableineitherSTANDBY
or TRANSMIT status.
OntheIMOtypetheanchorwatchfeatureisavailableonlyinSTANDBY
status.
Origin Mark
Youcanmarkanydangerouspoint,prominenttargetoraparticular
referencepointusingtheoriginmarkfeature.Thismarkisgeographically
fixed.
To use the origin mark:
1.Place the cursor (+) at a point where you want to place a reference mark
by operating the trackball.
2.Press the ORIGIN MARK key on the plotting keypad. The origin mark
appears at the cursor position of which range and bearing are indicated at
the lower left section of the screen.
3.To measure the range and bearing to a target of interest from the origin
mark, move the cursor to the target of interest. Then, the range and
bearing from the origin mark to the target are shown at the target data
display.
4.To erase the origin mark, press the ORIGIN MARK key once again.
Zoom
ThezoomfunctionisavailableontheR-typeradaronlytoenlargeanarea
of interest.
1.Place the cursor (+) close to the point of interest by operating the
trackball.
2.Press the X2 ZOOM key. The area around the cursor and own ship is
enlargedtwiceaslargeastheoriginalsizeandthelabelZOOMappearsat
the lower left corner of the screen.
3.To cancel zoom, press the X2 ZOOM key again.
Note: The zoom feature is inoperative when the display is off centered.
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MARKERS
Heading Marker
Theheadingmarkerindicatestheship’sheadinginallpresentation
modes.Itappearsatzerodegreesonthebearingscaleinheadupmode,in
anydirectiondependingontheshiporientationinnorthupandtruemotion
modes.
Temporarily Erasing Heading Marker
Totemporarilyextinguishtheheadingmarkertolookattargetsexisting
deadaheadofownship,presstheHMOFFkeyonthemodepanel.This
heading marker reappears when the key is released.
North Marker
Thenorthmarkerappearsasashortdashedline.Intheheadupmode,the
northmarkermovesaroundthebearingscaleinaccordancewiththe
compass signal.
Stern Marker
Thesternmarker(adot-and-dashline)appearsoppositetotheheading
marker.ThismarkercanbedisplayedontheRtypeonlyprovidedthatthe
STERN MARK ON is selected on the RADAR 2 menu.
Menu Keys
Threemenukeysareprovidedontheplottingkeypad:RADARMENU,
E-AUTO PLOT MENU and NAV MENU keys.
RADAR MENU: Permits setting of basic radar parameters.
E, AUTO PLOT MENU: Provides a choice of standard or large size of
plotting symbols for plot.
NAV MENU: Provides a choice of navigation data for on screen display.
Also select display for the Video Plotter.
219
FUNCTION KEYS
Thefourfunctionkeys(#1-4)onthecontrolpanel(figure5.17)worklike
theautodialingfeatureofatelephone,instantlycallingoutdesiredsettings
toperformspeciallyassignedfunctions.Thefunctionkeysprovideoptimum
radar settings for a specific purpose with a single key operation.
Eachfunctionkeycanbeassignedacombinationofparticularradar
settingsthatwillbemostsuitedtoyourspecificnavigatingpurpose,andan
adhesivelabel(suchasBUOY,HARBOR,COASTorthelike)isusually
attached to the key top for easy identification of the assigned purpose.
Theindividualfunctionkeysarepreset,orprogramed,forthefollowing
purposesbyqualifiedservicepersonnelatthetimeofinstallationusingthe
procedures described in the succeeding paragraphs;
Function key #1: Picture setup
Function keys #2 and #3: Picture setup and specific operation
Function key #4: Specific operation or watch alarm
Suppose that you have been navigating along a coast for hours and now
youareapproachingaharbor,yourfinaldestination.Youwillhavetoadjust
your radar to change from the settings for coastal navigation to those for
harbor approach. Every time your navigating environment or task changes,
you must adjust the radar, which can be a nuisance in a busy situation.
Instead of changing radar settings case by case, it is possible to assign the
function keys to provide optimum settings for often encountered situations.
The radar’s internal computer offers several picture setup options to be
assignedtoeachfunctionkeyforyourspecificnavigatingrequirements.For
instance, one of the functions keys may be assigned the buoy detecting
function and labeled BUOY on the key top. If you press this key, the radar
will be instantly set for optimum detection of navigation buoys and similar
objects and the label BUOY is shown at the left margin of the screen. If you
re-press the same key, the radar returns to the previous settings.
Figure 5.17 - Function keys
220
The radar’s internal computer offers several picture setup options to be
assignedtoeachfunctionkeyforyourspecificnavigatingrequirements.For
instance, one of the functions keys may be assigned the buoy detecting
function and labeled BUOY on the key top. If you press this key, the radar
will be instantly set for optimum detection of navigation buoys and similar
objects and the label BUOY is shown at the left margin of the screen. If you
re-press the same key, the radar returns to the previous settings.
Thepicturesetupoptionsassignabletoanyofthefunctionkeysareshown
in the table below.
LABELDESCRIPTION
RIVEROptimum setting for navigation on river.
BUOYOptimumsettingfordetectingnavigationbuoys,small
vessels and other small surface objects.
SHIPOptimum setting for detecting vessels.
SHORTOptimumsettingforshortrangedetectionusingarange
scale of 6 nm or larger.
CRUISINGFor cruising using a range scale of 1.5 nm or larger.
HARBOROptimumsettingforshortrangenavigationinaharbor
area using a range scale of 1.5 nm or less.
COASTFor coastal navigation using a range of 12 nm or less.
OCEANTransoceanicvoyageusingarangescaleof12nmor
larger.
ROUGH SEAOptimum setting for rough weather or heavy rain.
Eachpicturesetupoptiondefinesacombinationofseveralradarsettings
forachievingoptimumsetupforaparticularnavigatingsituation.Those
involvedareinterferencerejector,echostretch,echoaverage,automaticant
clutter, pulsewidth and noise rejector settings.
Adjustingthesefeaturesonafunctionkeymenuchangestheoriginal
functionkeysettings.Torestoretheoriginalsettingsforaparticularfunction
key,itisnecessarytodisplaytherelevantfunctionkeymenuandselect
appropriate menu options.
Note:Functionkeypresettingrequiresagoodknowledgeofoptimumradar
settings.Ifyouwanttochangetheoriginalfunctionkeysettings,consult
your Furuno representative or dealer.
Watch Alarm
Thewatchalarmsoundsanexternalbuzzerselectedtimeintervalstohelp
youkeepregularwatchoftheradarpictureforsafetyorotherpurposes.This
featurecanbeassignedtofunctionkey#4withachoiceofalarmintervalsof
3, 6, 10, 12, 15 and 20 minutes.
Providedthatfunctionkey#4isassignedthewatchalarmfeature,just
pressfunctionkey#4toactivatethefeature.ThelabelWATCHappearsat
thelowerleftcornerofthescreenassociatedwithawatchalarmtimer
counts down from the initial value (namely, “12:00”).
Whenanaudiblewatchalarmisreleasedthepresettimeintervalhas
elapsed,thescreenlabelWATCHturnsredandthewatchalarmtimer
freezes at “0:00”.
Tosilencethealarm,presstheAUDIOkey.ThelabelWATCHturnsto
normalcolorandthewatchalarmtimerisresettotheinitialvalueandstarts
the count down sequence again.
IfyoupresstheAUDIOOFFkeybeforetheselectedtimeintervalis
reached,thewatchalarmtimerisresettotheinitialvalueandstartsthe
countdown sequence again.
EPA Menu
EPAmenuappearsbypressingtheE,AUTOPLOTMENUkey.Youcan
set the following items.
1.COLLISIONALARM:YoucansetCPAandTCPAforthetrackedtarget.
Referto2.12settingCPA/TCPAalarmrange.NotethatTCPAsettingis
available over one minute.
2.MARK SIZE: Change the size of the plotting.
3.PLOTNO.:Displaysorhidesplotnumberinsideoftheplotsymbol
(circle and square).
4.TARGETDATA:SelectstargetvectormodebetweenTRUEorREL.
Selection of REL provides the target mode in REL on HU and HU TB.
221
NAVIGATION INFORMATION
Menu and Navigation Data Display
Various navigation data can be displayed on the radar screen. The data
includes,dependingonwhetherappropriateinformationisfedintotheradar,
own ship position, cursor position, waypoint data, wind data, water current
data,depthdata,watertemperature,rudderangle,rateofturnandnavigation
lane.
Notethatdatanotdirectlyrelatedwiththeradarpresentationisnot
available. Shown below id a typical navigational data display.
1.PresstheNAVMENUkeyontheplottingkeypadtoshowtheNAV
INFORMATION menu.
2.SelectnavigationdatainputdeviceandpresstheENTERkeytoconfirm
your selection.
3.Also,setothernavdataparametersasappropriatereferringtothe
operation flow diagram (not shown).
4.Press the NAV MENU key to close the NAV INFORMATION menu.
Notes:
1.OwnshippositiondisplayrequiresaninputfromanEPFS(elestrouis
positionfixingsystem)suchasaGPSreceiveroraLoran-Creceiver.Such
anEPFSshouldbeofthetypewhichprovidesoutputdatainaccordance
with IEC 1162.
2.Whenthesensorinusechanges(ex.fromGPSorDGPS),thenameof
sensorintheownshipcallturnsred,andEPFSlabelappears.Toerase,press
the CANCEL key.
Suppressing Second-trace Echoes
Incertainsituations,echoesfromverydistanttargetsmayappearasfalse
echoes(secondtraceechoes)onthescreen.Thisoccurswhenthereturn
echoisreceivedonetransmissioncyclelater,thatis,afteranextradarpulse
has been transmitted.
To activate or deactivate the second trace echo rejector:
1.PresstheRADARMENUkeyontheplottingkeypadtoshowthe
FUNCTIONS menu.
2.Press the (8) key to select menu item 8: 2ND ECHO REJ.
3.Furtherpressthe(8)keytoactivate(ON)ordeactivate(OFF)thesecond
trace echo rejector.
4.PresstheENTERkeytoconcludeselectionfollowedbytheRADAR
MENU key to close the FUNCTIONS menu.
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Adjusting Relative Brilliance Levels of Screen Data
Youcanadjustrelativebrilliancelevelsofvariousmarksand
alphanumericreadoutsdisplayedonthescreenbyfollowingthestepsshown
below:
1.PresstheRADARMENUkeyontheplottingkeypadtoshowthe
FUNCTIONS menu.
2.Press the (9) key to show the BRILLIANCE menu.
3.Selectadesiredmenuitembypressingthecorrespondingnumerickey.As
an example, press (4) if you want to change the brilliance of echo trails.
4.Furtherpressthesamenumerickeyasyoupressedinstep3aboveto
select or highlight a desired brilliance level.
5.PresstheENTERkeytoconcludeyourselectionfollowedbytheRADAR
MENU key to close the FUNCTIONS menu.
Set and Drift (Set and Rate)
Setthedirectioninwhichawatercurrentflows,canbemanuallyentered
on0.1-degreesteps.Drift(rate),thespeedofthetide,canalsobeentered
manually in 0.1 knot steps.
Setanddriftcorrectionsarebeneficialforincreasingtheaccuracyofthe
vectorsandtargetdata.Thecorrectionisbestmadeintheheadupmodewith
truevector,watchinglandmasses,orotherstationarytargets.Iftheyhave
vectors, set and drift values should be adjusted until they lose vectors.
Note:Setanddriftcorrectionisavailableonselectingthewatertracking
mode only.
Proceed as follows to enter set and drift (rate):
1.PresstheRADARMENUkeyontheplottingkeyboardtoshowthe
FUNCTIONS 1 menu.
2.Press the (8) key to select menu item 8; SET, DRIFT.
3.Further press the (8) key to select OFF or MAN option.
OFF: No correction against set and drift.
MAN: Manual entry of set and drift data.
4.If OFF is selected, press the ENTER key.
5.If you have selected MAN in step 3 above, the highlight cursor will
advanceonelinedownrequestingyoutoenterSETxxx.x.Enterthevalue
ofsetindegreesbyhittingnumerickeyswithoutomittingleadingzeroes,
if any, and press the ENTER key.
6.The highlight cursor will then advance to the next line DRIFT xx.x KT.
Enter the value of drift in knots by hitting numeric keys without omitting
leading zeroes, if any, and press the ENTER key. Set and drift have the
same effect on own ship and all targets.
7.Press the RADAR MENU key to close the menu.
223
OPERATION OF ARPA
GENERAL
TheFAR-2805serieswithARP-25boardprovidethefullARPA
functionscomplyingwithIMOA.823andIEC-60872-1aswellas
complying with the radar performance MSC.64(67) Annex 4.
PRINCIPAL SPECIFICATIONS
Acquisition and Tracking
Automaticacquisitionofupto20targetsplusmanualacquisitionof20
targets,orfullymanualacquisitionof40targetsbetween0.1and32nm(0.1
and 24 nm depending on initial setting)
Automatictrackingofallacquiredtargetsbetween0.1and32nm(0.1and
24 nm depending on initial setting)
Vectors
Vector length:30 sec, 1, 2, 3, 6, 12, 15, 30 min.
Orientation:True velocity or relative velocity
Motion trend:Displayed within 20 scans, full accuracy within 60scans
after acquisition.
Past positions:Choiceof5or10pastpositionsatintervalsof30sec,
1,2,3 or 6 min.
Alarms:VisualandaudiblealarmsagainsttargetsviolatingCPA/
TCPAlimits,losttargets,targetscrossingguardzone
(guard ring), system failure and target full status.
Trial maneuver:Predictedsituationappearsin1minafterselecteddelay
(1-60 minutes).
KEYS USED FOR ARPA
TheAutoPlotterusesthekeysontheplottingkeypadontherightsideof
theradarscreenandtwokeysonthecontrolpanel.Belowisabrief
description of these keys.
CANCEL:Terminatestrackingofasingletargetspecifiedbythetrackballif
thekeyispressedwithahit-and-releaseaction.Ifthekeyishelddepressed
for about 3 seconds, tracking of all targets is terminated.
ENTER: Registers menu options selected.
VECTORTRUE/REL:Selectsavectorlengthof30s1,2,3,6,12,15or
30min.
TARGETDATA:Displaysdataononeoftrackedtargetsselectedbythe
trackball.
TARGETBASEDSPEED:Ownship’sspeedismeasuredrelativetoafixed
target.
AUTO PLOT: Activates and deactivates the ARPA functions.
TRIAL:Showsconsequencesofownship’sspeedandcourseagainstall
tracked targets.
LOSTTARGET:Silencesthelosttargetauralalarmanderasesthelosttarget
symbol.
HISTORY: shows and erases pat positions of tracked targets.
ACQ: (on control panel): Manually acquires a target.
AUDIO OFF: (on control panel): Silences aural alarm.
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ARPA MENU OPERATION
VariousparametersortheAutoPlotteraresetontheARPA1andARPA2
menus. To do this, follow the steps shown below:
1.PresstheAUTOPLOTkeyiftheAutoPlotterisnotyetactivated.Note
that the label ARPPA appears in the upper right box on the screen.
2.Press the E, AUTO PLOT MENU key to show the ARPA 1 menu.
3.Press the (0) key once if you wish to go to the ARPA 2 menu.
4.Select a desired menu item by pressing the corresponding numeric key.
5.Selectamenuoptionbypressingthesamenumerickeyaspressedinstep
3above.Ifthereismorethanoneoptiononthecurrentmenuitem,you
mayneedtopressthenumerickeyseveraltimes.Pressituntilthedesired
optionishighlighted.(Notethatcertainmenuitemswillpromptyouto
enternumericdataortodefinepointsontheradarscreenwiththe
trackball).
6.Press the ENTER key to register settings.
7.Press the E, AUTO PLOT MENU key to close the menu.
START UP PROCEDURE
Activating the ARPA
To activate the ARPA:
1.AdjusttheA/CRAIN,A/CSEAandGAINcontrolsforproperradar
picture.
2.PresstheAUTOPLOTkey.ThelabelARPAappearsintheboxatthe
upper right on the screen.
Entering Own Ship’s Speed
TheARPArequiresownship’sspeedandheadingdata.Ofthese,the
speeddatacanbeenteredautomaticallyfromaspeedlog,navaid,or
manuallythroughthenumerickeysorbasedonaselectedreferencetarget
(such as a buoy or other prominent stationary target).
Automatic Speed Input
1.PresstheRADARMENUkeyontheplottingkeypadtoshowthe
FUNCTIONS menu.
2.Press the (6) key to select menu item 6 SHIP’S SPEED.
3.Press the (6) key to select (or highlight) LOG option.
4.PresstheENTERkeytoconcludeyourselectionfollowedbytheRADAR
MENUkeytoclosetheFUNCTIONSmenu.Theship’sspeedreadoutat
thetopofthescreenshowsownship’sspeedfedfromthespeedlog
preceded by the label “LOG”.
5.Whenthespeedlogisused,selectspeedreferencetoeitherofSEAor
GND (ground) on the ARPA 2 menu.
Notes:
1.IMOResolutionA.823:1995forARPArecommendsthataspeedlogtobe
interfacedwithanARPAshouldbecapableofprovidingthroughthewater
speed data rather than over the ground speed.
2.BesurenottoselectLOGwhenaspeedlogisnotconnected.Ifthelog
signalisnotprovided,theshipspeedreadoutatthetopofthescreenwillbe
blank.Intheeventofalogerror,youcancontinueplottingbyenteringa
manual speed.
3.Ifalogsignalintervalbecomesmorethan30secondswiththeship’s
speed5knotsormore,theradarregardsthespeedlogisintroubleandLOG
FAILappears,readingxx.xKT.ForR-type,ifnospeedinputispresentfor3
minutes at below 0.1 knots, the radar regards the log is in failure.
225
Manual Speed Input
To manually enter the ship’s speed with the numeric keys:
1.PresstheRADARMENUkeyontheplottingkeypadtoshowthe
FUNCTIONS menu.
2.Press the key (6) to select menu item 6 SHIP’S SPEED.
3.Press the key (6) to select (or highlight) MAN option.
4.PresstheENTERkeytoconcludeyourselection.Atthispoint,
“MAN=xx.xKT” appears at the bottom of the FUNCTIONS menu.
5.Entertheshipspeedbyhittingcorrespondingnumerickeysfollowedby
theENTERkeywithoutomittingleadingzeroes,ifany.Asanexample,if
theshipspeedis8knots,press(0)(8)ENTER.For4.5knots,(0)(4)(5)
ENTER.
6.PresstheRADARMENUkeytoclosetheFUNCTIONSmenu.Theship
speedreadoutatthescreentopshowsownship’sspeedyouentered
preceded by the label “MANU”.
Target Based Speed
The use of target based speed is recommended when:
1.The speed log is not operating properly. or not connected to the radar.
2.Thevesselhasnodevicewhichcanmeasureship’sleewardmovement
(dopplersonar,speedlog,etc.)thoughleewardmovementcannotbe
disregarded.
Ifyouselecttargetbasedspeed,theAutoPlottercalculatesownship’s
speed relative to a fixed reference target.
Note:Whenthetargetbasedspeedisadopted,automaticallyormanually
entered ship’s speed is disregarded.
To establish target based speed:
1.Selectasmallfixedislandoranyradarprominentpointlocatedat0.2to
24 nm from own ship.
2.Place the cursor (+) on the target by operating the trackball.
3.PresstheTARGETBASEDSPEEDkey.thereferencetargetmark
appearsatthecursorpositionandtheownshipdatalabelchangesfrom
“LOG”,“NAV”or“MENU”to“REF”.Notethatittakesoneminute
before a new speed is displayed.
Notes:
1.Whenthereferencetargetislostorgoesoutoftheacquisitionrange,the
reference target mark blinks and the speed reads “xx.x.”
2.Whenalltargetsaredeleted,thereferencetargetmarkisalsodeletedand
thetargetbasedspeedbecomesinvalid.thespeedisindicatedinKTBT
where BT means Bottom Track (speed over ground).
3.Thevectorofthereferencetargetcanbedisplayedbymenuoperation
(Auto Plot 1 menu).
Cancelling Target Based Speed
Tocancelthetargetbasedspeed,justpresstheTARGETBASEDSPEED
key.ThespeedisshownbyLOG,NAV*orMANUALasselected
previously. (NAV only on R-type).
Deactivating the ARPA
TodeactivatetheARPA,justpresstheAUTOPLOTkey.Targetplotting
symbols and the on-screen label ARPA will disappear.
Note:EvenwhentheARPAisturnedoff,targettrackingstillgoesonuntil
the radar id turned off.
AUTOMATIC ACQUISITION
TheARPAcanacquireupto40targets(20automaticallyand20
manuallyorall40manually).IfAUTOACQisselectedaftermorethan20
targetshavebeenmanuallyacquired,onlytheremainingcapacityoftargets
canbeautomaticallyacquired.Forexample,when30targetshavebeen
acquiredmanually,thentheARPAisswitchedtoAUTOACQ.Only10
226
targetscanbeacquiredautomatically.Atargetjustacquiredautomaticallyis
markedwithabrokensquareandavectorappearsaboutoneminuteafter
acquisitionindicatingthetarget’smotiontrend.Threeminutesafter
acquisition,theinitialtrackingstageisfinishedandthetargetbecomesready
forstabletracking.Atthispoint,thebrokensquaremarkchangestoasolid
circle.(Targetsautomaticallyacquiredaredistinguishedfromthoseacquired
manually, displayed by bold symbol).
Enabling and Disabling Auto Acquisition
1.PresstheE,AUTOPLOTkeyiftheARPAisnotyetactivated.Notethat
the label ARPA appears in the box at the upper right on the screen.
2.Press the E, AUTO PLOT MENU key to show the ARPA 1 menu.
3.Press the (1) key to select menu item 1 AUTO ACQ.
4.Furtherpressthe(1)keytoselect(orhighlight)ON(enableauto
acquisition) or OFF (disable auto acquisition) as appropriate.
5.PresstheENTERkeytoconcludeyourselectionfollowedbytheE,
AUTOPLOTMENUkeytoclosetheAUTOPLOT1menu.Notethatthe
labelAUTO+MANisdisplayedintheboxattheupperrightonthescreen
whenautoacquisitionisenabled;MANwhenautoacquisitionisdisabled.
Note:WhentheARPAhasacquired20targetsautomatically,themessage
AUTOTARGETFULLisdisplayedintheboxattherighthandsideofthe
screen.
Setting Auto Acquisition Areas
Insteadoflimitslines,autoacquisitionareasareprovidedinthesystem.
There are two setting methods:
3,6NauticalMiles:Twopredefinedautoacquisitionareas;onebetween
3.0 and 3.5 nautical miles and the other between 5.5 and 6.0 nautical miles.
SET:Twosectorshapedorfullcircleautoacquisitionareassetbyusing
the trackball.
To activate two predefined auto acquisition areas (3 & 6 NM):
1.Press the E, AUTO PLOT MENU key to show the ARPA 1 menu.
2.Press the (2) key to select menu item 2 AUTO ACQ AREA.
3.Furtherpressthe(2)keytoselect(orhighlight)menuoption3,6nautical
miles.
4.PresstheENTERkeytoconfirmyourselectionfollowedbytheE,AUTO
PLOT MENU key to close the ARPA 1 menu.
To set auto acquisition areas with trackball:
1.Press the E, AUTO PLOT MENU key to show the ARPA 1 menu.
2.Press the (2) key to select menu item 2 AUTO ACQ AREA.
3.Further press the (2) key to select (or highlight) SET option.
4.PresstheENTERkeytoconcludeyourselection.AtthispointtheAUTO
ACQ SETTING menu is displayed at the screen bottom.
5.Press the (2) key to select menu item 2 1/2 and press the ENTER key.
6.Placethecursorattheoutercounterclockwisecorneroftheareaandpress
the ENTER key.
7.PlacethecursorattheclockwiseedgeoftheareaandpresstheENTER
key.
Note:Ifyouwishtocreateanautoacquisitionareahavinga360degree
coveragearoundownship,setpointBinalmostthesamedirection(approx.
+/-3) as point A and press the ENTER key.
8.Repeatsteps5and7aboveifyouwanttosetanotherautoacquisitionarea
with the trackball.
9.Pressthe(1)keyfollowedbytheE,AUTOPLOTMENUkeytoclosethe
ARPA 1 menu.
Anautoacquisitionarealiketheexampleshownaboveappearsonthe
display.Notethateachautoacquisitionareahasafixedradialextension
width of 0.5 nautical miles.
Notethattheautoacquisitionareasarepreservedinaninternalmemory
oftheARPAevenwhenautoacquisitionisdisabledortheARPAisturned
off.
227
Terminating Tracking of Targets
WhentheARPAhasacquired20targetsautomatically,themessage
AUTOTARGETFULLisdisplayedintheboxatrighthandsideofthe
screenandnomoreautoacquisitionoccursunlesstargetsarelost.Youmay
findthismessagebeforeyousetanautoacquisitionarea.Shouldthis
happen,canceltrackingoflessimportanttargetsorperformmanual
acquisition.
Individual Targets
Placethecursor(+)onatargettocanceltrackingbyoperatingthe
trackball. Press the CANCEL key.
All Targets
PressandholdtheCANCELkeydownmorethan3seconds.Inthe
automatic acquisition mode, acquisition begins again.
Discrimination Between Landmass and True Targets
Atargetisrecognizedasalandmassandthusnotacquiredifitis800
meters or more in range or bearing direction.
MANUAL ACQUISITION
Inautoacquisitionmode(AUTOACQON),upto20targetscanbe
manuallyacquiredinadditionto20autoacquiredtargets.Whenauto
acquisitionisdisabled(AUTOACQOFF),upto40targetscanbemanually
acquired and automatically tracked.
To manually acquire a target:
1.Place the cursor (+) on a target of interest by operating the trackball.
2.PresstheACQkeyonthecontrolpanel.Theselectedplotsymbolis
marked at the cursor position.
Notethattheplotsymbolisdrawnbybrokenlinesduringtheinitialtracking
stage.Avectorappearsinaboutoneminuteafteracquisitionindicatingthe
target’smotiontrend.Ifthetargetisconsistentlydetectedforthreeminutes,
theplotsymbolchangestoasolidmark.Ifacquisitionfails,thetargetplot
symbol blinks and disappears shortly.
Notes:
1.Forsuccessfulacquisition,thetargettobeacquiredshouldbewithin0.1
to 32 nautical miles from own ship and not obscured by sea or rain clutter.
2.Whenyouhaveacquired40targetsmanually,themessageMAN
TARGETFULLisdisplayedatthescreenbottom.Canceltrackingofnon
threatening targets if you wish to acquire additional targets manually.
CHANGING PLOT SYMBOL SIZE
Youmayalsochooseplotsymbolsize.Tochoosealargeorstandardsize
for all plot symbols:
1.PresstheE,AUTOPLOTMENUkeyontheplottingkeypadfollowedby
the keys (0) to show the ARPA 2 menu.
2.Press the (3) key to select 3 MARK SIZE.
3.Furtherpressthe(3)keytoselect(orhighlight)STANDARDorLARGE
as appropriate.
4.PresstheENTERkeytoconcludeyourselectionfollowedbytheE,
AUTO PLOT MENU key to close the ARPA 2 menu.
ADJUSTING BRILLIANCE OF PLOT MARKS
1.PresstheRADARMENUkeyontheplottingkeypadtoshowthe
FUNCTIONS menu.
2.Press the (9) key to show the BRILLIANCE menu.
3.Press the (7) key to select 7 PLOT BRILL.
4.Further press the (7) key to select (or highlight) a desired brilliance level.
5.PresstheENTERkeytoconfirmyourselectionfollowedbytheRADAR
MENU key to close the FUNCTION menu.
228
Figure 5.18 - ARPA Symbols
229
Figure 5.19 - ARPA Symbols (continued)
230
DISPLAYING TARGET DATA
TheAutoPlottercalculatesmotiontrends(range,bearing,course,speed,
CPA and TCPA) of all plotted targets
Inheadupandheaduptruebearingmodes,targetbearing,courseand
speedshownintheupperrighttargetdatafieldbecometrue(suffix“T”)or
relative(suffix“R”)toownshipinaccordancewiththetrue/relativevector
setting.Innorthup,courseupandtruemotionmodes,thetargetdatafield
always displays true bearing, true course and speed over the ground.
PlacethecursoronthedesiredtargetandpresstheTARGETDATAkey
ontheplottingkeypad.Dataontheselectedtargetisdisplayedattheupper
rightcornerofthescreen.Atypicaltargetdatadisplayisshowninfigure
5.20.
RNG/BRG:Rangeandbearingfromownshiptotheselectedtargetwith
suffix “T” (True) or “R” (Relative).
CSE/SPD:Courseandspeedaredisplayedfortheselectedtargetwithsuffix
“T” or “R”.
CSE/SPD:CPA(ClosestPointofApproach)istheclosestrangeatargetwill
approachtoownship.TCPAisthetimetoCPA.BothCPAandTCPAare
automaticallycalculated.Whenatargetshiphaspassedclearofownship,
CPAisprefixedwithanasterisksuchas,CPA*1.5NM.TCPAiscountedto
99.9 min and beyond this, it reads TCPA.*99.9MIN.
BCR/BCT:Bowcrossingrangeisarangeofatargetwhichwillpassdead
aheadofownshipatacalculateddistance.BCTisthetimewhenBCR
occurs.
Figure 5.20 - Target Data
231
MODE AND LENGTH OF VECTORS
True or Relative Vector
Target vectors can be displayed relative to own ship’s heading (relative) or
with reference to the north (true).
Press the VECTOR TRUE/REL key to select true or relative vectors. This
feature is available in all presentation modes (gyrocompass must be working
correctly). The current vector mode is indicated at the upper right corner of
the screen.
True Vector
Inthetruemotionmode,allfixedtargetssuchasland,navigationalmarks
andshipsatanchorremainstationaryontheradarscreenwithvectorlength
zero.Butinthepresenceofwindand/orcurrent,truevectorsappearonfixed
targetsrepresentingthereciprocalofsetanddriftaffectingownshipunless
set and drift values are properly entered (see figure 5.21).
Relative Vector
Relativevectorsontargetswhicharenotmovingoverthegroundsuchas
land,navigationalmarksandshipsatanchorwillrepresentthereciprocalof
ownship’sgroundtrack.Atargetofwhichvectorextensionpassesthrough
ownshipisonthecollisioncourse.(Seefigure5.22-dottedlinesarefor
explanation only).
Vector Time
Vectortime(orlengthofvectors)canbesetto30seconds,1,2,3,6,12,
15or30minutesandtheselectedvectortimeisindicatedattheupperright
corner of the screen.
PresstheVECTORTIMEkeytoselectdesiredvectortime.Thevectortip
showsanestimatedpositionofthetargetaftertheselectedvectortime
elapses.Itcanbevaluabletoextendthevectorlengthtoevaluatetheriskof
collision with any target.
PAST POSITIONS
TheARPAdisplaysequallytimespaceddotsmarkingthepastpositions
of any targets being tracked.
Anewdotisaddedeveryminute(oratpresettimeintervals)untilthe
presentnumberisreached.Ifatargetchangesitspeed,thespacingwillbe
uneven. If it changes the course, its plotted course will not be a straight line.
Displaying and Erasing Past Positions
Todisplaypastpositions,presstheHISTORYkeytodisplaypast
positionsoftargetsbeingtracked.ThelabelHISTORYappearsattheupper
right corner of the screen.
To erase past positions, press the HISTORY key again.
Figure 5.21 - True vectors in head-up mode
Figure 5.22 - Relative vectors in head-up mode
232
Selecting the Number of Dots and Past Position Intervals
1.PresstheE,AUTOPLOTMENUkeyontheplottingkeyboardtoshow
the ARPA 1 menu.
2.Press the (7) key to select menu item 7 HISTORY POINTS.
3.Furtherpressthe(7)keytoselectadesirednumberofpastpositions(5,
10,20,30,100,150or200).TheIMO-typehastheselectionofonly5or
10.
4.Press the ENTER key to confirm your selection.
5.Press the (8) key to select menu item 8 HISTORY INTERVAL.
6.Furtherpressthe(8)keytoselectadesiredpastpositionplotinterval(30
seconds, 1, 2, 3 or 6 minutes).
7.Press the ENTER key to conclude your selection.
8.Press the E, AUTO PLOT MENU key to close the menu.
SETTING CPA/TCPA ALARMS RANGES
TheARPAcontinuouslymonitorsthepredictedrangeattheCPAand
predicted time to CPA (TCPA) of each tracked target to own ship.
WhenthepredictedCPAofanytargetbecomessmallerthanapresetCPA
alarmrangeanditspredictedTCPAlessthanapresetTCPAalarmlimit,the
ARPAreleasesanauralalarmanddisplaysthewarninglabelCOLLISION
onthescreen.Inaddition,theARPAsymbolchangestoatriangleand
flashes together with its vector.
Providedthatthisfeatureisusedcorrectly,itwillhelppreventtheriskof
collisionbyalertingyoutothreateningtargets.ItisimportantthatGAIN,A/
C SEA, A/C RAIN and other radar controls are properly adjusted.
CPA/TCPAalarmrangesmustbesetupproperlytakinginto
considerationthesize,tonnage,speed,turningperformanceandother
characteristics of own ship.
CAUTION:TheCPA/TCPAalarmfeatureshouldneverberelieduponasthe
solemeansfordetectingtheriskofcollision.Thenavigatorisnotrelievedof
theresponsibilitytokeepvisuallookoutforavoidingcollisions,whetheror
not the radar or other plotting aid is in use.
To set the CPA/TCPA alarm ranges:
1.PresstheE,AUTOPLOTMENUkeyontheplottingkeypadtoshowthe
ARPA 1 menu.
2.Pressthe(6)keytoselectmenuitem6CPA,TCPASET.Atthispoint,a
highlight cursor appears at the “CPAx.xNM” field.
