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Microwave propagation characteristics a study over Assam valley with respect to hydrometeors

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MICROWAVE PROPAGATION CHARACTERISTICS
A STUDY OVER ASSAM VALLEY
WITH RESPECT TO HYDROMETEORS
BY
itv
K. ISAIAH THIMOTHY
A THESIS SUBMITTED TO THE FACULTY OF S
GAUHATI UNIVERSITY
FOR THE AWARD OF THE DEGREE OF
DOCTOR OF PHILOSOPHY
IN PHYSICS
DEPARTMENT OF PHYSICS
1993
^
V*
t
ProQuest Number: 10115696
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CERTIFICATE
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RESEARCH
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Mr . K. ISAIAH THIMOTHY
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GUIDE
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Gu w a h a t i 781 014 •
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A B S T R A C T
p C t .£•«£•
ABSTRACT
The basic aim of
propagation
the research work
characteristics
(specially
rain)
and
microwave attenuation.
to
is to study
with
respect
analyse
to
system
This aim is built
the
microwave
hydrometeors
responses
towards
on the fact
that
the
radiowaves above 10GHz suffer attenuation due to absorption and
scattering by raindrops.
This study has been carried
out at Guwahati
(26° 12"N,
92°E ),
over a 1ine-of-sight (LOS), short haul
(3.2 kms) microwave
link
operating at 11 GHz by P?«T department.
A standard observational
procedure is adopted to impliment the aim.
Microwave
field
period
1991-93,
order
to
strength
measurements
are
taken
by tapping the AGC output of
associate
the
attenuation
raindrop size distribution,
the data
with
on
during
the
the receiver.
rainfall
rainfall
rate
and
In
and
raindrop
size and shape are collected simu 1teneous1y by the instruments
developed for the purpose.
raindrop size,
The observational
shape and microwave attenuation
Correlation between rainfall
computed.
of
is presented
rate and microwave attenuation
.
is
The observed results are then studied under the light
standard
Lognormal
study on rainfall,
existing
and
Global
models
model.
like
The
CCIR,
Negative
observed
exponential,
results
are
also
compared with those reported from various stations. Modelling on
thunderstorm
rain attenuation
is made with an
polarisation
in
different
observed
results
raindrops
on
of
rainfall,
the
aid
shapes.
rainfall
rate
of
Based
dipole
on
the
distribution
over Assam is model led in which the rain cl imatic zones of Assam
are classified.
The thesis is organized into six chapters:
CHAPTER 1: GENERAL INTRODUCTION
CHAPTER 2: THEORIES AND NUMERICAL MODELS ON RAIN ATTENUATION : A
REVIEW
ABSTRACT
CH APTER
Parte
C 11
>
3: E X P E R IM E N T A L T E C H N IQ U E S
C H A P T E R 4-: O B S E R V A T IO N S A N D A N A L Y S E S
CHAPTER 5 : COMPARISON OF OBSERVED RESULTS WITH STANDARD MODELS
AND MODELLING OF RAIN ATTEN UA TIO N AND RAINFALL RATE
CHAPTER 6: RESULTS- DISCUSSIONS AND CONCLUSIONS
Ch
Necessity
is
g ives
1
ap ter
a brief
introduction
for going to microwave
explained.
Troposphere,
Effects
of
the
to
the
present
frequencies
work.
for communication
intervening
medium
at these frequencies are described.
i.e
the
The physics of
Troposphere and the physics of rain attenuation are discussed. A
brief review and historical deve1opements in the field of study
are
presented..
The
importance
and
scope
of
the
present
investigation are given in the last section of the chapter.
CHAPTER 2
reviews
standard
models
distribution.
sections
of
the
on
theories
rain
attenuation
The absorption,
raindrops
are
on microwave
scattering
discussed
and
propagation
raindrop
and extinction
with
and
Rayleigh
size
cross-
scattering
theory as well as with Mie scattering theory. The derivation of
emperical
relation
between
attenuation
and
rainfall
rate
is
given.
deals
CH A P TER
3
adopted
and
The detailed
as
part
of
microwave
charts
with
suitably
experimental
designed
techniques
for convenient
information of the instruments
the
research
link and
for
the
the
work
the terrain
microwave
is
data
are
attenuation
are
col lection.
that are developed
presented.
profile
that
Details
given.
and
for
of
the
Calibration
the
above
instruments are presented.
CH AP TER
4
attenuation,
presents
the
observations
and
analyses
of
rain
rainfall and raindrop size data measured during the
period 1991-93. The following observational
rain attenuation,
rainfall and raindrop size.
A) On rain attenuation:
studies are made on
ABSTRACT
pageC u O
1. Pattern of rain attenuation
2. Seasonal variation
3. Cumulative probability distribution,
in which the percent
of
time exceedence of specified level of attenuation is studied.
4
Year
to year variability of CPD,
specially
attenuation that is exceeded for 0.01 %
5.
Percent
of
time
the
fade
margin
with
respect
to
of time in a year.
(45
dB
attenuation)
exceed ing.
B) On rainfall:
1. Seasonal variation of rainfall
2. Preferential time of occurrence
3. Occurrence percentage of thinderstorms
4. Cumulative probability distribution
5. worst month statistics
C) On raindrop size distribution (RSD):
1. RSD for thunderstorms
2. RSD for drizzle and
3. RSD for showers
CHAPTER 5
presents
the comparative
study
of
with the standard model and also with results
observed
results
repoted by other
workers in India. Observed attenuation is compared with the CC1R
and Lognormal
models.
Similarly
, observations
studied in coraparision with Global
rainfall
from
rate are compared with
tropical
Observations
countries
links
as
models.
guide
well
a t tenuation.
reported
(Malaysia,
The results
are given in the final chapter
a few
and CC1R models.
rainfall
Brezil
are
The CFD of
distribution
and
Nigeria).
on RSD are compared with the Negative exponential
and Lognormal
with
on rainfall
as
lines
global
towards
, discussins and conclusions
(chapter 65 of the thesis along
future
modelling
of
designing
rainfall
of
rate
microwave
and
rain
PREFACE
pCL
£?
C l UJ
PREFACE
The studies on the subject
"at tenuat ion of radiowaves by rain"
were originally started in 1Q47,
when severe rain attenuation
prob lem 'was faced by the 'radar■ operators during
II.
Since
then
, many
experimental
as
well
the World war
as
theoretical
investigations on the subject have been conducted all around the
globe.
As
a
result
of
these
investigations
it
is
now,
well
established that the radiowaves above lo OHz suffer attenuation
by rain and the depth of attenuation is a function of the radio
frequency,
polarisation
state,
intensity
of
rainfall
and
the
raindrop size distribution.
The
results
on
rain
attenuation
characteristics,
reported
recently from different climatic zones are at varience.
Though
radiowave attenuation by rain is a well established phenomenon,
the
characteristics
attenuation
another-.
etc. ,
In
attenuation
of
may
order
rain
vary
to
attenuation
from
one
understand
characteristics
on
like,
cliffiatic
the
depth
condition
dependence
climatic
of
of
to
rain
conditions,
many
research programmes are now being executed at different climatic
zones.
An experimental study on rain attenuation has been carried out
over Guwahati,
towards understanding the subject
,specially
in
view of dependence of rain attenuation on climatic conditions,
on seasons and on
type of rainfall.
This
thesis presents
the
observational results on rain attenuation, rainfall and raindrop
size distribution.
The results of this study may provide substantial input for the
system
s t udy
d&S t gTuEfFS t
is
future
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India
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it
links
As
triigh t
over
this
l{
Assam
j
in
and
ACKNOWLEDGEMENTS
pa.g& C roL>
ACKNOWLEDGEMENTS
The author would like to express extreme gratitude to his guide
Prof.
A. K. Barbara
brainstorm
during
which
the
for
his
enabled
course
of
guidance,
the
author
study.
He
affection,
to overcome
also
expresses
care
many
his
and
hurdles
profound
gratitude to Dr.(Miss) M.Devi for her inspiring superintendence,
constant care and stimulating encouragement.
He wishes
to express
his
Prof.D.Dutta Mazumadar,
Jois, Prof.
earnest
thanks
Dr. G.Swarup,
to Dr.
Dr.
J.Das and Dr. Narayan Rao
S .C .Mazumdar,
Govind,
Prof.
Sankar
for their many valuable
suggestions, encouragement and fruitful discussions.
He also
from
gratefully
the
acknowledges
Department
of
the financial
Electronics,
support
Government
of
received
India,
to
carry out this work.
The
author
department
is
grateful
of
Physics,
necessary
infrastructure
thankful
to
the
P&T
to
Prof.
D. K. Chowdhury, Head
Gauhati
for
the
university,for
research
Mic rowave
of
providing
works.
He
(maintenance)
is
the
calibrating
director,
field
the
1MD,
strength
recording
Guwahati,
data,
and
systems.
for
He
for
link to
co-operating
is
providing
also
department,
Guwahati for allowing him to work with their microwave
collect
the
indebted
supporting
in
to
the
data
on
for
the
rainfall and thunderstorms.
He
is
grateful
to
his
colleague
Mr.Sanjay
Sharma
co-operation, encouragement , help rendered and for constructive
criticism
and
discussions.
He
is
thankful
to
Mr.
Deepkamal,
Sukla and Bhasker for the help rendered by them.