3.EntertheCPAalarmrangeinnauticalmiles(max9.9min)without
omittingleadingzeroes,ifany,andpresstheENTERkey.Thehighlight
cursor now moves to the:TCPAxx.xMIN” field.
4.EntertheTCPAalarmlimitinminutes(max.99.0min)withoutomitting
leading zeroes, if any, and press the ENTER key.
5.Press the E, AUTO PLOT MENU key to close the menu.
Silencing CPA/TCPA Aural Alarm
PresstheAUDIOOFFkeytoacknowledgeandsilencetheCPA/TCPAaural
alarm.
ThewarninglabelCOLLISIONandtheflashingofthetriangleplot
symbolandvectorremainonthescreenuntilthedangeroussituationisgone
oryouintentionallyterminatetrackingofthetargetbyusingtheCANCEL
key.
Setting a Guard Zone
Whenatargettransitstheoperator-setguardzone,thebuzzersoundsand
theindicationGUARDRINGappearsatthescreenbottom.Thetarget
causing the warning is clearly indicated with an inverted flashing triangle.
CAUTION:TheGuardZone(GuardRing)shouldneverberelieduponasa
solemeansfordetectingtheriskofcollision.Thenavigatorisnotrelievedof
theresponsibilitytokeepavisuallookoutforavoidingcollisions,whetheror
not the radar or other plotting aid is in use.
233
Activating the Guard Zone
No.1GuardZoneisavailablebetween3and6nmwithafixedrange
depth of 0.5 nm. No. 2 GZ may be set anywhere when No. 1 GZ is valid.
To set and activate the guard zone:
1.PresstheE,AUTOPLOTMENUkeyontheplottingkeyboardtoshow
the ARPA 1 menu.
2.Press the (3) key to select menu item 3 GUARD RING.
3.Furtherpressthe(3)keytoselect(orhighlight)ONtoactivatetheguard
zone.
4.Press the ENTER key to conclude your selection.
5.Pressthe(4)keytoselectmenuitem4GUARDRINGSET.Atthispoint
the GUARD SETTING menu is displayed at the screen bottom.
6.Pressthe(2)keyandenterkey.(2)(2)(ENTER)whensettingtheno.2
ring.
7.Placethecursorattheouterleftcornerofthearea(point1)andpressthe
ENTER key.
8.Placethecursorattherightedgeofthearea(point2)andpressthe
ENTER key.
Note:Ifyouwishtocreateaguardzonehavinga360-degreecoverage
aroundownship,setpoint2inalmostthesamedirection(approx.+/-3°)as
point 1 and press the ENTER key.
9.Pressthe(1)keyfollowedbytheE,AUTOPLOTMENUkeytoclosethe
ARPA 1 menu.
Deactivating the Guard Zone
1. Press the E, AUTO PLOT MENU key on the plotting keyboard to show
the ARPA 1 menu.
2. Press the (3) key to select menu item 3 GUARD RING.
3. Further press the (3) key to select (or highlight) OFF to deactivate the
guard zone.
4. Press the ENTER key to conclude your selection followed by the E,
AUTO PLOT MENU key to close the ARPA 1 menu.
Silencing the Guard Zone Audible Alarm
PresstheAUDIOOFFkeytoacknowledgeandsilencetheguardzone
audible alarm.
Operational Warnings
TherearesixmainsituationswhichcausetheAutoPlottertotrigger
visual and aural alarms:
•CPA/TCPA alarm
•Guard zone alarm
•Lost target alarm
•Target full alarm for manual acquisition
•Target full alarm for automatic acquisition
•System failures
The audible alarm can be set to OFF through the AUTO PLOT 2 menu.
234
CPA/TCPA Alarm
VisualandauralalarmsaregeneratedwhenthepredictedCPAandTCPA
ofanytargetbecomelessthantheirpresetlimits.PresstheAUDIOOFFkey
to acknowledge and silence the CPA/TCPA aural alarm.
Guard Zone Alarm
Visual and audible alarms are generated when a target transmits the
operator-set guard zone. Press the AUDIO OFF key to acknowledge and
silence the guard zone audible alarm.
Lost Target Alarm
When the system detects a loss of a tracked target, the target symbol
becomes a flashing diamond. and the label “LOST” appears at the screen
bottom. At the same time, an aural alarm is produced for one second.
Press the LOST TARGET key to acknowledge the lost target alarm. Then,
the lost target mark disappears.
Target Full Alarm
When the memory becomes full, the memory full status is indicated and
the relevant indication appears on the screen and a short beep sounds.
Manually Acquired Targets
The indication “MAN TARGET FULL” appears at the screen bottom and
a short beep tone sounds when the number of manually acquired targets
reaches 20 or 40 depending on whether auto acquisition is activated or not.
Automatically Acquired Targets
The indication “AUTO TARGET FULL” appears at the screen bottom and
a short beep tone sounds when the number of automatically acquired targets
reaches 20.
System Failure Alarm
WhentheARPboardreceivesnosignalinputfromtheradarorexternal
equipment,thescreenshowsboth“SYSTEMFAIL”associatedwithan
indicationdenotingoffendingequipment,alsoreleasinganauralalarm.The
missing signals are denoted as shown below:
235
TRIAL MANEUVER
Trialsimulatestheeffectonalltrackedtargetsagainstownship’s
maneuver without interrupting the updating of target information.
There are two types of trial maneuvers: STATIC and DYNAMIC.
Dynamic Trial Maneuver
A dynamic trial maneuver displays predicted positions of the tracked
targets and own ship. You enter own ship’s intended speed and course with a
certain “delay time”. Assuming that all tracked targets maintain their present
speeds and courses, the targets’ and own ship’s future movements are
simulated in one second increments indicating their predicted positions in
one minute intervals.
The delay time represents the time lag from the present time to the time
when own ship will actually start to change her speed and/or course. You
should therefore take into consideration own ship’s maneuvering
characteristics such as rudder delay, turning delay and acceleration delay.
This is particularly important on large vessels. How much the delay is set the
situation starts immediately and ends in a minute.
Note that once a dynamic trial maneuver is initiated, you cannot alter own
ship’s trial speed, course or delay time until the trial maneuver is terminated.
Static Trial Maneuver
A static trial maneuver displays only the final situation of the simulation.
If you enter the same trial speed, course and delay time under the same
situation as in the aforementioned example of dynamic trial maneuver, the
screen will instantly show position OS7 for own ship, position A7 for target
A and position B7 for target B, omitting the intermediate positions. Thus, the
static trial maneuver will be convenient when you wish to know the
maneuver result immediately.
Note: For accurate simulation of ship movements in a trial maneuver, own
ship’s characteristics such as acceleration and turning performance should be
properly set in initial settings at the time of installation.
To perform a trial maneuver:
1. Press the E, AUTO PLOT MENU key on the plotting keypad followed by
the (0) key to show the ARPA 2 menu.
2. Press the (2) key to select 2 TRIAL MANEUVER.
3. Further press the (2) key to select (or highlight) STATIC or DYNAMIC
trial maneuver option as appropriate.
4. Press the ENTER key to conclude your selection followed by the E,
AUTO PLOT MENU key to close the ARPA 2 menu.
5. Press the VECTOR TRUE/REL key to select true or relative vector.
6. Press the TRIAL key. The TRIAL DATA SETTING menu appears at the
screen bottom associated with the current own ship’s speed and course
readouts.
Note: The second line reads (STATIC MODE) in the event of a static trial
maneuver.
7. Enter own ship’s intended speed, course and delay time in the following
manner:
Speed: Set with the VRM control.
Course: Set with the EBL control.
Delay time: Enter in minutes by hitting numeral keys. This is the time
after which own ship takes a new situation, not the time the
simulation begins. Change the delay time according to own
ship loading condition, etc.
8. Press the TRIAL key again to start a trial maneuver.
Trial maneuver takes place in three minutes with the letter “T” displayed
at the bottom of the screen. If any tracked target is predicted to be on a
collision course with own ship (that is, the target ship comes within preset
CPA/TCPA limits), the target plot symbol changes to a triangle and flashes.
If this happens, change own ship’s trial speed, course or delay time to obtain
a safe maneuver. The trial maneuver is automatically terminated and the
normal radar picture is restored three minutes later.
Terminating Trial Maneuver
Press the TRIAL key again at any time.
236
CRITERIA FOR SELECTING TARGETS FOR TRACKING
TheFURUNOARPAvideoprocessordetectstargetsinmidstofnoise
anddiscriminatesradarechoesonthebasisoftheirsize.Targetwhoseecho
measurementsaregreaterthanthoseofthelargestshipinrangeortangential
extentareusuallylandandaredisplayedonlyasnormalradarvideo.All
smallershipsizedechoeswhicharelessthanthisdimensionarefurther
analyzedandregardedasshipsanddisplayedassmallcirclessuperimposed
over the video echo.
When a target is first displayed, it is shown as having zero true speed but
develops a course vector as more information is collected. In accordance
with the International Maritime Organization Automatic Radar Plotting Aid
requirements, an indication of the motion trend should be available in 1
minute and full vector accuracy in 3 minutes of plotting. The FURUNO
ARPAs comply with these requirements.
Acquisition and Tracking
Atargetwhichishitby5consecutiveradarpulsesisdetectedasaradar
echo.Manualacquisitionisdonebydesigningadetectedechowiththe
trackball.Automaticacquisitionisdoneintheacquisitionareaswhena
targetisdetected5-7timescontinuouslydependinguponthecongestion.
Trackingisachievedwhenthetargetisclearlydistinguishableonthedisplay
for5outof10consecutivescanswhetheracquiredautomaticallyor
manually. Targets not detected in 5 consecutive scans become “lost targets”.
Quantization
Theentirepictureisconvertedtoadigitalfromcalled“QuantizedVideo”.
Asweeprangeisdividedintosmallsegmentsandeachrangeelementsis“1”
ifthereisradarechoreturnaboveathresholdlevel,or“0”ifthereisno
return.
The digital radar signal is then analyzed by a ship sized echo
discriminator. As the antenna scans, if there are 5 consecutive radar pulses
with l’s indicating an echo presence at the exact same range, a target “start”
is initiated. Since receiver noise is random, it is not three bang correlated,
and it is filtered out and not classified as an echo.
237
RADAR OBSERVATION
GENERAL
Minimum Range
The minimum range is defined by the shortest distance at which, using a
scale of 1.5 or 0.75 nm, a target having an echoing area of 10 square meters
is still shown separate from the point representing the antenna position.
It is mainly dependent on the pulse length, antenna height, and signal
processingsuchasmainbangsuppressionanddigitalquantization.Itisgood
practice to use a shorter range scale as far as it gives favorable definition or
clarityofpicture.TheIMOResolutionA.477(XII)andIEC936requirethe
minimum range to be less than 50m. All FURUNO radars satisfy this
requirement.
Maximum Range
The maximum detecting range of the radar, Rmax, varies considerably
depending on several factors such as the height of the antenna above the
waterline,theheightofthetargetabovethesea,thesize,shapeandmaterial
of the target, and the atmospheric conditions.
Undernormalatmosphericconditions,themaximumrangeisequaltothe
radar horizon or a little shorter. The radar horizon is longer than the optical
one about 6% because of the diffraction property of the radar signal. It
should be noted that the detection range is reduced by precipitation (which
absorbs the radar signal).
X-Band and S-Band
In fair weather, the above equation does not give a significant difference
betweenXandSbandradars.However,inheavyprecipitationcondition,an
S band radar would have better detection than X band.
Radar Resolution
Therearetwoimportantfactorsinradarresolution:bearingresolutionand
range resolution.
Bearing Resolution
Bearingresolutionistheabilityoftheradartodisplayasseparatepipsthe
echoesreceivedfromtwotargetswhichareatthesamerangeandclose
together.Itisproportionaltotheantennalengthandreciprocally
proportionaltothewavelength.Thelengthoftheantennaradiatorshouldbe
chosenforabearingresolutionbetterthan2.5°(IMOResolution).This
conditionisnormallysatisfiedwitharadiatorof1.2meters(4feet)orlonger
intheXband.TheSbandradarrequiresaradiatorofabout12feet(3.6
meters) or longer.
Range Resolution
Range resolution is the ability to display as separate pips the echoes
received from two targets which are on the same bearing and close to each
other. This is determined by pulselength only. Practically, a 0.08
microsecond pulse offers the discrimination better than 25 meters as do so
with all Furuno radars.
Test targets for determining the range and bearing resolution are radar
reflectors having an echo area of 10 square meters.
Bearing Accuracy
One of the most important features of the radar is how accurately the
bearing of a target can be measured. The accuracy of bearing measurement
basically depends on the narrowness of the radar beam. However, the
bearing is usually taken relative to the ship’s heading, and thus, proper
adjustment of the heading marker at installation is an important factor in
ensuring bearing accuracy. To minimize error when measuring the bearing of
a target, put the target echo at the extreme position on the screen by selecting
a suitable range.
Range Measurement
Measurement of the range to a target is also a very important function of
theradar.Generally,therearetwomeansofmeasuringrange:thefixedrange
ringsandthevariablerangemarker(VRM).Thefixedrangeringsappearon
thescreenwithapredeterminedintervalandprovidearoughestimateofthe
rangetoatarget.Thevariablerangemarker’sdiameterisincreasedor
decreasedsothatthemarkertouchestheinneredgeofthetarget,allowing
the operator to obtain more accurate range measurements.
238
FALSE ECHOES
Occasionallyechosignalsappearonthescreenatpositionswherethereis
notargetordisappeareveniftherearetargets.Theyare,however,
recognizedifyouunderstandthereasonwhytheyaredisplayed.Typical
false echoes are shown below.
Multiple Echoes
Multipleechoesoccurwhenatransmittedpulsereturnsfromasolid
objectlikealargeship,bridge,orbreakwater.Asecond,athirdormore
echoesmaybeobservedonthedisplayatdouble,tripleorothermultiplesof
theactualrangeofthetarget.Multiplereflectionechoescanbereducedand
oftenremovedbydecreasingthegain(sensitivity)orproperlyadjustingthe
A/C SEA control.
Sidelobe Echoes
Everytimetheradarpulseistransmitted,someradiationescapesoneach
sideofthebeam,called“sidelobes”.Ifatargetexistswhereitcanbe
detectedbythesidelobeaswellasthemainlobe,thesideechoesmaybe
representedonbothsidesofthetrueechoatthesamerange.Sidelobesshow
usuallyonlyonshortrangesandfromstrongtargets.Theycanbereduced
throughcarefulreductionofthegainorproperadjustmentoftheA/CSEA
control.
Virtual Image
Arelativelylargetargetclosetoyourshipmayberepresentedattwo
positionsonthescreen.Oneofthemisthetrueechodirectlyreflectedbythe
targetandtheotherisafalseechowhichiscausedbythemirroreffectofa
largeobjectonorclosetoyourship.Ifyourshipcomesclosetoalarge
metalbridge,forexample,suchafalseechomaytemporarilybeseenonthe
screen.
Shadow Sectors
Funnels,stacks,masts,orderricksinthepathoftheantennablockthe
radarbeam.Iftheanglesubtendedatthescannerismorethanafewdegrees,
anon-detectingsectormaybeproduced.Withinthissectortargetscannotbe
detected.
SEARCH AND RESCUE TRANSPONDER (SART)
ASearchandRescueTransponder(SART)maybetriggeredbyanyX-
Band(3cm)radarwithinarangeofapproximately8nauticalmiles.Each
radarpulsereceivedcausesittotransmitaresponsewhichisswept
repetitivelyacrossthecompleteradarfrequencyband.Wheninterrogated,it
firstsweepsrapidly(0.4microseconds)throughthebandbeforebeginninga
relativelyslowsweep(7.5microseconds)throughthebackbandtothe
startingfrequency.Thisprocessisrepeatedforatotaloftwelvecomplete
cycles.Atsomepointineachsweep,theSARTfrequencywillmatchthatof
theinterrogatingradarandbewithinthepassbandoftheradarreceiver.If
theSTRTiswithinrange,thefrequencymatchduringeachofthe12slow
239
POST-IT NOTE METHOD OF RADAR CONTACT THREAT AND ASPECT ASSESSMENT
Contributed by Mr. Eric K. Larsson
Rapidradarplottinghasbeenusefulfortheoceanmariner,buthasalways
beenviewedasaburdenbythecoastalorinlandmariner.Somecommon
complaints are listed below:
•I don’t have a reflection plotter!
•I don’t stay on course long enough to plot a target!
•Idon’thavetimetoplot-I’mtheonlyoneinthewheelhouseandI
have to steer!
Manyofthesestatementsarevalid,butifonedoesnotuseradarplotting
orsomeotherformofsystematicobservation,asrequiredbytheRulesofthe
Road,thatpersonismissingoutonvitalinformationandtheyareputting
themselvesandtheirvesselinanunfavorableposition.WhentheU.S.Coast
GuardN-VIConradartrainingfortugboatcaptains,matesandpilotswas
issued,itwasfeltthatsomesortofuseful,practicaltrainingshouldbeadded
totheplottingrequirementsthathavealwaysbeenpartofradarcourses.
BecausemostoftheindividualsaffectedbytheN-VICwereontugsor
towboats,thatpracticalmethodofplottingorobservationhadbeengearedto
the equipment found on board those vessels.
Radarontugshavesmallscreensandareusuallyarasterscanheadup
unstabilizedtypedisplay.thereisnoreflectionplotter.Becauseoflimited
spaceandtimeconstraints,transferplottingisnotpractical.Experience
showsthatwithoutuse,plottingskillsdeteriorate.Tokeeptheseskillssharp,
post-itnotesandtheuseofechotrailsortheplotfeatureoncertainradar
unitscanbeusedtosubstituteforplottingwithpencilsandrulers.Other
variationshavebeenutilizedinthepastsuchastonguedepressororaplastic
overlaybutthepost-itnotemethodseemstobequickerandeasiertouse.It
also deals with the four complaints stated above.
“Idon’thaveareflectionplotter.”Inexchangeforareflectionplotter,the
plotfeatureoncertainsmallscreenradarsallowstheoperatortoviewthe
relativetrackofthetargetatselectedintervalsof15,30or60secondsor
moreAcontinuoustrackofthetargetwithatimerthatcountsupinseconds
canalsobeselected.Infigure5.23,acontinuousechotrailhasbeenselected
andallowedtorunfor3minutes.Thisistheequivalentofathreeminute
“Idon’tstayoncourselongenoughtoplotatargetsthisstatementthe
questionisasked,“Doyoustayoncoursefor3minutes?”Theansweris
usually“Yes.”Theplotfeatureallowstheoperatortonotethetimethetarget
begantrackingandchooseatimeintervalthatisappropriateforthevessel,
the range scale used on the radar and the speed of the vessel.
Infigure5.24,ourvesselismovingataspeedof8knots.Atimeinterval
of3minutesisselected.Usingthe6minuterule,avesselmoving8milesin
60minuteswillmove0.8milesin6minutes(1/10thetimeand1/10the
distance).Inordertofindthedistancetraveledin3minutes,thedistancefor
6minutesiscutinhalfandavesselmoving0.8milesin6minuteswillmove
0.4 miles in 3 minutes (1/2 the time and 1/2 the distance).
Figure 5.23
240
Theradarrangescaleinuseis3miles.Adistanceof0.4milesis
measuredontheradarusingtheVariableRangeMarker(VRM).Placethe
post-itnoteparalleltotheheadingflasherandtheupperleftorrightcorner
touchingthe0.4nmVRM.Markthepost-itnoteatthecornerandatthestart
pointoftheheadingflasher.Thismeasureddistanceonthepot-itnoteisthe
equivalentofa3-minutesegmentofourvessel’smovement.Itisthe
equivalent of the “er” vector in rapid radar plotting.
Repeattheprocessfortheothercorner/sideofthepost-itnote.Once
made,thepost-itnotewillworkforthatrangescaleandspeed,andcanbe
stucktothesideoftheradarreadyforuseatanytime.Otherscalescanbe
modefordifferentspeedsorrangesasneeded.Thisprocessonlytakesafew
seconds and can be done “on the spot.”
“Idon’thavetimetoplot-I’mtheonlyoneinthewheelhouseandIhave
tosteer!”Theechotrailallowsthesingleofficerinthewheelhouseto
“systematicallyobserve”themovementofvessels.Theechotrailsalone,
howeverwillnotgivetheofficermuchmoreinformationthanwhichtargets
arecollisionthreats.Thepost-itnotewillallowtheofficertoobtainmore
information.Thisincludestheaspectofthetargetaswellastheabilityto
obtain the approximate course and speed of the target.
Assumeinthisexample(figure5.25)thatourcourseis270degreesata
speedof8knots.Toobtainthecourseandspeedofthetargetplacethe
cornerwiththefirstmarkonthepost-itnoteatthebeginningofthetarget
trailorplotechoparalleltotheheadingflasher.Observethedirectionofa
linethatwouldconnectthesecondmarkonthepost-itnotewiththetarget.
Thislineindicatesthecourseofthetarget(indicatedbyaredline).The
speedofthetargetoverthe3-minutetimeperiodcanbecomparedwiththe
distancewewouldtravelover3minutesasindicatedbythetwomarkson
the post-it note.
Ifyoudrewalinedrawnfromthesecondmarktothetargetattheendofa
3minuteintervalyoucandeterminethetargetscourserelativetoour
headingof270degrees.ThedashedEBLlineshownaboveisparalleltothe
linedrawnfromthepost-itnotetothetargetpositionatminute3.00.Ithasto
bereadinthedirectionfromthepost-itnotetothetarget(hencethesolid
lineinthedirectionof260).Withourheadingof270degreestherelative
bearingwillread260degrees.Ifyouadd260and270(530)andthen
subtract 360 the target’s true course is found to be 170 degrees.
This is shown on the compass rose in figure 5.26.
Thelengthofthelineisalittleshorterthanthedistancebetweenmarkson
thepost-itnote.Thislengthcouldbemeasuredatabout0.35nminthree
Figure 5.24
Figure 5.25
241
minuteswhichtranslatestoabout7knots.Thislineistheequivalentofthe
target course and speed vector “em” in rapid radar plotting.
Asecondexampleisshowninfigure5.27foratargetonareciprocal
course at a speed approximately equal to our own.
Becauseofthevalidstatementslistedaboveabouttheabilitytoreflection
plot,andrulesoftheroadrequirementtoplot,apracticalmethodofplotting
needstobeused.Itishopedthepot-itmethodwillassistthemarinerinhis
efforts to “systematically observe” all targets.
Figure 5.26
Figure 5.27
243
CHAPTER 6 - MANEUVERING BOARD MANUAL
PART ONE
OWN SHIP AT CENTER
244
EXAMPLE 1
CLOSEST POINT OF APPROACH
Situation:
Other shipM is observed as follows:
Required:
(1) Direction of Relative Movement (DRM).
(2) Speed of Relative Movement (SRM).
(3) Bearing and range at Closest Point of Approach (CPA).
(4) Estimated time of Arrival at CPA.
Solution:
(1)PlotandlabeltherelativepositionsM
1
,M
2
,etc.Thedirectionoftheline
M
1
M
4
through them is the direction of relative movement (DRM): 130˚.
(2)Measuretherelativedistance(MRM)betweenanytwopointsonM
1
M
4
.
M
1
toM
4
=4,035yards.Usingthecorrespondingtimeinterval(0920-0908=
12
m
),obtainthespeedofrelativemovement(SRM)fromtheTime,Distance,
and Speed (TDS) scales: 10 knots.
(3)ExtendM
1
M
4
.Providedneithershipalterscourseorspeed,thesuccessive
positionsofMwillplotalongtherelativemovementline.Dropaperpendicular
fromRtotherelativemovementlineatM
5
.ThisistheCPA:220˚,6,900yards.
(4)MeasureM
1
M
5
:9,800yards.WiththisMRMandSRMobtaintimeinter-
val to CPA from TDS scale: 29 minutes. ETA at CPA= 0908 + 29 = 0937.
Answer:
(1) DRM 130˚.
(2) SRM 10 knots.
(3) CPA 220˚, 6,900 yards.
(4) ETA at CPA 0937.
TimeBearingRange (yards)Rel. position
0908........................275˚12,000M
1
0913........................270˚10,700M
2
0916........................266˚.510,000M
3
0920........................260˚9,000M
4
245
OWN SHIP AT CENTER
EXAMPLE 1
Scale: Distance 2:1 yd.
246
EXAMPLE 2
COURSE AND SPEED OF OTHER SHIP
Situation:
OwnshipRisoncourse150˚,speed18knots.ShipMisobservedasfollows:
Required:
(1) Course and speed ofM.
Solution:
(1)PlotM
1
,M
2
,M
3
,andR.Drawthedirectionofrelativemovementline
(RML)fromM
1
throughM
3
.WiththedistanceM
1
M
3
andtheintervaloftime
betweenM
1
andM
3
,findtherelativespeed(SRM)byusingtheTDSscale:21
knots.Drawthereferenceshipvectorercorrespondingtothecourseandspeed
ofR.ThroughrdrawvectorrmparalleltoandinthedirectionofM
1
M
3
witha
lengthequivalenttotheSRMof21knots.Thethirdsideofthetriangle,em,is
the velocity vector of the shipM: 099˚, 27 knots.
Answer:
(1) Course 099˚, speed 27 knots.
TimeBearingRange (yards)Rel. position
1100........................255˚20,000M
1
1107........................260˚15,700M
2
1114........................270˚11,200M
3
247
OWN SHIP AT CENTER
EXAMPLE 2
Scale: Speed 3:1;
Distance 2:1 yd.
248
EXAMPLE 3
COURSE AND SPEED OF OTHER SHIP USING RELATIVE PLOT AS RELATIVE VECTOR
Situation:
OwnshipRisoncourse340˚,speed15knots.Theradarissetonthe12-mile
range scale. ShipM is observed as follows:
Required:
(1) Course and speed ofM.
Solution:
(1)PlotMandM
2
.Drawtherelativemovementline(RML)fromM
1
through
M
2
.
(2)FortheintervaloftimebetweenM
1
andM
2
,findthedistanceownshipR
travelsthroughthewater.Sincethetimeintervalis6minutes,thedistancein
nautical miles is one-tenth of the speed ofR in knots, or 1.5 nautical miles.
(3)UsingM
1
M
2
directlyastherelativevectorrm,constructthereferenceship
truevectorertothesamescaleasrm(M
1
-M
2
),or1.5nauticalmilesinlength.
(4)Completethevectordiagram(speedtriangle)toobtainthetruevectorem
ofshipM.Thelengthofemrepresentsthedistance(2.5nauticalmiles)traveled
by shipM in 6 minutes, indicating a true speed of 25 knots.
Note:
Insomecasesitmaybenecessarytoconstructownship’struevectororigi-
natingattheendofthesegmentoftherelativeplotuseddirectlyastherelative
vector.Thesameresultsareobtained,buttheadvantagesoftheconventional
vector notation are lost.
Answer:
(1) Course 252˚, speed 25 knots.
Note:
Althoughatleastthreerelativepositionsareneededtodeterminewhetherthe
relativeplotformsastraightline,forsolutionandgraphicalclarityonlytworel-
ative positions are given in examples 3, 6, and 7.
TimeBearingRange (mi.)Rel. position
1000........................030˚9.0M
1
1006........................025˚6.3M
2
249
OWN SHIP AT CENTER
EXAMPLE 3
Scale: 12-mile range setting
250
EXAMPLE 4
CHANGING STATION WITH TIME, COURSE, OR SPEED SPECIFIED
Situation:
Formationcourseis010˚,speed18knots.At0946whenordersarereceived
tochangestation,theguideMbears140˚,range7,000yards.Whenonnewsta-
tion, the guide will bear 240˚, range 6,000 yards.
Required:
(1) Course and speed to arrive on station at 1000.
(2)Speedandtimetostationoncourse045˚.Uponarrivalonstationorders
are received to close to 3,700 yards.
(3) Course and minimum speed to new station.
(4) Time to station at minimum speed.
Solution:
(1)PlotM
1
140˚,7,000yardsandM
2
240˚,6,000yardsfromR.Drawemcor-
respondingtocourse010˚andspeed18knots.Thedistanceof5.0milesfrom
M
1
toM
2
mustbecoveredin14minutes.TheSRMistherefore21.4knots.Draw
r
1
mparalleltoM
1
M
2
and21.4knotsinlength.Thevectorer
1
denotesthere-
quired course and speed: 062˚, 27 knots.
(2)Drawer
2
,course045˚,intersectingr
1
mtherelativespeedvectoratthe21-
knotcircle.Byinspectionr
2
mis12.1knots.ThusthedistanceM
1
M
2
of5.0miles
will be covered in 24.6 minutes.
(3)TomdrawalineparalleltoandinthedirectionofM
2
M
3
.Dropaperpen-
dicularfrometothislineatr
3
.Vectorer
3
isthecourseandminimumspeedre-
quired to complete the final change of station: 330˚, 13.8 knots.
(4)Bymeasurement,thelengthofr
3
misanSRMof11.5knotsandtheMRM
fromM
2
toM
3
is2,300yards.TherequiredmaneuvertimeMRM/r
3
m=6min-
utes.
Answer:
(1) Course 062˚, speed 27 knots.
(2) Speed 21 knots, time 25 minutes.
(3) Course 330˚, speed 13.8 knots.
(4) Time 6 minutes.
Explanation:
Insolutionstep(1)themagnitude(SRM)oftherequiredrelativespeedvector
(r
1
m)isestablishedbytherelativedistance(M
1
M
2
)andthetimespecifiedto
completethemaneuver(14
m
).Insolutionstep(2),however,themagnitude
(12.1knots)oftheresultingrelativespeedvector(r
2
m)isdeterminedbythedis-
tancefromtheheadofvectoremalongthereciprocaloftheDRMtothepoint
wheretherequiredcourse(045˚)isintersected.Suchintersectionalsoestablish-
esthemagnitude(21knots)ofvectorer
2
.Thetime(25
m
)tocompletethema-
neuverisestablishedbytheSRM(12.1knots)andtherelativedistance(5
miles).
Insolutionstep(3)thecourse,andminimumspeedtomaketheguideplot
alongM
2
M
3
areestablishedbytheshortesttruevectorforownship’smotion
thatcanbeconstructedtocompletethespeedtriangle.Thisvectorisperpendic-
ular to the relative vector(r
3
m).
Insolutionstep(4)thetimetocompletethemaneuverisestablishedbythe
relative distance (2,300 yards) and the relative speed (11.5 knots).
251
OWN SHIP AT CENTER
EXAMPLE 4
Scale: Speed 3:1;
Distance 1:1 yd.
252
EXAMPLE 5
THREE-SHIP MANEUVERS
Situation:
OwnshipRisinformationproceedingoncourse000˚,speed20knots.The
guideMbears090˚,distance4,000yards.ShipNis4,000yardsaheadofthe
guide.
Required:
RandNaretotakenewstationsstartingatthesametime.Nistotakestation
4,000yardsontheguide’sstarboardbeam,usingformationspeed.Ristotake
N’s old station and elects to use 30 knots.
(1)N’s course and time to station.
(2)R’s course and time to station.
(3) CPA ofN andR to guide.
(4) CPA ofR toN.
(5) Maximum range ofR fromN.
Solution:
(1)PlotR,M
1
,M
2
,andN
1
.Drawem.FromM
1
plotN’snewstationNM,bear-
ing090˚,distance4,000yards.FromM
2
plotN
3
bearing090˚,distance4,000
yards(N’sfinalrangeandbearingfromM).DrawN
1
NM,theDRMofNrelative
toM.Fromm,drawmnparalleltoandinthedirectionofN
1
NMintersectingthe
20-knotspeedcircleatn.N’scoursetostationisvectoren:090˚.Timetostation
N
1
NM/mn is 6 minutes.
(2)Tom,drawalineparalleltoandinthedirectionofM
1
M
2
intersectingthe
30-knotspeedcircleatr.R’scoursetostationisvectorer:017˚.Timetostation
M
1
M
2
/rm is 14 minutes.
(3)FromM
1
dropaperpendiculartoN
1
NM.AtCPA,Nbears045˚,2,850
yardsfromM.FromRdropaperpendiculartoM
1
M
2
.AtCPA,Rbears315˚,
2,850 yards fromM.
(4)Fromrdrawrn.ThisvectoristhedirectionandspeedofNrelativetoR.
FromN
1
drawaDRMlineofindefinitelengthparalleltoandinthedirectionof
rn.FromRdropaperpendiculartothisline.AtCPA,Nbears069˚,5,200yards
fromR.
(5)TheintersectionoftheDRMlinefromN
1
andthelineNMN
3
isN
2
,the
pointatwhichNresumesformationcourseandspeed.MaximumrangeofN
fromR is the distanceRN
2
, 6,500 yards.
Answer:
(1)N’s course 090˚, time 6 minutes.
(2)R’s course 017˚, time 14 minutes.
(3) CPA ofN toM 2,850 yards at 045˚.R toM 2,850 yards at 315˚.
(4) CPA ofN toR 5,200 yards at 069˚.
(5) Range 6,500 yards.
Solution Key:
(1)SolutionsforchangingstationbyownshipRandshipNareeffectedsep-
aratelyinaccordancewiththesituationandrequirements.TheCPAsofNand
R to guide are then obtained.
(2)TwosolutionsforthemotionofshipNrelativetoownshipRarethenob-
tained:relativemotionwhileNisproceedingtonewstationandrelativemotion
afterN has taken new station and resumed base course and speed.
Explanation:
Insolutionstep(4)themovementofNinrelationtoRisparalleltothedirec-
tionofvectorrnandfromN
1
untilsuchtimethatNreturnstobasecourseand
speed.Afterwards,themovementofNinrelationtoRisparalleltovectorrm
andfromN
2
towardthatpoint,N
3
,thatNwilloccupyrelativetoRwhenthema-
neuver is completed.
253
OWN SHIP AT CENTER
EXAMPLE 5
Scale: Speed 3:1;
Distance 1:1 yd.
254
EXAMPLE 6
COURSE AND SPEED TO PASS ANOTHER SHIP AT A SPECIFIED DISTANCE
Situation 1:
OwnshipRisoncourse190˚,speed12knots.OthershipMisobservedas
follows:
Required:
(1) CPA.
(2) Course and speed ofM.
Situation 2:
It is desired to pass ahead ofM with a CPA of 3,000 yards.
Required:
(3) Course ofR at 12 knots if course is changed when range is 13,000 yards.
(4) Bearing and time of CPA.
Solution:
(1)PlotM
1
andM
2
at153˚,20,000yardsand153˚,16,700yards,respectively,
fromR.Drawtherelativemovementline,M
1
M
2
,extended.Sincethebearingis
steady and the line passes throughR, the two ships are on collision courses.
(2)Drawownship’svelocityvectorer
1
190˚,12knots.MeasureM
1
M
2
,the
relativedistancetraveledbyMfrom1730to1736:3,300yards.FromtheTDS
scaledeterminetherelativespeed,SRM,using6minutesand3,300yards:16.5
knots.Drawtherelativespeedvectorr
1
mparalleltoM
1
M
2
and16.5knotsin
length. The velocity vector ofM isem: 287˚, 10 knots.
(3)PlotM
3
bearing153˚,13,000yardsfromR.WithRasthecenterdescribe
acircleof3,000yardsradius,thedesireddistanceatCPA.FromM
3
drawaline
tangenttothecircleatM
4
.ThisplacestherelativemovementlineofM(M
3
M
4
)
therequiredminimumdistanceof3,000yardsfromR.Throughm,drawr
2
m
paralleltoandinthedirectionofM
3
M
4
intersectingthe12-knotcircle(speedof
R) atr
2
. Own ship velocity vector iser
2
: course 212˚, speed 12 knots.
(4)Measuretherelativedistance(MRM),M
2
M
3
:3,700yards.FromtheTDS
scaledeterminethetimeintervalbetween1736andthetimetochangetonew
courseusingM
2
M
3
,3,700yards,andanSRMof16.5knots:6.7minutes.Mea-
suretherelativedistanceM
3
M
4
:12,600yards.Measuretherelativespeedvector
r
2
m:13.4knots.UsingthisMRMandSRM,theelapsedtimetoCPAafter
changingcourseisobtainedfromtheTDSscale:28minutes.ThetimeofCPA
is 1736 + 6.7 + 28 = 1811.
Note:
IfM’sspeedwasgreaterthanR’s,twocourseswouldbeavailableat12knots
to produce the desired distance.
Answer:
(1)M andR are on collision courses and speeds.
(2) Course 287˚, speed 10 knots.
(3) Course 212˚.
(4) Bearing 076˚, time of CPA 1811.
TimeBearingRange (yards)Rel. position
1730...................153˚20,000M
1
1736...................153˚16,700M
2
255
OWN SHIP AT CENTER
EXAMPLE 6
Scale: Speed 2:1;
Distance 2:1 yd.
256
EXAMPLE 7
COURSE AND SPEED TO PASS ANOTHER SHIP AT A SPECIFIED
DISTANCE USING RELATIVE PLOT AS RELATIVE VECTOR
Situation 1:
OwnshipRisoncourse190˚,speed12knots.OthershipMisobservedas
follows:
Required:
(1) CPA.
(2) Course and speed ofM.
Situation 2:
It is desired to pass ahead ofM with a CPA of 1.5 nautical miles.
Required:
(3)CourseofRat12knotsifcourseischangedwhenrangeis6.5nautical
miles.
(4) Bearing and time of CPA.
Solution:
(1)PlotM
1
andM
2
at153˚,10.0nauticalmilesand153˚,8.3nauticalmiles,
respectivelyfromR.Drawtherelativemovementline,M
1
M
2
,extended.Since
thebearingissteadyandthelinepassesthroughR,thetwoshipsareoncollision
courses.
(2)FortheintervaloftimebetweenM
1
andM
2
,findthedistanceownshipR
travelsthroughthewater.Sincethetimeintervalis6minutes,thedistancein
nautical miles is one-tenth of the speed ofR in knots, or 1.2 nautical miles.
(3)UsingM
1
M
2
directlyastherelativevectorr
1
m,constructthereference
shiptruevectorer
1
tothesamescaleasr
1
m(M
1
M
2
),or1.2nauticalmilesin
length.
(4)Completethevectordiagram(speedtriangle)toobtainthetruevectorem
ofshipM.Thelengthofemrepresentsthedistance(1.0nauticalmiles)traveled
by shipM in 6 minutes, indicating a true speed of 10 knots.
(5)PlotM
3
bearing153˚,6.5nauticalmilesfromR.WithRasthecenterde-
scribeacircleof1.5nauticalmilesradius,thedesireddistanceatCPA.From
M
3
drawalinetangenttothecircleatM
4
.Thisplacestherelativemovementline
ofM (M
3
M
4
) the required minimum distance of 1.5 nautical miles fromR.
(6)ConstructthetruevectorofshipMasvectore'm',terminatingatM
3
.From
e'describeacircleof1.2milesradiuscorrespondingtothespeedofRof12
knotsintersectingthenewrelativemovementline(M
3
M
4
)extendedatpointr
2
.
OwnshipRtruevectorrequiredtopassshipMatthespecifieddistanceisvector
e'r
2
: course 212˚, speed 12 knots.
(7)Forpracticalsolutions,thetimeatCPAmaybedeterminedbyinspection
orthroughsteppingofftherelativevectorsbydividersorspacingdividers.Thus
the time of CPA is 1736 + 6.5 + 28 = 1811.
Note:
IfthespeedofshipMisgreaterthanownshipR,therearetwocoursesavail-
able at 12 knots to produce the desired distance.
Answer:
(1)M andR are on collision courses and speeds.
(2) Course 287˚, speed 10 knots.
(3) Course 212˚.
(4) Bearing 076˚, time of CPA 1811.
TimeBearingRange (mi.)Rel. position
1730...................153˚10.0M
1
1736...................153˚8.3M
2
257
OWN SHIP AT CENTER
EXAMPLE 7
Scale: 12-mile range setting
258
EXAMPLE 8
COURSE AT SPECIFIED SPEED TO PASS ANOTHER SHIP AT MAXIMUM
AND MINIMUM DISTANCES
Situation:
ShipMoncourse300˚,speed30knots,bears155˚,range16milesfromown
shipR whose maximum speed is 15 knots.
Required:
(1)R’scourseat15knotstopassMat(a)maximumdistance(b)minimum
distance.
(2) CPA for each course found in (1).
(3) Time interval to each CPA.
(4) Relative bearing ofM fromR when at CPA on each course.
Solution:
(1)PlotM
1
155˚,16milesfromR.Drawthevectorem300˚,30knots.With
easthecenter,describeacirclewithradiusof15knots,thespeedofR.From
mdrawthetangentsr
1
mandr
2
mwhichproducethetwolimitingcoursesfor
R.ParalleltothetangentsplottherelativemovementlinesthroughM
1
.Course
ofownshiptopassatmaximumdistanceiser
1
:000˚.Coursetopassatmini-
mum distance iser
2
: 240˚.
(2)ThroughRdrawRM
2
andRM'
2
perpendiculartothetwopossiblerelative
movementlines.PointM
2
bearing180˚,14.5milesistheCPAforcourseof
000˚. PointM'
2
bearing 240˚, 1.4 miles is the CPA for course 240˚.
(3)MeasureM
1
M
2
:6.8miles,andM
1
M'
2
:15.9miles.Mmusttraveltheserel-
ativedistancesbeforereachingtheCPAoneachlimitingcourse.Therelative
speedofMisindicatedbythelengthofthevectorsr
1
mandr
2
m:26knots.From
theTDSscalethetimesrequiredtoreachM
2
andM'
2
arefound:15.6minutes
and 36.6 minutes, respectively.
(4)Bearingsaredeterminedbyinspection.M
2
bears180˚relativebecause
ownship’scourseisalongvectorer
1
formaximumCPA.M'
2
bears000˚relative
when own ship’s course iser
2
for minimum passing distance.
Note:
ThissituationoccursonlywhenownshipRis(1)aheadoftheothershipand
(2)hasamaximumspeedlessthanthespeedoftheothership.Underthesecon-
ditions,ownshipcanintercept(collisioncourse)onlyifRliesbetweenthe
slopesofM
1
M
2
andM
1
M'
2
.Notethatforlimitingcourses,andonlyforthese,
CPAoccurswhenothershipisdeadaheadordeadastern.Thesolutiontothis
problemisapplicabletoavoidingatropicalstormbytakingthatcoursewhich
results in maximum passing distance.
Answer:
(1) Course (a) 000˚; (b) 240˚.
(2) CPA (a) 180˚, 14.5 miles; (b) 240˚, 1.4 miles.
(3) Time (a) 16 minutes; (b) 37 minutes.
(4) Relative bearing (a) 180˚; (b) 000˚.
259
OWN SHIP AT CENTER
EXAMPLE 8
Scale: Speed 3:1;
Distance 2:1 mi.
260
EXAMPLE 9
COURSE CHANGE IN COLUMN FORMATION ASSURING LAST SHIP IN
COLUMN CLEARS
Situation:
OwnshipD1istheguideinthevanofadestroyerunitconsistingoffourde-
stroyers(D1,D2,D3,andD4)incolumnastern,distance1,000yards.D1ison
stationbearing090˚,8milesfromtheformationguideM.Formationcourseis
135˚,speed15knots.Theformationguideisatthecenterofaconcentriccircu-
lar ASW screen stationed on the 4-mile circle.
Thedestroyerunitisorderedtotakenewstationbearing235˚,8milesfrom
theformationguide.TheunitcommanderinD1decidestouseawheelingma-
neuverat27knots,passingaheadofthescreenusingtwocoursechangessothat
the CPA of his unit on each leg is 1,000 yards from the screen.
Required:
(1) New course to clear screen commencing at 1000.
(2) Second course to station.
(3) Bearing and range ofM fromD1 at time of coming to second course.
(4) Time of turn to second course.
(5) TimeD1 will reach new station.
Solution:
(1)PlotownshipD1atthecenteroncourse135˚withtheremainingthree
destroyersincolumnasD2,D3,D4.(D2andD3notshownforgraphicalclar-
ity.)Distancebetweenships1,000yards.PlottheformationguideMatM
1
bear-
ing270˚,8milesfromD1.Drawem,thespeedvectorofM.Itisrequiredthat
thelastshipincolumn,D4,clearMby9,000yards(screenradiusof4milesplus
1,000yards).Attheinstantthesignalisexecutedtochangestation,onlyD1
changesbothcourseandspeed.Theotherdestroyersincreasespeedto27knots
butremainonformationcourseof135˚untileachreachestheturningpoint.
D4’smovementof3,000yardsat27knotstotheturningpointrequires3min-
utes,20seconds.Duringthisintervalthereisa12knottruespeeddifferential
betweenD4andtheformationguideM.Thustoestablishtherelativeposition
ofD4toMattheinstantD4turns,advanceD4toD4'(3
m
20
S
x12knots=1,350
yards).WithD4'asacenter,describeaCPAcircleofradius9,000yards.Draw
alinefromM
1
tangenttothiscircle.Thisistherelativemovementlinerequired
forD4toclearthescreenby1,000yards.DrawalinetomparalleltoM
1
M
2
in-
tersectingthe27-knotcircleatr
1
.Thispointdeterminestheinitialcourse,er
1
:
194˚.2.
(2)PlotthefinalrelativepositionofMatM
3
bearing055˚,8milesfromD1.
DrawalinefromM
3
tangenttotheCPAcircleandintersectingthefirstrelative
movementlineatM
2
.DrawalinetomparalleltoandinthedirectionofM
2
M
3
.
Theintersectionofthislineandthe27-knotcircleatr
2
isthesecondcoursere-
quired,er
2
: 252˚.8.
(3)BearingandrangeofM
2
fromD1isobtainedbyinspection:337˚at11,250
yards.
(4)TimeintervalforMtotraveltoM
2
isM
1
M
2
/r
1
m=7.8miles/23.2knots=
20.2 minutes. Time of turn 1000 + 20 = 1020.
(5)TimeintervalforthesecondlegisM
2
M
3
/r
2
m=8.8miles/36.5knots=14.2
minutes.D1 will arrive at new station at 1034.
Answer:
(1) Course 194˚.
(2) Course 253˚.
(3) Bearing 337˚, range 11,250 yards.
(4) Time 1020.
(5) Time 1034.
261
OWN SHIP AT CENTER
EXAMPLE 9
Scale: Speed 3:1;
Distance 1:1 mi.
262
EXAMPLE 10
DETERMINATION OF TRUE WIND
Situation:
Ashipisoncourse240˚,speed18knots.Therelativewindacrossthedeckis
30 knots from 040˚ relative.
Required:
Direction and speed of true wind.
Solution:
Ploter,theship’svectorof240˚,18knots.Converttherelativewindtoap-
parentwindbyplottingrw040˚relativetoship’sheadwhichresultsinatrue
directionof280˚T.Plottheapparentwindvector(reciprocalof280˚T,30knots)
fromtheendofthevectorer.Labeltheendofthevectorw.Theresultantvector
ewisthetruewindvectorof135˚,20knots(wind’scourseandspeed).Thetrue
wind, therefore, isfrom 315˚.
Answer:
True wind from 315˚, speed 20 knots.
Note:
Asexperiencedonamovingship,thedirectionoftruewindisalwaysonthe
samesideandaftofthedirectionoftheapparentwind.Thedifferenceindirec-
tionsincreasesasship’sspeedincreases.Thatis,thefasterashipmoves,the
more the apparent wind draws ahead of true wind.
263
OWN SHIP AT CENTER
EXAMPLE 10
Scale: Speed 3:1
264
EXAMPLE 11a
DESIRED RELATIVE WIND
(First Method)
Situation:
Anaircraftcarrierisproceedingoncourse240˚,speed18knots.Truewind
has been determined to be from 315˚, speed 10 knots.
Required:
Determinealaunchcourseandspeedthatwillproducearelativewindacross
the flight deck of 30 knots from 350˚ relative (10˚ port).
Solution:
Setapairofdividersfor30knotsusinganyconvenientscale.Placeoneend
ofthedividersattheorigineofthemaneuveringboardandtheotheronthe350˚
line,markingthispointa.Setthedividersforthetruewindspeedof10knots
andplaceoneendonpointa,theotheronthe000˚line(centerlineoftheship).
Markthispointonthecenterlineb.Drawadashedlinefromorigineparallelto
ab.Thisproducestheangularrelationshipbetweenthedirectionfromwhichthe
truewindisblowingandthelaunchcourse.Inthisproblemthetruewindshould
befrom32˚offtheportbow(328˚relative)whentheshipisonlaunchcourse
and speed. The required course and speed is thus 315˚ + 32˚ = 347˚, 21 knots.
Answer:
Course 347˚, speed 21 knots.
Note:
Asexperiencedonamovingship,thedirectionoftruewindisalwaysonthe
samesideandaftofthedirectionoftheapparentwind.Thedifferenceindirec-
tionsincreasesasship’sspeedincreases.Thatis,thefasterashipmoves,the
more the apparent wind draws ahead of true wind.
265
OWN SHIP AT CENTER
EXAMPLE 11a
Scale: Speed 3:1
266
EXAMPLE 11b
DESIRED RELATIVE WIND
(Second Method)
Situation:
Ashipisoncourse240˚,speed18knots.Truewindhasbeendeterminedto
be from 315˚, speed 10 knots.
Required:
Determineacourseandspeedthatwillproduceawindacrossthedeckof30
knots from 350˚ relative (10˚ port).
Solution:
(1)Apreliminarystepinthedesiredrelativewindsolutionistoindicateon
thepolarplottingsheetthedirectionfromwhichthetruewindisblowing.The
direction of the true wind is along the radial from 315˚.
(2)Thesolutionistobeeffectedbyfirstfindingthemagnitudeoftherequired
ship’strue(course-speed)vector;knowingthetruewind(direction-speed)vec-
torandthemagnitude(30knots)oftherelativewindvector,andthattheship’s
courseshouldbetotherightofthedirectionfromwhichthetruewindisblow-
ing, the vector triangle can then be constructed.
(3) Construct the true wind vectorew.
(4)Withapencilcompassadjustedtothetruewind(10knots),setthepoint
ofthecompassonthe30-knotcircleatapoint10˚clockwisefromtheintersec-
tionofthe30-knotcirclewiththeradialextendinginthedirectionfromwhich
thewindisblowing.Strikeanarcintersectingthisradial.Thatpartoftheradial
fromthecenteroftheplottingsheettotheintersection
*
representsthemagni-
tudeoftherequiredship’struevector(21knots).Thedirectionofalineextend-
ingfromthisintersectiontothecenterofthearcisthedirectionoftheship’s
true vector.
(5)Fromeatthecenteroftheplottingsheet,strikeanarcofradiusequalto
21knots.Fromwattheheadofthetruewindvector,strikeanarcofradiusequal
to30knots.Labelintersectionr.Thisintersectionistotherightofthedirection
from which the true wind is blowing.
(6)Alternatively,theship’strue(course-speed)vectorcanbeconstructedby
drawingvectorerparalleltothedirectionestablishedin(4)andtothemagni-
tudealsoestablishedin(4).Oncompletingthevectortriangle,thedirectionof
the relative wind is 10˚ off the port bow.
Answer:
Course 346˚, speed 21 knots.
Note:
Ifthepointofthecompasshadbeensetatapointonthe30-knotcircle10˚
counterclockwisefromtheradialextendinginthedirectionfromwhichthetrue
windisblowingin(4),thesamemagnitudeoftheship’struevectorwouldhave
beenobtained.However,thedirectionestablishedforthisvectorwouldhave
been for a 30-knot wind across the deck from 10˚ off the starboard bow.
*Use that intersection closest to the center of the polar diagram.
267
OWN SHIP AT CENTER
EXAMPLE 11b
Scale: Speed 3:1
268
EXAMPLE 11c
DESIRED RELATIVE WIND
(Third Method)
Situation:
Ashipisoncourse240˚speed18knots.Truewindhasbeendeterminedto
be from 315˚ speed 10 knots.
Required:
Determineacourseandspeedthatwillproduceawindacrossthedeckof30
knots from 350˚ relative (10˚ port).
Solution:
(1)Apreliminarystepinthedesiredrelativewindsolutionistoindicateon
thepolarplottingsheetthedirectiontowardwhichthetruewindisblowing.The
direction of the true wind is along the radial from 315˚.
(2)Thesolutionistobeeffectedbyfirstfindingthemagnitudeoftherequired
ship’strue(course-speed)vector;knowingthetruewind(direction-speed)vec-
torandthemagnitude(30knots)oftherelativewindvector,andthattheship’s
courseshouldbetotherightofthedirectionfromwhichthetruewindisblow-
ing, the vector triangle can then be constructed.
(3) Construct the true wind vectorew.
(4)Withapencilcompassadjustedtothetruewind(10knots),setthepoint
ofthecompassonthe30-knotcircleatapoint10˚clockwisefromtheintersec-
tionofthe30-knotcirclewiththeradialextendinginthedirectiontowardwhich
thewindisblowing.Strikeanarcintersectingthisradial.Thatpartoftheradial
fromthecenteroftheplottingsheettotheintersection
*
representsthemagni-
tudeoftherequiredship’struevector(21knots).Thedirectionofalineextend-
ingfromthecenterofthearctotheintersectionwiththeradialisthedirection
of the ship’s true vector.
(5)Fromeatthecenteroftheplottingsheet,strikeanarcofradiusequalto
21knots.Fromwattheheadofthetruewindvector,strikeanarcofradiusequal
to30knots.Labelintersectionr.Thisintersectionistotherightofthedirection
from which the true wind is blowing.
(6)Alternatively,theship’strue(course-speed)vectorcanbeconstructedby
drawingvectorerparalleltothedirectionestablishedin(4)andtothemagni-
tudealsoestablishedin(4).Oncompletingthevectortriangle,thedirectionof
the relative wind is 10˚ off the port bow.
Answer:
Course 346˚, speed 21 knots.
Note:
Ifthepointofthecompasshadbeensetatapointonthe30-knotcircle10˚
counterclockwisefromtheradialextendinginthedirectionfromwhichthetrue
windisblowingin(4),thesamemagnitudeoftheship’struevectorwouldhave
beenobtained.However,thedirectionestablishedforthisvectorwouldhave
been for a 30-knot wind across the deck from 10˚ off the starboard bow.
*Use that intersection closest to the center of the polar diagram.
269
OWN SHIP AT CENTER
EXAMPLE 11c
Scale: Speed 3:1
270
PRACTICAL ASPECTS OF MANEUVERING BOARD SOLUTIONS
Theforegoingexamplesandtheiraccompanyingillustrationsarebasedupon
thepremisethatshipsarecapableofinstantaneouschangesofcourseandspeed.
Itisalsoassumedthatanunlimitedamountoftimeisavailablefordetermining
the solutions.
Inactualpractice,theintervalbetweenthesignalforamaneuveranditsexe-
cutionfrequentlyallowsinsufficienttimetoreachacompletegraphicalsolu-
tion.Nevertheless,undermanycircumstances,safetyandsmartseamanship
bothrequirepromptanddecisiveaction,eventhoughthisactionisdetermined
fromaquick,mentalestimate.Theestimatemustbebasedupontheprinciples
ofrelativemotionandthereforeshouldbenearlycorrect.Courseandspeedcan
bemodifiedenroutetonewstationwhenamoreaccuratesolutionhasbeenob-
tained from a maneuvering board.
Allowancemustbemadeforthosetacticalcharacteristicswhichvarywidely
betweentypesofshipsandalsoundervaryingconditionsofseaandloading.
Experiencehasshownthatitisimpracticaltosolvefortherelativemotionthat
occursduringaturnandthatacceptablesolutionscanbefoundbyeyeandmen-
tal estimate.
BycarefulappraisalofthePPIandmaneuveringboard,therelativemove-
mentofownshipandtheguideduringaturncanbeapproximatedandanesti-
matemadeoftherelativepositionuponcompletionofaturn.Ship’s
characteristiccurvesandafewsimplethumbrulesapplicabletoownshiptype
serveasabasisfortheseestimates.Duringthefinalturntheshipcanbebrought
ontostationwithsmallcompensatoryadjustmentsinenginerevolutionsand/or
course.
EXAMPLE 12
ADVANCE, TRANSFER, ACCELERATION, AND DECELERATION
Situation:
OwnshipRisadestroyeronstationbearing020˚,8,000yardsfromtheguide
M.Formationcourseis000˚,speed15knots.Risorderedtotakestationbearing
120˚, 8,000 yards from guide, using 25 knots.
Required:
(1) Course to new station.
(2)BearingofMwhenorderisgiventoresumeformationcourseandspeed.
(3) Time to complete the maneuver.
Solution:
(1)PlotRatthecenterwithM
1
bearing200˚,8,000yardsandM
2
bearing
300˚, 8,000 yards. Draw the guide’s speed vectorem 000˚, 15 knots.
Byeye,itappearsRwillhavetomakeaturntotherightofabout150˚,accel-
eratingfrom15to25knotsduringtheturn.Priortoreachingthenewstationa
reverseturnofaboutthesameamountanddecelerationto15knotswillbere-
quired. Assume thatR averages 20 knots during each turn.
Using30˚rudderat20knots,aDDcalibrationcurveindicatesapproximately
2˚turnpersecondanda600yardtacticaldiameter.Thus,a150˚turnwillre-
quireabout75secondsandwillproduceanoff-setofabout600yards.During
theturn,Mwilladvance625yards(1
1
/
4
minutesat15knots).Plottingthisap-
proximateoff-setdistanceonthemaneuveringboardgivesanewrelativeposi-
tionofM
3
atthetimetheinitialturniscompleted.Similarly,anewoff-set
positionatM
4
isdeterminedwhereRshouldorderaleftturntoformationcourse
and reduction of speed to 15 knots.
DrawalinetomparalleltoandinthedirectionofM
3
M
4
andintersectingthe
25-knot speed circle atr. Vectorer is the required course of 158˚.
(2)WhenMreachespointM
4
bearing299˚,turnlefttoformationcourseusing
30˚ rudder and slow to 15 knots.
(3)TimetocompletethemaneuverisM
3
M
4
/SRM+2.5minutes=11,050
yards/39.8 knots + 2.5 minutes = 11 minutes.
Answer:
(1) Course 158˚.
(2) Bearing 299˚.
(3) Time 11 minutes.
271
OWN SHIP AT CENTER
EXAMPLE 12
Scale: Speed 3:1;
Distance 1:1 yd.
272
COLLISION AVOIDANCE
Numerousstudiesandtheinventivegeniusofmanhaveprovidedthemariner
withadequatemeansforvirtuallyeliminatingcollisionsatsea.Oneofthemost
significantoftheseisradar.However,radarismerelyanaid,andisnosubstitute
forgoodjudgmentcoupledwithgoodseamanship.Itsusegrantsnospecialli-
censeinapplyingtheRulesoftheRoadinagivensituation.Properlyinterpret-
ed,however,theinformationitdoesprovidethemarinercanbeofinestimable
value in forewarning him of possible danger.
Thefollowingexampleisapracticalproblemencounteredintheapproaches
to many of the world’s busy ports.
EXAMPLE 13
AVOIDANCE OF MULTIPLE CONTACTS
Situation:
Ownshipisproceedingtowardaharborentranceabout30milestothesouth-
east.Ownship’scourse145˚,speed15knots.Visibilityisestimatedtobe2
miles.Numerousradarcontactsarebeingmade.Atthepresenttime,2235,six
pips are being plotted on the PPI scope.
Problems:
(1)ByvisualinspectionofthePPI(Fig.1),whichofthecontactsappeardan-
gerousandrequireplottingonamaneuveringboard?(Radarisseton20-mile
range scale.)
(2)Afterplottingthecontactsselectedin(1),whataretheirCPA’s,true
courses and speeds? (Fig. 2 is an example.)
(3)AssumethePPIplotsindicateallcontactshavemaintainedasteady
courseandspeedduringyoursolutionin(2).Whatmaneuveringaction,ifany,
do you recommend? (Fig. 2 shows one possibility.)
(4)Assumethatyoumaneuverat2238andallothershipsmaintaintheir
coursesandspeeds.WhatarethenewCPA’softhedangerouscontactsin(2)
above? (Fig. 2 shows a possible solution.)
(5)Assumethatallshipsmaintaincourseandspeedfrom2238until2300.
What will be the PPI presentation at 2300? (Fig. 3 is an example.)
(6)Atwhattimewouldyoureturntooriginalcourseandspeedormakeother
changes?
Solutions:
(1)ShipsEandFlookdangerous.Theirbearingsarealmoststeadyandrange
isdecreasingrapidly.Fwillreachthecenterinaboutonehalfhour.Allother
contactsappearsafeenoughtomerelytrackonthescope.Aisclosing,buttoo
slowlytobeofconcernforseveralhours.Bisovertakingataveryslowrate.C
shouldcrosswellclearasterninaboutanhour.Disharmlessandneedsonly
cursory checks.
(3) Change course to 180˚, maintain 15 knots.
(5) See Fig. 3.D has faded from the scope.
(6)WithFwellclearat2300,areturntooriginalcourseappearsdesirable.
ApparentlyA,B,andCalsoaremakingthesameapproachandshouldcauseno
trouble.TheintentionsofEareunknownbutyouhaveaboutanhour’stimebe-
fore convergence.
CPATimeCourseSpeed
(2)ShipF ... 1,700 yds.2258069˚7.5 knots
ShipE ... 1,900 yds.2338182˚14.0 knots
CPATime
(4)ShipF ... 6,300 yds.2250
ShipE ... 17,700 yds.(Both own ship andE are now
on about the same course with
E drawing very slowly astern.
CPA thus has little meaning.)
273
OWN SHIP AT CENTER
EXAMPLE 13Figure 1
PPI SCOPE (20-mile scale)
274
OWN SHIP AT CENTER
EXAMPLE 13 Figure 2
Scale: Speed 2:1;
Distance 3:1 yd.
275
OWN SHIP AT CENTER
EXAMPLE 13 Figure 3
PPI SCOPE (20-mile scale)
276
EXAMPLE 14
AVOIDANCE OF MULTIPLE CONTACTS WITHOUT FIRST DETERMINING
THE TRUE COURSES AND SPEEDS OF THE CONTACTS
Situation:
OwnshipRisoncourse000˚,speed20knots.Withtherelativemotionpre-
sentationradarsetatthe12-milerangesetting,radarcontactsareobservedas
follows:
Required:
(1)DeterminethenewrelativemovementlinesforcontactsA,B,andCwhich
wouldresultfromownshipchangingcourseto065˚andspeedto15knotsat
time 1006.
(2)Determinewhethersuchcourseandspeedchangewillresultindesirable
or acceptable CPA’s for all contacts.
Solution:
(1)Withthecenteroftheradarscopeastheirorigin,drawownship’struevec-
torserander'forthespeedineffectortobeputineffectattimes1000and
1006,respectively.Usingthedistancescaleoftheradarpresentation,draweach
vectoroflengthequaltothedistanceownshipRwilltravelthroughthewater
duringthetimeintervaloftherelativeplot(relativevector),6minutes.Vector
er,havingaspeedof20knots,isdrawn2.0milesinlengthintruedirection
000˚;vectorer',havingaspeedof15knots,isdrawn1.5milesinlengthintrue
direction 065˚.
(2) Draw a dashed line betweenr andr'.
(3)ForContactsA,B,andC,offsettheinitialplots(A
1
,B
1
,andC
1
)inthe
samedirectionanddistanceasthedashedliner-r';labeleachsuchoffsetplotr'.
(4)Ineachrelativeplot,drawastraightlinefromtheoffsetinitialplot,r',
throughthefinalplot(A
2
orB
2
orC
2
).Thelinesr'A
2
,r'B
2
,andr'C
2
representthe
newRML’swhichwouldresultfromacoursechangeto065˚andspeedchange
to 15 knots at time 1006.
Answer:
(1) New DRM of ContactA 280˚.
New DRM of ContactB 051˚.
New DRM of ContactC 028˚.
(2)Inspectionofthenewrelativemovementlinesforallcontactsindicates
thatifallcontactsmaintaincourseandspeed,allcontactswillplotalongtheir
respectiverelativemovementlinesatsafedistancesfromownshipRoncourse
065˚, speed 15 knots.
Explanation:
Thesolutionmethodisbasedupontheuseoftherelativeplotastherelative
vectorasillustratedinExample4.Witheachcontactmaintainingtruecourse
andspeed,theemvectorforeachcontactremainsstaticwhileownship’svector
isrotatedaboutetothenewcourseandchangedinmagnitudecorrespondingto
the new speed.
Bearing
Time 1000
Range (mi.)Rel. position
ContactA050˚9.0A
1
ContactB320˚8.0B
1
ContactC235˚8.0C
1
Bearing
Time 1006
Range (mi.)Rel. position
ContactA050˚7.5A
2
ContactB333˚6.0B
2
ContactC225˚6.0C
2
277
OWN SHIP AT CENTER
EXAMPLE 14
Scale: 12-mile range setting
278
EXAMPLE 15
DETERMINING THE CLOSEST POINT OF APPROACH FROM THE GEOGRAPHICAL PLOT
Situation:
Ownshipisoncourse000˚,speed10knots.Thetruebearingsandrangesof
anothershipareplottedfromownship’ssuccessivepositionstoformageo-
graphical (navigational) plot:
Required:
(1) Determine the Closest Point of Approach.
Solution:
(1)Sincethesuccessivetimedpositionsofeachshipofthegeographicalplot
indicaterateofmovementandtruedirectionoftravelforeachship,eachline
segmentbetweensuccessiveplotsrepresentsatruevelocityvector.Equalspac-
ingoftheplotstimedatregularintervalsandthesuccessiveplottingofthetrue
positionsinastraightlineindicatethattheothershipismaintainingconstant
course and speed.
(2)Thesolutionisessentiallyareversaloftheprocedureinrelativemotion
solutionsinwhich,fromtherelativeplotandownship’struevector,thetrue
vectoroftheothershipisdetermined.Accordingly,thetruevectorsfromthe
twotrueplotsforthesametimeinterval,0206-0212forexample,aresubtracted
to obtain the relative vector.
(3)Therelative(DRM-SRM)vectorrmisextendedbeyondownship’s0212
position to form the relative movement line (RML).
(4)Theclosestpointofapproach(CPA)isfoundbydrawingalinefromown
ship’s 0212 plot perpendicular to the relative movement line.
Answer:
(1) CPA 001˚, 2.2 miles.
Explanation:
Thissolutionisessentiallyareversaloftheprocedureinrelativemotionso-
lutionsinwhich,fromtherelativeplotandownship’struevector,thetruevec-
tor of the other ship is determined. See Example 3.
Notes:
(1)Eitherthetime0200,0206,or0212plotsoftheothershipcanbeusedas
theoriginofthetruevectorsofthevectordiagram.Usingthetime0200plotas
theoriginandatimeintervalof6minutesforvectormagnitude,thelineper-
pendiculartotheextendedrelativemovementlinewouldbedrawnfromthe
time 0206 plot of own ship.
(2)ApracticalsolutionforCPAinthetruemotionmodeofoperationofara-
darisbasedonthefactthattheendoftheInterscan(electronicbearingcursor)
movesfromthepoint,atwhichinitiallyset,inthedirectionofownship’scourse
atarateequivalenttoownship’sspeed.Withthecontactatthispoint,initially,
thecontactmovesawayfromthepointinthedirectionofitstruecourseatarate
equivalenttoitsspeed.Thus,astimepasses,avectortriangleisbeingcontinu-
ouslygenerated.Atanyinstant,theverticesaretheinitialpoint,thepositionof
thecontact,andtheendoftheInterscan.Thesideofthetrianglebetweenthe
endoftheInterscanandthecontactisthermvector,theoriginofwhichisatthe
end of the Interscan.