He would
to
his
like to express, with a sense of appreciation,
brother
motivation
and
and
sister-in-law
understanding
have not been formed.
for
.without
their
which
moral
this
thanks
support,
thesis
would
CONTENTS
PAGE
Ab s t r a c t
i
Pr e f a c e
iv
Ack n o w led g em en ts
V
C h a p t e r 1 GENERAL INTRODUCTION
i
i
.i
Complusion
to c h o o s e
microwaves
for
commun-
n icat i on
1
.1
. 1
i
Fundamentals
of
communication
: Band-width
•t
r eq u i r e m en t
1. 1. 2
Allocation
of
frequencies
for
different
u s er
a pp l i c at i ons
1.2
Microwave
1.3
Effects
2
system
of
: A brief
intervening
description.
medium
on
4
microwave
p r o p a g a t ion
1.3.1
5
Transparency
of
the
Ionosphere
and
Ionospheric
effects
1.3.2
5
Physics
of
the
Troposphere
and
Tropospheric
effects
5
1.3 .3
Clear
sky
i n du c e d
1.3.4
Rain
1.3.5
Physics
1.3.6
Prediction
i n du c e d
of
1. 4
Historical
propagation
rain
and
attenuation
propagation
7
effects
effects
10
attenuation
11
computational
methods
of
rain
:A r e v i e w
12
developments
in r a i n d r o p
size
measurements
1.5
Scope
of
the
15
present
research
work
16
REFERENCES
16
C h a p t e r 2: T h e o r ie s a n d n u m e r ic a l m o d e l s : A r e v ie w
2. 1
Theory
2.1.1
Scattering
cross-section
of
a
raindrop
(from
RST )
2 2
j[
Absorption
cross-section
of
a
raindrop
(from
R ST )
25
Extinction
cross-section
of
a
raindrop
(from
RST )
25
C1
2.1.3
on
r ai n
2 2
attenuation
2 2
2.1.4
E x t i n c ti o n c r o s s - s e c t i o n
2. 2
M odels on
2. 2. 1
N e g a t i v e exponential
2.2.2
Gamm a d i st r i b u t i o n
32
2.2.3
Lognormal
32
2.3
Attenuation,
of a raindrop
(from M S T )
r ai nd rop size d i s t r i b u t i o n
29
9q
model
d i st r i b u t i on
rainfall
26
rate
relation
33
RE F ER E NC ES
37
C h a p t e r 3: EXPERIMENTAL TECHNIQUES
40
3. 1
Introduction
40
3. 2
M i c r o w a v e field
3. 2. 1
Microwave
O . z,. z.
Experi me nt al
s trength
meas ur em en ts
link c h a r a c t e r i s t i c s
o r ga n iz a t i o n
41
41
for mic ro wa ve
a t t e n u a t i o n m e a s ur em en ts
44
3.2.3
Calibration
44
3. 3.
Collection
3.3. 1
Non
3. 3.2
Self
recording
3.3. 3
Fast
response rain
3. 4
R a i n d r o p size m e a s su re me nt s
52
3. 5
C i rc u it d e v el o pm en t
53
3.5. 1
V o lt ag e to current
3.5.2
Development
3.5.3
A nalog
of
the system
of s up po rt in g
recording
of
data
46
rain gauge
s y phoning
fast
r ecording
49
49
rain gauge
49
gauge
c on verter
response
53
rain
tec hn iq ue for
gauge
fast
response
rain gauge
3.5.4
3. 6
Development
56
63
of
r aind ro p size measuring
i ns trument
69
C on e 1us i ons
76
R E FE R E N C E S
80
C h a p t e r 4-: OBSERVATIONS AND ANALYSES
82
4. 1
Introduction
82
4.2
Data base
84
4.3
Observations on microwave attenuation over
the link under test
85
4.3.1
Various pattern of attenuation
85
4.3.2
Seasonal variation of attenuation
85
4.3.3
Cumulative probability distribution of
attenuation
4.4
93
Observation on rainfall over Guwahati
4.4.1
Seasonal variation of rainfall
4.4.2
Preferential time of occurrence of rainfall
93
97
and its seasonal variation
97
4.4.3
Percentage of occurrence of thunderstorms
97
4.4.4
Cumulative probability distribution of
rainfall rate
4.4.5
4.5
4.5. 1
1 00
Correlative study of attenuation and
rainfall rate
105
Observations on raindrop size
111
RSD for thunderstorms
111
4.5.2
RSD for drizzle
114
4.5.3
RSD for showers
117
4.6
Rainfall rate and RSD: A correlative study
119
4.7
RSD and attenuation
121
4.8
Summary
125
REFERENCES
127
Chapter 5: Comparision
of observed results with
S T A N D A R D M ODELS A N D MODEL C O M P U T A T IO N
129
5.1
Introduction
5.2
Comparision of observed attenuationwith models
5.2.1
With the CCIR model
129
5.2.2
With the Log normal model
131
5.2.3
With reported results
133
5.3
Comparision of rainfall rate withmodels
136
5.3.1
5.3.2
With the Global model
With the CCIR model
136
138
5.3.3
With reported results from tropicalcountries
141
5.4
Comparision of RSD with
129
models
129
141
5.4.1
Uith the Negative exponential model
141
5.4.2
With the Lognormal model
143
5.5.
Raindrop size, shape and microwave attenuation:
A model
5.6
5.7
147
Modelling of rainfall rate distribution over
Assam valley
160
conclusions
166
REFERENCES
168
C h a p t e r 6: RESULTS, DISCUSSION AND CONCLUSIONS
m
6.1.
172
Introduction
6.2
Observational results
173
6.3
Results of comparative study
177
6.4
Assessment of the microwave link
178
6.5
Prediction of microwave attenuation at
various frequencies
179
6.6
results on
180
6.7
Future guide lines
circuit developed
181
APPENDIX A: DEVELOPEMENT OF RADIO ANEMOMETER
182
APPENDIX B: SODAR
186
OBSERVATIONS OF THE ATMOSPHERE
BEFORE AND AFTER RAIN EVENT
TO MICROWAVE FADING
HN RELATION
GENERAL INTRODUCTION
M
l
M
CHAPTER 1
pag& i
GENERAL INTRODUCTION
1.1 COMPULSION TO CHOOSE MICROWAVES FOR COMMUNICATION:
1.1.1 FUNDAMENTALS OF COMMUNICATION: BANDWIDTH REQUIREMENT:
Communication by electrical means began with the introduction of
telegraphy
in
1844,
systems electrical
lines
followed
by
telephony
in
the sender
and
recipient.
During
period when these systems were being developed,
for
In
these
signals are sent over two-wire transmission
that connect
foundation
1878.
electromagnetic
Maxwell and others. However,
radiation
the same
the theoretical
was
being
laid
by
it was not until 1897 that Marconi
first patented a complete wireless
telegraphy
system based
on
the use of electromagnetic radiation (Radio waves) that had been
predicted theoretically by Maxwell.
The early transmitters were
of the spark-gap variety and served the purpose of sending the
on-off
pulse
characteristics
of
telegraphy.
The
actual
transmission of voice by means of electromagnetic radiation did
not
occur
before
the
invention
of
the
vacuum
tubes,
in
the
period 1904-1915.
Any specific communication channel
is typically associated with
a particular frequency range called as bandwidth. The process of
modulation in which amplitude, frequency or phase of the carrier
signal
is
varied
transmitted,
in
accordance
with
the
information
to
be
requires a specific bandwidth Transmission of human
voice requires a bandwidth of 4 KHz where as high fidelity music
demands
at
least
15
KHz.
Television
transmission
of
colour
picture requires about 7 MHz bandwidth. This requirement has put
a limitation on the number of channels that can be transmitted
over a specified carrier frequency.
To overcome
this
problem,
the techniques like multiplexing , in which transmission of more
than one signal over a single carrier band have been developed.
There
are
frequency
multiplexing
various
division
types
of
multiplexing
multiplexing!FDM)
(TDM ) , etc.
In FDM,
the
techniques,
and
individual
time
voice
like
division
signals,
c h a p te r- i
page
which are overlapping in frequency,
have
their
own
frequency
content shifted through sinusoidal amplitude modulation so
the spectra
of modulated
signals
be
band
can
channel.
signals
transmitted
In
other
no
longer
simultaneously
words
FDM
overlap
over
provides
2
and
that
these
a single
sharing
wide
of
the
frequency by the individual signals or sub channels, where as in
TDM,
the individual sub channels are assigned a specified
slot
by
means
of
pulse
amplitude
modulation
C
Fredie,
time
1981;
Oppenheim and. WiLlsh.y, 1987; Roddy and Coolin, 19875.
1.1.2 ALLOCATION OF FREQUENCIES FOR DIFFERENT USER APPLICATIONS:
The world spectrum utilization bodies, namely, The International
Telecommunication
Conference
(CC1R)
Union
(WARC)
have
and
(ITU),
World
International
allocated
the
Administrative
Radio
Radio Consultive Commettee
available
frequencies
in
the
electromagnetic spectrum for different user applications on the
basis
of
power
efficiencies,
reduced noise and
allocated
minimal
propagation
interference effects.
frequency
bands
along
with
distortion,
Table 1.1.1.
the
shows the
application
of
each
band.
The
demand
proportion
for
to
additional
the
growth
channel
of
capacity
information.
has
This
grown
demand
in
has
compelled the system designers to look into the higher frequency
>VHF, side of the spectrum.
Thus the ever increasing demand for additional channel capacity
and congestion at lower frequencies has provided the necessary
impetus to explore the possibilities of using higher frequencies
i.e microwaves C3-300 GHz) in telecommunications.
c h a p t er 1
page? 3
T able 1.1.1
Frequency
band allocation :
Frequency band
Designation
Typical
service
3-30 KHz
Very Low Frequency
Navigation,
sonar
(VLF )
30-300 KHz
300-3000 KHz
Low Frequency
Radio beacons and
(LF)
Navigational aids
Medium Frequency
AM broadcasting,
(MF )
maritime radio,
coast
gaurd communication and
direction finding
3-30 MHz
High f requency
Telephone,
telegraph,
(HF )
Facsimile,
amateur
radio, citizens band,
ship-aircraft communi­
cation
30-300 MHz
Very high frequency
TV, FM broadcasting,
(VHF )
air traffic control,
police,
taxicab mobile
radi o
300-3000 MHz
Ultra High Frequency
Television,
radio sonde,
(UHF )
surveillance radar
chapter- i
page 4
The table
1.1.2 shows m i c r o w a v e frequency
for d i f ferent
types
band d e s i g n a t i o n used
of communication.