TheCPAisfoundbysettingtheendoftheInterscanatthecontact,and,after
thevectortrianglehasbeengenerated,extendingthermvectorbeyondown
ship’s position of the PPI.
TimeBearingRange (mi.)True position
0200074˚7.3T
1
0206071˚6.3T
2
0212067˚5.3T
3
rmemer–=
()
279
OWN SHIP AT CENTER
EXAMPLE 15
Scale: Distance: 1:1 mi.
280
EXAMPLE 16
COURSE AND SPEED BETWEEN TWO STATIONS, REMAINING WITHIN A
SPECIFIED RANGE FOR SPECIFIED TIME INTERVAL ENROUTE
Situation:
OwnshipRisonstationbearing280˚,5milesfromtheguideMwhichison
course 190˚, speed 20 knots.
Required:
At1500proceedtonewstationbearing055˚,20miles,arrivingat1630.Re-
mainwithina10-milerangefor1hour.Thecommandingofficerelectstopro-
ceed directly to new station adjusting course and speed to comply.
(1) Course and speed to remain within 10 miles for 1 hour.
(2) Course and speed required at 1600.
(3) Bearing ofM at 1600.
Solution:
(1)Plotthe1500and1630positionsofMatM
1
andM
3
,respectively.Draw
therelativemotionline,M
1
M
3
,intersectingthe10-milecircleatM
2
.Drawem.
MeasureM
1
M
2
:13.6miles.Thetimerequiredtotransitthisdistanceis1hour
atanSRMof13.6knots.Throughmdrawr
1
m13.6knotsinlength,parallelto
and in the directionM
1
M
3
. Vectorer
1
is 147˚.5, 16.2 knots.
(2)MeasureM
2
M
3
,10.3miles,whichrequiresanSRMof20.6knotsforone
half hour. Throughm drawr
2
m. Vectorer
2
is 125˚.5, 18.2 knots.
(3) By inspection,M
2
bears 226˚ fromR at 1600.
Answer:
(1) Course 148˚, speed 16.2 knots.
(2) Course 126˚, speed 18.2 knots.
(3) Bearing 226˚.
Explanation:
SinceownshipRmustremainwithin10milesoftheguidefor1hour,Mmust
notplotalongM
1
M
2
fartherthanM
2
priorto1600.Therequiredmagnitudesof
therelativespeedvectorsfortimeintervals1500to1600and1600to1630to-
getherwiththeircommondirectionarecombinedwiththetruevectorofthe
guide to obtain the two true course vectors for own ship.
281
OWN SHIP AT CENTER
EXAMPLE 16
Scale: Speed 3:1;
Distance 2:1 mi.
282
EXAMPLE 17
COURSE AT MAXIMUM SPEED TO OPEN RANGE TO A SPECIFIED DISTANCE
IN MINIMUM TIME
Situation:
OwnshipRhasguideMbearing240˚,range12miles.Theguideisoncourse
120˚, speed 15 knots. Own ship’s maximum speed is 30 knots.
Required:
Open range to 18 miles as quickly as possible.
(1) Course at 30 knots.
(2) Time to complete the maneuver.
(3) Bearing of guide upon arrival at specified range.
Solution:
Thekeytothissolutionistofindthatrelativeposition(M')oftheguidethat
couldexistbeforetheproblemsstartsinordertobeabletodrawtheRML
throughthegivenrelativeposition(M
1
)andM'tointersectthespecifiedrange
circle.
(1)PlotRandM
1
.AboutRdescribeacircleofradius18miles.Drawem.On
the reciprocal ofM’s course plotM'9 miles fromR.
DrawalinethroughM'andM
1
andextendittointersectthe18-milerange
circle atM
2
.
ThroughmdrawrmparalleltoandinthedirectionM
1
M
2
.Theintersectionof
rmandthe30-knotspeedcircleisthecourserequiredtocompletethemaneuver
in minimum time. Vectorer is 042˚.6, 30 knots.
(2)SRMis30.5knots.MRMis7.5miles.Timetocompletethemaneuver:
14.8 minutes.
(3)Uponreachingthe18-milerangecircle,MisdeadasternofRbearing
222˚.6.
Answer:
(1) Course 043˚.
(2) Time 15 minutes.
(3) Bearing 223˚.
Explanation:
ForRtoopenorclosetoaspecifiedrangeinminimumtime,Rmusttravel
theshortestgeographicaldistanceatmaximumspeed.Theshortestdistanceis
alongtheradiusofacirclecenteredatthepositionoccupiedbyMattheinstant
R reaches the specified range circle.
Inthe“openingrange”problem,determinehypotheticalrelativepositionsof
MandRthatcouldexistbeforetheproblemstarts.Referringtothegeograph-
icalplot,assumeRstartsfrompositionR'andproceedsoutwardalongsomera-
dius18milesinlengthonanunknowncourseat30knots.IfMmovestoward
itsfinalpositionatM
2
alongthegivencourseof120˚,speed15knots,itshould
arriveatM
2
theinstantRreachesthe18-milecircle.Atthisinstant,theproblem
conditionsaresatisfiedbyRbeing18milesdistantfromM.However,own
ship’scourserequiredtoreachthispositionisnotyetknown.Duringthetime
intervalRopens18milesat30knots,Mmoves9milesat15knotsfromM'on
M’strack.ThisprovidestheneededsecondrelativepositionofM'fromR',9
miles bearing 300˚. This position is then transferred to therelative plot.
Speed ofM
Speed ofR
----------------------------
18 miles×9 miles=
283
OWN SHIP AT CENTER
EXAMPLE 17
Scale: Speed 3:1;
Distance 2:1 mi.
284
EXAMPLE 18
COURSE AT MAXIMUM SPEED TO CLOSE RANGE TO A SPECIFIED DISTANCE
IN MINIMUM TIME
Situation:
OwnshipRhastheguideMbearing280˚,range10miles.Theguideison
course 020˚, speed 15 knots. Own ship’s maximum speed is 24 knots.
Required:
Close range to 2 miles as quickly as possible.
(1) Course at 24 knots.
(2) Time to complete the maneuver.
(3) Bearing of guide upon arrival at the specified range.
Solution:
Thekeytothissolutionistofindthatrelativeposition(M')oftheguidethat
couldexistaftertheproblemstartsinordertobeabletodrawtheRMLthrough
the given relative position (M
1
) andM' to intersect the specified range circle.
(1)PlotRandM
1
.AboutRdescribeacircleofradius2miles.Drawem.On
M’s course plotM' 1.25 miles fromR.
DrawalinethroughM'andM
1
.Theintersectionofthislineandthe2-milerange
circle isM
2
.
TomdrawalineparalleltoandinthedirectionM
1
M
2
.Theintersectionofthis
lineandthe24-knotspeedcircleisthecourserequiredtocompletethemaneu-
ver in minimum time. Vectorer is 309˚.8, 24 knots.
(2)SRMis23.6knots.MRMis8.3miles.Timetocompletethemaneuver:
21.1 minutes.
(3)Uponreachingthe2-milerangecircle,MisdeadaheadofRonabearing
309˚.8.
Answer:
(1) Course 310˚.
(2) Time 21 minutes.
(3) Bearings 310˚.
Explanation:
ForRtoopenorclosetoaspecifiedrangeinminimumtime,Rmusttravel
theshortestgeographicaldistanceatmaximumspeed.Theshortestdistanceis
alongtheradiusofacirclecenteredatthepositionoccupiedbyMattheinstant
R reaches the specified range circle.
Inthe“closingrange”problem,determinehypotheticalrelativepositionsof
MandRthatcouldexistaftertheproblemends.Referringtothegeographical
plot,assumeRstartsfrompositionR
1
andproceedsinwardalongsomeradius
onanunknowncourseat24knots.IfMmovestowarditsfinalpositionatM
2
alongthegivencourse020˚,speed15knots,itshouldarriveatM
2
theinstantR
reachesthe2-milecircle.Atthisinstanttheproblemconditionsaresatisfiedal-
thoughthesolutionforownship’scourseisnotyetknown.AssumethatRcon-
tinuesonthesamecourseandspeedthroughthe2milestothecenterofthe
circlewhileMmovesawayfromthecenteroncourse020˚,speed15knots.
DuringthetimeintervalRmovesthese2milesat24knots,Mopens1.25miles.
ThisprovidestheneededsecondrelativepositionofM'fromR':1.25miles,
bearing 020˚. This position is then transferred to therelative plot.
Speed ofM
Speed ofR
-----------------------
2miles
×
1.25miles
=
285
OWN SHIP AT CENTER
EXAMPLE 18
Scale: Speed 3:1;
Distance 1:1 mi.
286
EXAMPLE 19
COURSE AT MAXIMUM SPEED TO REMAIN WITHIN A SPECIFIED RANGE
FOR MAXIMUM TIME
Situation:
ShipMbears110˚,4milesfromR.Misoncourse230˚,18knots.Maximum
speed ofR is 9 knots.
Required:
Remain within a 10-mile range ofM for as long as possible.
(1) Course at maximum speed.
(2) Bearing ofM upon arrival at specified range.
(3) Length of time within specified range.
(4) CPA.
Solution:
(1)PlotRandM.AboutRdescribecirclesofradius9knotsandrange10
miles. Drawem. OnM’s course, plotM' 20 miles fromR.
DrawalinethroughM'andM
1
.Theintersectionofthe10-milerangecircleand
M'M
1
isM
2
,thepointbeyondwhichthespecifiedorlimitingrangeisexceeded.
ThroughmdrawrmparalleltoandinthedirectionM
1
M
2
.Theintersectionof
rmandthe9-knotspeedcircleisthecourserequiredforR,at9knots,toremain
within 10 miles ofM. Vectorer is 220˚.8, 9 knots.
(2)UponarrivalatlimitingrangeatM
2
,MisdeadaheadofRbearing220˚.8.
(3) The time interval within specified range is:
(4) Drop a perpendicular fromR toM
1
M
2
. CPA is 148˚.9, 3.1 miles.
Note:
WhenR’sspeedisequaltoorgreaterthanthatofM,aspecialcaseexistsin
which there is no problem insofar as remaining within a specified range.
Answer:
(1) Course 221˚.
(2) Bearing 221˚.
(3) Time 79 minutes.
(4) CPA 149˚, 3.1 miles.
Explanation:
Asinthe“closingrange”problem,example18,determinehypotheticalrela-
tivepositionsofMandRthatcouldexistaftertheproblemends.Referringto
thegeographicalplot,assumeRstartsfrompositionR
1
andproceedsinward
alongsomeradiusonanunknowncourseat9knots.Misoncourse230˚at18
knots.AttheinstantMpassesthroughM
2
,Rreachesthe10-milelimitingrange
atR
2
.Atthisinstanttheproblemconditionsaresatisfiedalthoughthesolution
isnotyetknown.AssumethatRcontinuesonthesamecourseandspeedthe10
milestothecenterofthecirclewhileMmovesawayfromthecenteroncourse
230˚,speed18knots.DuringthetimeintervalRcloses10milesat9knots,M
opens20milesat18knots.Thisprovidestheneededsecondrelativepositionof
M'fromR',20milesbearing230˚.Thispositionisthentransferredtotherela-
tive plot.
Speed ofM
Speed ofR
-----------------------
10 miles
×
20miles
=
M
1
M
2
rm
-------------
12miles
9.1knots------------------
78.8minutes
==
287
OWN SHIP AT CENTER
EXAMPLE 19
Scale: Speed 2:1;
Distance 2:1 mi.
288
EXAMPLE 20
COURSE AT MAXIMUM SPEED TO REMAIN OUTSIDE OF A SPECIFIED
RANGE FOR MAXIMUM TIME
Situation:
ShipMbears020˚,14milesfromownshipR.Misoncourse210˚,speed18
knots. Maximum speed ofR is 10 knots.
Required:
Remain outside a 10-mile range fromM for as long as possible.
(1) Course at maximum speed.
(2) Bearing ofM upon arrival at specified range.
(3) Time interval before reaching specified range.
Solution:
(1)PlotRandM
1
.AbouteandR,describecirclesofradius10knotsand10
miles. Drawem. On the reciprocal ofM’s course, plotM' 18 miles fromR.
DrawalinethroughM'andM
1
intersectingthe10-milerangecircleatM
2
and
M
3
.
TomdrawalineparalleltoandinthedirectionofM
1
M
2
intersectingthe10-
knotspeedcircleatr
1
andr
2
.M
2
ander
1
areselectedforuseincompletingthe
solution.M
2
isthefirstpointatwhichlimitingrangeisreachedandr
1
misthe
minimumrelativespeedvectorwhichgivesthemaximumtime.Vectorer
1
is
175˚.9, 10 knots.
(2)UponarrivalatlimitingrangeatpointM
2
,MisdeadasternofRbearing
355˚.9.
(3) The time interval outside of specified range is:
Note:
OwnshipcanremainoutsidethelimitingrangeindefinitelyifM
1
fallsoutside
the area between two tangents drawn to the limiting range circle fromM'.
Answer:
(1) Course 176˚.
(2) Bearing 356˚.
(3) Time 34 minutes.
Explanation:
Todetermineacoursetoremainoutsideofagivenrangeformaximumtime,
determinehypotheticalrelativepositionsofMandRthatcouldexistbeforethe
problemstarts.Referringtothegeographicalplot,assumeRstartsfromposi-
tionR'andproceedsoutwardalongsomeradiusonanunknowncourseat10
knots.IfMmovestowarditsfinalpositionatM
2
alongthegivencourse210˚,
speed18knots,itshouldarriveatM
2
theinstantRreachesthe10milecircleat
R
2
.Atthisinstanttheproblemconditionsaresatisfiedalthoughthesolutionfor
ownship’scourseisnotyetknown.DuringthetimeintervalrequiredforRto
movefromR'toR
2
,10milesat10knots,MmovesfromM'toM
2
,18milesat
18knotsalongthegivencourse210˚.Thisprovidestheneededsecondrelative
positions.M'bears030˚,18milesfromR'.Thispositionisthentransferredto
therelative plot.
Speed ofM
Speed ofR
----------------------------
10 miles×18miles=
M
1
M
2
r
1
m
-------------
6.3miles
11.1knots---------------------
34.2minutes
==
289
OWN SHIP AT CENTER
EXAMPLE 20
Scale: Speed 2:1;
Distance 2:1 mi.
290
USE OF A FICTITIOUS SHIP
Theexamplesgiventhusfarhavebeenconfinedtoshipsthathaveeithermain-
tainedconstantcoursesandspeedsduringamaneuverorelsehaveengagedina
successionofsuchmaneuversrequiringonlyrepeatedapplicationofthesame
principles.Whenoneoftheshipsalterscourseand/orspeedduringamaneuver,
a preliminary adjustment is necessary before these principles can be applied.
Thisadjustmentconsists,ineffect,ofsubstitutingafictitiousshipforthe
ship making the alteration. This fictitious ship is presumed to:
(1)maintainaconstantcourseandspeedthroughouttheproblem(thisisthe
final course and speed of the actual ship).
(2)startandfinishitsrunattimesandpositionsdeterminedbytheconditions
established in the problem.
Forexample,thecourseandspeedofadvanceofashipzig-zaggingarecon-
sideredtobetheconstantcourseandspeedofafictitiousshipwhichdeparts
fromagivenpositionatagiventimesimultaneouslywiththeactualship,and
arrivessimultaneouslywiththeactualshipatthesamefinalposition.Theprin-
ciplesdiscussedinpreviousexamplesarejustasvalidforafictitiousshipasfor
anactualship,bothintherelativeplotandspeedtriangle.Ageographicalplot
facilitates the solution of problems of this type.
EXAMPLE 21
ONE SHIP ALTERS COURSE AND/OR SPEED DURING MANEUVER
Situation:
At0630shipMbears250˚,range32miles.Misoncourse345˚,speed15
knots but at 0730 will change course to 020˚ and speed to 10 knots.
Required:
OwnshipRtakesstation4milesonthestarboardbeamofMusing12knots
speed.
(1) Course to comply.
(2) Time to complete maneuver.
Solution:
Thekeytothissolutionistodeterminethe0630positionofafictitiousship
thatbysteeringcourse020˚,speed10knots,willpassthroughtheactualship’s
0730position.Inthiswaythefictitiousshiptravelsonasteadycourseof020˚
and speed 10 knots throughout the problem.
(1) PlotR,M
1
, andM
3
. Drawem
1
andem
2
/emf.
ConstructageographicalplotwithinitialpositionM
1
.PlotM
1
andM
2
,M’s
0630-0730travelalongcourse345˚,distance15miles.PlotMF
1
,thefictitious
ship’sinitialposition,onbearing200˚,10milesfromM
2
.MF
1
toMF
2
isthefic-
titious ship’s 0630-0730 travel.
TransfertherelativepositionsofM
1
andMF
1
totherelativeplot.MF
1
MF
3
is
therequiredDRMandMRMforproblemsolution.Drawrm
2
paralleltoandin
thedirectionofMF
1
MF
3
.Theintersectionofrm
2
andthe12-knotspeedcircle
is the course,er: 303˚, required byR in changing stations whileM maneuvers.
(2)ThetimetocompletethemaneuverisobtainedfromtheTDSscaleusing
fictitious ship’s MRM fromMF
1
toMF
3
and the SRM ofrmf.
Answer:
(1) Course 303˚.
(2) Time 2 hours 29 minutes.
291
OWN SHIP AT CENTER
EXAMPLE 21
Scale: Speed 2:1;
Distance 4:1 mi.
292
EXAMPLE 22
BOTH SHIPS ALTER COURSE AND/OR SPEED DURING MANEUVER
Situation:
At0800Misoncourse105˚,speed15knotsandwillchangecourseto350˚,
speed18knotsat0930.OwnshipRismaintainingstationbearing330˚,4miles
fromM.Risorderedtotakestationbearing100˚,12milesfromM,arrivingat
1200.
Required:
(1) Course and speed forR to comply if maneuver is begun at 0800.
(2)CourseforRtocomplyifRdelaysthecoursechangeaslongaspossible
and remains at 15 knots speed throughout the maneuver.
(3) Time to turn to course determined in (2).
Solution:
SincetherelativepositionsofRandMatthebeginningandendofthema-
neuverandthetimeintervalforthemaneuveraregiven,thesolutionfor(1)can
beobtaineddirectlyfromageographicalplot.Solvetheremainderoftheprob-
lem using arelative plot.
(1)Usingageographicalplot,layoutM’s0800-1200trackthroughpointsM
1
,
M
2
,andM
3
.PlotR
1
andR
3
relativetoM
1
andM
3
,respectively.Thecourseof
040˚fromR
1
toR
3
canbemeasureddirectlyfromtheplot.Rwillrequireaspeed
of 10.8 knots to move 43.4 miles in 4 hours.
(Thissolutioncanbeverifiedontherelativeplot.First,usingageographical
plot,determinethe0800positionofafictitiousship,MF
1
,suchthatbydepart-
ingthispointat0800oncourse350˚,18knotsitwillarriveatpointMF
2
simul-
taneouslywiththemaneuveringshipM.MF
1
bears141˚,41.7milesfromM
1
.
TransferthepositionsofM
1
andMF
1
totherelativeplot.PlotRandM
2
.Draw
thefictitiousship’svector,emf
1
.Tomf
1
constructtheSRMvectorparallelto
MF
1
MF
2
and 13.8 knots in length. Vectorer
1
is the required course of 040˚.)
(2)TofindthetwolegsofR’s0800-1200track,usearelativeplot.Drawer
2
,
ownship’sspeedvectorwhichisgivenas105˚,15knots.Atthisstageofthe
solution,disregardMandconsiderownshipRtomaneuverrelativetoanew
fictitiousship.Ownshiponcourse040˚,10.8knotsfrompart(1)isthefictitious
shipused.Labelvectorer
1
asemf
2
,thefictitiousship’svector.Frompointr
2
drawalinethroughmf
2
extendedtointersectthe15-knotspeedcircleatr
3
.
Drawer
3
, the second course of 012˚ required byR in changing station.
(3)Tofindthetimeoneachlegdrawatimelinefromr
2
usinganyconvenient
scale.Throughr
3
drawr
3
X.Throughr
1
drawr
1
Yparalleltor
3
X.Similartrian-
glesexist;thus,thetimelineisdividedintoproportionaltimeintervalsforthe
twolegs:XYisthetimeonthefirstleg:1hour22minutes.Theremainderof
the 4 hours is spent on the second leg.
Answer:
(1) Course 040˚, 10.8 knots.
(2) Course 012˚.
(3) Time 0922.
Note:
Intheaboveexample,analternativeconstructionofthetimelineasdefined
intheglossaryisusedsothatthelinecanbedrawntoaconvenientscale.The
proportionalityismaintainedbyconstructingsimilartriangles.SeeNotewith
example 24.
293
OWN SHIP AT CENTER
EXAMPLE 22
Scale: Speed 2:1;
Distance 4:1 mi.
294
EXAMPLE 23
COURSES AT A SPECIFIED SPEED TO SCOUT OUTWARD ON PRESENT
BEARING AND RETURN AT A SPECIFIED TIME
Situation:
OwnshipRismaintainingstationonMwhichbears110˚,range5miles.For-
mation course is 055˚, speed 15 knots.
Required:
Commencingat1730,scoutoutwardonpresentbearingandreturntopresent
station at 2030. Use 20 knots speed.
(1) Course for first leg.
(2) Course for second leg.
(3) Time to turn.
(4) Maximum distance from the guide.
Solution:
(1)PlotRandM
1
.Drawem.TheDRM“out”isalongthebearingofMfrom
R.TheDRM“in”isalongthebearingofRfromM.Throughmdrawalinepar-
alleltotheDRM’sintersectingthe20-knotcircleatr
1
andr
2
.Vectorr
1
misthe
DRM “out”. Vectorer
1
is 327˚.8, the course “out”.
(2) Vectorr
2
m is the DRM “in”. Vectorer
2
is 072˚, the course “in”.
(3)Tofindthetimeoneachleg,drawatimelinefromr
1
usinganyconvenient
scale.Throughr
2
drawr
2
X.ThroughmdrawmYparalleltor
2
X.Similartrian-
glesexist;thus,thetimelineisdividedintoproportionaltimeintervalsforthe
twolegs.XYisthetimeonthefirstleg,41minutes.Theremainderofthetime
is spent on the second leg returning to station.
(4)RangeofMwhencourseischangedto“in”legis21.7miles.Initialrange
+ (r
1
m xtime on “out” leg).
Answer:
(1) Course 328˚.
(2) Course 072˚.
(3) Time 1811.
(4) Distance 21.7 miles.
Explanation:
SinceownshipRreturnstopresentstation,relativedistancesoutandinare
equal. In going equal distances, time varies inversely as speed:
Therefore,thetimeoutpartofthespecifiedtime(3
h
)isobtainedbysimplepro-
portion or graphically.
Asdefinedintheglossary,thetimelineisthelinejoiningtheheadsofvectors
er
1
ander
2
.Thislineisdividedbytheheadofvectoremintosegmentsinversely
proportionaltothetimesspentbyownshipRonthefirst(out)andsecond(in)
legs.Intheaboveexampleanalternativeconstructionisusedsothatthelinecan
bedrawntoaconvenientscale.Theproportionalityismaintainedbyconstruct-
ing similar triangles.
time (out)
time (in)
-------------------
relative speed (in)
relative speed (out)
--------------------------------------
r
1
m(in)
r
2
m(out)
-------------------==
295
OWN SHIP AT CENTER
EXAMPLE 23
Scale: Speed 2:1;
Distance 2:1 mi.
296
EXAMPLE 24
COURSES AND MINIMUM SPEED TO CHANGE STATIONS WITHIN
A SPECIFIED TIME, WHILE SCOUTING ENROUTE
Situation:
OwnshipRbears130˚,8milesfromtheguideMwhichisoncourse040˚,
speed 12 knots.
Required:
Proceedtonewstationbearing060˚,10milesfromtheguide,passingthrough
apointbearing085˚,25milesfromtheguide.Completethemaneuverin4.5
hours using minimum speed.
(1) First and second courses forR.
(2) Minimum speed.
(3) Time to turn to second course.
Solution:
(1)PlotM
1
,M
2
andM
3
.Drawem.Frommdrawlinesofindefinitelengthpar-
alleltoandinthedirectionofM
1
M
2
andM
2
M
3
.Assumethatafictitiousship,
MF,departsM
1
simultaneouslywithMandproceedsdirectlytoM
3
arrivingat
thesametimeasMwhichtraveledthroughM
2
enroute.Thefictitiousshipcov-
ersarelativedistanceof10.5milesin4.5hours.SRMofthefictitiousshipis
2.3knots.Tomdrawmfm2.3knotsinlengthparalleltoandinthedirectionof
M
1
M
3
.Vectoremfisthetruecourseandspeedvectorofthefictitiousship.With
mfasapivot,rotateastraightlinesothatitintersectsthetwopreviouslydrawn
linesonthesamespeedcircle.Thepointsofintersectionarer
1
andr
2
.Vector
er
1
is the course out: 049˚. Vectorer
2
is the course in: 316˚.9.
(2)Vectorsr
1
andr
2
lieonthe17.2knotcirclewhichistheminimumspeed
to complete the maneuver.
(3)Fromr
2
layoffa4.5hourtimelineusinganyconvenientscale.Drawr
1
X.
DrawmfYparalleltor
1
X.ThepointYdividesthetimelineintopartsthatare
inverselyproportionaltotherelativespeedsr
2
mfandr
1
mf.XYthetime“in”is
51minutes.Yr
2
thetime“out”is3hours39minutes.Timeoneachlegmayalso
be determined mathematically by the formula MRM/SRM=time.
Answer:
(1) First course 049˚, second course 317˚.
(2) Speed 17.2 knots.
(3) Time 3 hours and 39 minutes.
Note:
Thetimeline,asdefinedintheglossary,isthelinejoiningtheheadsofvec-
torser
1
ander
2
andtouchingtheheadofthefictitiousshipvectoremf.Thistime
lineisdividedbytheheadofthefictitiousshipvectorintosegmentsinversely
proportional to the times spent by the unit on the first and second legs.
Intheaboveexample,analternativeconstructionofthetimelineisusedso
thatthelinecanbedrawntoaconvenientscale.Theproportionalityismain-
tained by constructing similar triangles.
297
OWN SHIP AT CENTER
EXAMPLE 24
Scale: Speed 2:1;
Distance 3:1 mi.
298
EXAMPLE 25
COURSE, SPEED, AND POSITION DERIVED FROM BEARINGS ONLY
Situation:
Ownshipisoncourse090˚,speed15knots.Thetruebearingsofanothership
are observed as follows:
At1600ownshipchangescourseto050˚andincreasesspeedto22knots.The
following bearings of shipM are then observed:
Required:
(1) Course and speed of shipM.
(2) Distance ofM at time of last bearing.
Solution:
(1) Draw own ship’s vectorer
1
.
(2) Plot first three bearings and label in order observed,B
1
,B
2
, andB
3
.
(3)AtanypointonB
1
,constructperpendicularwhichintersectsB
2
andB
3
.
Label these pointsP
1
,P
2
, andP
3
.
(4)MeasurethedistanceP
1
toP
2
andplotpointXatthesamedistancefrom
P
2
towardsP
3
.
(5)FromXdrawalineparalleltoB
1
untilitintersectsB
3
.Labelthisintersec-
tionY.
(6) FromY draw a line throughP
2
until it intersectsB
1
atZ.
(7)Fromheadofownship’svectorer
1
,drawalineparalleltoYZ.Thisestab-
lishestheDRMontheoriginalcourseandspeed.Theheadoftheemvectorof
shipMliesonthelinedrawnparalleltoYZ.ItisnownecessarytofindtheDRM
followingacourseand/orspeedchangebyownship.Theintersectionofthetwo
linesdrawninthedirectionofrelativemovementfromtheheadsofownship’s
vector establishes the head of vectorem.
(8)Followingcourseandspeedchangemadetoproduceagoodbearingdrift,
threemorebearingsareplotted;thenewdirectionofrelativemovementisob-
tainedfollowingtheproceduregiveninsteps(3)through(7).Thelinesdrawn
inthedirectionsofrelativemovementfromtheheadsofvectorer
1
ander
2
in-
tersect at the head of the vectorem. ShipM is on course 170˚ at 10 knots.
(9)Fromrelativevectorr
2
m,theSRMisfoundas28.4knotsduringthesec-
ond set of observations.
(10)Computetherelativedistancetraveledduringthesecondsetofobserva-
tions (MRM 56.8 mi.).
(11)OnthelineZYforthesecondsetofobservations,layofftherelativedis-
tanceZA.FromAdrawalineparalleltoB
4
untilitintersectsB
6
.Labelthispoint
B. This is the position ofM at the time of the last bearing.
Answer:
(1) Course 170˚, speed 10 knots.
(2) Position ofM at 1830: 274˚.5 at 61 miles.
Note:
Theseproceduresarebasedonbearingsobservedatequalintervals.Forun-
equal intervals, use the following proportion:
TimeBearing
1300010˚
1430358˚
1600341˚
TimeBearing
1630330˚
1730302˚
1830274˚.5
Time difference betweenB
1
andB
2
Distance fromP
1
toP
2
--------------------------------------------------------------------------------------
Time difference betweenB
2
andB
3
Distance fromP
2
toX
------------------------------------------------------------------------------------
--
.
=
299
OWN SHIP AT CENTER
EXAMPLE 25
Scale: Speed 3:1;
Distance 10:1 mi.
300
EXAMPLE 26
LIMITING LINES OF APPROACH
(single ship)
Situation:
OwnshipR’scourseandspeedis000˚,20knots.At0930,bothsonarandra-
darreportacontactbearing085˚,distance22,500.At0931,radarlosescontact
andat0932sonarlosescontact.Lastknownpositionwas085˚,distance20,000.
Datum error is 1,000 yards.
Required:
(1) Advanced position.
(2)Limitinglinesofapproachforsubmarinewithmaximumquietspeedof
15 knots.
Solution:
(1)PlotRatcenterofmaneuveringboardanddrawthevector“er”000˚,20
knots.
(2) Plot datum position from own ship (085˚, 20,000 yards).
(3) Plot datum error (circle of radius 1,000 yards) around datum.
(4) Compute own ship’s advanced position using the formula:
(5) Plot advanced position along own ship’s course and speed vector.
(6)PlotTorpedoDangerZone(10,000yardcircle)aroundadvancedposition.
(7)From“r”,describeanarcwitharadiusof15nauticalmiles(theassumed
quiet speed of the submarine).
(8)Drawthetangentvector“eMq”untilitintersectstheedgeofthemaneu-
veringboardplottingcircle.Dothisonbothsidesoftheship’shead.Thetrue
bearing of the tangent lines are the limiting lines of approach.
(9)Parallelthetangentvectors“eMq”untiltheyaretangenttotheTorpedo
Danger Zone to complete the plotting picture.
Answer:
(1) Advanced position = 4,444 yards.
(2) Left side limiting line = 310˚.
Right side limiting line = 050˚.
Limiting lines of approach = 310˚ - 050˚.
Notes:
(1) Limiting lines of approach are read clockwise.
(2)Thisexampleassumesthesubmarinemaintainsaconstantspeedthrough-
out the approach.
(3)Thesubmarineandtorpedodatawerechosenforexamplepurposesand
shouldNOTbeusedasrealestimates.Consultappropriateintelligencepublica-
tions for correct data.
Torpedo Firing Range
Torpedo Speed
-----------------------------------------------------
Vessel Speed×
10,000 yds
45 kts
--------------------------
20 kts×4,444 yd
s
==
301
OWN SHIP AT CENTER
EXAMPLE 26
Scale: Speed 1:1;
Distance 1:1 mi.
302
EXAMPLE 27a
CONES OF COURSES
Solution: 1
Situation:
OwnshipRisoncourse000˚,15knots.At1600,submarineMisreported
bearing 325˚, 40 miles fromR. Maximum assumed speed forM is 10 knots.
Required:
(1) Courses at 10 knots the submarineM will steer to interceptR.
(2)TimeofthefirstandlastinterceptopportunitiesforsubmarineMagainst
R at the assumed speed of 10 knots.
Solution:
(1)Plotthe1600positionofthesubmarineM325˚,40milesfromR.Draw
thevector“er”000˚,15knots.FromM,drawaDRMlinetoRandfrom“r”
drawthevector“rm”parallelandinthesamedirectionastheDRM.With“e”
asthecenter,describeanarcwithradiusof10knots,theassumedspeedofM.
Thepointsem
1
andem
2
wherethearcintersectsthe“rm”vector,definethe
coursesat10knotsthatthesubmarinewillsteertointerceptR.Coursesbetween
“em
1
”and“em
2
”arelowerassumedspeedinterceptsand“em
L
”,theperpendic-
ularlinefromRto“rm”,isthecourseforthelowestpossibleassumedspeedat
which the submarine can move and still interceptR.
(2)Parallelthe“em
1
”and“em
2
”linesasvectorstothe1600positionatMand
extend“er”untilitcrossesthesevectors;theareaenclosedbythese3vectors
representsthetruegeographicareathroughwhichthesubmarinewillmoveat
orbelow10knotstointerceptR.Theelapsedtimestothefirst(“t
1
”)andthelast
(“t
2
”)interceptopportunitiesisobtainedbydividingtherelativedistanceat
1600 (RM) by the respective relative speed (“rm
1
” and “rm
2
”).
Answer:
(1) Courses 024˚ to 086˚.
Note:
Ifthesubmarine’spositioninvolvesanerror(i.e.,datumerror)andamain
bodyorconvoyformationispresent(withanassociatedTorpedoDangerZone
(TDZ)aroundit)theDRMfromMtoRbecomestangentiallinesdrawnfrom
“r”withahighspeedandlowspeedlegcorrespondingtoaforwardoraftDRM
on the formation.
(2)''t
1
''
RM
''rm
1
''
--------------
40 miles
17.5 knots
-------------------------
2 hrs 17 mins===
T
1
1600''t
1
''1817=+=
''t
2
''
RM
''rm
2
''
--------------
40 miles
7knots
--------------------
5hrs 43 mins===
T
2
1600''t
2
''2143=+=
303
OWN SHIP AT CENTER
EXAMPLE 27a
Scale: Speed 3:1;
Distance 10:1 mi.
304
EXAMPLE 27b
CONES OF COURSES
Solution: 2
Situation:
OwnshipRisoncourse000˚,15knots.At1600,submarineMisreported
bearing 325˚, 40 miles fromR. Maximum assumed speed forM is 10 knots.
Required:
(1) Courses at 10 knots the submarineM will steer to interceptR.
(2)TimeofthefirstandlastinterceptopportunitiesforsubmarineMagainst
R at the assumed speed of 10 knots.
Solution:
(1)Plotthe1600positionofthesubmarineM325˚,40milesfromR.Draw
thevector“er”000˚,15knots.FromM,drawaDRMlinetoRandfrom“r”
drawthevector“rm”parallelandinthesamedirectionastheDRM.With“e”
asthecenter,describeanarcwithradiusof10knots,theassumedspeedofM.
ThepointsEM
1
andEM
2
wherethearcintersectsthe“rm”vector,definethe
coursesat10knotsthatthesubmarinewillsteertointerceptR.Coursesbetween
“em
1
”and“em
2
”arelowerassumedspeedinterceptsand“em
2
”,theperpendic-
ularlinefromRto“rm”,isthecourseforthelowestpossibleassumedspeedat
which the submarine can move and still interceptR.
(2)Parallelthe“em
1
”and“em
2
”linesasvectorstothe1600positionatMand
extend“er”untilitcrossesthesevectors;theareaenclosedbythese3vectors
representsthetruegeographicareathroughwhichthesubmarinewillmoveat
orbelow10knotstointerceptR.Theelapsedtimestothefirst(“t
1
”)andthelast
(“t
2
”)interceptopportunitiesisobtainedbydividingtherelativedistanceat
1600 (RM) by the respective relative speed (“rm
1
” and “rm
2
”).
Answer:
(1) Courses 024˚ to 086˚.
(2)
Note:
Ifthesubmarine’spositioninvolvesanerror(i.e.,datumerror)andamain
bodyorconvoyformationispresent(withanassociatedTorpedoDangerZone
(TDZ)aroundit)theDRMfromMtoRbecomestangentiallinesdrawnfrom
“r”withahighspeedandlowspeedlegcorrespondingtoaforwardoraftDRM
on the formation.
''t
1
''
RM
''rm
1
''
--------------
40 miles
17.5 knots
-------------------------
2 hrs 17 mins===
T
1
1600''t
1
''1817=+=
''t
2
''
RM
''rm
2
''
--------------
40 miles
7knots
--------------------
5hrs 43 mins===
T
2
1600''t
2
''2143=+=
305
OWN SHIP AT CENTER
EXAMPLE 27b
Scale: Speed 3:1;
Distance 10:1 mi.
306
EXAMPLE 28
EVASIVE ACTION AGAINST A TARGET MOVING AT SLOW SPEED
Situation:
Avesselpossessingaspeedadvantageisalwayscapableoftakingevasiveac-
tionagainstaslow-movingenemy.Itmaybenecessarytotakeevasiveaction
againstaslow-movingenemy.Forexample,whenasurfacevesselisattempting
to evade attack by a submarine.
Required:
Theessenceoftheproblemistofindthecourseforthemaneuveringshipat
whichnomatterhowtheenemymaneuvershewillnotbeabletocomeanyclos-
erthandistanceD(Torpedo/MissileDangerZone)tothemaneuveringship.In
ordertoaccomplishthis,themaneuveringshipshouldpresstheslow-moving
enemy at a relative bearing greater than critical.
Solution:
Evasiveactionisgraphicallycalculatedinthefollowingmanner.Theposi-
tionoftheslow-movingenemyvesselK
0
isplottedonamaneuveringboardand
thedistanceittravelsfromthemomentofdetectiontothebeginningofevasive
action is calculated:
whereT
1
= time at which evasive action begins;
T
0
= time of detection of the enemy.
Theaccuracyofdeterminationofthepositionoftheenemy,assumedtobe
withinthedatumerrorzone,(r)isalsoverified.Thentheminimumdivergence
fromtheenemy(d)isdetermined(e.g.,2-3timestherangeoffireoftorpedoes
or1.5to2timesthesonardetectionrange).Addinguptheselectedvalues,with
a radius of:
we have a circle about the initial position of the enemy K
0
.
Constructingatangenttothiscirclefromthepositionofthemaneuveringship
(pointM
0
)and,constructingaspeedtriangleatthepointoftangency,weobtain
thecourseofthemaneuveringvesselKm
1
orKm
2
whichthelattermuststeerin
order to avoid meeting the enemy.
Note:
Asarule,thepointofturntothepreviouscourseaftertakingevasiveaction
isnotcalculatedandtheturnisusuallyexecutedafterthebearingonthepoint
of detection of the slow-moving enemy vessel changes more than 90˚.
S
V
k
T
1
T
0
–
()=
D
1
rSd,
++=
307
OWN SHIP AT CENTER
EXAMPLE 28
Scale: Speed 1:1;
Distance 1:1 mi.
309
PART TWO
GUIDE AT CENTER
310
EXAMPLE 29
CHANGING STATION WITH TIME, COURSE, OR SPEED SPECIFIED
Situation:
Formationcourseis010˚,speed18knots.At0946whenordersarereceived
tochangestation,theguideRbears140˚,range7,000yards.Whenonnewsta-
tion, the guide will bear 240˚, range 6,000 yards.
Required:
(1) Course and speed to arrive on station at 1000.
(2)Speedandtimetostationoncourse045˚.Uponarrivalonstationorders
are received to close to 3,700 yards.
(3) Course and minimum speed to new station.
(4) Time to station at minimum speed.
Solution:
(1)PlotM
1
320˚,7,000yardsandM
2
060˚,6,000yardsfromR.Drawercor-
respondingtocourse010˚andspeed18knots.Therelativedistanceof10,000
yardsfromM
1
toM
2
mustbecoveredin14minutes.SRMistherefore21.4
knots.Drawrm
1
paralleltoM
1
M
2
,and21.4knotsinlength.Oncompletingthe
vectordiagram,thevectorem
1
denotestherequiredcourseandspeed:062˚,27
knots.
(2)Drawem
2
,course045˚,intersectingtherelativespeedvectorrm
1
atthe
21-knotcircle.Thelengthrm
2
is12.1knots.ThustherelativedistanceM
1
M
2
of
10,000 yards will be covered in 24.6 minutes.
(3)PlotM
3
060˚,3,700yardsfromRafterclosing.Throughrdrawalinepar-
alleltoandinthedirectionofM
2
M
3
.Dropaperpendicularfrometothislineat
m
3
.Vectorem
3
isthecourseandminimumspeedrequiredtocompletethefinal
change of station: 330˚, 13.8 knots.
(4)Bymeasurement,thelengthofrm
3
isanSRMof11.5knots;thedistance
fromM
2
toM
3
is2,300yards.M
2
M
3
/rm
3
istherequiredmaneuvertime:6min-
utes.
Answer:
(1) Course 062˚, speed 27 knots.
(2) Speed 21 knots, time 25 minutes.
(3) Course 330˚, speed 13.8 knots.
(4) Time 6 minutes.
311
GUIDE AT CENTER
EXAMPLE 29
Scale: Speed 3:1;
Distance 1:1 yd.
312
EXAMPLE 30
THREE-SHIP MANEUVERS
Situation:
OwnshipMisinformationproceedingoncourse000˚,speed20knots.The
guideRbears090˚,distance4,000yards.ShipNis4,000yardsaheadofthe
guide.
Required:
MandNaretotakenewstationsstartingatthesametime.Nistotakestation
4,000yardsontheguide’sstarboardbeamusingformationspeed.Mistotake
N’s old station and elects to use 30 knots.
(1)N’s course and time to station.
(2)M’s course and time to station.
(3) CPA ofM andN to guide.
(4) CPA ofM toN.
(5) Maximum range ofM fromN.
Solution:
(1)PlotRatthecenterwithM
1
at270˚,4,000yards;M
2
andN
1
at000˚,4,000
yards.Drawer000˚,20knots.FromRplotN’snewstationNR,bearing090˚,
distance4,000yards.InrelationtoR,NmovesfromN
1
toNR.Fromr,drawa
lineparalleltoandinthedirectionofN
1
NRandintersectingthe20-knotspeed
circleatn.N’scoursetostationisvectoren:090˚.TimetostationN
1
NR/rnis
6 minutes.
(2)InrelationtoR,MmovesfromM
1
toM
2
.Fromr,drawrmparalleltoand
inthedirectionofM
1
M
2
andintersectingthe30-knotspeedcircleatm.M’s
course to station is vectorem: 017˚. Time to stationM
1
M
2
/rm is 14 minutes.
(3)FromRdropaperpendiculartoN
1
NR.AtCPA,Nbears045˚,2,850yards
fromR.FromRdropaperpendiculartoM
1
M
2
.AtCPA,Mbears315˚,2,850
yards fromR.
(4)InrelationtoM,NtravelsfromN
1
toN
2
toN
3
.PlotN
3
bearing135˚,5,700
yardsfromM
1
.Frompointmdrawtherelativespeedvectormn.Drawarelative
movementlinefromN
1
paralleltoandinthesamedirectionasmn.WhenNar-
rivesonnewstationandreturnstobasecoursetherelativespeedbetweenMand
Nisthesameasrm.FromN
3
drawarelativemovementlineparalleltoandin
thesamedirectionasrm.TheselinesintersectatN
2
.FromM
1
dropaperpendic-
ular to lineN
1
N
2
. At CPA,N bears 069˚, 5,200 yards fromM.
(5)ThepointatwhichNresumesformationcourseandspeedN
2
,isthemax-
imum range ofN fromM; 6,500 yards.
Answer:
(1)N’s course 090˚, time 6 minutes.
(2)M’s course 017˚, time 14 minutes.
(3) CPA:N toR 2,850 yards at 045˚;M toR 2,850 yards at 315˚.
(4) CPA ofN toM 5,200 yards at 069˚.
(5) Range 6,500 yards.
Explanation:
Insolutionstep(4),themovementofNinrelationtoMisparalleltothedi-
rectionofvectormnandfromN
1
untilsuchtimethatNreturnstobasecourse
andspeed.Afterwards,themovementofNinrelationtoMisparalleltovector
rmandfromN
2
towardthatpoint,N
3
,thatNwilloccupyrelativetoMwhenthe
maneuver is completed.
313
GUIDE AT CENTER
EXAMPLE 30
Scale: Speed 3:1;
Distance 1:1 yd.
314
EXAMPLE 31
COURSE AND SPEED TO PASS ANOTHER SHIP AT A SPECIFIED DISTANCE
Situation:
At1743ownshipMisoncourse190˚,speed12knots.AnothershipRisob-
servedbearing153˚,13,000yardsoncourse287˚,speed10knots.Itisdesired
to pass ahead ofR with a CPA of 3,000 yards.
Required:
(1) Course ofM at 12 knots.
(2) Bearing ofR and time at CPA.
Solution:
(1)PlotRatthecenterofM
1
bearing333˚,13,000yardsfromR.Drawthe
othership’svectorer287˚,10knots.WithRasacenter,describeacircleofra-
dius3,000yards.FromM
1
drawalinetangenttothecircleatM
2
.Thissatisfies
therequirementofpassingwithaCPAof3,000yardsfromR.Fromrdrawa
lineparalleltoandinthesamedirectionasM
1
M
2
,intersectingthe12-knotspeed
circle atm. Drawem, own ship’s vector 212˚, 12 knots.
(2)FromRdropaperpendiculartoM
2
.WhenownshipreachesM
2
,Rwill
bear076˚.MeasuretherelativedistanceM
1
M
2
,12,600yards,andtherelative
speedvectorrm,13.4knots.Usingthisdistanceandspeed,theelapsedtimeto
CPAisobtainedfromtheTDSscale:28minutes.ThetimeatCPAis1743+28
= 1811.
Answer:
(1) Course 212˚.
(2) Bearing 076˚, time at CPA 1811.
315
GUIDE AT CENTER
EXAMPLE 31
Scale: Speed 2:1;
Distance 2:1 yd.
316
EXAMPLE 32
COURSE AT SPECIFIED SPEED TO PASS ANOTHER SHIP AT MAXIMUM
AND MINIMUM DISTANCES
Situation:
ShipRoncourse300˚,speed30knots,bears155˚,range16milesfromown
shipM whose maximum speed is 15 knots.
Required:
(1)M’scourseat15knotstopassRat(a)maximumdistance,(b)minimum
distance.
(2) CPA for each course found in (1).
(3) Time interval to each CPA.
(4) Relative bearing ofR fromM when at CPA on each course.
Solution:
(1)PlotM
1
335˚,16milesfromR.Drawthevectorer300˚,30knots.Withe
asthecenter,drawacirclewithradiusof15knots,thespeedofM.Fromrdraw
thetangentsrm
1
andrm
2
whichproducethetwolimitingcoursesforM.Parallel
tothetangentsplottherelativemovementlinesfromM
1
.Courseofownshipto
passatmaximumdistanceisem
1
:000˚.Coursetopassatminimumdistanceis
em
2
: 240˚.
(2)ThroughRdrawRM
2
andRM'
2
perpendiculartothetwopossiblerelative
movementlines.Rbearing180˚,14.5milesfromM
2
istheCPAforcourseof
000˚.R bearing 240˚, 1.4 miles fromM'
2
is the CPA for course 240˚.
(3)MeasureM
1
M
2
:6.8miles,andM
1
M'
2
:15.9miles.Mmusttraveltheserel-
ativedistancesbeforereachingtheCPAoneachlimitingcourse.Therelative
speedofMisindicatedbythelengthofthevectorsrm
1
andrm
2
:26knots.From
theTDSscalethetimesrequiredtoreachM
2
andM'
2
arefound:15.6minutes
and 36.6 minutes, respectively.
(4)Bearingsaredeterminedbyinspection.Rbears180˚relativebecauseown
ship’scourseisalongvectorem
1
formaximumCPA.Rbears000˚relativewhen
own ship’s course isem
2
for minimum passing distance.
Note:
ThissituationoccursonlywhenownshipMis(1)aheadoftheothershipand
(2)hasamaximumspeedlessthanthespeedoftheothership.Underthesecon-
ditions,ownshipcanintercept(collisioncourse)onlyifRliesbetweenthe
slopesofM
1
M
2
andM
1
M'
2
.Notethatforlimitingcourses,andonlyforthese,
CPAoccurswhenothershipisdeadaheadordeadastern.Thesolutiontothis
problemisapplicabletoavoidingatropicalstormbytakingthatcoursewhich
results in maximum passing distance.
Answer:
(1) Course (a) 000˚; (b) 240˚.
(2) CPA (a) 180˚, 14.5 miles; (b) 240˚, 1.4 miles.
(3) Time (a) 16 minutes; (b) 37 minutes.
(4) Relative bearing (a) 180˚; (b) 000˚.
317
GUIDE AT CENTER
EXAMPLE 32
Scale: Speed 3:1;
Distance 2:1 mi.
318
EXAMPLE 33
COURSE CHANGE IN COLUMN FORMATION ASSURING LAST SHIP IN
COLUMN CLEARS
Situation:
OwnshipD1istheguideinthevanofadestroyerunitconsistingoffourde-
stroyers(D1,D2,D3,andD4)incolumnastern,distance1,000yards.D1ison
stationbearing090˚,8milesfromtheformationguideR.Formationcourseis
135˚,speed15knots.Theformationguideisatthecenterofaconcentriccircu-
lar ASW screen stationed on the 4-mile circle.
Thedestroyerunitisorderedtotakenewstationbearing235˚,8milesfrom
theformationguide.TheunitcommanderinD1decidestouseawheelingma-
neuverat27knots,passingaheadofthescreenusingtwocoursechangessothat
the CPA of his unit on each leg is 1,000 yards from the screen.
Required:
(1) New course to clear screen commencing at 1000.
(2) Second course to station.
(3) Bearing and range ofR andD1 at time of coming to second course.
(4) Time of turn to second course.
(5) TimeD1 will reach new station.
Solution:
(1)PlottheformationguideRatthecenter.PlotownshipD1bearing090˚,8
milesfromR.PlottheremainingthreedestroyersincolumnasternofD1,dis-
tancebetweenships1,000yards.Drawer,thespeedvectorofR,135˚,15knots.
ItisrequiredthatthedestroyercolumnclearRbyaminimumof9,000yards
(screenradiusof4milesplus1,000yards).Attheinstantthesignalisexecuted,
onlyD1changesbothcourseandspeed.Theotherdestroyersincreasespeedto
27knotsbutremainonformationcourseof135˚untileachreachestheturning
point.AdvanceRalongtheformationcoursethedistanceRwouldmoveat15
knotswhileD4advancestotheturningpointat27knots.Thedistanceisequal
to:
Drawacircleofradius9,000yardsabouttheadvancedpositionoftheguide
R'.DrawalinefromD1(theturningpoint)tangenttothecircle.Thisistherel-
ativemovementlinerequiredforD4toclearthescreenby1,000yardsonthe
firstleg.Drawalinefromrparalleltothislineandintersectingthe27-knotcir-
cle atm
1
. This producesem
1
, the initial course of 194˚.2.
(2)PlotthefinalrelativepositionofD1atD1'bearing235˚,8milesfromR.
DrawalinefromD1'tangenttothe9,000yardcircleandintersectingthefirst
relativemovementlineatD1".Drawalineparalleltoandinthedirectionof
D1"D1'fromr.Theintersectionofthislineandthe27-knotcircleatm
2
isthe
second course required,em
2
252˚.8.
(3) Bearing and range ofR fromD1" is 337˚ at 11,250 yards.
(4)TimeintervalforD1totraveltoD1"is:D1D1"/rm
1
=7.8miles/23.2knots
= 20.2 minutes. Time of turn 1000 + 20 = 1020.
(5)Timeintervalforthesecondlegis:D1"D1'/rm
2
=8.8miles/36.5knots=
14.2 minutes.D1 will arrive at new station at 1034.
Answer:
(1) Course 194˚.
(2) Course 253˚.
(3) Bearing 337˚, range 11,250 yards.
(4) Time 1020.
(5) Time 1034.
Speed ofR
Speed ofD4
-------------------------
3000yards,
×
1666yards,
=
319
GUIDE AT CENTER
EXAMPLE 33
Scale: Speed 3:1;
Distance 1:1 mi.
320
PRACTICAL ASPECTS OF MANEUVERING BOARD SOLUTIONS
Theforegoingexamplesandtheiraccompanyingillustrationsarebasedupon
thepremisethatshipsarecapableofinstantaneouschangesofcourseandspeed.
Itisalsoassumedthatanunlimitedamountoftimeisavailablefordetermining
the solutions.
Inactualpractice,theintervalbetweenthesignalforamaneuveranditsexe-
cutionfrequentlyallowsinsufficienttimetoreachacomplete,graphicalsolu-
tion.Nevertheless,undermanycircumstances,safetyandsmartseamanship
bothrequirepromptanddecisiveaction,eventhoughthisactionisdetermined
fromaquick,mentalestimate.Theestimatemustbebasedupontheprinciples
ofrelativemotionandthereforeshouldbenearlycorrect.Courseandspeedcan
bemodifiedenroutetonewstationwhenamoreaccuratesolutionhasbeenob-
tained from a maneuvering board.
Allowancemustbemadeforthosetacticalcharacteristicswhichvarywidely
betweentypesofshipsandalsoundervaryingconditionsofseaandloading.
Experiencehasshownthatitisimpracticaltosolvefortherelativemotionthat
occursduringaturnandthatacceptablesolutionscanbefoundbyeyeandmen-
tal estimate.
BycarefulappraisalofthePPIandmaneuveringboard,therelativemove-
mentofownshipandtheguideduringaturncanbeapproximatedandanesti-
matemadeoftherelativepositionuponcompletionofaturn.Ships’
characteristiccurvesandafewsimplethumbrulesapplicabletoownshiptype
serveasabasisfortheseestimates.Duringthefinalturntheshipcanbebrought
ontostationwithsmallcompensatoryadjustmentsinenginerevolutionsand/or
course.
EXAMPLE 34
ADVANCE, TRANSFER, ACCELERATION, AND DECELERATION
Situation:
OwnshipMisadestroyeronstationbearing020˚,8,000yardsfromtheguide
R.Formationcourseis000˚,speed15knots.Misorderedtotakestationbearing
120˚, 8,000 yards from guide, using 25 knots.
Required:
(1) Course to new station.
(2) Bearing ofR when order is given to resume formation course and speed.
(3) Time to complete the maneuver.
Solution:
(1)PlotRatthecenterwithM
1
bearing020˚,8,000yardsandM
2
bearing
120˚, 8,000 yards. Draw guide’s vector,er, 000˚, 15 knots.
Byeye,itappearsMwillhavetomakeaturntotherightofabout150˚,ac-
celeratingfrom15to25knotsduringtheturn.Priortoreachingthenewstation
areverseturnofaboutthesameamountanddecelerationto15knotswillbere-
quired. Assume thatM averages 20 knots during each turn.
Using30˚rudderat20knots,aDDcalibrationcurveindicatesapproximately
2˚turnpersecondanda600yarddiameter.Thus,a150˚turnwillrequireabout
75secondsandwillproduceatransferofabout600yards.Duringtheturn,R
willadvance625yards(1
1/4
minutesat15knots).Plottingthisapproximateoff-
setdistanceonthemaneuveringboardgivesanewrelativepositionofM
3
atthe
timetheinitialturniscompleted.Similarly,anewoff-setpositionatM
4
isde-
terminedwherealeftturntoformationcourseandreductionofspeedto15
knots should be ordered.
DrawalinefromrparalleltoM
3
M
4
andintersectingthe25-knotspeedcircle
atm. Vectorem is the required course of 158˚.
(2)WhenMreachespointM
4
withRbearing299˚,turnlefttoformation
course using 30˚ rudder and slow to 15 knots.
(3)TimetocompletethemaneuverisM
3
M
4
/SRM+2.5minutes=11,050
yards/39.8 knots + 2.5 minutes = 11 minutes.
Answer:
(1) Course 158˚.
(2) Bearing 299˚.
(3) Time 11 minutes.
321
GUIDE AT CENTER
EXAMPLE 34
Scale: Speed 3:1;
Distance 1:1 yd.
322
MANEUVERING BY SEAMAN’S EYE
Inmanycircumstancesitisimpossibletouseamaneuveringboardinthesolution
ofrelativemovementproblems.Whenthedistancebetweenoldandnewstationsis
shortandwellabaftthebeam,itmaybeimpracticaltoattempttocompletethethe-
oreticallyrequiredturnsandtravelalonganM
1
M
2
path.Insuchcases,areduction
in speed, fishtailing, or various modifications of a fishtail may be required.
Inthefollowingexample,itisassumedthatadestroyertypeshipisproceed-
ingatformationspeedandusingstandardrudderwhichyieldsaperfectturning
circleof1,000yardsdiameterand3,150yardscircumference.Itisalsoassumed
that a 13% reduction in speed is produced by large turns.
Basedupontheseassumptions,ashipusinga45˚fishtaileithersideoffor-
mationcoursewillfallbehindoldstationbyabout400yards.Byusinga60˚
fishtail,itwilldropbackabout700yards.Approximatedistancesforany
amountofcoursechangecanbecomputedifdesired;however,theabovequan-
titiesusedasthumbrulesshouldbesufficient.Repeatedapplicationofeither
willproducelarger“dropbacks”andalsooffertheadvantageofnotusingex-
cessive sea room.
Ifitisdesiredtomovelaterallyaswellasfallback,aturnof45˚tooneside
onlyandthenimmediatereturntooriginalcoursewillproducea300yardtrans-
fer and a 200 yard drop back.
Iftimeisnotaconsiderationandtherelativemovementlineisrelativelyvery
short, a reduction in speed may prove most desirable.
EXAMPLE 35
Situation:
OwnshipMisonformationcourse225˚,speed15knots,withguideRbear-
ing 000˚, 3,000 yards.
Required:
Take station 2,000 yards broad on the port beam of the guide.
Solution:
Anattempttosolvethisproblembynormalmaneuveringboardprocedures
willproveimpractical.M
2
isdirectlyasternofM
1
atadistanceof2,150yards.
AnycombinationofcoursechangesinanattempttotravelalinefromM
1
to
M
2
willresultinownshipfallingfarasternofthenewstation.Evenasimple
360˚turnwilldropownshipback3,600yards,almosttwicethedesiredmove-
ment.
Byfishtailing60˚toeithersideusingcoursesof165˚and285˚threetimesper
side,ownshipwilldropstraightbackapproximately2,000yards,within150
yardsofstation.Finaladjustmenttostationcanbeeffectedbynormalstation
keepingmaneuverssuchasrapidlyshiftingtherudderbetweenmaximumposi-
tions or reduction in engine revolutions.
323
CHANGING STATIONS BY
FISHTAIL METHOD
EXAMPLE 35
324
EXAMPLE 36
FORMATION AXIS ROTATION—GUIDE IN CENTER
Situation:
Theformationisoncourse240˚,speed15knots.Theformationaxisis130˚.
TheguideisinstationZeroandownshipisinstation6330.TheOTCrotates
the formation axis to 070˚. Stationing speed is 20 knots.
Required:
(1)Courseat20knotstoregainstationrelativetothenewformationaxis,
070˚.
Solution:
(1)Marktheinitialandnewformationaxesat130˚and070˚,respectively.
Plottheguide’sstationinthecenter(stationZero)andlabelasR.Plotown
ship’sinitialpositionM
1
oncircle6inadirectionfromtheformationcenter
330˚relativetotheinitialformationaxis.Drawercorrespondingtoguide’s
course 240˚ and speed 15 knots.
(2)Plotownship’snewpositionM
2
orientedtothenewaxis.Theoriginalsta-
tionassignmentsareretained,exceptthestationsarenowrelativetothenewax-
is.
(3)Drawthedirectionofrelativemovementline(DRM)fromM
1
throughM
2
.
(4)Throughrdrawalineinthedirectionofrelativemovementintersecting
the 20-knot circle atm.
(5) Own ship’s true vector isem: course 293˚, speed 20 knots.
Answer:
(1) Course 293˚ to regain station relative to the new axis.
325
GUIDE AT CENTER
EXAMPLE 36
Scale: Speed 3:1;
Distance 1:1 thousands of yds.
326
EXAMPLE 37
FORMATION AXIS ROTATION—GUIDE OUT OF CENTER
FORMATION CENTER KEPT IN CENTER OF PLOT
Situation:
Theformationisoncourse275˚,speed18knots.Theformationaxisis190˚.
Theguideisinstation3030andownshipisinstation7300.TheOTCrotates
the formation axis to 140˚. Stationing speed is 20 knots.
Required:
(1)Courseat20knotstoregainstationrelativetothenewformationaxis,
140˚.
Solution:
(1)Marktheinitialandnewformationaxesat190˚and140˚,respectively.
Plottheguide’sinitialstationR
1
oncircle3inadirectionfromtheformation
center30˚relativetotheinitialformationaxis.Plotownship’sinitialstationS
1
oncircle7inadirectionfromtheformationcenter300˚relativetotheinitial
formationaxis.Drawercorrespondingtoguide’scourse275˚andspeed18
knots.
(2)Plottheguide’snewstationR
2
orientedtothenewformationaxis;plot
own ship’s new stationS
2
oriented to the new formation axis.
(3)MeasurethebearingsanddistancesofS
1
andS
2
fromR
1
andR
2
,respec-
tively.
(4)Fromthecenter,plotthebearinganddistanceofS
1
fromR
1
asM
1
andthe
bearing and distance ofS
2
fromR
2
asM
2
.
(5)SincethelinefromM
1
toM
2
representstherequiredDRMforownshipto
regainstationrelativetothenewaxis,drawalinethroughrinthedirectionof
relative movement.
(6) Own ship’s true vector isem: course 291˚, speed 20 knots.
Answer:
(1) Course 291˚ to regain station relative to the new axis.
327
GUIDE OUT OF CENTER
EXAMPLE 37
Scale: Speed 3:1;
Distance 1:1 thousands of yds.
328
EXAMPLE 38
FORMATION AXIS ROTATION—GUIDE OUT OF CENTER
Situation:
Theformationisoncourse275˚,speed18knots.Theformationaxisis190˚.
Theguideisinstation3030andownshipisinstation7300.TheOTCrotates
the formation axis to 140˚. Stationing speed is 20 knots.
Required:
(1)Courseat20knotstoregainstationrelativetothenewformationaxis,
140˚.
Solution:
(1)Marktheinitialandnewformationaxesat190˚and140˚,respectively.
Plottheguide’sstationR
1
oncircle3inadirectionfromtheformationcenter
30˚relativetotheinitialformationaxis.Plotownship’sstationM
1
oncircle7
inadirectionfromtheformationcenter300˚relativetotheinitialformationax-
is. Drawer corresponding to guide’s course 275˚ and speed 18 knots.
(2)Plottheguide’sstation,R
2
,orientedtothenewformationaxis.Plotown
ship’spositionM
3
orientedtothenewaxis.Theoriginalstationassignmentsare
retained, except the stations are now relative to the new axis.
(3)Shifttheinitialpositionofownship’sstationatM
1
inthedirectionand
distanceofthefictitiousshiftoftheguidetoitspositionrelativetothenewaxis.
Mark the initial position so shifted asM
2
.
(4)Drawthedirectionofrelativemovementlines(DRM)fromM
2
through
M
3
.
(5)Throughrdrawalineinthedirectionofrelativemovementintersecting
the 20-knot circle atm.
(6) Own ship’s true vector isem: course 291˚, speed 20 knots.
Answer:
(1) Course 291˚ to regain station relative to the new axis.
Explanation:
Sincetheguidedoesnotactuallymoverelativetotheinitialformationcenter
whilemaintainingcourseandspeedduringtheformationmaneuver,allinitial
positionsofstationsintheformationmustbemovedinthesamedirectionand
distance as the fictitious movement of the guide to its new position.
329
GUIDE OUT OF CENTER
EXAMPLE 38
Scale: Speed 3:1;
Distance 1:1 thousands of yds.
330
EXAMPLE 39
COURSE AND SPEED BETWEEN TWO STATIONS, REMAINING WITHIN A
SPECIFIED RANGE FOR SPECIFIED TIME INTERVAL ENROUTE
Situation:
OwnshipMisonstationbearing280˚,5milesfromtheguideRonformation
course 190˚, speed 20 knots.
Required:
At1500ownshipMisorderedtoproceedtonewstationbearing055˚,20
miles,arrivingat1630andtoremainwithina10-milerangefor1hour.The
commandingofficerelectstoproceeddirectlytonewstation,adjustingcourse
and speed as necessary to comply with the foregoing requirements.
(1) Course and speed to remain within 10 miles for 1 hour.
(2) Course and speed required at 1600.
(3) Bearing ofR at 1600.
Solution:
(1)Plotthe1500and1630positionsofMatM
1
andM
3
,respectively.Draw
therelativemotionline,M
1
M
3
,intersectingthe10-milecircleatM
2
.Drawer.
MeasureM
1
M
2
:13.6miles.Thetimerequiredtotransitthisdistanceis1hour
atanSRMof13.6knots.Throughrdrawrm
1
13.6knotsinlength,parallelto
and in the directionM
1
M
3
. Vectorem
1
is 147˚.5, 16.2 knots.
(2)MeasureM
2
M
3
,10.3miles,whichrequiresanSRMof20.6knotsforone
half hour. Throughr drawrm
2
. Vectorem
2
is 125˚.5, 18.2 knots.
(3) By inspection,R bears 226˚ fromM
2
at 1600.
Answer:
(1) Course 148˚, speed 16.2 knots.
(2) Course 126˚, speed 18.2 knots.
(3) Bearing 226˚.
Explanation:
SinceownshipMmustremainwithin10milesoftheguidefor1hour,M
mustnotplotalongM
1
M
2
fartherthanM
2
priorto1600.Therequiredmagni-
tudesoftherelativespeedvectorsfortimeintervals1500to1600and1600to
1630togetherwiththeircommondirectionarecombinedwiththetruevectorof
the guide to obtain the two true course vectors for own ship.
331
GUIDE AT CENTER
EXAMPLE 39
Scale: Speed 3:1;
Distance 2:1 mi.
332
EXAMPLE 40
COURSE AT MAXIMUM SPEED TO OPEN RANGE TO A SPECIFIED
DISTANCE IN MINIMUM TIME
Situation:
OwnshipMhasguideRbearing240˚,range12miles.Theguideisoncourse
120˚, speed 15 knots. Own ship’s maximum speed is 30 knots.
Required:
Open range to 18 miles as quickly as possible.
(1) Course at 30 knots.
(2) Time to complete the maneuver.
(3) Bearing of guide upon arrival at specified range.
Solution:
Thekeytothissolutionistofindthatrelativeposition(M')oftheguidethat
couldexistbeforetheproblemstartsinordertobeabletodrawtheRML
throughthegivenrelativeposition(M
1
)andM'tointersectthespecifiedrange
circle.
(1)PlotRandM
1
.AboutRdescribeacircleofradius18miles.Drawer.
AlongR’s course plotM' 9 miles fromR.
DrawalinethroughM'andM
1
andextendittointersectthe18-milerange
circle atM
2
.
FromrdrawrmparalleltoandinthedirectionM
1
M
2
.Theintersectionofrm
andthe30-knotspeedcircleisthecourserequiredtocompletethemaneuverin
minimum time. Vectorem is 042˚.6, 30 knots.