T able 1.1.2
Microwave
frequency designation :
F r e quency
ccoitn
,i >
988
mi c r o w a v e band design a t i o n
ol d
new
5 00-1000 MHz
VHF
C
1-2 GHz
L
D
2-3 GHz
S
E
3-4 GHz
S
F
4-6 GHz
C
G
6-8 GHz
C
H
8-10 GHz
X
I
10-12.4 GHz
X
J
12.4-18 GHz
Ku
J
18-20
GHz
K
J
20-26.5 GHz
K
K
26.5-40 GHz
Ka
K
1.2 MICROWAVE COMMUNICATION SYSTEM: A BRIEF DESCRIPTION
Microwave
frequencies
1 i ne-of - s i gh t
ground
Since
of
to
or
allow
over
the
the
the
or
radio
same
route
the
f requenc i es
in
transmi tter
ou tput
directional
antennas
to
of
p r o pagate
whether
s a t ellite
systems
many
b a n dwidth
thousands
the
same
range
3-18
GHz
are
are used.
not
be
in
are
the
systems.
us i ng
need
they
become
and
power
mainly
used
communication
have
communication
trans m i s s i o n
transmission
GHz
mode,
ground
telephone
needed
1
space
m i c rowave
long d istance
provide
free
ground
1950s,
above
and
of
a
in
links.
workhorses
These
systems
reliability
to
te l e p h o n e
channels
facilities.
Carr i er
generally
high
used.
because
The
highly
chapter 1
page 5
1.3 EFFECTS OF INTERVENING MEDIUM ON MICROWAVES
1.3.1 TRANSPARENCY OF THE IONOSPHERE AND IONOSPHERIC EFFECTS:
In
the
satellite
to
ground
communication
signals have to pass
through upper
while
to
in
travel
the
only
atmosphere
40
ground
MHz
through
ground
lower
(Ionosphere),
is well-known.
LOS
links,
microwave
to lower atmospheric media
links,
atmosphere.
the
The
signal
role
are
of
to
upper
towards communication of signals below
In
ideal
situation,
the
ionosphere
is
transperent to frequencies higher than that and has practically,
no control
towards transmission and reception qualities at VHF
and microwave frequencies. But the irregularities in the medium,
generated
in
various
solar,
geomagnetic
situations
and
by
different electric current systems are proved to be nuisecence
at VHF.
while
But
Even GHz
passing
the
signals
through
suffer
small
fluctuations
scale
magnitude
of
fluctuations
signals
in
the
microwave
env ironmentf/4ctr-e>7us, 1987;
Kel Ley,i977J>. A brief
ionospheric
or
irregularities.
attenuation
ionospheric
comparison to that experienced
(scintillations)
media
is
suffered
much
by
low
in
in tropospheric and near earth
Franhe and Liu>
description
of
the
t984; Sunan.da.bctsu and
troposphere
along
with
its variabilities is presented below.
1.3.2 PHYSICS OF THE TROPOSPHERE AND TROPOSPHERIC EFFECTS:
Troposphere
is
the
extended from ground
region
level
in
earth’s
atmosphere
to the altitude of about
which
is
15 kms at
the equator and 8 kms at the poles.
Physical
processes
evaporation,
like
advection,
radiation,
convective
absorption,
turbulent
diffusion
number of complex and interacting meteorological
the
troposphere
a
cauldron
processes.
The
physics
microwave
propagation
of
are
of
the
non-linear
troposphere
well
dealt
condensation,
in
a
phenomena make
and
and
and
unpredictable
its
the
effects
on
literature
ch a p te r
1
page 6
C M i t r a , 1 975;
R e d d y , 1 9 87 ;
O y i n i o y e , 1987;
D o lx ik f ia n o v ,
Bean and Dxit t o n , 1 9 6 6 ; H a l l , 19795.
19 7 1 ;
a)
C r a n e , 197Q;
T emperature
structure of the troposphere :
The thermal structure of the troposphere is important not only
from the considerations
radiowave
propagation
of atmospheric
point
of
view.
dynamics
The
but also from
temperature
troposphere gradually decreases from ground
in
the
level upwards,
at a
rate of approximately 6°c/km. The mean temperature as a function
of
altitude
b)
Inversions :
The
is
shown
temperature
modified
in
in
fig
structure
some
1.3.1.
of
the
situations
troposphere,
when,
the
some
normal
times,
is
decrease
in
temperature is inverted, giving rise to increase of temperature
with
height.
This
phenomenon
is
called
the
temperature
inversion. Temperature inversions increase the stability of the
atmosphere
and
situation
that
cooling.
Dry
inhibit
radiation.
causes
air
sun* s radiation.
Air
less density,
mixing.
temperature
absorbs
very
little
It is the earth's
touching
conduction only
vertical
One
inversion
heat
the process
thin
of
layer.
As
convection
the
is
obvious
radiative
directly
surface which
the earth’s surface
in a very
of
from
the
is heated by
is heated by heat
this
starts
heated
air
which
has
takesthe
heat up to higher levels. An opposite effect takes place during
night.
When
radiation,
the earth's
the
conduction.
earth's
very thin
neighboring
In this
surface
case,
and
increases
is cooled because of outgoing
air
is
the denser
cannot
layer of air
temperature
surface
move
also
air
with height.
by
is trapped
upwards.
is cooled.
cooled
near
Consequently
Above this cooled
Another
typical
heat
the
only
level
process
a
the
is
the advection. Hot and dry air from land, for example, may blow
over
cold
D\Lttan,
wet
1 9 66 ;
air
causing
B re m m e r, 1 9 4 9 5 .
temperature
inversions
CBean
and
chapter t
page 7
c) Radio Refractive Index CRRI):
The
troposphere,
for
all
practical
purposes
is considered
to
consist of dry air and water vapour. Molecules of dry air do not
have a significant permanent dipole moment.
influence of an external
moment is induced.
is
to
increase
electromagnetic field,
The net effect of all
the
However,
dielectric
hence the refractive index.
such
permittivity
a small
dipole
induced dipoles
of
On the other hand,
under the
the
medium
and
water molecules
possess permanent dipole moment and these dipoles are randomly
distributed.
In response to the field associated with a passing
radiowave, the permanent dipoles partially align with such field
adding to the polarization of the medium.
The mean value of the refractive
unity.
Since
index of air,
its departure from unity
n is close to
is so small
that
it
is
expressed in parts per million for the sake of convenience and a
parameter N is defined as followes:
N = (n-1) x 106 ....
....
....
....
...(1.3.1)
where, N is the Radio refractive index.
Smith and Weintrub
depends on pressure,
in 1Q53,
have shown
that
the
refractivity
temperature and humidity of the atmosphere
by the following relation.
N =
77.6 xP
-----T
+
373000xe
--T
....
....
....
...(1.3.2)
where, P is the atmospheric pressure in millibars,
e is the water vapour pressure in millibar and
T is absolute temperature in degree Kelvin.
The above equation is valid upto 100 GHz, with an error of less
than 0.5% . It is obvious from the equation that the important
parameter which plays a dominant role in RRI is the water vapour
pressure.
1.3.3 CLEAR SKY INDUCED PROPAGATION EFFECTS:
chapter 1
STAN D A R D
XJ
ft
K
!«
H E IG H T A b O v E SEA LE V E L , Km
A T M o r r u iM F
FIO.1.3.1 V A R IA TIO N OF TE M P E R A TU R E WITH A L TITU D E
chapt&r- i
pag& 9
The path between transmitting and receiving antenna, for most of
the time, will be occupied by atmosphere or free space which is
clear to eye, i.e, free from obstructions, clouds, rain or other
hydrometeors
atmosphere
and
has
dust
two
or
main
other
particulates.
constituents
This
namely,
gaseous
Nitrogen
and
Oxygen. The proportion of these constituents are fixed and their
effects for a particular link are constant. But, it is the third
constituent, water vapour pressure whose proportion varies, with
terrain and season that causes the most of clear sky impairments.
Water vapour distribution and gravity induced variations in the
density and pressure of the atmosphere as a function of altitude
contribute to systematic ray bending through variation of RR1.
The
average
RRI,
for
an
exponential
atmosphere
is
given
by
CCC1R, t97S>,
n (h ) = 1+Nx 10~6
= 1+315x 10~6 x exp(-0.138h) ---
...(1.3.3)
where h is altitude above sea level and
N is the RRI.
At any point on the ray path,
and Dutton, i
,
1/p
x
=
cosfi/n
ray curvature is given by CB&an
dn/dh
= dn/dh (for low elevation angles) ....
....
(1.3.4)
The refractivity N, decreases with height by 3.9 units per 100
meters or 40N/km.
This
is called standard value of refractive
gradient.
When the RRI gradient is greater than the standard (40N/km), the
situation is named as super refraction and if dn/dh is <40N/km,
it is subrefraction.
chapter- 1
The
page 10
local changes
in RRI,
which arise due to
development of
local inhomogenities in the atmosphere, cause severe impairments
to
microwave
inversion
propagation.
of
radiowaves),
refractive
Among
these
gradient
fluctuations
effects,
giving
in angle of
rise
ducting
to
arrival,
lor
trapping, of
fluctuation
of
amplitude and phase and antenna defocussing are important.
Longer the path through
the atmosphere,
the more severe these
impairments tend_' to be. For the line of sight (LOS) paths,
the
atmosphere can behave in such a manner that for some percentage
of time, rays are bent so that earth's surface appears to block
the
LOS
path
for .certain
terrain
and
atmospheric
situations
CVingant, 19812. Another serious source of signal fluctuation is
interference
of
signals
travelling
different
paths
from
transmitter to receiver which is known as multipath propagation.
This
condition
arises
because
of
local
variations
in
water
vapour content and temperature or because of formation of layers
with varying RRI.
Extensive
effects
investigations
have
led
to
on
the
clear
sky
induced
development
of
propagation
techniques
for
counteracting the imposed limitations. These techniques are now
available
in
literature
. One
frequency
diversity.
separated,
to overcome the fading
The
approach
use
of
is
two
to
use
space
antennas,
and
spatially
is called space diversity.
A
vertical separation of 5 to 15 meters has been found to improve
the fading characteristics of LOS links OVigant, 19712.
1.3.4 RAIN INDUCED PROPAGATION EFFECTS:
Microwave propagation networks above
10
GHz,
suffer
major
limitation due to presence of hydrometeors, specially rain CRyde
and Ryde,1944;
Radiowaves
various
Medhxix-s t,196(5; Hogg and Chu, 1976;
propagating
physical
discussed below.
through
processes.
rain
Some
are
of
Ogucgi ,19812.
attenuated
these
through
processes
are
chapter 1
page 11
1.3.5. PHYSICS OF RAIN ATTENUATION:
Absorption
cause
and
microwave
scattering
are
attenuation
the
two
physical
(Dolukhanov,
factors
1971;
that
Johnson
and
Jesik, 19842. To begin with, each rain droplet may be treated as
an imperfect conductor,
displacement
in which the advancing radiowaves induce
currents.
The
density
of
these
currents
considerable because the dielectric constant of water
is
is about
80 times that of air. On the other hand, density of displacement
currents
is proportional
to frequency,
so
that
heavy
currents
appear at higher frequencies in the centimetric and millimetric
bands. The resultant absorption of energy by raindrops manifest
itself as the attenuation of radiowaves.
Besides,
the currents
induced in droplets of rain or fog are the sources of scattered
or secondary radiation.
attenuation of
Practically this scattering
radiowaves
in the direction
of
results
propagation
in
and
the waves instead of being propagated in the right direction are
partly scattered in all directions. The most important effects of
absorption
and
scattering
are
CJohnson
and
Jesik,
1984;
Ippolito, 19812 are the following:
1)
Attenuation
caused
by
dissipation
of
radiowave
energy
as
heat,
2) Scattering results in loss of radiowave energy in the desired
direction
and
consequently
causing
interference
to
other
systems. Scatter loss will dominate when the wave length is much
smaller than the size of hydrometeor,
3) Depolarisation of radio waves due to non-spherica1 nature of
raindrops,
4) Rapid amplitude and phase scintillations caused by equivalent
multipath propagation,
5) Antenna gain degradation due to phase dispersion of ray paths
reaching the antenna and
6)
Band
systems.
width
coherence
reduction,
especially
in
digital
cha.ptex- t
page
12
1.3.6 PREDICTION AND COMPUTATIONAL METHODS OF RAIN ATTENUATION :
A REVIEW:
Extensive investigations on the subject during past two decades
have come out with a number of methods to estimate or predict
rain
attenuation
over
a
given
link.
These
methods
can
be
classified in two broad groups:
a) Prediction based on rainfall
1979;
Olsen
et.al,
1978;
statistics CCrane,1971
Moup/owna, 1985;
; Fedi,
Tattlemann
and
Gran them., 19855 and
b) Frequency scaling in which attenuation statistics at desired
frequency are predicted from the attenuation statistics at other
frequencies CDrofxica, 1974; 5
A fundamental
quantity
in calculation of rain attenuation over
terrestrial or earth-to-space paths is the specific attenuation
(A)
defined
as
attenuation
per
unit
distance
or
dB/KM.
Two
general approaches for calculating A are found in literature
COlsen et.al 19785:
1)
A
theoretical
distribution
of
method
raindrops
employing
modelled
as
a
uniformly
water
spheres
random
or
more
complex shapes and
2)
An
empirical procedure
approximate
relation
between specific attenuation (A), dB/km, and rainfall
rate <R),
mm/hr which is
a
and
on
the
given by,
A = aRb ....
where,
based
....
b
are
....
functions
....
....
..(1.3.1)
of
frequency,
dropsize
distribution and rain temperature.
An
excellent
theoretical
treatment
of
the
above
relation
towards computing a set of values for coefficients a , b in the
frequency range of 1-1000 GHz is given by Olsen in 1978. (Chaper 2
presents the theory).
Since not all
the
raindrops
are
spherical
and
their
shape
chapter 1
evolves
p a g e
through
spherical
to
spheroids of flattened base,
oblate
spheroidal,
to
oblate
to almost cardoids of revolution ,
for sizes larger than 0.5 mm CPruppecher and. pit ter, 19712,
rain attenuation
is polarisation sensitive COgichi
19742.
Attenuation perpendicular
to the
major
axis
parallel
radiowaves
is
less
which
than
are
1 3
that
vertically
and Hosoya,
inclination
to
polarised
of
them.
are
the
raindrop
Thus,
less
the
attenuated
than horizontally polarised waves CHarries and Hyde, 19772. But,
there remains the matter of path length through the rain medium.
The problem is that rain may not fall
whether
the path
links terrestrial
everywhere on the path,
systems
or
is a slant path
into the space. Moreover the rainfall rate on the path may not be
uniform. This has led to the concept of effective or equivalent
path length, such that,
of R mm/hr,
over
if rain were failing uniformly at a rate
an effective
portion
of
the
path,
then
the
product of specific attenuation , A and effective path length, L
would give values of attenuation that are in consistence with the
measured
data
published
CGoddard,1977;
nomograms/curves
Lin,
19772.
for estimating
In
1978,
CCIR,
has
the rain attenuation
as a function of rainfall rate and frequency (fig.1.3.2).
Many
workers
have
conducted
attenuation
both
links
compared
and
CCrane,1971;
over
Lin,1979;
experimental
terrestrial
their
and
results
studies
satellite
with
Manabeet.a l ,1984;
on
rain
communication
CCIR
Carassa
predictions
et.al,
1987;
Anthony et.al, 19842.
A
number
of
experimental
studies
conducted at various places of the
on
this
subject
are
also
Indian sub-continent CRaina
et.al, 1984; Te-wari et.al, 1986; Sen et.al ,19852.
However,
the greatest uncertainty in prediction of attenuation ,
when the theoretical formulae are used as basis,
knowledge
varying
on
rates
raindrop
over
size
different
distribution
climatic
is the limited
(RSD),
and weather
in
rains
of
conditions.
IxCLp t &T' 1
fig.1.3.2
Nomograms for estimatimg rain a t t e n u a tio n
AS A FUNCTION OF FREQUENCY C C C I R , 19792
C } 'cC l p t
1
Recent
experimental
p a -g *
c ountries
show
c li matic
....
zone
____
•_ii ' - ’ . j .
j.
_
<S=*rr.
t
that
the
- i ••
c o n d uc te d
RSD
on
RSD
charater is t ics
CAJoyi,1990;
to other
-V
t.
\
studies
over
i5
tropical
differ
from
one
Ma.cia.1 and Maura., 1990;
Din
»
1.4 HISTORICAL DEVELOPMENTS IN RSD MEASUREMENTS:
In
an
attempt
a t t en u at i on
ccr. duo ted
,p 45,
of
with
rainfall
to
measured.
In
i air.
The
this
and
u; ...e.,.:r. of
.i
j_j.
j 1.- a
..res
_f.t 1■.o !
subsequent
detec tor
a
a
include
C Joss
<?t. al
the
i::e of
on
as
optical
workers
fine
produced
used
more
the
which
the
diaphragm
sensor
C lamiuers,
charges
19&1 ;
raindrops
on
size
was
rain
was
by
so ph is ti ca te d
sensor
transforms
into
the
electrical
i9 6 9 0 ,
which
the drops
Barbara
that
In
flour
electr om ec ha ni ca l
,i9680
have
(RSD).
to m ea s u r e
c o n t a in in g
an
Jones,
the
of
method
depend an t elec tr ic
and
number
pellets
may
TTroa
well
distribution
investigators
falling
as
size
pan
of
e ’e t tr o s t a t i c
the size
wl. i wl. .r.e c
size
a
flour
method,
raindrops
an
raindrop
the
•.!i s 1 1 omet ei
as
intensity,
on
insl. umeutat ior.. Thes e
awn
microwave
and Pal/ruer used
drops.
“ Apooed
c o r r e la te
e xp eriments
Marshall
rain
to
or
an
e t .a l ,1yOHJ
pass
through
made
at
a
- - <5h : beam.
ai
ana
l'net
from
ou r
thod,
propo sed
?'
r .r
Joss
idrn.
n n n
-
L
p rer
£
e,
T
-
ted
a
set
dial
r
their
a
mea su re me nt s
negative
et.al
■a
shown
w idespread
rain
of
for
values
m o d e l , namely
o
have
the
the
:me. 1 pattern.
They
:a aide la are reviewed
the
made
shown
have
and
total
m e a su re me nt s
i v d c > have
exponential
that
that
Lacorno,
model
the
RSD
is
thunderstorm.
parame te rs
number
at
the
proposed
in chapter
They
in
the
of drops
Ile-Ife
for
and
Nigeria,
tropical
a tropical
RSD
mode!
2).
c : r _a ed a new technique of
inferring
111e
page 16
RSD
indirectly
rain
from
attenuation
the
and
carried out in Japan.
simultaneous
rainfall
rate
measurements
at
of
mu 1tifrequencies,
Their RSD also varies significantly from
negative exponential model.
1.5 SCOPE OF THE PRESENT RESEARCH WORK:
The basic aim of the research work
propagation
charateristics
is to study
with
respect
the microwave
to
hydrometeors,
specially with rain and to analyse the system responses towards
rain attenuation.
(26.
12^
N,
This
92^
study has been carried
E),
over
a
out at Guwahati
1ine-of-s igh t
(LOS)
short
hau1(3.2kms) link, operating at 11 GHz. The plans and programs
that
are
taken
to
realise
the
aim
are
of
its
kind
already
described
in
preface of the thesis.
As
this
since,
study
is the
first
over
this
region and
the microwave communication is planned in a big way for
the NE region of India, this study may provide substantial
for
the
system
designers,
engineers
for
designing
a
input
reliable
network. This study also provides a good amount of observational
information
to
the
meteorologists.