(2)SRMis30.5knots.MRMis7.5miles.Timetocompletethemaneuver:
14.8 minutes.
(3)Uponreachingthe18-milerangecircle,RisdeadasternofMbearing
222˚.6.
Answer:
(1) Course 043˚.
(2) Time 15 minutes.
(3) Bearing 223˚.
Explanation:
ForMtoopenorclosetoaspecifiedrangeinminimumtime,Mmusttravel
theshortestgeographicaldistanceatmaximumspeed.Theshortestdistanceis
alongtheradiusofacirclecenteredatthepositionoccupiedbyRattheinstant
M reaches the specified range circle.
Inthe“openingrange”problem,determinehypotheticalrelativepositionsof
MandRthatcouldexistbeforetheproblemstarts.Referringtothegeograph-
icalplot,assumeMstartsfrompositionM'andproceedsoutwardalongsome
radius18milesinlengthonanunknowncourseat30knots.IfRmovestoward
itsfinalpositionatR
2
alongthegivencourseof120˚,speed15knots,itshould
arriveatR
2
theinstantMreachesthe18-milecircle.Atthisinstant,theproblem
conditionsaresatisfiedbyMbeing18milesdistantfromR.However,own
ship’scourserequiredtoreachthispositionisnotyetknown.Duringthetime
intervalMopened18milesat30knots,Rmoved9milesat15knotsfromR'to
R
2
.
ThisprovidestheneededsecondrelativepositionofM'fromR',9milesbear-
ing 120˚. This position is then transferred to therelative plot.
Speed ofR
Speed ofM
----------------------------
18miles9 miles=×
Speed ofM
Speed ofR
----------------------------
18miles9 miles=×
333
GUIDE AT CENTER
EXAMPLE 40
Scale: Speed 3:1;
Distance 2:1 mi.
334
EXAMPLE 41
COURSE AT MAXIMUM SPEED TO CLOSE RANGE TO A SPECIFIED DISTANCE IN MINIMUM TIME
Situation:
OwnshipMhastheguideRbearing280˚,range10miles.Theguideison
course 020˚, speed 15 knots. Own ship’s maximum speed is 24 knots.
Required:
Close range to 2 miles as quickly as possible.
(1) Course at 24 knots.
(2) Time to complete the maneuver.
(3) Bearing of guide upon arrival at the specified range.
Solution:
Thekeytothissolutionistofindthatrelativeposition(M')oftheguidethat
couldexistaftertheproblemstartsinordertobeabletodrawtheRMLthrough
the given relative position (M
1
) andM' to intersect the specified range circle.
(1)PlotRandM
1
.AboutRdescribeacircleofradius2miles.Drawer,
guide’sspeedvector020˚,15knots.OnreciprocalofR’scourseplotM'1.25
miles fromR.
DrawalinethroughM'andM
1
.Theintersectionofthislineandthe2-mile
range circle isM
2
.
FromrdrawalineparalleltoandinthedirectionM
1
M
2
.Theintersectionof
thislineandthe24-knotspeedcircleatmisthecourserequiredtocompletethe
maneuver in minimum time. Vectorem 309˚.8, 24 knots.
(2)SRMis23.6knots.MRMis8.3miles.Timetocompletethemaneuver:
21.1 minutes.
(3)Uponreachingthe2-milerangecircle,RisdeadaheadofMonabearing
309˚.8.
Answer:
(1) Course 310˚.
(2) Time 21 minutes.
(3) Bearing 310˚.
Explanation:
Referringtothegeographicalplot,assumeMstartsfrompositionM
1
and
proceedsinwardalongsomeradiusonanunknowncourseat24knots.IfR
movestowarditsfinalpositionatR
2
alongthegivencourse020˚,speed15
knots,itshouldarriveatR
2
theinstantMreachesthe2-milecircle.Atthisin-
stanttheproblemconditionsaresatisfiedalthoughthesolutionforownship’s
courseisnotyetknown.AssumethatMcontinuesonthesamecourseandspeed
throughthe2milestoM'atthecenterofthecirclewhileRmovesawayfrom
thecenteroncourse020˚,speed15knots.DuringthetimeintervalthatM
moves these 2 miles at 24 knots,R opens 1.25 miles.
ThisprovidestheneededsecondrelativepositionofM'fromR':1.25miles,
bearing 200˚. This position is then transferred to therelative plot.
Speed ofR
Speed ofM
----------------------------
2miles1.25miles=×
Speed ofR
Speed ofM
----------------------------
2miles1.25miles=×
335
GUIDE AT CENTER
EXAMPLE 41
Scale: Speed 3:1;
Distance 1:1 mi.
336
EXAMPLE 42
COURSE AT MAXIMUM SPEED TO REMAIN WITHIN A SPECIFIED RANGE
FOR MAXIMUM TIME
Situation:
ShipRbears110˚,4milesfromM.Risoncourse230˚,18knots.Maximum
speed ofM is 9 knots.
Required:
Remain within a 10-mile range ofR for as long as possible.
(1) Course at maximum speed.
(2) Bearing ofR upon arrival at specified range.
(3) Length of time within specified range.
(4) CPA.
Solution:
(1)PlotRatthecenterandM
1
bearing290˚,4milesfromR.AboutRdescribe
arcsofradius9knotsand10miles.Drawer230˚,18knots.Alongtherecipro-
cal ofR’s course, plotM' 20 miles fromR.
DrawalinethroughM'andM
1
.TheintersectionofM'M
1
andthe10-mile
rangecircleisM
2
,thepointbeyondwhichthespecifiedorlimitingrangeisex-
ceeded.ThroughrdrawalineparalleltoandinthedirectionM
1
M
2
.Theinter-
sectionofthislineatpointmonthe9-knotspeedcircleistherequiredcourse
to remain within 10 miles ofR. Vectorem is 220˚.8, 9 knots.
(2)UponarrivalatlimitingrangeatM
2
,RisdeadaheadofMbearing220˚.8.
(3) The time interval within specified range is:
(4) Drop a perpendicular fromR toM
1
M
2
. CPA is 148˚.9, 3.1 miles.
Note:
WhenM’sspeedisequaltoorgreaterthanthatofR,aspecialcaseexistsin
which there is no problem insofar as remaining within a specified range.
Answer:
(1) Course 221˚.
(2) Bearing 221˚.
(3) Time 79 minutes.
(4) CPA 149˚, 3.1 miles.
Explanation:
Asinthe“closingrange”problem,example39,determinehypotheticalrela-
tivepositionsofMandRthatcouldexistaftertheproblemends.Referringto
thegeographicalplot,assumeMstartsfrompositionM
1
andproceedsinward
alongsomeradiusonanunknowncourseat9knots.Risoncourse230˚at18
knots.AttheinstantRpassesthroughR
2
,Mreachesthe10-milelimitingrange
atM
2
.Atthisinstanttheproblemconditionsaresatisfiedalthoughthesolution
isnotyetknown.AssumethatMcontinuesonthesamecourseandspeedfor10
milestothecenterofthecirclewhileRmovesawayfromthecenteroncourse
230˚,speed18knots.DuringthetimeintervalMcloses10milestowardthecen-
ter,R opens 20 miles at 18 knots.
ThisthengivesustheneededsecondrelativepositionofR'fromM',20miles
bearing 230˚. This position is then transferred to therelative plot.
Speed ofR
Speed ofM
----------------------------
10 miles×20 miles=
M
1
M
2
rm
---------------
12 miles
9.1 knots
----------------------
78.8 minutes==
Speed ofR
Speed ofM
----------------------------
10 miles×20 miles=
337
GUIDE AT CENTER
EXAMPLE 42
Scale: Speed 2:1;
Distance 2:1 mi.
338
EXAMPLE 43
COURSE AT MAXIMUM SPEED TO REMAIN OUTSIDE OF A SPECIFIED
RANGE FOR MAXIMUM TIME
Situation:
ShipRbears020˚,14milesfromownshipM.Risoncourse210˚,speed18
knots. Maximum speed ofM is 9 knots.
Required:
Remain outside a 10-mile range fromR for as long as possible.
(1) Course at maximum speed.
(2) Bearing ofR upon arrival at specified range.
(3) Time interval before reaching specified range.
Solution:
(1)PlotRatthecenterandM
1
bearing200˚,14milesfromR.AboutR,de-
scribecirclesofradius9knotsand10miles.Drawer210˚,speed18knots.
AlongR’s course, plotM' 20 miles fromR.
DrawalinethroughM'andM
1
intersectingthe10-milerangecircleatM
2
.
ThroughrdrawalineparalleltoandinthedirectionofM
1
M
2
intersectingthe
9-knotspeedcircleatm.Completionofthespeedtriangleproducesem,there-
quired course of 184˚.2 at 9 knots.
(2)UponarrivalatlimitingrangeatpointM
2
,RisdeadasternofMbearing
004˚.2.
(3) The time interval outside of specified range is:
Note:
OwnshipcanremainoutsidethelimitingrangeindefinitelyifM
1
fallsoutside
theareabetweentwotangentsdrawntothelimitingrangecirclefromM'andif
R remains on the same course and speed.
Answer:
(1) Course 184˚.
(2) Bearing 004˚.
(3) Time 30 minutes.
Explanation:
Todetermineacoursetoremainoutsideofagivenrangeformaximumtime,
determinehypotheticalrelativepositionsofMandRthatcouldexistbeforethe
problemstarts.Referringtothegeographicalplot,assumeMstartsfromposi-
tionM'andproceedsoutwardalongsomeradiusonanunknowncourseat9
knots.IfRmovestowarditsfinalpositionR
2
alongthegivencourse210˚,speed
18knots,itshouldarriveatR
2
theinstantMreachesthe10-milecircleatM
2
.At
thisinstanttheproblemconditionsaresatisfiedalthoughthesolutionforown
ship’scourseisnotyetknown.DuringthetimeintervalrequiredforMtomove
fromM'toM
2
,10milesat9knots,RmovesfromR'toR
2
,20milesat18knots
along the given course 210˚.
Thisprovidestheneededsecondrelativeposition,M'bearing210˚,20miles
fromR'. This position is then transferred to therelative plot.
Speed ofR
Speed ofM
----------------------------
10 miles×20 miles=
M
1
M
2
rm
---------------
5.2miles
10.7knots
-------------------------
30minutes==
Speed ofR
Speed ofM
----------------------------
10 miles×20 miles=
339
GUIDE AT CENTER
EXAMPLE 43
Scale: Speed 2:1;
Distance 2:1 mi.
340
USE OF A FICTITIOUS SHIP
TheexamplesgiventhusfarinPARTTWOhavebeenconfinedtoshipsthat
haveeithermaintainedconstantcoursesandspeedsduringamaneuverorelse
haveengagedinasuccessionofsuchmaneuversrequiringonlyrepeatedappli-
cationofthesameprinciples.Whenoneoftheshipsalterscourseand/orspeed
duringamaneuver,apreliminaryadjustmentisnecessarybeforetheseprinci-
ples can be applied.
Thisadjustmentconsists,ineffect,ofsubstitutingafictitiousshipforthe
ship making the alteration. This fictitious ship is presumed to:
(1)maintainaconstantcourseandspeedthroughouttheproblem(thisisthe
final course and speed of the actual ship).
(2)startandfinishitsrunattimesandpositionsdeterminedbytheconditions
established in the problem.
Forexample,thecourseandspeedofadvanceofashipzig-zaggingarecon-
sideredtobetheconstantcourseandspeedofafictitiousshipwhichdeparts
fromagivenpositionatagiventimesimultaneouslywiththeactualship,and
arrivessimultaneouslywiththeactualshipatthesamefinalposition.Theprin-
ciplesdiscussedinpreviousexamplesarejustasvalidforafictitiousshipasfor
anactualship,bothintherelativeplotandspeedtriangle.Ageographicalplot
facilitates the solution of this type.
EXAMPLE 44
ONE SHIP ALTERS COURSE AND/OR SPEED DURING MANEUVER
Situation:
At0630shipRbears250˚,range32miles.Risoncourse345˚,speed15knots
but at 0730 will change course to 020˚ and speed to 10 knots.
Required:
Own shipM take station 4 miles ahead ofR using 12 knots speed.
(1) Course to comply.
(2) Time to complete maneuver.
Solution:
Determinethe0630positionofafictitiousshipFthat,bysteeringcourse020˚
atspeed10knots,willpassthroughthe0730positionsimultaneouslywiththe
actualship.Inthiswaythefictitiousshiptravelsonasteadycourseof020˚,
speed 10 knots throughout the problem.
(1)ConstructageographicalplotwithRandR
1
the0630and0730positions
respectivelyofshipRmovingalongcourse345˚at15knots.PlotF,the0630
positionofthefictitiousshipbearing200˚,10milesfromR
1
.Bymeasurement,
Fbears304˚,8.8milesfromR.TransferthispositiontoarelativeplotwithRat
the center.
PlotownshipatM
1
bearing070˚,32milesfromR.Drawerf,thefictitious
ship’svector,020˚,10knots.Layoffownship’sfinalposition,M
2
,4miles
aheadofFalongitsfinalcourse020˚.DrawtherelativemovementlineM
1
M
2
and,paralleltoit,constructtherelativespeedvectorfromrftoitsintersection
with the 12-knot circle atm. This producesem the required course of 316˚.
(2)ThetimetocompletethemaneuvercanbeobtainedfromtheTDSscale
usingMRMof36.4milesandSRMof11.8knotswhichgivesatimeof3.1
hours.
Answer:
(1) Course 316˚.
(2) Time 3 hours 6 minutes.
341
GUIDE AT CENTER
EXAMPLE 44
Scale: Speed 2:1;
Distance 4:1 mi.
342
EXAMPLE 45
BOTH SHIPS ALTER COURSE AND/OR SPEED DURING MANEUVER
Situation:
At0800Risoncourse105˚,speed15knotsandwillchangecourseto350˚,
speed18knotsat0930.OwnshipMismaintainingstationbearing330˚,4miles
fromR.Misorderedtotakestationbearing100˚,12milesfromR,arrivingat
1200.
Required:
(1) Course and speed forM to comply if maneuver is begun at 0800.
(2)CourseforMtocomplyifMdelaysthecoursechangeaslongaspossible
and remains at 15 knots speed throughout the maneuver.
(3) Time to turn to course determined in (2).
Solution:
SincetherelativepositionsofRandMatthebeginningandendofthema-
neuverandthetimeintervalforthemaneuveraregiven,thesolutionfor(1)can
beobtaineddirectlyfromageographicalplot.Solvetheremainderoftheprob-
lem using arelative plot.
(1)Usingageographicalplot,layoutR’s0800-1200trackthroughpointsR
1
,
R
2
,andR
3
.PlotM
1
andM
3
relativetoR
1
andR
3
,respectively.Thecourse040˚
fromM
1
toM
3
canbemeasureddirectlyfromtheplot.Mwillrequireaspeedof
10.8 knots to move 43.4 miles in 4 hours.
Thissolutionmaybeverifiedonarelativeplotbymeansofafictitiousship.
First,usingageographicalplot,determinethe0800positionofafictitiousship
that,bysteering350˚,speed18knots,willpassthroughthe0930positionsi-
multaneouslywithR.At0800ownshipatM
1
bears322˚,45.7milesfromthe
fictitiousshipatF
1
.Transferthesepositionstoarelativeplot,placingFatthe
center.Plotownship’s1200positionatM
3
bearing100˚,12milesfromF.Draw
thefictitiousship’svectorerf
1
350˚,18knots.Fromrf
1
,constructtherelative
speedvectorparalleltoM
1
M
3
and13.8knotsinlength.(MRMof55.2miles/4
hours = 13.8 knots.) Drawem
1
, the required course of 040˚, 10.8 knots.
(2)TofindthetwolegsofM’s0800-1200track,usearelativeplot.Drawem
2
,
ownship’svectorwhichisgivenas105˚,15knots.Atthisstageofthesolution,
disregardRandconsiderownshipMtomaneuverrelativetoanewfictitious
ship.Ownshiponcourse040˚,10.8knotsfrompart(1)isthefictitiousship
used.Labelvectorem
1
aserf
2
,thefictitiousship’svector.Frompointm
2
draw
alinethroughrf
2
extendedtointersectthe15-knotspeedcircleatm
3
.Drawem
3
,
the second course of 012˚ required byM in changing station.
(3)Tofindthetimeoneachlegdrawatimelinefromm
2
usinganyconve-
nientscale.Throughm
3
drawm
3
X.Throughm
1
drawm
1
Yparalleltom
3
X.Sim-
ilartrianglesexist;thus,thetimelineisdividedintoproportionaltimeintervals
fortwolegs.XYisthetimeonthefirstleg:1hour22minutes.Theremainder
of the 4 hours is spent on the second leg.
Answer:
(1) Course 040˚, 10.8 knots.
(2) Course 012˚.
(3) Time 0922.
Note:
Intheaboveexample,analternativeconstructionofthetimelineasdefined
intheglossaryisusedsothatthelinecanbedrawntoaconvenientscale.The
proportionalityismaintainedbyconstructingsimilartriangles.SeeNotewith
example 47.
343
GUIDE AT CENTER
EXAMPLE 45
Scale: Speed 2:1;
Distance 4:1 mi.
344
EXAMPLE 46
COURSES AT A SPECIFIED SPEED TO SCOUT OUTWARD ON PRESENT
BEARING AND RETURN AT A SPECIFIED TIME
Situation:
OwnshipMismaintainingstationontheguideRwhichbears110˚,range5
miles. Formation course is 055˚, speed 15 knots.
Required:
Commencingat1730,scoutoutwardonpresentbearingandreturntopresent
station at 2030. Use 20 knots speed.
(1) Course for first leg.
(2) Course for second leg.
(3) Time to turn.
(4) Maximum distance from the guide.
Solution:
(1)PlotRatthecenterandM
1
bearing290˚,5milesfromR.Drawer055˚,
15knots.TheDRM“out”isalongthebearingofMfromR.TheDRM“in”is
alongthebearingofRfromM.ThroughrdrawalineparalleltotheDRM’sand
intersectingthe20-knotcircleatm
1
andm
2
.Vectorrm
1
istheDRM“out”.Vec-
torem
1
is 327˚.8, the course “out”.
(2) Vectorrm
2
is the DRM “in”. Vectorem
2
is 072˚, the course “in”.
(3)Tofindthetimeoneachleg,drawatimelinefromm
1
usinganyconve-
nientscale.Throughm
2
drawm
2
X.ThroughrdrawrYparalleltom
2
X.Similar
trianglesexist;thus,thetimelineisdividedintoproportionaltimeintervalsfor
thetwolegs.XYisthetimeonthefirstleg,41minutes.Theremainderofthe
time is spent on the second leg returning to station.
(4)RangeofRwhencourseischangedto“in”legis21.7miles.Initialrange
+ (rm
1
x
time on “out” leg).
Answer:
(1) Course 328˚.
(2) Course 072˚.
(3) Time 1811.
(4) Distance 21.7 miles.
Explanation:
SinceownshipRreturnstopresentstation,relativedistancesoutandinare
equal. In going equal distances, time varies inversely as speed:
Therefore,thetimeoutpartofthespecifiedtime(3
h
)isobtainedbysimplepro-
portion or graphically.
Asdefinedintheglossary,thetimelineisthelinejoiningtheheadsofvectors
em
1
andem
2
.Thislineisdividedbytheheadofvectorerintosegmentsinverse-
lyproportionaltothetimesspentbyownshipRonthefirst(out)andsecond
(in)legs.Intheaboveexampleanalternativeconstructionisusedsothattheline
canbedrawntoaconvenientscale.Theproportionalityismaintainedbycon-
structing similar triangles.
time (out)
time (in)
-----------------------
relative speed (in)
relative speed (out)
----------------------------------------------
rm
1
(in)
rm
2
(out)
---------------------==
345
GUIDE AT CENTER
EXAMPLE 46
Scale: Speed 2:1;
Distance 2:1 mi.
346
EXAMPLE 47
COURSES AND MINIMUM SPEED TO CHANGE STATIONS WITHIN A
SPECIFIED TIME, WHILE SCOUTING ENROUTE
Situation:
OwnshipMbears130˚,8milesfromtheguideRwhichisoncourse040˚,
speed 12 knots.
Required:
Proceedtonewstationbearing060˚,10milesfromtheguide,passingthrough
apointbearing085˚,25milesfromtheguide.Completethemaneuverin4.5
hours using minimum speed.
(1) First and second courses forM.
(2) Minimum speed.
(3) Time to turn to second course.
Solution:
(1)PlotM
1
,M
2
andM
3
at130˚,8miles;085˚,25miles;and060˚,10miles
fromR,respectively.Drawer040˚,12knots.Fromrdrawlinesofindefinite
lengthparalleltoandinthedirectionofM
1
M
2
andM
2
M
3
.Assumethataficti-
tiousship,F,departsM
1
simultaneouslywithMandproceedsdirectlytoM
3
ar-
rivingatthesametimeasMwhichtraveledthroughM
2
enroute.Thefictitious
shipcoversarelativedistanceof10.5milesin4.5hours.SRMofthefictitious
shipis2.3knots.Throughrdrawrrf,therelativespeedvector,2.3knotsparallel
toandinthedirectionofM
1
M
3
.Vectorerfisthetruecourseandspeedvector
ofthefictitiousship.Withrfasapivot,rotateastraightlinesothatitintersects
thetwopreviouslydrawnlinesonthesamespeedcircle.Thepointsofintersec-
tionarem
1
andm
2
.Vectorem
1
isthecourseout:049˚.Vectorem
2
isthecourse
in: 316˚.9.
(2)Pointsm
1
andm
2
lieonthe17.2knotcirclewhichistheminimumspeed
to complete the maneuver.
(3)Fromm
2
layoffa4.5hourtimelineusinganyconvenientscale.Draw
m
1
X.DrawrfYparalleltom
1
X.ThepointYdividesthetimelineintopartsthat
areinverselyproportionaltotherelativespeedsrfm
1
andrfm
2
.XYthetime“in”
is51minutes.Ym
2
thetime“out”is3hours39minutes.Timeoneachlegmay
also be determined mathematically by the formula MRM/SRM = time.
Answer:
(1) First course 049˚, second 317˚.
(2) Speed 17.2 knots.
(3) Time 3 hours and 39 minutes.
Note:
Thetimeline,asdefinedintheglossary,isthelinejoiningtheheadsofvec-
torsem
1
andem
2
andtouchingtheheadofthefictitiousshipvectorerf.Thistime
lineisdividedbytheheadofthefictitiousshipvectorintosegmentsinversely
proportional to the times spent by the unit on the first and second legs.
Intheaboveexample,analternativeconstructionofthetimelineisusedso
thatthelinecanbedrawntoaconvenientscale.Theproportionalityismain-
tained by constructing similar triangles.
347
GUIDE AT CENTER
EXAMPLE 47
Scale: Speed 2:1;
Distance 3:1 mi.
348
EXAMPLE 48
LIMITING LINES OF APPROACH
Situation:
Acircularformationofships4milesacross,withguideRthecenterispro-
ceedingoncourse000˚,15knots.Anenemytorpedofiringsubmarineissus-
pectedtobeinapositionsomedistanceaheadoftheformationwithamaximum
speed capability corresponding to modes of operation of:
Note:
Themaximumspeedsabovewerechosenforexamplepurposesandshould
NOTbeusedasrealestimates.Consultappropriateintelligencepublicationson
individual submarines for correct speeds.
Required:
(1) Construct Limiting Lines of Submerged Approach (LLSUA).
(2) Construct Limiting Lines of Quiet Approach (LLQA).
(3) Construct Limiting Lines of Snorkel Approach (LLSNA).
(4) Construct Limiting Lines of Surfaced Approach (LLSA).
Solution:
(1)PlotRatthecenterofthemaneuveringboardanddrawthevector“er”
000˚,15knots.ConstructtheTDZfortheassumedeffectivetorpedofiring
range(e.g.,5miles)andtorpedospeed(e.g.,30knots).From“r”describean
arc(withradiusof5knots),theassumedsubmergedspeed.Drawthetangent
vector“emsu”tothearcandparallelthisvectortotheTDZ.Byextendingthe
parallelvectoruntilitintersectstheformationcoursevector,theotherlimiting
linetotheTDZcanbeconstructed(theareaenclosedbytheLimitingLinesof
SubmergedApproach(LLSUA)andtheaftperimeteroftheTDZdefinesthe
submarineDangerZone).Solutions(2)through(4)usethesimilarconstruction
principlesasinsolution(1)toconstructtheLLQA,LLSNAandLLSAusing
their respective assumed speeds.
Note:
Thisconstructionassumesthesubmarinemaintainsaconstantspeedthrough-
out the approach.
Submerged (SU) speed:5knots
Quiet (Q) speed:8knots
Snorkel (SN) speed:10knots
Surfaced (S) speed:12knots
349
GUIDE AT CENTER
EXAMPLE 48
Scale: Speed 2:1;
Distance 4:1 mi.
350
EXAMPLE 49
TORPEDO DANGER ZONE (TDZ)
Situation:
Acircularformationofships4milesacross,withguideRatthecenterispro-
ceedingoncourse000˚,at15knots.Anenemytorpedocarryingsubmarineis
suspected of being in the area with weapon parameters of:
Required:
Torpedo Danger Zone (TDZ)
Solution:
PlotRatthecenterofthemaneuveringboard.Calculatetheformation’sad-
vancedposition(i.e.,R’sfuturepositionalongtheformationdirectionofad-
vance if a torpedo is fired whenR was located at board center) by:
LabelthispositionAPandplottheformationaroundAP.ConstructtheTDZ
outerboundarybyplottingpointsatadistanceequaltothemaximumeffective
torpedofiringrange(e.g.,5miles)fromtheperimeteroftheformation.Thearea
enclosed is the TDZ relative to the formation in its original position aroundR.
Note:
Thetorpedorangeandspeedwerechosenforexamplepurposesonlyand
shouldnotbeusedasrealestimates.Consultappropriateintelligencepublica-
tions on individual submarine torpedoes for correct ranges and speeds.
Maximum effective torpedo firing range:5 miles
Speed:30 knots
Advanced Position
Maximum Effective Torpedo
Firing RangeFormation Speed
×
Torpedo Speed
-----------------------------------------------------------------------=
351
GUIDE AT CENTER
EXAMPLE 49
Scale: Speed 3:1;
Distance 2:1 mi.
352
EXAMPLE 50
MISSILE DANGER ZONE (MDZ)
Situation:
Acircularformationofships4milesacrosswithguideRatthecenterispro-
ceedingoncourse000˚at15knots.Anenemymissilecarryingsubmarineis
suspected of being in the area with weapon parameters of:
Required:
Missile Danger Zone (MDZ)
Solution:
PlotRatthecenterofthemaneuveringboard.Sincetheenemy’smissiletrav-
elsat40timestheformation’sspeed,theformationwillnotappreciablyad-
vanceduringthemissile’stimeofflight.Themissile’smaximumeffective
firingrange(20miles)isaddedtotheperimeteroftheformationandplotted
around the formation. The area enclosed is the MDZ.
Note:
Themissilerangeandspeedwerechosenforexamplepurposesonlyand
shouldnotbeusedasrealestimates.Consultappropriateintelligencepublica-
tions on individual submarine missiles for correct ranges and speeds.
Maximum effective missile firing range:20 miles
Speed:600 mph
353
GUIDE AT CENTER
EXAMPLE 50
Scale: Speed 3:1;
Distance 8:1 mi.
354
Space is provided for user’s insertion of example according to his needs
Situation:
Required:
Solution:
Answers:
355
Scale: Speed :1;
Distance :1 thousands of yds.
356
Space is provided for user’s insertion of example according to his needs
Situation:
Required:
Solution:
Answers:
357
Scale: Speed :1;
Distance :1 thousands of yds.
358
Space is provided for user’s insertion of example according to his needs
Situation:
Required:
Solution:
Answers:
359
Scale: Speed :1;
Distance :1 thousands of yds.
360
Space is provided for user’s insertion of example according to his needs
Situation:
Required:
Solution:
Answers:
361
Scale: Speed :1;
Distance :1 thousands of yds.
362
Space is provided for user’s insertion of example according to his needs
Situation:
Required:
Solution:
Answers:
363
Scale: Speed :1;
Distance :1 thousands of yds.
364
Space is provided for user’s insertion of example according to his needs
Situation:
Required:
Solution:
Answers:
365
Scale: Speed :1;
Distance :1 thousands of yds.
367
APPENDIX A
EXTRACT FROM REGULATION 12, CHAPTER V OF THE IMO-SOLAS (1974) CONVENTION AS AMENDED TO 1983
THE REQUIREMENT TO CARRY RADAR AND ARPA
Shipsof500grosstonnageandupwardsconstructedonorafter1
September1984andshipsof1600grosstonnageandupwardsconstructed
before 1 September 1984 shall be fitted with a radar installation.
Shipsof1000grosstonnageandupwardsshallbefittedwithtworadar
installations, each capable of being operated independently of the other.
Facilitiesforplottingradarreadingsshallbeprovidedonthenavigating
bridgeofshipsrequiredbyparagraph(g)or(h)tobefittedwitharadar
installation.Inshipsof1600grosstonageandupwardsconstructedonor
after1September1984,theplottingfacilitiesshallbeatleastaseffectiveas
a reflection plotter.
An automatic radar plotting aid shall be fitted on:
1.Shipsof10,000grosstonnageandupwards,constructedonor
after 1 September 1984;
2.Tankers constructed before 1 September 1984 as follows:
(a)Ifof40,000grosstonnageandupwards,by1January
1985;
(b)Ifof10,000grosstonnageandupwards,butlessthan
40,000 gross tonnage, by 1 September 1986;
3.Shipsconstructedbefore1September1984,thatarenottankers,
as follows:
(a)Ifof40,000grosstonnageandupwards,by1September
1986;
(b)Ifof20,000grosstonnageandupwards,butlessthan
40,000 gross tonnage, by 1 September 1987;
(c)Ifof15,000grosstonnageandupwards,butlessthan
20,000 gross tonnage, by 1 September 1998.
(ii)Automaticradarplottingaidsfittedpriorto1September1984which
donotfullyconformtotheperformancestandardsadoptedbythe
organizationmay,atthediscretionoftheadministration,beretaineduntil1
January 1991.
(iii)theadministrationmayexemptshipsfromtherequirementsofthis
paragraph,incaseswhereitconsidersitunreasonableorunnecessaryfor
suchequipmenttobecarried,orwhentheshipswillbetakenpermanently
out of service within two years of the appropriate implementation date.
368
EXTRACT FROM IMO RESOLUTIONS A222(VII), A278(VII), A477(XII)
Performance Standards for Navigational Radar equipment installed before 1 September 1984
INTRODUCTION
TheradarequipmentrequiredbyRegulation12ofChapterVshould
provideanindicationinrelationtotheshipofthepositionofothersurface
craftandobstructionsofbuoys,shorelinesandnavigationalmarksina
manner which will assist in avoiding collision and navigation.
It should comply with the following minimum requirements:
Range Performance
Theoperationalrequirementundernormalpropagationconditions,when
theradaraerialismountedataheightof15metersabovesealevel,isthat
the equipment should give a clear indication of:
Coastlines:
At 20 nautical miles when the ground rises to 60 meters,
At 7 nautical miles when the ground rises to 6 meters.
Surface objects:
At7nauticalmilesashipof5,000grosstonnage,whateverher
aspect,
At2nauticalmilesanobjectsuchasanavigationalbuoyhavingan
effective echoing area of approximately 10 square meters,
At 3 nautical miles a small ship of length 10 meters.
Minimum Range
Thesurfaceobjectsspecifiedinparagraph2(a)(ii)shouldbeclearly
displayedfromaminimumrangeof50metersuptoarangeof1nautical
mile, without adjustment of controls other than the range selector.
Display
Theequipmentshouldprovidearelativeplandisplayofnotlessthan180
mm effective diameter.
Theequipmentshouldbeprovidedwithatleastfiveranges,thesmallest
ofwhichisnotmorethan1nauticalmileandthegreatestofwhichisnotless
than24nauticalmiles.Thescalesshouldpreferablyof1:2ratio.Additional
ranges may be provided.
Positiveindicationshouldbegivenoftherangeofviewdisplayedandthe
interval between range rings.
Range Measurement
Theprimarymeansprovidedforrangemeasurementshouldbefixed
electronicrangerings.Thereshouldbeatleastfourrangeringsdisplayedon
eachoftherangesmentionedinparagraph2(c)(ii),exceptthatonranges
below1nauticalmilerangeringsshouldbedisplayedatintervalsof0.25
nautical mile.
Fixedrangeringsshouldenabletherangeofanobject,whoseecholieson
arangering,tobemeasuredwithanerrornotexceeding1.5percentofthe
maximum range of the scale in use, or 70 meters, whichever is greater.
Anyadditionalmeansofmeasuringrangeshouldhaveanerrornot
exceeding2.5percentofthemaximumrangeofthedisplayedscaleinuse,
or 120 meters, whichever is the greater.
Heading Indicator
Theheadingoftheshipshouldbeindicatedbyalineonthedisplaywitha
maximumerrornotgreaterthan+/-1°.Thethicknessofthedisplayheading
line should not be greater than 0.5°.
Provisionshouldbemadetoswitchofftheheadingindicatorbyadevice
which cannot be left in the “heading marker off” position.
369
Bearing Measurement
Provisionshouldbemadetoobtainquicklythebearingofanyobject
whose echo appears on the display.
Themeansprovidedforobtainingbearingsshouldenablethebearingofa
targetwhoseechoappearsattheedgeofthedisplaytobemeasuredwithan
accuracy of +/- 1°or better.
Discrimination
The equipment should display as separate indications, on the shortest
range scale provided, two objects on the same azimuth separated by not
more than 50 meters in range.