As
per
the
present
day
planning the microwave frequencies 20-30GHz are preferred
communication purpose and to that extent
limited. However,
guide lines,
extending
other
for
the scope of work
is
taking the fallouts of this research work as
the presentgroup has already taken up programs for
the work to 20-30 GHz.
measurements
like
In connection with
radiometric
sky
temperature
this,
are
the
also
p1anned.
Moreover,
recent publication by Crane CiQQiy indicatesthat,
there are 317 rain attenuation data bases available with CC1R,
out of which only 18 are from tropical
importance
of
such data,
been now made or proposed
several
region.
measurement
throughout
In view of the
programmes
the world.
have
Therefore,
in
chapter t
page
connection with this,
be
added
to
the
17
the results of the present study may also
common
understanding of the subject.
pool
of
knowledge
for
better
chapter- 1
REFERENCES
p a g e 18
REFERENCES:
1.
AJoyi.
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GO and
size
RL.
Olsen
distribution
0985}
for
"Modelling
microwave
of
and
a
tropical
millimeter
wave
applications"; Radio science, Vol.20, pp 193-202.
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IEEE
transactions on antenna and propagation, Vol.AP-25, pp 729.
3. Barbara.
size
AK, Devi.M,
measurements"
Timothy.
Technical
KI, S. Sharma 0 9 9 3 }
report
No.5-94,
"raindrop
Submitted
to
Department of electronics, Govt, of India.
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Basu.
S
and
MC.
Kelley
0977}
"
Review
of
eqitorial
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theory and measurement of equitorial
in the
irregularities" Journal
of
Atmospheric and terrestrial physics, Vol. 39, pp 1229-1242.
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Bean.
BR
and
EJ.
Dutton
0960}
"
Radio
meteorology
NBS
monographs 92, March 1, US Govt, printing press.
6. Bennet. JA; RC. Bostan and PA. Barclay 0 9 7 9 }
study",
Report
MEE79-2,
Electronic
* Raindrop size
engineering
department,
Monash university, Melbourne.
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Bremmer.
H
0949}
"
Terrestrial
radiowaves";
Elsevier
pub 1icat ions
8. Brierley. HG 0 9 8 6 } " Telecommunication
engineering"
Edward
Arnold publishers, London.
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K 0961}
" Radiowaves
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University
press, Cambridge.
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Capsonl.
C,
Pawl ilia 0 9 8 7 }
Fedl.
"
Data
F,
Maglstronl.
and
theory
C,
for
a
Parabonl.
new
A
model
and
of
A.
the
horizontal structure of rain cells for propagation applications"
Radio science, Vol.22, pp 395-404.
11. CCIR 0 9 7 8 }
" Propagation in non-ionized media" Report 72-1
pp 107-113.
12.
Collin.
RE
0988}
" Antenna
and
radiowave
propagation";
McGraw Hill book company, pp 339-473.
13.
Crane.
centimeter
RK 097 1 }
and
" Propagation
millimeter
wave
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length
bands";
in the
Proceedings
of
chapter 1
page 19
IEEE , Vo 1.59, pp 173-188.
14. Crane. RK C19901 " Modelling attenuation by rain in tropical
regions";
International
Journal
of
satellite
communication,
Vo 1.8 No.3, pp 197-210.
15.
Dolukhanov.
M
C19711
"
Propagatopn
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Mir
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16. Drufuea. G C1974} Journal of research atmosphere, vol. 8, pp
399-411.
17. Fang. DJ and CH. Chen C19821 " Propagation of centimeter and
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science, Vol. 17, No.5 pp 989-1005.
18. Fedi. F C19791 " Alta frequenza, Vol. 48 pp 167-184.
19.
Franke.
SJ
mu 1tifrequency
and CH. Liu C1985I)
scintillations";
"
modelling
Radio
of equitorial
science,
Vol.30,
pp
403-415.
20. Gardiol. F C19811 " Introduction to microwaves* Artech house
pub 1ishers, MA.
21.
Hall.
MPM
C19791
"
Effects
of
troposphere
on
radio
communication ’ Peter peregrinus Ltd (IEE) publication.
22.
Har den.
BN,
Nor bury.
JRL
and
WJK.
White
Estimation of attenuation by rain on terrestrial
the
UK
at
frequencies for
10-100GHz"
Cl978
bl
"
radio links in
Microwaves optics
and
acoustics, Vol.2(4), pp 97-104.
23.
Harries.
COMSTAR
JR and G. Hyde C19771
19/29 GHz
beacon
"
measurements
Preliminary results
at Clarkberg,
of
Maryland"
COMSAT technical report, vol.7 No.2 pp 579-623.
24. Hogg. DC and TS. Chu Cl9751 " The role of rain in satellite
comminications" Proceeding of IEEE , Vol. 63, pp 1308-1331.
25. Ishlmaru. A and JC. Lin C19731 " Multiple scattering effects
on wave propagation through rain"
NATO/AGARD conf.
proceedings
No. 107, pp 1- 13. (Be 1giurn).
26.
Ippolito.
LJ
C19811
’
Radio
prpagation
for
space
comminication systems" Proceedings of IEEE Vol. 69 .
27. Johnson. RC and H. Jesik C1984J
book" McGraw-Hill book company.
" Antenna engineering hand
c h a p t& r 1
pa.g& SO
28. Joss. J, Thants. JC arid A. Waldvogel Ci9685
" The variation
of raindrop size distribution at Lacarno" Proceeding of conf. on
cloud physics (canada) pp 369-373.
29. Laws. JO and DA. Parsons Cl9431) " The relation of raindrop
size to intensity* Transactions of Amerrican Geophysical Union,
Vol. 24, pp 452-460.
30.
Lin.
SH 0 9 7 3 5
*A method for calculating
rain attenuation
distribution on microwave paths, BSTJ, pp 1051-1086.
31. Lin. SH 0 9 7 7 5
B Nationwide long term rain rate statistics
and emperical calculations of 11 GHz microwave rain attenuation*
BSTJ, pp 1581-1604.
32.
Marshall.
JS and
WM.
Palmer
0948}
"The
distribution
of
raindrops with size* Journal of Meteorology, Vol.5 pp 165-166.
33. Medhurst. RG 0 9 6 3 5 "rainfall attenuation of centimeterwaves
comparision
of
theoty
and
measurement*
IEEE
transactions
on
antenna and propagation, Vol.AP-13, pp 550-564.
34.
Mitra.
AP 0 9 7 8 5
first advance
course
"Tropospheric
on
propagation
tropospheric
mechanism"
propagation
and
in
antenna
measurements, NPL, Delhi.
35. Moupfouma. F 0 9 8 3 5 "Model of rainfall rate distribution for
radio system design* Proceedings of IEE, Vol. 132(H) pp 39-43.
36.
Muller.
EA
and
AL.
Sims
09665
"Investigation
quantitative determination of point and areal
radar echo measurements",
Technical
of
the
precipitation by
report. ECOM-OOO,
Vol.32-F,
state water survey, Urbana.
37.
Oguchi. T and
raindrops
Part-II:
and
Y.
cross
calculation
Hosoya
09735
polarization
at
"Scattering
of
microwave
radiowaves
and
properties
due
to
millimeter
of
rain
region",
Journal of radio research labs (Japan), Vol.21, pp 191-259.
38. Oguchi. T 0 9 8 1 5
"Scattering from hydrometeors: A survey ,
Radio science, Vol.16, No.5, pp691~730.
t,
39.
Olsen.
RL»
Rogers.
DV
and
DB.
Hogde
09785
*
The
aR
relation in calculation of rain attenuation * IEEE transactions
on antenna and propagation, Vol.AP-26, No.2 pp
chapter t
pag& Si
40. Openhienu AV and AS. Wlllsky 0 9 8 7 5
■ Signals and Systems",
Prentice hall of India publication.
41.
Oyihloye.
subtropical
JO
09875
countries"
propagation
for
*
The
cited
tropical
troposphere
in
the
and
hand
in
tropical
book
subtropical
on
and
radiowave
countries,
URSI
publication.
42.
Pruppacher.
determination
HR
and
of cloud
PL.
and
Fitter
C19715
raindrops"
* A
Journal
semiemperical
of
atmonspheric
sciences, Vol.28, No. 1, pp 86-94.
43.
book
Reddy.
on
BM C19875
radiowave
" Physics
propagation
of
troposphere*
for
tropical
cited
and
in hand
subtropical
countries, URSI publication.
44.
Roddy.
D and J. Collin 09881)
Prentice Hall
" Electronic communication*
India publication.
45. Roggers. DV and RL.Olsen 0 9 7 6 5
" calculation of radiowave
attenuation due to rain at frequencies upto
1000 GHZ",
Report
1299,communication rasearch center, Department of communication,
Ottawa, Canada.
46.
Ryde.
JW and D.
Ryde 0944!)
* attenuation
of
centimeter
waves by rain, hail and clouds", Repott 8516, General electric.
Co. research labs, Wembley, England.
47. Stratton.
JA C1941 !> " Electromagnetic theory",
McGraw-Hill
book company, New York.
48. Tettlemann.
P and DD. Granthem 0 9 8 5 5
for estimating 1 min.
calculations"
49.
Van
De
rainfall
rates for microwave attenuation
IEEE transactions on
Hulst.
HC
09575
* A review of models
"
commun., Vol.COM-23, pp 361.
Light
scattering
by
small
BSTJ.
Vol.
raindrop
size
particles" Weiley publications, New York.
50.
Vigant.
A 09755
"space
diversity
engineering"
No.54 pp 103-142
51.
Ulbrich.
CV 0 9 8 3 5
" Natural
variations
in
distribution" Journal of climate and applied meteorology, Vol.22
pp 1764-1775.
52. Watson. PA 0 9 7 6 5 * Survey of measurements of attenuation by
rain and other hydrometeors" Proceedings of IEE, Vol.123,pp 863.
ms
bse subs
ni
eh
«
m
n
ih b
a
mm
bb h
ebu i b h s
«ra n a i n se ra
kbs
m s m
in
mss
pag& ££
CHAPTER 2
THEORIES ON RAIN ATTENUATION: A REVIEW
2.1.THE0RY ON RAIN ATTENUATION
Radiowaves propagating through rain
are attenuated
absorption of power in lossy dielectric
medium
because
of
represented
by
water. There is also some loss in the transmitted power
due
to
scattering of the energy by rain droplets. Therefore, the theory
of rain attenuation is based on calculation
scattering cross sections of a
single
absorption cross sections
be
can
of
absorption
and
raindrop.scattering
and
derived
from
the
Rayleigh
scattering theory (RST) where wave length is many times the size
of
the
scattering
particle
(in
such
coefficient is inverse,ly proportional to
cases,
scattering
wavelength
raised
the fourth power) and from {lie’s scattering theory (MST),
icable to the cases where size of
the
scattering
to
appl­
particle
is
many times the wave length.
2.11 SCATTERING CROSS SECTION OF A RAINDROP C FROM
R S T)
Let us consider a spherical drop of water with a radius a,
smaller than the wavelength of incident plane wave as
the fig.2.1.1fa). The drop
is
characterised
sphere with a complex dielectric constant
as
much
shown
a
in
dielectric
k = k4-jk2.
and
the
incident electric field is chosen as
E4 = E0az exp(-jk0x) ....
....
....
....
where, E0 = maximum electric field intensity,
E0az
=
(2.1.1)
electric
field intensity along z-direction,
k0 = 2n/X0 is a free space wavelength.
Over the extent of
the
drop,
incident
field
is
essentially
uniform and equal to Eeaz. The polarisation produced in the drop
is thus same as that would be produced in
a
dielectric
sphere
under the action of uniform static electric field. Therefore the
dipole polarisation P,per unit volume in the drop
is
given
by
19852
P = 3 [k-l/k-21 ^0E0az ---
---
---
---
(2.1.2)
^
chapter
5
'*
----------
X
f io
.2.1.1 a
s c a t t e r in g
by
S P H E R IC A L R A IN D R O P
FIG.2.1.1B
R A D IA T IO N B Y S M A L L C U R R E N T
C A R R Y IN G E L E M E N T
chapter
page 24
where,
-
10
-9
/36n
farad/meter,
is
the
free
space
permittivity.
The total dipole moment of
the
water
sphere
is
obtained
by
multiplying P with the volume of the sphere.
P0 = 4/3 ria3 P = 411a3 (k-l/k-2) £0E0as ...
......... (2.1.3)
since a << \of the far- zone scattered electric field
from
the
sphere is the same as that radiated by a small electric field of
total strength PQ. The far-zone electric
field
radiated
by
a
small current element (fig.2.1.1(b) is given by
Ea = jzQ Idl k0sin© (exp( - jk0r )/4FIr ) a0 ....
....
..(2.1.4)
where, z0 = intrinsic impedance of free space = 4tJa/'so,
Idl = current through the element of length dl,
r = distance to the observation point,
a0 = unit vector along the scattered field .
replacing Idl with joaP0, in the equation 2.1.4, we have,
Es = -0i>zokoPo sin© (exp( - jkGr )/4I1r ) a0 ....
....
similarly Hs = -wk0P0 sin©
(exp( - jk0r 3/4TIr )
The total scattered
can
power
-(2.1.5(a)
a0 ...... <2.1.5(b->
be
calculated
f0
f0
by
taking
the
poynting vector, i.e
Pa =
1/2
E
X
H*
=
1/2
Y0
|Ea |
r2
sin©
....
2
or, P g " |p^ | ,
kq Zq / 1ZTl ....
....
....
....
....
d©
d<£
(2.1.6)
(2. 1.7)
where Y0 = l/zc_
Substituting the equation 2.1.3 in equation 2.1.7 we get,
Pa = 4/3 n a2 <k0a) Y0 |E0 |2 Jk-l/k-2|2 ...
The scattering
cross
section
is
defined
......... (2.1.8)
as
the
total
scattered power divided by the incident power per unit area,
cra = Pa/ 1/2 Y0 |E0 }2 =8/3 Ha2 (k0a )4 |k-l/k+2 J2 ___
___ (2.1.9)
ch a p ter
page 25
2
The above expression for cfl
section varies
length,
and 3)
1)
shows
inversely
that
with
the
the
scattering
fourth
power
of
2) directly with the sixth power of the raindrop
is a function of complex
refractive
cross
wave
radius
index of water.
2.1.2 ABSORPTION CROSS SECTION C FROM RST)
The absorption cross section can similarly
evaluating
the
power
absorbed
polarisation current density
is defined by the equation
in the sphere
is related
P = (k-1) £0E ....
(Pa )
be
by
found
the
by
first
sphere.
The
in the sphere is jp = j6>P. Where,
(2.1.2).
The total
electric field
....
....
....
---
is given by
Pa = 1/2 Re
d<fi dr = 2/3 ria3Re E.JP*
Jp*r2 sin0
....
Re E Jp*
therefore,
E,
to P by the equation
The time averaged absorbed power
where,
P
(2.1.10)
......... (2.1.11)
= 9k0 Y0 |k-1/k +2 12 k2/ (k4- 1)2 + (k2 )2
Pa = 6 ria3 k0Y0 |k - 1/ k +2 12 kz |E0 j2 / (k4- 1 !2 + <k2 )2
....
......... (2. 1. 12)
The absorption cross section aa is given by
o-a = Pa/ ( ^ 2
Y0 |Ec |2 )
= 12na2 (kea)
|k-l/k +2|2 k2/(k1-l)2 + (k2 )2 ........ (2.1.13)
The absorption cross section given by the above equation,
inversely with the wavelength and
complex refractive
varies
is a function of radius a
and
index k.
2.1.3 EXTINCTION CROSS SECTION (FROM RST)
The extinction cross section t?s of the raindrop
the sum of scattering and
given by
(Saxton,
absorption
cross
is
defined
as
sections,which
is
1962)
o* = [8/3 na^( k0a)4 +12P.a2 (k0a) k2/ (kt- 1 )2 + (k2 )2 ] |k - 1/ k + 2 |2
. . . (2. 1. 15 ).
page 26
chapter 2
The
cross
sections
of
the
raindrop
viz.,absorption,scattering and
as
extinction
described
cross
above
sections
are
derived from Rayleigh scattering theory(RST), with an assumption
that the radius of the raindrop is not larger than h0 /10.
Since
the radius of raindrop ranges from a fraction of millimeter upto
several millimeters,
the Rayleigh scattering theory is generally
valid down to wavelengths of the order of
3
cm
less. The assumption of spherical droplets is
since ,the raindrops take an oblate
However,
at
spheroidal
longer
some
also
not
or
shape under the influence of aerodynamical and
as they fall.
or
valid
flattened
pressure
wavelengths
what
an
forces
equivalent
spherical radius can be assumed. At millimeter
wavelengths,
is important to
raindrop,
consider
the
shape
of
the
it
(which
depends on the size of drop and determination of cross sections
has to be done from the Mie's
scattering theory(MST).
2.1.4 EXTINCTION CROSS SECTION CFROM M ST)
Ue have seen in
the
previous
section
that
extinction
cross
section is a sum of absorption and scattering cross sections, so
the Mie's scattering theory will be discussed here with
respect
to the derivation of the extinction cross section only.
In 1941, Stratton has derived an expression for
cross section from the Mie's scattering
widely
used
by
many
workers.
amplitude was considered to be the
which is defined as a function of
Here
the
theory,
the
prime
extinction
which
is
now
forward
scattering
important
parameter,
frequency,
size,
material of the hydrometeor and also a function of
shape
and
polarisation
of the incident wave.
A hydrometeor is placed at the origin of
cartessian
coordinate
system as illustrated in the fig.2.1.4. A plane wave incident on
the hydrometeor with an angle a, induces a transmitted field
the interior of the drop and a scattered field. E ^
denotes
electric field of the incident wave, the polarisation
state
in
the
of
27
chapter 2
point
.
FIG 2 1.4 S C A T T E R IN G B Y R A IN D R O P M lE 'S S C A T T E R IN G TH E O R Y
chapter 2
page 28
which is given by a unit vector, e. K, is a unit vector directed
towards the polarisation direction of the incident wave
is a unit vector directed from the
point.
In the
far-field
origin
region,
the
to
the
electric
and
observation
field
of
the
scattered wave may be written as €Oguchi,z9833,
E
~ f (i<1,K2 ) r”1 e"lkr ___
where, unit incident wave
propagation constant, r
....
is
is
___
assumed.
the
observation point and ftK^.Kg)
k
is
distance
of
quantity of interest is the vector
the
from
is a vector
amplitude and polarisation state
...(2.1.16)
free
origin
function
the
to the
denoting
scattered
scattering
space
wave.
amplitude.
The
This
function is obtained from solution of boundary value problem
the
surface
of
hydrometeor.
techniques have been devised
boundary
value
problem.
Quite
by
a
many
number
of
workers
Which
analytical
to
include:
at
solve
this
method
of
moments {Marrington, 19&83, the perturbation method<Ty<&h., 19&43,
the
point matching method€Morrison arid Cross, 1974; Oguchi ,19773, the
fredholm equation
method
CHolt,19803,
the
extended
boundary
method CBarber and. Yeh,19753. An excellent review and assessment
of
the
above
methods
are
given
by
Oguchi €19813, €19883
and Yeh et.al (19883.
Once
the
scattering
amplitude
flK^.Kg)
is
scattering and extinction cross section
can
terms of f(K,,K_). The extinction cross
section
l
2
obtained,
be
the
formulated
as
in
calculated
from extended boundary method is given by,
Q
= -<4fr/k> Im Ee.
....
---
---
.(2.1.17)
The above equation can be written in terms of S(o,a), where o is
indication of polarisation state
and
a
is
the
drop
radius,
€Stratton,1941; Ishamaru and Lin, 19733 as
Q t = 4rr/k2 Re CS(o,a>] ___
When
electromagnetic
wave
___
propagates
___
through
___ (2.1.18)
rain,
it
chapter-
page 29
2
encounters a great many water
droplets
with
different
radii.