The equipment should display as separate indications two objects at the
same range separated by not more than 2.5° in azimuth.
The equipment should be designed to avoid, as far as is practicable, the
display of spurious echoes.
Roll
The performance of the equipment should be such that when the ship is
rolling +/- 10° the echoes of the targets remain visible on the display.
Scan
The scan should be continuous and automatic through 360° of azimuth.
The target data rate should be at least 12 per minute. The equipment should
operate satisfactorily in relative wind speeds of 100 knots.
Azimuth Stabilization
Means should be provided to enable the display to be stabilized in
azimuth by a transmitting compass. The accuracy of alignment with the
compasstransmissionshouldbewithin0.5withacompassrotationrateof2
r.p.m.
Theequipmentshouldoperatesatisfactorilyforrelativebearingswhenthe
compass control is inoperative or not fitted.
Performance Check
Means should be available, while the equipment is used operationally, to
determine readily a significant drop in performance relative to a calibration
standard established at the time of installation.
Anti-clutter Devices
Meansshouldbeprovidedtominimizethedisplayofunwantedresponses
from precipitation and the sea.
Operation
Theequipmentshouldbecapableofbeingswitchedonandoperatedfrom
the main display position.
Operational controls should be accessible and easy to identify and use.
After switching on from the cold, the equipment should become fully
operational within 4 minutes.
Astandbyconditionshouldbeprovidedfromwhichtheequipmentcanbe
brought to a fully operational condition within 1 minute.
Interference
After installation and adjustment on board, the bearing accuracy should
be maintained without further adjustment irrespective of the variation of
external magnetic fields.
Sea or Ground Stabilization
Sea or ground stabilization, if provided, should not degrade the accuracy
of the display below the requirements of these performance standards, and
the view ahead on the display should not be unduly restricted by the use of
this facility.
Siting of the Aerial
The aerial system should be installed in such a manner that the efficiency
of the display is not impaired by the close proximity of the aerial to other
objects. In particular, blind sectors in the forward direction should be
avoided.
370
Performance Standards for Navigational Radar equipment installed on or after 1 September 1984
Application
ThisRecommendationappliestoallships’radarequipmentinstalledon
orafter1September1984incompliancewithRegulation12,ChapterVof
theInternationalConventionfortheSafetyofLifeatSea,1974,asamended.
Radarequipmentinstalledbefore1September1984shouldcomplyat
leastwiththeperformancestandardsrecommendedinresolution
A.222(VII).
General
Theradarequipmentshouldprovideanindication,inrelationtotheship,
ofthepositionoftheothersurfacecraftandobstructionsandofbuoys,
shorelinesandnavigationalmarksinamannerwhichwillassistin
navigation and in avoiding collision.
All radar installations
Allradarinstallationsshouldcomplywiththefollowingminimum
requirements.
Range performance
Theoperationalrequirementundernormalpropagationconditions,when
theradarantennaismountedataheightof15metersabovesealevel,isthat
the equipment should in the absence of clutter give a clear indication of:
Coastlines:
At 20 nautical miles when the ground rises to 60 meters
At 7 nautical miles when the ground rises to 6 meters.
Surface objects:
At 7 nautical miles a ship of 5000 gross tonage, whatever her aspect
At 3 nautical miles a small ship of 10 meters in length
At2nauticalmilesanobjectsuchasanavigationalbuoyhavingan
effective echoing area of approximately 10 square meters.
Minimum Range
Thesurfaceobjectsspecifiedinparagraph3.1.2shouldbeclearly
displayedfromaminimumrangeof50metersuptoarangeof1nautical
mile, without changing the setting of controls other than the range selector.
Display
Theequipmentshouldwithoutexternalmagnificationprovidearelative
plandisplayintheheadupunstabilizedmodewithaneffectivediameterof
not less than:
180millimetersonshipsof500grosstonnageandmorebutlessthan
1600 gross tonnage;
250millimetersonshipsof1600grosstonnageandmorebutlessthan
10000 gross tonnage;
340millimetersinthecaseofonedisplayand250millimetersinthe
case of the other on ships of 10000 gross tonnage and upwards.
Note:Displaydiametersof180,250and340millimeterscorrespond
respectively to 9, 12 and 16 inch cathode ray tubes.
Theequipmentshouldprovideoneofthetwofollowingsetsofrange
scales of display:
1.5,3,6,12,and24nauticalmilesandonerangescaleofnotlessthan
0.5 and not greater than 0.8 nautical miles; or
1, 2, 4, 8, 16, and 32 nautical miles.
Additional range scales may be provided.
Therangescaledisplayedandthedistancebetweenrangeringsshouldbe
clearly indicated at all times.
371
Range measurement
Fixedelectronicrangeringsshouldbeprovidedforrangemeasurements
as follows:
Whererangescalesareprovidedinaccordancewithparagraph3.3.2.1,
ontherangescaleofbetween0.5and0.8nauticalmilesatleasttwo
rangeringsshouldbeprovidedandoneachoftheotherrangescalessix
range rings should be provided; or
Whererangescalesareprovidedinaccordancewithparagraph3.3.2.2,
four range rings should be provided on each of the range scales.
Avariableelectronicrangemarkershouldbeprovidedwithanumeric
readout of range.
Thefixedrangeringsandthevariablerangemarkershouldenablethe
rangeofanobjecttobemeasuredwithanerrornotexceeding1.5percentof
the maximum range of the scale in use, or 70 meters, whichever is greater.
Itshouldbepossibletovarythebrillianceoftherangeringsandthe
variable range marker and to remove them completely from the display.
Heading indicator
Theheadingindicatoroftheshipshouldbeindicatedbyalineonthe
displaywithamaximumerrornotgreaterthan+/-1°.Thethicknessofthe
displayed heading line should not be greater than 0.5°.
Provisionshouldbemadetoswitchofftheheadingindicatorbyadevice
which cannot be left in the “heading marker off” position.
Bearing measurement
Provisionshouldbemadetoobtainquicklythebearingofanyobject
whose echo appears on the display.
Themeansprovidedforobtainingbearingshouldenablethebearingofa
targetwhoseechoappearsattheedgeofthedisplaytobemeasuredwithan
accuracy of +/-°or better.
Discrimination
Theequipmentshouldbecapableofdisplayingasseparateindicationson
arangescaleof2nauticalmilesorless,twosimilartargetsatarangeof
between50%and100%oftherangescaleinuse,andonthesameazimuth,
separated by not more than 50 meters in range.
Theequipmentshouldbecapableofdisplayingasseparateindications
twosmallsimilartargetsbothsituatedatthesamerangebetween50percent
and100%ofthe1.5or2milerangescales,andseparatedbynotmorethan
2.5° in azimuth.
Roll or pitch
Theperformanceoftheequipmentshouldbesuchthatwhentheshipis
rollingorpitchingupto+/-10°therangeperformancerequirementsof
paragraphs 3.1 and 3.2 continue to be met.
Scan
Thescanshouldbeclockwise,continuousandautomaticthrough360°of
azimuth.Thescanrateshouldbenotlessthan12r.p.m.Theequipment
should operate satisfactorily in relative wind speed of up to 100 knots.
Azimuth stabilization
Meansshouldbeprovidedtoenablethedisplaytobestabilizedin
azimuthbyatransmittingcompass.Theequipmentshouldbeprovidedwith
acompassinputtoenableittobestabilizedinazimuth.Theaccuracyof
alignmentwiththecompasstransmissionshouldbewithin0.5°witha
compass rotation rate of 2 r.p.m.
Theequipmentshouldoperatesatisfactorilyintheunstabilizedmode
when the compass control is inoperative.
Performance check
Meansshouldbeavailable,whiletheequipmentisusedoperationally,to
determinereadilyasignificantdropinperformancerelativetoacalibration
standardestablishedatthetimeofinstallation,andthattheequipmentis
correctly tuned in the absence of targets.
372
Anti-clutter devices
Suitablemeansshouldbeprovidedforthesuppressionofunwanted
echoesfromseaclutter,rainandotherformsofprecipitation,cloudsand
sandstorms.Itshouldbepossibletoadjustmanuallyandcontinuouslythe
anti-cluttercontrols.Anti-cluttercontrolsshouldbeinoperativeinthefully
anti-clockwisepositions.Inaddition,automaticanti-cluttercontrolsmaybe
provided; however, they must be capable of being switched off.
Operation
Theequipmentshouldbecapableofbeingswitchedonandoperatedfrom
the display position.
Operationalcontrolsshouldbeaccessibleandeasytoidentifyanduse.
Wheresymbolsareusedtheyshouldcomplywiththerecommendationsof
theorganizationonsymbolsforcontrolsonmarinenavigationalradar
equipment.
Afterswitchingonfromcoldtheequipmentshouldbecomefully
operational within 4 minutes.
Astandbyconditionshouldbeprovidedfromwhichtheequipmentcanbe
brought to an operational condition within 15 seconds.
Interference
Afterinstallationandadjustmentonboard,thebearingaccuracyas
prescribedintheseperformancestandardsshouldbemaintainedwithout
furtheradjustmentirrespectiveofthemovementoftheshipintheearth’s
magnetic field.
Sea or ground stabilization (true motion display)
Whereseaorgroundstabilizationisprovidedtheaccuracyand
discriminationofthedisplayshouldbeatleastequivalenttothatrequiredby
these performance standards.
Themotionofthetraceoriginshouldnot,exceptundermanualoverride
conditions,continuetoapointbeyond75percentoftheradiusofthe
display. Automatic resetting may be provided.
Antenna system
Theantennasystemshouldbeinstalledinsuchamannerthatthedesign
efficiency of the radar system is not substantially impaired.
Operation with radar beacons
Allradarsoperatinginthe3cmbandshouldbecapableofoperatingina
horizontally polarized mode.
Itshouldbepossibletoswitchoffthosesignalprocessingfacilitieswhich
might prevent a radar beacon from being shown on the radar display.
Multiple radar installations
Wheretworadarsarerequiredtobecarriedtheyshouldbesoinstalled
thateachradarcanbeoperatedindividuallyandbothcanbeoperated
simultaneouslywithoutbeingdependentupononeanother.Whenan
emergencysourceofelectricalpowerisprovidedinaccordancewiththe
appropriaterequirementsofChapterII-1ofthe1974SOLASconvention,
both radars should be capable of being operated from this source.
Wheretworadarsarefitted,interswitchingfacilitiesmaybeprovidedto
improvetheflexibilityandoverallradarinstallation.Theyshouldbeso
installedthatfailureofeitherradarwouldnotcausethesupplyofelectrical
energy to the other radar to be interrupted or adversely affected.
373
APPENDIX B
GLOSSARY AND ABBREVIATIONS
across-the-scope
Aradarcontactwhosedirectionofrelativemotionisperpendicularto
thedirectionoftheheadingflashindicatoroftheradar.Alsocalled
LIMBO CONTACT.
advance
Thedistanceavesselmovesinitsoriginaldirectionafterthehelmisput
over.
AFC
Automatic frequency control.
aerial
Antenna.
afterglow
Theslowlydecayingluminescenceofthescreenofthecathode-raytube
after excitation by an electron beam has ceased. See PERSISTENCE.
amplify
To increase the strength of a radar signal or echo.
antenna
Aconductororsystemofconductorsconsistingofhornandreflector
used for radiating or receiving radar waves. Also called AERIAL.
anti-clutter control
Ameansforreducingoreliminatinginterferencesfromseareturnand
weather.
apparent wind
See RELATIVE WIND.
ARPA
Automatic radar plotting aid.
attenuation
Thedecreaseinthestrengthofaradarwaveresultingfromabsorption,
scattering,andreflectionbythemediumthroughwhichitpasses
(waveguide,atmosphere)andbyobstructionsinitspath.Also
attenuationofthewavemaybetheresultofartificialmeans,suchasthe
inclusionofanattenuatorinthecircuitryorbyplacinganabsorbing
device in the path of the wave.
automatic frequency control (AFC)
Anelectronicmeansforpreventingdriftinradiofrequencyor
maintainingthefrequencywithinspecifiedlimits.TheAFCmaintains
thelocaloscillatoroftheradaronthefrequencynecessarytoobtaina
constantornearconstantdifferenceinthefrequencyoftheradarecho
(magnetron frequency) and the local oscillator frequency.
azimuth
Whilethistermisfrequentlyusedforbearinginradarapplications,the
termazimuthisusuallyrestrictedtothedirectionofcelestialbodies
among marine navigators.
azimuth-stabilized PPI
See STABILIZED PPI.
beam width
Theangularwidthofaradarbeambetweenhalf-powerpoints.See
LOBE.
bearing
Thedirectionofthelineofsightfromtheradarantennatothecontact.
SometimescalledAZIMUTHalthoughinmarineusagethelatterterm
is usually restricted to the directions of celestial bodies.
bearing cursor
Theradiallineinscribedonatransparentdiskwhichcanberotated
manuallyaboutanaxiscoincidentwiththecenterofthePPI.Itisused
forbearingdetermination.Otherlinesinscribedparalleltotheradial
line have many useful purposes in radar plotting.
blind sector
Asectorontheradarscopeinwhichradarechoescannotbereceived
because of an obstruction near the antenna. See SHADOW SECTOR.
cathode-ray tube (CRT)
Theradarscope(picturetube)withinwhichastreamofelectronsis
directedagainstafluorescentscreen(PPI).Onthefaceofthetubeor
screen (PPI), light is emitted at the points where the electrons strike.
374
challenger
See INTERROGATOR.
circle spacing
Thedistanceinyardsbetweensuccessivewholenumberedcircles.
Unless otherwise designated, it is always 1,000 yards.
clutter
Unwantedradarechoesreflectedfromheavyrain,snow,waves,etc.,
which may obscure relatively large areas on the radarscope.
cone of courses
Mathematicallycalculatedlimits,relativetodatum,withinwhicha
submarine must be in order to intercept the torpedo danger zone.
contact
Anyechodetectedontheradarscopenotevaluatedasclutterorasa
false echo.
contrast
Thedifferenceinintensityofilluminationoftheradarscopebetween
radar images and the background of the screen.
corner reflector
See RADAR REFLECTOR.
CPA
Closest point of approach.
course
Direction of actual movement relative to true north.
cross-band racon
Araconwhichtransmitsatafrequencynotwithinthemarineradar
frequencyband.Tobeabletousethistypeofracon,theship'sradar
receivermustbecapableofbeingtunedtothefrequencyofthecross-
bandraconorspecialaccessoryequipmentisrequired.Ineithercase,
theradarscopewillbeblankexceptfortheraconsignal.SeeIN-BAND
RACON.
CRT
Cathode-ray tube.
crystal
Acrystallinesubstancewhichallowselectriccurrenttopassinonlyone
direction.
datum
InAnti-submarineWarfare(ASW),thelastknownpositionofanenemy
submarineataspecifiedtime.(Lackingotherknowledgethisistheposition
and time of torpedoing.)
definition
Theclarityandfidelityofthedetailofradarimagesontheradarscope.
Acombinationofgoodresolutionandfocusadjustmentisrequiredfor
good definition.
distance circles
Circlesconcentrictotheformationcenter,withradiiofspecified
distances,usedinthedesignationofmainbodystationsinacircular
formation.Circlesaredesignatedbymeansoftheirradii,inthousands
of yards from the formation center.
double stabilizationThestabilizationofaHeading-UpwardPPIdisplayto
North.Thecathode-raytubewiththePPIdisplaystabilizedtoNorthis
rotated to keep ship’s heading upward.
down-the-scope
Aradarcontactwhosedirectionofrelativemotionisgenerallyinthe
opposite direction of the heading flash indicator of the radar.
DRM
Directionofrelativemovement.Thedirectionofmovementofthe
maneuveringshiprelativetothereferenceship,alwaysinthedirection
of M
1
→ M
2
→ M
3
→...
duct
Alayerwithintheatmospherewhererefractionandreflectionresultsin
thetrappingofradarwaves,andconsequentlytheirpropagationover
abnormallylongdistances.Ductsareassociatedwithtemperature
inversions in the atmosphere.
EBL
Electronic bearing line.
echo
Theradarsignalreflectedbacktotheantennabyanobject;theimageof
the reflected signal on the radarscope. Also called RETURN.
echo box
Acavity,resonantatthetransmittedfrequencywhichproducesan
artificialradartargetsignalfortuningortestingtheoverallperformance
ofaradarset.Theoscillationsdevelopedintheresonantcavitywillbe
greater at higher power outputs of the transmitter.
375
echo box performance monitor
Anaccessorywhichisusedfortuningtheradarreceiverandchecking
overallperformancebyvisualinspection.Anartificialechoasreceived
fromtheechoboxwillappearasanarrowplumefromthecenterofthePPI.
Thelengthofthisplumeascomparedwithitslengthwhentheradaris
knowntobeoperatingatahighperformancelevelisindicativeofthe
current performance level.
face
Theviewingsurface(PPI)ofacathode-raytube.Theinnersurfaceof
thefaceiscoatedwithafluorescentlayerwhichemitslightunderthe
impact of a stream of electrons. Also called SCREEN.
fast time constant (FTC) circuit
Anelectroniccircuitdesignedtoreducetheundesirableeffectsof
clutter.WiththeFTCcircuitinoperation,onlytheneareredgeofan
echohavingalongtimedurationisdisplayedontheradarscope.The
useofthiscircuittendstoreducesaturationofthescopewhichcouldbe
caused by clutter.
fictitious ship
Animaginaryship,presumedtomaintainconstantcourseandspeed,
substituted for a maneuvering ship which alters course and speed.
fluorescenceEmissionoflightorotherradiantenergyasaresultofandonly
duringabsorptionofradiationfromsomeothersource.Anexampleis
theglowingofthescreenofacathode-raytubeduringbombardmentby
astreamofelectrons.Thecontinuedemissionoflightafterabsorption
of radiation is called PHOSPHORESCENCE.
formation axis
Anarbitrarilyselecteddirectionfromwhichallbearingsusedinthe
designationofmainbodystationsinacircularformationaremeasured.
Theformationaxisisalwaysindicatedasatruedirectionfromthe
formation center.
formation center
Thearbitrarilyselectedpointoforiginforthepolarcoordinatesystem,
aroundwhichacircularformationisformed.Itisdesignated“station
Zero”.
formation guide
AshipdesignatedbytheOTCasguide,andwithreferencetowhichall
shipsintheformationmaintainposition.Theguidemayormaynotbe
at the formation center.
FTCFast time constant.
gain (RCVR) control
Acontrolusedtoincreaseordecreasethesensitivityofthereceiver
(RCVR).Thiscontrol,analogoustothevolumecontrolofabroadcast
receiver,regulatestheintensityoftheechoesdisplayedonthe
radarscope.
geographical plot
Aplotoftheactualmovementsofobjects(ships)withrespecttothe
earth. Also called NAVIGATIONAL PLOT.
heading flashAnilluminatedradiallineonthePPIforindicatingownship’s
heading on the bearing dial. Also called HEADING MARKER.
heading-upward display
See UNSTABILIZED DISPLAY.
in-band racon
Araconwhichtransmitsinthemarineradarfrequencyband,e.g.,the3-
centimeterband.Thetransmittersweepsthrougharangeoffrequencies
withinthebandtoinsurethataradarreceivertunedtoaparticular
frequencywithinthebandwillbeabletodetectthesignal.SeeCROSS-
BAND RACON.
intensity control
Acontrolforregulatingtheintensityofbackgroundilluminationonthe
radarscope. Also called BRILLIANCE CONTROL.
interference
Unwantedandconfusingsignalsorpatternsproducedontheradarscope
byanotherradarortransmitteronthesamefrequency,andmorerarely,
bytheeffectsofnearbyelectricalequipmentormachinery,orby
atmospheric phenomena.
interrogator
Aradartransmitterwhichsendsoutapulsethattriggersatransponder.
Aninterrogatorisusuallycombinedinasingleunitwitharesponsor,
whichreceivesthereplyfromatransponderandproducesanoutput
suitableforfeedingadisplaysystem;thecombinedunitiscalledan
INTERROGATOR-RESPONSOR.
IRP
Image retaining panel.
kilohertz (kHz)
A frequency of one thousand cycles per second. See MEGAHERTZ.
376
limbo contacts
See ACROSS-THE-SCOPE.
limited lines of approach
Mathematicallycalculatedlimits,relativetotheforce,withinwhichan
attackingsubmarinemustbeinorderthatitcanreachthetorpedo
danger zone
lobeOfthethree-dimensionalradiationpatterntransmittedbyadirectional
antenna,oneoftheportionswithinwhichthefieldstrengthorpoweris
everywheregreaterthanaselectedvalue.Thehalf-powerlevelisused
frequentlyasthisreferencevalue.Thedirectionoftheaxisofthemajor
lobeoftheradiationpatternisthedirectionofmaximumradiation.See
SIDE LOBES.
maneuvering ship (M)
Any moving unit except the reference ship.
MCPAMinutes to closest point of approach.
megacycle per second (Mc)
Afrequencyofonemillioncyclespersecond.Theequivalentterm
MEGAHERTZ (MHz) is now coming into more frequent use.
megahertz
A frequency of one million cycles per second. See KILOHERTZ.
microsecond
One millionth of 1 second.
microwaves
Commonly,veryshortradiowaveshavingwavelengthsof1millimeterto
30centimeters.Whilethelimitsofthemicrowaveregionarenotclearly
defined,theyaregenerallyconsideredtobetheregioninwhichradar
operates.
minor lobes
Side lobes.
missile danger zone
Anareawhichthesubmarinemustenterinordertobewithinmaximum
effective missile firing range.
MRMMilesofrelativemovement.Thedistancealongtherelative
movementlinebetweenanytwospecifiedpointsortimes.Alsocalled
RELATIVE DISTANCE.
nanosecond
One billionth of 1 second.
north-upward display
See STABILIZED DISPLAY.
NRML
New relative movement line.
paint
ThebrightareaonthePPIresultingfromthebrighteningofthesweep
bytheechoes.Also,theactofformingthebrightareaonthePPIbythe
sweep.
persistence
Ameasureofthetimeofdecayoftheluminescenceofthefaceofthe
cathode-raytubeafterexcitationbythestreamofelectronshasceased.
Relativelyslowdecayisindicativeofhighpersistence.Persistenceisthe
length of time during which phosphorescence takes place.
phosphorescence
Emissionoflightwithoutsensibleheat,particularlyasaresultof,but
continuingafter,absorptionofradiationfromsomeothersource.An
exampleistheglowingofthescreenofacathode-raytubeafterthe
beamofelectronshasmovedtoanotherpartofthescreen.Itisthis
propertythatresultsinthechartlikepicturewhichgivesthePPIits
principalvalue.PERSISTENCEisthelengthoftimeduringwhich
phosphorescencetakesplace.Theemissionoflightorotherradiant
energyasaresultofandonlyduringabsorptionofradiationfromsome
other source is called FLUORESCENCE.
plan position indicator (PPI)
Thefaceorscreenofacathode-raytubeonwhichradarimagesappearin
correctrelationtoeachother,sothatthescopefacepresentsachartlike
representationoftheareaabouttheantenna,thedirectionofacontactor
targetbeingrepresentedbythedirectionofitsechofromthecenterandits
range by its distance from the center.
plotting head
Reflection plotter.
polarization
Theorientationinspaceoftheelectricaxis,ofaradarwave.This
electricaxis,whichisatrightanglestothemagneticaxis,maybeeither
horizontal,vertical,orcircular.Withcircularpolarization,theaxis
rotate,resultinginaspiraltransmissionoftheradarwave.Circular
polarization is used for reducing rain clutter.
377
PPI
Plan position indicator.
pulse
Anextremelyshortburstofradarwavetransmissionfollowedbya
relatively long period of no transmission.
pulse duration
Pulse length.
pulse length
Thetimeduration,measuredinmicroseconds,ofasingleradarpulse.
Also called PULSE DURATION.
pulse recurrence rate (PRR)
Pulse repetition rate.
pulse repetition rate (PRR)
The number of pulses transmitted per second.
racon
Aradarbeaconwhich,whentriggeredbyaship’sradarsignal,transmits
areplywhichprovidestherangeandbearingtothebeacononthePPI
displayoftheship.Thereplymaybecodedforidentificationpurposes;
inwhichcase,itwillconsistofaseriesofconcentricarcsonthePPI.
TherangeisthemeasurementonthePPItothearcnearestitscenter;the
bearingisthemiddleoftheraconarcs.Ifthereplyisnotcoded,the
raconsignalwillappearasaradiallineextendingfromjustbeyondthe
reflectedechooftheraconinstallationorfromjustbeyondthepoint
wheretheechowouldbepaintedifdetected.SeeIN-BANDRACON,
CROSS-BAND RACON, RAMARK.
radar indicator
Aunitofaradarsetwhichprovidesavisualindicationofradarechoes
received,usingacathode-raytubeforsuchindication.Besidesthe
cathode-raytube,theradarindicatoriscomprisedofsweepand
calibration circuits, and associated power supplies.
radar receiver
Aunitofaradarsetwhichdemodulatesreceivedradarechoes,amplifies
theechoes,anddeliversthemtotheradarindicator.Theradarreceiver
differsfromtheusualsuperheterodynecommunicationsreceiverinthat
itssensitivityismuchgreater;ithasabettersignaltonoiseratio,andit
is designed to pass a pulse type signal.
radar reflector
Ametaldevicedesignedforreflectingstrongechoesofimpingingradar
signalstowardstheirsource.Thecornerreflectorconsistsofthree
mutuallyperpendicularmetalplates.Cornerreflectorsaresometimes
assembled in clusters to insure good echo returns from all directions.
radar repeater
AunitwhichduplicatesthePPIdisplayatalocationremotefromthe
mainradarindicatorinstallation.AlsocalledPPIREPEATER,
REMOTE PPI.
radar transmitter
Aunitofaradarsetinwhichtheradio-frequencypowerisgenerated
andthepulseismodulated.Themodulatorofthetransmitterprovides
the timing trigger for the radar indicator.
ramark
Aradarbeaconwhichcontinuouslytransmitsasignalappearingasa
radiallineonthePPI,indicatingthedirectionofthebeaconfromthe
ship.Foridentificationpurposes,theradiallinemaybeformedbya
seriesofdotsordashes.Theradiallineappearsevenifthebeaconis
outsidetherangeforwhichtheradarisset,aslongastheradarreceiver
iswithinthepowerrangeofthebeacon.UnliketheRACON,theramark
does not provide the range to the beacon.
range markers
EquallyspacedconcentricringsoflightonthePPIwhichpermitthe
radarobservertodeterminetherangetoacontactinaccordancewiththe
rangesettingortherangeoftheouterrings.SeeVARIABLERANGE
MARKER.
range selector
A control for selecting the range setting for the radar indicator.
RCVR
Short for RECEIVER.
reference ship (R)
The ship to which the movement of others is referred.
reflection plotterAnattachmentfittedtoaPPIwhichprovidesaplotting
surfacepermittingradarplottingwithoutparallaxerrors.Anymark
madeontheplottingsurfacewillbereflectedontheradarscopedirectly
below. Also called PLOTTING HEAD.
378
refraction
Thebendingoftheradarbeaminpassingobliquelythroughregionsof
the atmosphere of different densities.
relative motion display
Atypeofradarscopedisplayinwhichthepositionofownshipisfixed
atthecenterofthePPIandalldetectedobjectsorcontactsmoverelative
to own ship. See TRUE MOTION DISPLAY.
relative movement line
Thelocusofpositionsoccupiedbythemaneuveringshiprelativetothe
reference ship.
relative plot
Theplotofthepositionsoccupiedbythemaneuveringshiprelativeto
the reference ship.
relative vector
Avelocityvectorwhichdepictstherelativemovementofanobject
(ship)inmotionwithrespecttoanotherobject(ship),usuallyinmotion.
relative wind
Thespeedandrelativedirectionfromwhichthewindappearstoblow
with reference to a moving point. See APPARENT WIND.
remote PPI
Radar repeater.
resolution
Thedegreeofabilityofaradarsettoindicateseparatelytheechoesof
two contacts in range, bearing, and elevation. With respect to:
range-theminimumrangedifferencebetweenseparatecontactsat
thesamebearingwhichwillallowbothtoappearasseparate,
distinct echoes on the PPI.
bearing-theminimumangularseparationbetweentwocontactsat
thesamerangewhichwillallowbothtoappearasseparate,distinct
echoes on the PPI.
elevation-theminimumangularseparationinaverticalplane
betweentwocontactsatthesamerangeandbearingwhichwill
allow both to appear as separate, distinct echoes on the PPI.
responder beacon
Transponder beacon.
RML
Relative movement line.
scan
Toinvestigateanareaorspacebyvaryingthedirectionoftheradar
antennaandthustheradarbeam.Normally,scanningisdoneby
continuous rotation of the antenna.
scanner
Aunitofaradarsetconsistingoftheantennaanddriveassemblyfor
rotating the antenna.
scope
Short for RADARSCOPE.
screen
The face of a cathode-ray tube on which radar images are displayed.
screen axis
Anarbitrarilyselecteddirectionfromwhichallbearingsusedinthe
designationofscreenstationsinacircularformationaremeasured.The
screenaxisisalwaysindicatedasatruedirectionfromthescreencenter.
screen center
Theselectedpointoforiginforthepolarcoordinatesystem,around
whichascreenisformed.Thescreencenterusuallycoincideswiththe
formationcenter,butmaybeaspecifiedtruebearinganddistancefrom
it.
screen station numbering
Screeningstationsaredesignatedbymeansofa“stationnumber”,
consistingoffourormoredigits.Thelastthreedigitsarethebearingof
thescreeningstationrelativetothescreenaxis,whiletheprefixeddigits
indicatetheradiusofthedistancecircleinthousandsofyardsfromthe
screen center.
sea return
Clutterontheradarscopewhichistheresultoftheradarsignalbeing
reflected from the sea, especially near the ship.
sensitivity time control (STC)
Anelectroniccircuitdesignedtoreduceautomaticallythesensitivityof
the receiver to nearby targets. Also called SWEPT GAIN CONTROL.
shadow sector
Asectorontheradarscopeinwhichtheappearanceofradarechoesis
improbablebecauseofanobstructionneartheantenna.Whileboth
blindandshadowsectorshavethesamebasiccause,blindsectors
379
generallyoccuratthelargeranglessubtendedbytheobstruction.See
BLIND SECTOR.
side lobes
Unwantedlobesofaradiationpattern,i.e.,lobesotherthanmajorlobes.
Also called MINOR LOBES.
speed triangle
TheusualdesignationoftheVECTORDIAGRAMwhenscaledin
knots.
SRMSpeedofrelativemovement.Thespeedofthemaneuveringship
relative to the reference ship.
stabilized display (North-Upward)
APPIdisplayinwhichtheorientationoftherelativemotion
presentationisfixedtoanunchangingreference(North).Thisdisplayis
North-Upward,normally.InanUNSTABILIZEDDISPLAY,the
orientationoftherelativemotionpresentationchangeswithchangesin
ship’s heading. See DOUBLE STABILIZATION.
stabilized PPI
See STABILIZED DISPLAY.
station numbering
Positionsinacircularformation(otherthantheformationcenter)are
designatedbymeansofa“stationnumber,”consistingoffourormore
digits.Thelastthreedigitsarethebearingofthestationrelativetothe
formationaxis,whiletheprefixeddigitsindicatetheradiusofthe
distancecircleinthousandsofyards.Thus,station4090indicatesa
positionbearing90degreesrelativetotheformationaxisonadistance
circle with a radius of 4,000 yards from the formation center.
STC
Sensitivity time control.
strobe
Variable range marker.
sweep
Asdeterminedbythetimebaseorrangecalibration,theradial
movementofthestreamofelectronsimpingingonthefaceofthe
cathode-raytube.Theoriginofthesweepisthecenterofthefaceofthe
cathode-raytubeorPPI.Becauseoftheveryhighspeedofmovementof
thepointofimpingement,thesuccessivepointsofimpingementappear
asacontinuouslyluminousline.Thelinerotatesinsynchronismwith
theradarantenna.Ifanechoisreceivedduringthetimeofradialtravel
oftheelectronstreamfromthecentertotheouteredgeofthefaceofthe
tube,thesweepwillbeincreasedinbrightnessatthepointoftravelof
theelectronstreamcorrespondingtotherangeofthecontactfrom
whichtheechoisreceived.Sincethesweeprotatesinsynchronismwith
theradarantenna,thisincreasedbrightnesswilloccuronthebearing
fromwhichtheechoisreceived.Withthisincreasedbrightnessandthe
persistenceofthetubeface,paintcorrespondingtotheobjectbeing
“illuminated” by the radar beam appears on the PPI.
swept gain control
Sensitivity time control.
TCPA
Time to closest point of approach.
time line
Alinejoiningtheheadsoftwovectorswhichrepresentsuccessive
coursesandspeedsofaspecificunitinpassingfromaninitialtoafinal
positioninknowntime,viaaspecifiedintermediatepoint.Thislinealso
touchestheheadofaconstructiveunitwhichproceedsdirectlyfromthe
initialtothefinalpositioninthesametime.Bygeneralusagethis
constructiveunitiscalledthefictitiousship.Theheadofitsvector
dividesthetimelineintosegmentsinverselyproportionaltothetimes
spentbytheunitonthefirstandsecondlegs.Thetimelineisusedin
two-course problems.
torpedo danger zone
Anareawhichthesubmarinemustenterinordertobewithinmaximum
effective torpedo firing range.
traceTheluminouslineresultingfromthemovementofthepointsof
impingementoftheelectronstreamonthefaceofthecathode-raytube.
See SWEEP.
transfer
Thedistanceavesselmovesperpendiculartoitsinitialdirectionin
making a turn.
transponderAtransmitter-receivercapableofacceptingthechallenge
(radarsignal)ofaninterrogatorandautomaticallytransmittingan
appropriate reply. See RACON.
transponder beacon
A beacon having a transponder. Also called RESPONDER BEACON.
trigger
Asharpvoltagepulseusuallyoffrom0.1to0.4microsecondsduration,
whichisappliedtothemodulatortubestofirethetransmitter,andwhich
380
isappliedsimultaneouslytothesweepgeneratortostarttheelectron
beammovingradiallyfromthesweeporigintotheedgeofthefaceof
the cathode-ray tube.
true motion display
Atypeofradarscopedisplayinwhichownshipandothermoving
contactsmoveonthePPIinaccordancewiththeirtruecoursesand
speed.Thisdisplayissimilartoanavigational(geographical)plot.See
RELATIVE MOTION DISPLAY.
true vector
Avelocityvectorwhichdepictsactualmovementwithrespecttothe
earth.
true wind
True direction and force of wind relative to a fixed point on the earth.
unstabilized display (Heading-Upward)
APPIdisplayinwhichtheorientationoftherelativemotion
presentationissettoship’sheadingand,thus,changeswithchangesin
ship’sheading.InthisHeading-Upwarddisplay,radarechoesareshown
attheirrelativebearings.Atruebearingdialwhichiscontinuouslysetto
ship’scourseatthe000degreesrelativebearingisnormallyusedwith
thisdisplayfordeterminingtruebearings.Thistruebearingdialmaybe
eithermanuallyorautomaticallysettoship’scourse.Whenset
automaticallybyacourseinputfromthegyrocompass,thetruebearing
dialissometimescalledaSTABILIZEDAZIMUTHSCALE.Thelatter
termwhichappearsinmanufacturer'sinstructionbooksandoperating
manualsismoreinconformitywithairnavigationratherthanmarine
navigation usage. See DOUBLE STABILIZATION.
up-the-scope
Aradarcontactwhosedirectionofrelativemotionisgenerallyinthe
same direction as the heading flash indicator of the radar.
variable range marker
AluminousrangecircleorringonthePPI,theradiusofwhichis
continuouslyadjustable.Therangesettingofthismarkerisreadonthe
range counter of the radar indicator.
vector
A directed line segment representing direction and magnitude.
vector diagram
Agraphicalmeansofaddingandsubtractingvectors.Whenthevector
magnitudeisscaledinknots,thisdiagramisusuallycalledSPEED
TRIANGLE.
velocity vector
Avectorthemagnitudeofwhichrepresentsrateofmovement;a
velocityvectormaybeeithertrueorrelativedependinguponwhetherit
depictsactualmovementwithrespecttotheearthortherelative
movementofanobject(ship)inmotionwithrespecttoanotherobject
(ship).
VRM
Variable range marker.
VTS
Vessel traffic system.
XMTR
Short for TRANSMITTER.
381
APPENDIX C
RELATIVE MOTION PROBLEMS
RAPID RADAR PLOTTING PROBLEMS
1.Ownship,oncourse311˚,speed17knots,obtainsthefollowingradar
bearings and ranges at the times indicated, using a radar setting of 24 miles:
Required:
(1) Range at CPA.
(2) Time at CPA.
(3) Direction of relative movement (DRM)
Solution:
(1) R 8.2 mi., (2) T 1204.5, (3) DRM 131˚.
2.Ownship,oncourse000˚,speed12knots,obtainsthefollowingradar
bearingsandrangesatthetimesindicated,usingaradarrangesettingof12
miles:
Required:
(1)Distance at which the contact will cross dead ahead.
(2)Direction of relative movement (DRM).
(3)Speed of relative movement (SRM); relative speed.
(4)Range at CPA.
(5)Bearing of contact at CPA.
(6)Relative distance (MRM) from 0422 position of contact to the CPA.
(7)Time at CPA.
(8)Distanceownshiptravelsfromthetimeofthefirstplot(0410)tothe
time of the last plot (0422) of the contact.
(9)True course of the contact.
(10)Actual distance traveled by the contact between 0410 and 0422.
(11)True speed of the contact.
Solution:
Assumingthatthecontactmaintainscourseandspeed:(1)D4.3.mi.,(2)
DRM234˚,(3)SRM20kn.,(4)R3.5mi.,(5)B324˚,(6)MRM6.5mi.,
(7) T 0441, (8) D 2.4 mi., (9) C 270˚, (10) D 3.2 mi., (11) S 16 kn.
TimeBearingRange (mi.)
1136280˚16.0
1142274˚13.6
1148265˚11.4
TimeBearingRange (mi.)
0410035˚11.1
0416031˚9.2
0422025˚7.3
382
3.Ownship,oncourse030˚,speed23knots,obtainsthefollowingradar
bearingsandrangesatthetimesindicated,usingaradarrangesettingof12
miles:
Required:
(1)Range at CPA.
(2)Bearing of contact at CPA.
(3)Speed of relative movement (SRM); relative speed.
(4)Time at CPA.
(5)Distanceownshiptravelsfromthetimeofthefirstplot(1020)tothe
timeofthelastplot(1026)ofthecontact;distanceownshiptravels
in 6 minutes.
(6)True course of the contact.
(7)Actual distance traveled by the contact between 1020 and 1026.
(8)True speed of the contact.
(9)Assumingthatthecontacthasturnedonitsrunninglightsduring
daylighthoursbecauseofinclementweather,whatsidelight(s)
might be seen at CPA?
Solution:
Assumingthatthecontactmaintainscourseandspeed:(1)R1.0mi.,(2)
B167˚,(3)SRM32kn.,(4)T1041,(5)D2.3mi.,(6)C304˚,(7)D2.2
mi., (8) S 22 kn., (9) starboard (green) side light.
4.Ownship,oncourse000˚,speed11knots,obtainsthefollowingradar
bearingsandrangesatthetimesindicated,usingaradarrangesettingof12
miles:
Required:
(1)Range at CPA.
(2)Speed of relative movement (SRM); relative speed.
(3)Time at CPA.
(4)True course of contact.
Decision:
Whentherangetothecontactdecreasesto6miles,ownshipwillchange
course so that the contact will pass safely ahead with a CPA of 2.0 miles.
Required:
(5)New course for own ship.
(6)New SRM after course change.
Solution:
Assuming that the contact maintains course and speed:(1)Nil;riskof
collisionexists,(2)SRM12kn.,(3)T1200,(4)307˚,(5)063˚,(6)New
SRM 22 kn.
TimeBearingRange (mi.)
1020081˚10.8
1023082˚9.2
1026083˚7.7
TimeBearingRange (mi.)
1100080˚12.0
1106080˚10.8
1112080˚9.6
383
5.Ownship,oncourse220˚,speed12knots,obtainsthefollowingradar
bearingsandrangesatthetimesindicated,usingaradarrangesettingof12
miles:
Required:
(1)Range at CPA.
(2)Speed of relative movement (SRM); relative speed.
(3)Time at CPA.
(4)True course of contact.
Decision:
Whentherangetothecontactdecreasesto6miles,ownshipwillchange
coursesothatthecontactwillclearahead,inminimumtime,withaCPA
of 3.0 miles.
Required:
(5)New course for own ship.
(6)New SRM after course change.
Solution:
Assumingthatthecontactmaintainscourseandspeed:(1)R1.2mi.,(2)
SRM16.5kn.,(3)T0343,(4)C161˚,(5)Comerightto290˚,(6)New
SRM 28 kn.
6.Ownship,oncourse316˚,speed21knots,obtainsthefollowingradar
bearingsandrangesatthetimesindicated,usingaradarrangesettingof12
miles:
Required:
(1)Range at CPA.
(2)Speed of relative movement (SRM); relative speed.
(3)True course of contact.
(4)True speed of contact.
Decision:
Whentherangetothecontactdecreasesto6miles,ownshipwillchange
coursesothatthecontactwillclearahead,inminimumtime,withaCPA
of 3 miles.
Required:
(5)New course for own ship.
Solution:
Assumingthatthecontactmaintainscourseandspeed:(1)R1.1mi.,(2)
SRM 15.5 kn., (3) C 269˚, (4) S 12.5 kn., (5) C 002˚.
TimeBearingRange (mi.)
0300297˚11.7
0306296˚10.0
0312295˚8.5
TimeBearingRange (mi.)
1206357˚11.8
1212358˚10.2
1218359˚8.7
7.Ownship,oncourse000˚,speed10knots,obtainsthefollowingradar
bearingsandrangesatthetimesindicated,usingaradarrangesettingof12
miles:
Required:
(1)Range at CPA.
(2)Speed of relative movement (SRM); relative speed.
(3)Time at CPA.
(4)True course of contact.
(5)True speed of contact.
Decision:
Ownshipwillchangecourseat0418sothatthecontactwillclearahead
(on own ship's port side), with a CPA of 2 miles.
Required:
(6)New course for own ship.
Solution:
Assumingthatthecontactmaintainscourseandspeed:(1)Nil.,(2)SRM
20 kn., (3) T 0433, (4) C 200˚, (5) S 10 kn., (6) C 046˚.
8.Ownship,oncourse052˚,speed15knots,obtainsthefollowingradar
bearingsandrangesatthetimesindicated,usingaradarrangesettingof24
miles:
Required:
(1)Range at CPA.
(2)True course of contact.
(3)Assumingthattherearenoothervesselsintheareaandthatthe
contactisalargepassengership,clearlyvisibleat0352,isthisa
crossing, meeting, or overtaking situation?
(4)True speed of contact.
Decision:
Adecisionismadetochangecoursewhentherangetothecontact
decreases to 6 miles.
(5)NewcourseofownshiptoclearthecontactporttoportwithaCPA
of 3 miles.
Solution:
Assumingthatthecontactmaintainscourseandspeed:(1)Nil;riskof
collision exists, (2) C 232˚, (3) Meeting, (4) S 18 kn., (5) C 119˚.
TimeBearingRange (mi.)
0400010˚11.1
0406010˚9.0
0412010˚7.1
TimeBearingRange (mi.)
0340052˚14.9
0346052˚11.6
0352052˚8.3
385
9.Ownship,oncourse070˚,speed16knots,obtainsthefollowingradar
bearingsandrangesatthetimesindicated,usingaradarrangesettingof12
miles:
Required:
(1)Range at CPA.
(2)Time at CPA.
(3)True course of the contact.
(4)True speed of the contact.
Decision:
Whentherangetothecontactdecreasesto5miles,ownshipwillchange
speed only so that contact will clear ahead at a distance of 3 miles.
Required:
(5)New speed of own ship.
Solution:
Assumingthatthecontactmaintainscourseandspeed:(1)R0.5mi.,(2)T
0333., (3) C 152˚, (4) S 21 kn., (5) S 3
1
/
4
kn.
10.Ownship,oncourse093˚,speed18knots,obtainsthefollowingradar
bearingsandrangesatthetimesindicated,usingaradarrangesettingof12
miles:
Required:
(1)Range at CPA.
(2)Relative distance (MRM) from 0452 to 0504 position of contact.
(3)Speed of relative movement (SRM); relative speed.
(4)Direction of relative movement (DRM).
(5)Distanceownshiptravelsfromthetimeofthefirstplot(0452)tothe
time of the last plot (0504) of the contact.
(6)True course and speed of the contact.
Solution:
Assumingthatthecontactmaintainscourseandspeed:(1)R1.9mi.,(2)
MRM3.6mi.,(3)SRM18kn.,(4)DRM273˚,(5)D3.6mi.,(6)The
contactiseitherastationaryobjectoravesselunderwaybutwithnoway
on.
TimeBearingRange (mi.)
0306015˚10.8
0312016˚8.3
0318017˚5.9
TimeBearingRange (mi.)
0452112˚5.9
0458120˚4.2
0504137˚2.7
386
11.Ownship,oncourse315˚,speed11knots,obtainsthefollowingradar
bearingsandrangesatthetimesindicated,usingaradarrangesettingof24
miles:
Required:
(1)Range at CPA.
(2)True course and speed of the contact.
Decision:
Whentherangetothecontactdecreasesto8miles,ownshipwillchange
coursesothatthecontactwillpasssafelytostarboardwithaCPAof3
miles.
Required:
(3)New course for own ship.
Solution:
Assumingthatthecontactmaintainscourseandspeed:(1)R1.6mi.,(2)
Thecontactiseitherstationaryoravesselwithlittleornowayon.(3)C
303˚.
12.Ownship,oncourse342˚speed11knots,(halfspeed),obtainsthe
followingradarbearingsandrangesatthetimesindicated,usingaradar
range setting of 12 miles:
Required:
(1)Range at CPA.
(2)True course of the contact.
(3)True speed of the contact.
(4)Is this a crossing, meeting, or overtaking situation?
Decision:
Ownshipisacceleratingtofullspeedof18knotsandwillchangecourse
at0924whenthespeedis15knotssothatthecontactwillclearastern
with a CPA of 2 miles.
Required:
(5)New course for own ship.
Solution:
Assumingthatthecontactmaintainscourseandspeed:(1)R0.5mi.,(2)
C 067˚, (3) S 15 kn., (4) Crossing, (5) C 006˚.
TimeBearingRange (mi.)
0405319˚17.8
0417320˚15.6
0429321˚13.4
TimeBearingRange (mi.)
0906287˚12.0
0912287˚10.2
0918288˚8.4
387
13.Ownship,oncourse350˚,speed18knots,obtainsthefollowingradar
bearingsandrangesatthetimesindicated,usingaradarrangesettingof12
miles:
Required:
(1)Range at CPA.
(2)True course of the contact.
(3)True speed of the contact.
Decision:
Whentherangetothecontactdecreasesto6miles,ownshipchanges
course to 039˚.
Required:
(4)New range at CPA.
(5)Describe how the new time at CPA would be computed.
(6)New time at CPA.
(7)Atwhatbearingandrangetothecontactcanownshipsafelyresume
the original course of 350˚ and obtain a CPA of 3 miles?
(8)Whatwouldbethebenefit,ifany,ofbringingownshipslowlyback
totheoriginalcourseof350˚oncethepointreferredtoin(7)above
is reached?
Solution:
Assumingthatthecontactmaintainscourseandspeed:(1)R1.0mi.,(2)
C252˚,(3)S18.5kn.,(4)R3.0mi.,(5)Determinetheoriginalrelative
speed(SRM);thenusingit,determinethetimeatMx.Next,determinethe
newSRM;thenusingit,determinehowlongitwilltakeforthecontactto
moveinrelativemotiondownthenewRMLfromMxtothenewCPA.(6)
T0219,(7)Whenthecontactbears318˚,range3.0miles.(8)Theslow
returntotheoriginalcoursewillservetoinsurethatthecontactwill
remainoutsidethe3-miledangerorbufferzoneafterownshipissteady
on 350˚.
14.Ownship,oncourse330˚,speed20knots,obtainsthefollowingradar
bearingsandrangesatthetimesindicated,usingaradarrangesettingof12
miles:
Required:
(1)Range at CPA.
(2)Time at CPA.
(3)True course of the contact.
(4)True speed of the contact.
(5)Whatdanger,ifany,wouldbepresentifownshipmaintainedcourse
and speed and contact changed course to 120˚ at 0620?
Decision:
Assumethatthecontactmaintainsitsoriginalcourseandspeedandthat
ownship'sspeedhasbeenreducedto11.5knotswhentherangetothe
contact has decreased to 6 miles.
Required:
(6)New range at CPA.
(7)Will the contact pass ahead or astern of own ship?
Solution:
(1)Nil;riskofcollisionexists.(2)T0644,(3)C045˚,(4)S10.5kn.,(5)
None, (6) R 2.0 mi., (7) Ahead.
TimeBearingRange (mi.)
0200030˚10.0
0203029˚8.7
0206028˚7.4
TimeBearingRange (mi.)
0608300˚12.0
0614300˚10.0
0620300˚8.0
388
15.Ownship,oncourse022˚,speed32knots,obtainsthefollowingradar
bearingsandrangesatthetimesindicated,usingaradarrangesettingof24
miles:
Theobservationsaremadeonawarm,summermorning.Theweatheris
calm;theseastateis0.Fromseawatertemperaturemeasurementsand
weatherreports,itisdeterminedthatthetemperatureoftheairimmediately
abovetheseais12˚Fcoolerthantheair300feetabovetheship.Also,the
relativehumidityimmediatelyabovetheseais30%greaterthanat300feet
above the ship.
Required:
(1)Sincethecontactsaredetectedatrangeslongerthannormal,towhat
do you attribute the radar's increased detection capability?
(2)Ranges at CPA for the three contacts.
(3)True courses of the contacts.
(4)True speeds of the contacts.
(5)Which contact presents the greatest threat?
(6)Ifownshiphasadequatesearoom,shouldownshipcomeleftor
right of contact A?
Decision:
WhentherangetocontactAdecreasesto12miles,ownshipwillchange
course so that no contact will pass within 4 miles.
Required:
(7)New course for own ship.
Solution:
Assumingthatthecontactsmaintaincourseandspeed:(1)Super-
refraction,(2)ContactA-nil;ContactB-R23.8mi.;ContactC-R9.2mi.,
(3)ContactA-C299˚;ContactB-C022˚;ContactC-C282˚,(4)Contact
A-S30kn;ContactB-S32kn.;ContactC-S19kn.,(5)ContactA;itison
collision course, (6) Come right, (7) C 063˚.
16.Ownship,oncourse120˚,speed12knots,obtainsthefollowingradar
bearingsandrangesatthetimesindicated,usingaradarrangesettingof12
miles:
Required:
(1)Ranges at CPA for the three contacts.
(2)True courses of the contacts.
(3)Which contact presents the greatest danger?
(4)Which contact, if any, might be a lightship at anchor?
Decision:
WhentherangetocontactBdecreasesto6miles,ownshipwillchange
course to 190˚.
Required:
(5)At what time will the range to contact B be 6 miles?
(6)New CPA of contact C after course change to 190˚.
Solution:
Assumingthecontactsmaintaincourseandspeed:(1)ContactA-R3.0
mi.;contactB-nil;contactC-R4.3mi.,(2)contactA-C138˚;contactB-C
329˚;contactC-C101˚,(3)ContactB;itisoncollisioncourse,(4)None,
(5) T 0314, (6) R 3.2 mi.
TimeContact AContact BContact C
0423070˚-23.2 mi.170˚-23.8 mi.025˚-22.6 mi.
0426070˚-21.1 mi.170˚-23.8 mi.023˚-21.2 mi.
0429070˚-19.1 mi.170˚-23.8 mi.020˚-19.0 mi.
TimeContact AContact BContact C
0300095˚-8.7 mi.128˚-10.0 mi.160˚-7.7 mi.
0306093˚-7.8 mi.128˚-8.3 mi.164˚-7.0 mi.
0312090˚-7.0 mi.128˚-6.6 mi.170˚-6.3 mi.
389
MANEUVERING BOARD PROBLEMS
17.Ownship,oncourse298˚,speed13knots,obtainsthefollowingradar
bearingsandrangesatthetimesindicated,usingaradarrangesettingof20
miles:
Required:
(1)Range at CPA as determined at 0729.
(2)Time at CPA as determined at 0729.
(3)Course of other ship as determined at 0729.
(4)Speed of other ship as determined at 0729.
(5)Range at CPA as determined at 0741.
(6)Time at CPA as determined at 0741.
(7)Course of other ship as determined at 0741.
(8)Speed of other ship as determined at 0741.
Solution:
(1)R1.0mi.,(2)T0755,(3)C030˚,(4)S7.0kn.,(5)R2.0mi.,(6)T
0749.5, (7) C 064˚, (8) S 7.0 kn.
18.Ownship,oncourse073˚,speed19.5knots,obtainsthefollowingradar
bearingsandrangesatthetimesindicated,usingaradarrangesettingof20
miles:
Required:
(1)Range at CPA as determined at 1558.
(2)Time at CPA as determined at 1558.
(3)Course of other ship as determined at 1558.
(4)Speed of other ship as determined at 1558.
(5)Range at CPA as determined at 1624.
(6)Time at CPA as determined at 1624.
(7)Course of other ship as determined at 1624.
(8)Speed of other ship as determined at 1624.
(9)Range at CPA as determined at 1657.
(10)Time at CPA as determined at 1657.
(11)Course of other ship as determined at 1657.
(12)Speed of other ship as determined at 1657.
Solution:
(1)R0.0mi.,(2)T1718,(3)C098˚,(4)S21.5kn.,(5)R2.0mi.,(6)T
1721,(7)C098˚,(8)S20.0kn.,(9)R3.7mi.,(10)T1718,(11)C098˚,
(12) S 18.0 kn.
TimeBearingRange (mi.)
0639267˚19.0
0651266.5˚16.0
0709265˚11.5
0729261˚ 6.5
0735255.5˚ 4.9
0737252˚ 4.3
0741242.5˚ 3.3
TimeBearingRange (mi.)
1530343˚16.2
1540343˚14.7
1546343˚13.8
1558343˚12.0
1606342.5˚10.9
1612341.5˚10.1
1624339.5˚8.4
1632.5336˚7.3
1644328.5˚6.0
1657315˚4.7
390
19.Ownship,oncourse140˚,speed5knots,obtainsthefollowingradar
bearingsandrangesatthetimesindicated,usingaradarrangesettingof12
miles:
Required:
(1)Range at CPA as determined at 0308.
(2)Time at CPA as determined at 0308.
(3)Course of other ship as determined at 0308.
(4)Speed of other ship as determined at 0308.
(5)Range at CPA as determined at 0317.
(6)Time at CPA as determined at 0317.
(7)Course of other ship as determined at 0317.
(8)Speed of other ship as determined at 0317.
Solution:
(1)R0.2mi.,(2)T0322,(3)C325˚,(4)S20.0kn.,(5)R3.0mi.,(6)T
0320, (7) C 006˚, (8) S 20.0 kn.
20.Ownship,oncourse001˚,speed15knots,obtainsthefollowingradar
bearingsandrangesatthetimesindicated,usingaradarrangesettingof15
miles:
Required:
(1)Range at CPA as determined at 2318.
(2)Time at CPA as determined at 2318.
(3)Course of other ship as determined at 2318.
(4)Speed of other ship as determined at 2318.
(5)Predictedrangeofothervesselasitcrossesdeadaheadofownship
as determined at 2318.
(6)Predicted time of crossing ahead as determined at 2318.
(7)Course of other ship as determined at 2351.
(8)Speed of other ship as determined at 2351.
(9)Predictedrangeofothervesselasitcrossesdeadasternofownship
as determined at 2351.
(10)Predicted time of crossing astern as determined at 2351.
(11)Direction of relative movement between 0002.5 and 0008.
(12)Relative speed between 0002.5 and 0008.
(13)Course of other ship as determined at 0026.
(14)Speed of other ship as determined at 0026.
Solution:
(1)R1.2mi.,(2)T0042,(3)C349˚,(4)S21.0kn.,(5)R2.0mi.,(6)T
0056,(7)C326˚,(8)S21.0kn.,(9)R5.1mi.,(10)T2358,(11)DRM
281.5˚, (12) SRM 12.0 kn., (13) C 349˚, (14) S 21.0 kn.
TimeBearingRange (mi.)
0257142˚10.5
0303141.5˚8
0308141˚6
0312135˚4.5
0314126.5˚4
0317110.5˚3.2
TimeBearingRange (mi.)
2243138˚14.0
2255137.5˚12.6
2318136˚9.9
2332140˚8.0
2351166.5˚5.5
0002.5191.5˚5.0
0008204˚5.1
0014214˚5.1
0020222˚4.95
0026230˚4.85
391
21.Ownship,oncourse196˚,speed8knots,obtainsthefollowingradar
bearingsandrangesatthetimesindicated,usingaradarrangesettingof12
miles:
Required:
(1)Range at CPA as determined at 2318.
(2)Time at CPA as determined at 2318.
(3)Course of other ship as determined at 2318.
(4)Speed of other ship as determined at 2318.
(5)Range at CPA as determined at 2400.
(6)Time at CPA as determined at 2400.
(7)Course of other ship as determined at 2400.
(8)Speed of other ship as determined at 2400.
(9)Course of other ship as determined at 0026.
(10)Speed of other ship as determined at 0026.
Solution:
(1)R0.0mi.,(2)T0009,(3)C196˚,(4)S18.0kn.,(5)R2.0mi.,(6)T
0006, (7) C 207˚, (8) S 18.0 kn., (9) C 196˚, (10) S 18.0 kn.
22.Ownship,oncourse092˚,speed12knots,obtainsthefollowingradar
bearingsandrangesatthetimesindicated,usingaradarrangesettingof16
miles:
Required:
(1)Range at CPA as determined at 1830.
(2)Time at CPA as determined at 1830.
(3)Course of other ship as determined at 1830.
(4)Speed of other ship as determined at 1830.
(5)Course of other ship as determined at 1906.
(6)Speed of other ship as determined at 1906.
(7)Course of other ship as determined at 1950.
(8)Speed of other ship as determined at 1950.
Solution:
(1)R0.5mi.,(2)T1935.5,(3)C114˚,(4)S16.0kn.,(5)C147˚,(6)S
16.0 kn., (7) C 124˚, (8) S 20.0 kn.
TimeBearingRange (mi.)
2303016˚11.0
2309016˚10.0
2318016˚8.5
2330016˚6.5
2340011.5˚4.9
2350359.5˚3.4
2400333.5˚2.2
0010.5286˚2.0
0020247.5˚2.5
0026233.5˚3.2
TimeBearingRange (mi.)
1720335˚15.0
1750334.5˚11.7
1830333˚7.2
1854325.5˚4.5
1858315.5˚4.0
1902303.5˚3.6
1906289.5˚3.4
1914263.5˚3.3
1930212.5˚3.8
1950184.5˚6.8
392
23.Ownship,oncourse080˚,speed12.5knots,obtainsthefollowingradar
bearingsandrangesatthetimesindicated,usingaradarrangesettingof16
miles:
Required:
(1)Range at CPA.
(2)Time at CPA.
(3)Course of other ship.
(4)Speed of other ship.
Decision:
Whentherangedecreasesto8.0miles,ownshipwillturntotheleftto
increase the CPA distance to 3.0 miles.
Required:
(5)Predicted time of change of course.
(6)Predicted bearing of other ship when own ship changes course.
(7)New course for own ship.
(8)Time at new CPA.
(9)Time at which own ship is dead astern of other ship.
Solution:
(1)R1.0mi.,(2)T0215,(3)C124˚,(4)S9.0kn.,(5)T0120,(6)B
041.5˚, (7) C 064˚, (8) T 0200, (9) T 0204.
TimeBearingRange (mi.)
0035038˚14.5
0044038.5˚13.2
0106040˚10.0
393
24.Ownship,oncourse251˚,speed18.5knots,obtainsthefollowingradar
bearingsandrangesatthetimesindicated,usingaradarrangesettingof20
miles:
Required:(As determined at 0401.)
(1)Range at CPA.
(2)Time at CPA.
(3)Course of other ship.
(4)Speed of other ship.
Decision:
Ownshipwillpassasternofothervessel,withaCPAof4.0milesand
newdirectionofrelativemovementperpendiculartoownship'soriginal
course,maintainingaspeedof18.5knots.Theoriginalcoursewillbe
resumed when the other ship is dead ahead of this course.
Required:
(5)New direction of relative movement.
(6)Predicted time of change of course.
(7)Predicted bearing of other ship when own ship changes course.
(8)Predicted range of other ship when own ship changes course.
(9)New course for own ship.
(10)Predicted new relative speed.
(11)Predicted time at which other ship is dead ahead of own ship.
(12)Predicted range of other ship when it is dead ahead of own ship.
(13)Predicted time at CPA, as determined at 0422.
(14)Bearingofothershipwhenitisdeadaheadofownship'soriginal
course.
(15)Predicted time of resuming original course.
Solution:
(1)R1.0mi.,(2)T0515,(3)C222˚,(4)S16.0kn.,(5)DRM161˚,(6)T
0411,(7)B316.5˚,(8)R9.6mi.,(9)C292˚,(10)SRM19.8kn.,(11)T
0428, (12) R 5.3 mi., (13) T 0438.5, (14) B 251˚, (15) T 0438.5.
TimeBearingRange (mi.)
0327314˚16.2
0337314.5˚14.7
0351315˚12.6
0401315.5˚11.1
0413.5315˚9.1
0422305˚6.7
394
25.Ownship,oncourse035˚,speed20knots,obtainsthefollowingradar
bearingsandrangesatthetimesindicated,usingaradarrangesettingof15
miles:
Required:(As determined at 1915.)
(1)Range at CPA.
(2)Time at CPA.
(3)Course of other ship.
(4)Speed of other ship.
Decision:
Whentherangedecreasesto5.0miles,ownshipwillchangecoursetothe
right,maintainingaspeedof20knots,topasstheothershipwithaCPA
of1.0mile.Originalcourseof035˚willberesumedwhentheothership
is broad on the port quarter.
Required:
(5)Predicted time of change of course to the right.
(6)New course for own ship.
(7)Bearing of CPA as determined at 1935.
(8)Predicted time at 1.0 mile CPA as determined at 1935.
(9)Bearingofothershipwhenownshipcommencesturntooriginal
course.
(10)Predicted time of resuming original course.
Solution:
(1)R0.0mi.,(2)T1954,(3)C035˚,(4)S4.0kn.,(5)T1935,(6)C044˚,
(7) B 314˚, (8) T 1952, (9) B 269˚, (10) T 1957.
TimeBearingRange (mi.)
1900035˚14.4
1906035˚12.8
1915035˚10.4
1924035˚8.0
1933035˚5.6
1941030˚3.5
1947015˚1.9
395
26.Ownship,oncourse173˚,speed16.5knots,obtainsthefollowingradar
bearingsandrangesatthetimesindicated,usingaradarrangesettingof20
miles:
Required:(As determined at 2142.)
(1)Range at CPA.
(2)Time at CPA.
(3)Predicted range other ship will be dead ahead.
(4)Predicted time of crossing ahead.
(5)Course of other ship.
(6)Speed of other ship.
Decision:
Whenrangedecreasesto10milesownshipwillchangecoursetothe
righttobearingofsternofothervessel(assume0.5˚rightofradar
contact).
Required:
(7)Range at new CPA.
(8)Time at new CPA.
(9)Direction of new relative movement line.
(10)New relative speed.
(11)New course of own ship.
Decision:
Ownshipwillresumeoriginalcoursewhenbearingofothervesselisthe
same as the original course of own ship.
Required:
(12)Predicted time of resuming original course.
(13)Distance displaced to right of original course line.
(14)Additional distance steamed in avoiding other vessel.
(15)Time lost in avoiding other vessel.
Solution:
(1)R2.5mi.,(2)T2233,(3)R3.0mi.,(4)T2225.5,(5)C120˚,(6)S
14.7kn.,(7)R6.3mi.,(8)T2211.5,(9)DRM075˚,(10)SRM23.2kn.,
(11)C216˚,(12)T2209.5,(13)D3.4mi.,(14)D1.3mi.,(15)tlessthan
5 min.
TimeBearingRange (mi.)
2125.5221˚16.0
2130220.5˚15.0
2137.5219˚13.2
2142218˚12.2
2151.5215.5˚10.0
2158205.5˚8.3
2206185˚6.7
396
27.Ownship,oncourse274˚,speed15.5knots,obtainsthefollowingradar
bearingsandrangesatthetimesindicated,usingaradarrangesettingof20
miles:
Required:
(1)Range at CPA.
(2)Time at CPA.
(3)Course of other ship.
(4)Speed of other ship.
Decision:
Whentherangedecreasesto6.0miles,ownshipwillcommenceactionto
obtainaCPAdistanceof4.0miles,withownshipcrossingasternofother
vessel.
Required:
(5)Predicted bearing of other ship when at a range of 6.0 miles.
(6)Predictedtimewhenothershipisat6.0milerange,andownship
must commence action to obtain the desired CPA of 4.0 miles.
Decision:
Ownshipmay(1)altercoursetorightandmaintainspeedof15.5knots,
or (2) reduce speed and maintain course of 274˚.
Required:
(7)New course if own ship maintains speed of 15.5 knots.
(8)Predictedtimewhenothervesselbears274˚andownship’soriginal
course can be resumed.
(9)New speed if own ship maintains course of 274˚.
(10)Predictedtimewhenothervesselcrossesaheadofownshipand
original speed of 15.5 knots can be resumed.
Solution:
(1)R1.1mi.,(2)T0935,(3)C242˚,(4)S20.0kn.,(5)B002˚,(6)T
0902, (7) C 019˚, (8) T 0916, (9) S 8.2 kn., (10) T 0936.
TimeBearingRange (mi.)
0815008˚14.4
0839006˚10.1
0853004˚7.6
397
28.Ownship,oncourse052˚,speed8.5knots,obtainsthefollowingradar
bearingsandrangesatthetimesindicated,usingaradarrangesettingof20
miles:
Required:
(1)Range at CPA.
(2)Time at CPA.
(3)Course of other ship.
(4)Speed of other ship.
Decision:
At0555,ownshipistoaltercoursetorighttoprovideaCPAdistanceof
2.0 miles on own ship’s port side.
Required:
(5)Predicted bearing of other ship when own ship changes course.
(6)Predicted range of other ship when own ship changes course.
(7)New course for own ship.
Ownshipcontinuestotrackothershipandobtainsthefollowingradar
bearingsandrangesatthetimesindicated,usingaradarrangesettingof20
miles:
Required:
(8)Course of other ship as determined at 0609.
(9)Speed of other ship as determined at 0609.
(10)Range at CPA as determined at 0609.
Solution:
(1)R0.0mi.,(2)T0619,(3)C232˚,(4)S21.5kn.,(5)B052˚,(6)R12.0
mi., (7) C 086˚, (8) C 241˚, (9) S 21.5 kn., (10) R 3.0 mi.
TimeBearingRange (mi.)
0542052˚18.5
0544052˚17.5
0549052˚15.0
0550052˚14.5
TimeBearingRange (mi.)
0559050˚10.0
0604.5043.5˚7.4
0606.5040˚6.5
0609034˚5.5
398
APPENDIX D
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