Since the extinction cross section is a strong function of the
radius a, it is necessary to take into account of the drop
distribution. The total power removed from a wave ,
size
with
power
2
dens ity P = 1/2 Y , lE I- by the drops in a volume element of
unit cross sectional area and thickness dz along the z-direction
is
dP/dz
= -1/2 Y o |E2
| u Q t, (a ) N(a) da
1 1
= “P / o CLt (a ) N (a ) da ....
....
....
....
(2.1.19)
As a result of this loss in power, the power decays at a rate of
2a where
A = 2a /“Q (a) N(a) da ....
The equation 2.1.20 defines the
....
....
....
specific attenuation
(2.1.20!
i.e,
the
attenuation per unit length (dB/km) along the propagation path.
2.2. MODELS ON RAINDROP SIZE DISTRIBUTION:
2.2.1 NEGATIVE EXPONENTIAL MODEL:
In the year 1943, Laws and Parsons CLaws and Pax-sons, 19439 have
made extensive measurements on raindrop size, at Washington for
various types of rainfall using ''flour method* *.
The
raindrop
size distribution given by them is of the form
n (a ) da = 103 R m(a) da/(4.8n a3 V(a))....
where m(a) is the percent of total volume
....
reaching
(2.2.1)
the
ground
contributed by drops of all ranges,
R is the rainfall rate in mm/hr,
V(a) is the terminal velocity in meters/sec.
and a is the drop diameter.
Based on their own and Laws and Parsons*
,
measurements,
Marshal
c h a p te r 2
and
p a g e 30
P a ltrie r
C
have
19435,
proposed
a
negative
exponential
relation for raindrop size distribution which is given by
N (D ) = N
e'XD....
....
4-3-1
where N o = 1,6 X 10 m mm
___
....
....
(2.2.2)
°
X
= 8.2 X R
In 1968, J o e s
-
et.al
0. 21
___
___
....
measured
C196S5
Switzerland using distrometer, This
drop
....
size
instrument
at
(2.2.3)
Lacarno,
transforms
the
momentum of the raindrops falling on a diaphragm into electrical
pulses.
It has been shown by them that the
considerably from one type of
constant N q for all types of
model.
The
parameters
(
rainfall
rainfall
N q ,X)
of
values
to
in
of
other,
vary
unlike
Marshal
the
Nq
and
average
the
Palmer
negative
exponential model obtained by them are shown in table 2.2.1.
T a b l e 2.2.1:
T he PARAMETERS OF NEGATIVE EXPONETIAL MODEL
OJo
s s
e t .a l,
i 9 & S5
. (mm
, -1 )
.
X
Type of rain
.. (mm
, "1 m "3,)
N
o
Drizzle
6 x 10
W idespread rain
1.4xl04
8.2R~°"21
Thunderstorm
2.8xl03
6R"0 *21
Extensive investigations
different values for
19325.
Table
the
on
4
RSD
11.4R-0 -21
at
parameters
2.2.2 presents these
33
places,
(N ,X)
values
('F a n g
yielded
33
and
C hen,
hap ter £
page 31
Table 3.2.2 :
Values of N f X. for­ negati ve exponent1al RSD
es
S. No.
1
.
N
X
o
Rain rate
type
stratiform
ref erence
Marshal &
palmerC19485
Joss et. al
8, 000
4.1R"0 -21
2.
30000
5.7R
3.
7000
4.1R-°*21
widespread
C19635
4.
1400
3R-0'21
thunder
-do-
5.
135000
5.2
0-1
a l 1types
Geo ttsC1968.
6.
3400
3. 15
1-2
-do-
7.
2500
2. 25
2-5
-do-
-do-
8.
2700
2.2
5-10
-do-
-do-
9.
2100
17.2
10-25
-do-
-do-
10.
4000
15.5
>25
-do-
-do-
11.
182000
3. 18
>51
-do-
BarelayC1977
12.
313000
3.11
50-60
-do-
-do-
13.
373000
3. 11
60-70
-do-
-do-
14.
825000
2.45
70-80
-do-
-do-
15.
20000
5.25
drizzle
Barelay c 197B
16.
10700
4. 94
drizzle
-do-
17.
10000
3. 5
widespread
-do-
18.
6590
3.37
-do-
-do-
19.
3000
2.84
shower
-do-
20.
3020
2.76
-do-
-do-
21.
22000
4. 11
1
shower
Muller&Sims
22.
22000
3. 41
3.2
shower
€19665
23.
18000
4.8
shower
-do-
24.
7R-°'3
3.08
3R -0.41
thunder
SehhoriC 19715
25.
25200
4.21
3.8
widespread
Waldvogel
26.
27900
2. 19
8. 1
-do-
C19745
27.
6347
2. 97
5.6
-do-
-do-
28.
6571
3.35
2. 6
-do-
-do-
29.
16500
3.51
5.7
-do-
-do-
30.
3804
2.6
5
-do-
-do-
31.
22800
4. 16
3.5
-do-
-do-
32.
4000
2. 5
5.8
-do-
-do-
33.
8000
2. 6
8
-do-
-do-
-0 9 1
drizzle
t&T' 2
From the simultaneous measurements
11.5,
34.5 and 81.8 GHz,
of attenuation at frequencies
and for the rainfall
rates below
70
techn ique
hown
that
N
R _0,16and X = 5.11 R °-253;
for
N
o
17300
=
o
fit
for
and
ra inf a 11
the
rates <70 mm/hr.
2.2.2 GAMMA DISTRIBUTION MODEL:
In 1983,
Ulbrich showed that a gamma distribution for the RSD can
describe many of
variation
in N
o
the
natural
variabilities,
within a event of rainfall
such
as
sudden
etc,.This model
gives
the RSD in the following form
N(D)
= N q D m exp ( -X D )
where N(D)
size
....
(D to D+SD),
m
is
= (Nt >/{ r (a ) ft* }
where,
an
This distribution can be
number of drops per unit volume
N(D)
....
....
is the number of raindrops per unit volume
interval
constants.
....
ft >0, a >0 and
integer
written
and
interms
(2.2.4)
per
N^,
unit
X
of
are
total
(N^) as
D01" 1exp(-D/ft)
....
....
(2.2.5)
0
2.2.3 LOGNORMAL MODEL:
The
log normal
preferred
working
by
at
C hen,19823
model,
many
proposed by
researchers
tropical
to
applications.
Muller
places
exponential
Their model
model
describes
D (2rr)°‘5 1n(c
Sims,
meteorologists
CHarden
-■{exp- [0.5
N (D )
g
and
and
for
et.al. ,1973-,
microwave
19883
is
specially
Fang
and
and
radar
the RSD in the form,
In (D/D
)/ 1n (£/) ] >.
(2 . 2.6
chapter 2
where,
page 33
N g (D)= n umber d e n s i t y
“3
(mm
m
-
1
)
N^= total number of drops of all sizes per cub i c meter,
D = g e o m e t r i c mean diameter,
mm,
&= s t a n d a r d d e v i a t i o n of d r o p diameter.
The numerical values of the p a r a m e t e r s in the
lognormal
p r e s e n t e d by Bennet et.a.1 Ci9790,
in tab l e 2,2,3.
T a b le
are
2. 2.
s h own
model
3
Numerical values of lognormal parameters.
para m e t e r
*
nt
e
thunderstorm
thunderstorm
(5<R<50mm/hr)
5<R<50mm/hr
50<R<200mm/hr
4 0 R 0 *64
4 6 R 0 -55
8. SR
1. 1 4 + 0 . 1 8 1 nR
dm
et
showers
(0. 2 9 - 0 . 0 0 1 I n R )
1 . 7 6 + 7 . 3 3 x l 0 ~ 4 lnR
0 . 2 2 + 0 . 3 9 1 nR
(0.5-0.003R)
i .37
M a k i n g use of the r a i n d r o p s i z e m e a s u r e m e n t s
made
at
Nigeria,
„4j’
oy£ and Olsen Ct9S3y* h a v e sho w n that the tropical d a t a
best be f itted w i t h Lognormal model RSD.
They
p r o p o s e d the f o l l o w i n g tropical m o d e 1CAj'oyi
have
and
could
latter
Olsen*
on
£9850*
nt
N C D ) = --------------------- e x p i - 0 . 5 C t i n D - p i / o ) 2} --------( 2 . 2 , 7 )
& D (Zn)
w h e r e , p = m e a n of In D = - 0 . 1 9 5 + 0 . 1 9 9 In R
0. 5
a = s t a n d a r d d e v i a t i o n = ( 0 . 1 3 7 - 0 . 0 1 3 In R)
N,j, = total n umber of dro p s = 108 R
O
oo o
2.3 ATTENUATION AND RAINFALL RATE RELATION:
C o m p u t a t i o n of a t t e n u a t i o n c a n be d o n e by two general a p p r o a c h e s
page 34
chapter £
A) A theoretical method employing uniformly random
distribution
of raindrops and
B) An empirical procedure
based
on
the
approximate
relation
between attenuation (A) and rainfall rate (R), which is given by
€ Olsen et.al* t978>
A = a R
___
(2.3.1)
where, a,b are functions of frequency and rain temperature.
The
problem of determining the theoretical basis of A = a R ^is best
approached initially from low frequency end of the spectrum CVaax
de Hulst, i957 J. Here the
infinite
series
expansion
for
the
forward scattering amplitude S(0,D) of a spherical particle
can
be approximated by first few terms ,
S (0, D ) = 0.5 [3( a 1 + bj^ ) + 5 (a2 + b2 ) + 7 (a3 + b3 )+.... ]
....
....
.(2.3.2)
where a,b etc,, are Mie's coefficients.
The first few
coefficients
to
the
order
8
of
parameter x (x = tcD/X), as proposed by Penndorf
x in
the
size
CPenndorf,i9&£5
are
S (0,D ) = j x (M. + M_ x2
1 2
+
MQ
a
x3
+
M.
4
••••
x4
+
Mc
5
* ** *
x5
+...)
...(2.3.3)
where, M^= (m2-l)/m2+2
M2= M 1[ (3/5)
m2 -l/(m2 +2)+ (1/30) (m2 +2) +(1/6)(m2 +2/2m2 43 )
V -JHj2
M4 * M 1[ (3/350) ( m6+20 m4 + 200 m2
+ 200)/(m2+2 )2+
(1/315)(m2 +2)(m2-2) - (5/42) (m2+2)/!2m2 +3)2+
(2/225) (m2+2)/(3m4+4)] ,
Mg = -j (4/5)M12(m2-2/m2+l)
(2.3.4)
chapter 2
page 35
where m is frequency and temperature dependant refractive
index
of water.
Olsen et.al have solved the integral equation (eq,2.1.20)
by making use of negative exponential
drop
size
distribution.
They have shown that
A = J
R b ( i+ Ec
n
fn R n^/q) ___
___
___
(2.3.5)
where,
y
a
60 n2 N
f Im (M.)
o
i.
=----------------4
In (10) c a
l/ = 4/3? (3= 0.21
c =
n
(n+3)inim(M )
n
------------ ,
3 !cotIm (M , )
1
n = 1, 2, 3, 4 and 5.
The importance of the expression for A in terms of frequency (f)
and rainfall rate (R) is that, it helps to show both the
mathematical significance and limitations of relation A = a R
Substituting the values of a^, b^, c^, (3 in
equation
2.3.5
h
we
have
A = a R k , with
a * G a f ^a and b = G,b f ^b ....
where , G = 6.39 X 10~5 ;
’ a
V
V
V
4.21 X 10
0.851?
1.41
-5
E = 2.03
a
..(2.3.6)
for f < 2.9 GHz
?
E a 2.42 for 2.9GHz < f < 54 GHz
a
E = 0.158 for f < 8.5 GHz
b
• Kb
F
= -0.07 for 8.9 GHz < f < 25 GHz.
,
The representative values of a, b c'Olsen et.al iS780
computed by utilising various drop size distributions
that are
Viz,.Laws
and Parsons*(LP ), Marshal and Palmer's (HP), Joss' drizzle
(JD)
chapter 2
page 3€>
and Joss thunder storm
2.2.4.
(JT)
models,
are
presented
It has been highlighted by them that the
are least
sensitive
to
the
drop
frequency range of 10-30 GHz and
size
in
this
in
table
values
of
a,b
distribution
in
the
frequency range
the
relation A = a R ^t is most accurate. But latter on by utilising
the lognormal RSD , Fang and Chen €iQS25
have
values of a,b are sensitive to the raindrop
shown
size
that
the
distribution,
which varies considerably from one type of rainfall to other and
also varies from location to location.
T he
values of coefficients
T able 2.2.4
a , b .(Freouency «11GHz , Horizontal
polarisation )
Mode l
Rain temperatue
coeff icients
a
b
0.015
1. 117
o
o
o
for widespread rain
0.012
1.259
20°c
Laws and Parsons' model
0.0152
1. 167
0°c
for convective rain
0.0167
1. 181
Marshal & Palmer model
0.0173
1. 143
! 0
o
o
0.0137
1.244
20°c
Joss model for
0.021
1.065
no
0 c
thunderstorm
0.024
1.060
20°c
Joss drizzle model
0.014
0. 977
no
0 c
0.009
1.047
20°c
1. widespread rain
0.015
1.207
0 c
2. Showers
0.0188
1.138
0 c
3. thunderstorm
0.0436
1. 113
0 c
O
C"4
o
o
Laws and Parsons’ model
!
Lognormal model:
c N < lp t z * 'f
.
REFERENCE:
2
.
...
C .K Z g + ?
JD>
/
REFERENCES:
1. Aj oyi. GO and RL. Olsen C19833 , "Measurements and analysis of
raindrop
size
distribution
in
southwest
Nigeria";
commission F ,Symposs iurn on wave propagation and remote
Belgium,
sensing,
pp 173-184.
2. Ajoyi. GO C19853,
"Rain induced attenuation and
of cm and mm waves using
model"
URSI
International
propagation,ISAP,
3. Barber.
tropical
raindrop
sympossium
on
Kyoto, Japan,
phase
size
shift
distribution
antennas
and
pp 1095-1098.
Band C. Yeh, C19753,
"Scattering
of
waves by orbitrarily shaped dielctric bodies"
electromagnetic
;
Applied
Optics
Vo 1. 14, pp 2861-2872.
4. Bennett. JA, Boston. RC, PA. Barclay
C19793,
study *;
Engineering
Rep.
MEE
Monash University,
79-2,
Electronic
Melbourne,
5. Col Lin. RE ci9S5j,
"Raindrop
size
department,
Australia.
"Antennas and rad iowave propagation McGraw
Hill publication New york.
6.
Fang.
DJ,
CH. Chen
centimeter/mi 11imeter
waves
Cl 9823,
along
precipitation"
; Radio science Vol.
7.
BN,
Harden.
"Attenuation/rain
Norbury.
rate
JR,
10-40
slant
path
of
through
17, pp 989-1005.
WJK.
relationship
links in the frequency range
Vol.
a
"Propagation
White
on
C1978
aj ,
terrestrial microwave
GHz";
Electronic
lette'rs,
14, pp 154-155.
8. Harrington.
RF 119683 "Field computation by moment
method
"
Macmillan publication, New York.
9. Hoit. AR C19803,
computat ion
with
" The f r edho1m integral
T-Matr ix
approach" ;
equat ion method
Pergamon
and
publishers,
NewYork.
10. Ishmaru.
A and JC. Lin C19733,
propagation through rain";
No.
"Multiple scattering on
in NATO/AGARD
conference
107, pp 1-13, North Atlantic Treaty organigation,
Belg iurn.
11. Jcss.J,
Thane. JC and A. Waidvogel C19683,
wave
procedings
Brussels,
"The variation of
raindrop size distribution at Lacorno" ; in proceeding of
Inter-
chapter 2
REFERENCES
page 33
national conference on cloud Physics, pp 369-373.
12. Laws. JO and DA. Parsons C19431, " The relation of
raindrop
size to intensity"; Transactions of American Geophysical
Union,
Vo1. 24, pp 452-460.
13. Manabe. T, Ihara. T and Y. Faruhama (119841), "
raindrop size distribution from attenuation
measurements";
IEEE transactions on
and
Antennas
Inference
rainfall
and
of
rate
propagation,
Vo 1. AP-32, pp474-478.
14. Marshal. JS and WM.
Palmer
raindrops
Journal
with
size";
C1948},
of
"The
distribution
Meteorology,
Vol.
of
5,
pp
165-166.
15. Mei. KK (119741, " Unimoment method of
scattering problems";
solving
antenna
and
IEEE transactions on Antennas and propaga­
tion, Vol. AP-22 pp 760-766.
16. Morgan. MA C1Q8QJ, "Finite element computation of
scattering by raindrops"; Radio science, Vol.
microwave
15, ppll09-1119.
17. Morrison. JA and MJ. Cross C19741, "Scattering
of
electromagnetic wave by axisymmetric raindrops"; BSTJ,
a
plane
Vol.
53
of
the
pp 955-1019.
18. Muller. EA
and
AL.
Sims
(.'19661,
"Investigation
quantitative determination of point and areal precipitation by
radar echo measurements": Technical report EC0M-000, 32-F, State
water survey, Urbana.
19. Oguchi. T (.'19771, "Scattering properties of Pruppercher
Fitter form
raindrops and
cross
polarization
due
Calculation
at 11, 13, 19.3, 34.8GHz"; Radio science,
to
and
rain.
Vol.12,
pp 41-51.
20. Oguchi. T Cl 9811, "Scattering from hydrometeor,
Radio science, Vol.
21. Oguchi.
T
A
survey";
16, pp 691-730.
Cl 9831, "Electromagnetic
scattering in rain and
other
wave
hydrometeors";
propagation
Proceedings
and
of
IEEE, Vol. 71, PP 1029-1078.
22. Olsen. RL,
Rogers. DV
and
DB.
Hogde
Cl 9781,
"
The
aR
relation in calculation of rain attenuation"; IEEE transactions
on Antennas and propagation, Vol. AP-26, pp 318-329.
23. Olser:. RL C198S1 , " Review of theories of coherent radiowave
ch a p ter £
REFERENCES
page 39
propagation through precipitation
media
of
randomly
oriented
scatterers and the role of multiple scatering"; Radio science,
Vol.17 pp 913-928.
24. Penndorf. RB C19623, "Scattering and extinction coefficients
for small absorbing
and
non-absorbing
aerosols";
Journal
of
Optical society of America, Vol. 52, pp 896-904.
25.
Saxton.
JA
Cl9620,
"Radiowave
propagat ion
in
the
McGraw
Hill
troposphere" Elsevier publication, New York.
26. Stratton.JA C19410
E 1ectromagnetic
theory
,
publcations, New York.
27. Ulbrich. CW €19833, " Natural variation
in
the
analytical
form of the raindrop size distribution"; Journal of Climate
and
applied meteorology, Vol. 39, pp 174-1775.
28. Van de Hulst. HC €19573, Light scattering by smal1 partic1es
Weily Publications, New York.
29. Yeh. C C1984,3,
electromagnetic
"Purterbation approach to the difraction
waves
by
arbitrarily
shaped
of
dielectric
obstacles": Physical reviews, series A, Vol. 135, pp 1193-1206.
n tj rJt A
l
\ m rr
tX
^
ims ta mm mm ra mm um mm mm 8ra asm mm ma mm mm m sra mm us n
n
bks&
n
raa
CHAPTER 3
cj'
Ji&&
*iL''
EXPERIMENTAL TECHNIQUES
3 .1
Ever
IN TR O D U C TIO N :
since the
problem
during the world war
I!
carried out almost all
ding of the subject.
can be categorised
of
rain
attenuation
{R y d e ,1
over
All
,
many
under
three major
1. Rain attenuation studies over
experienced
investigations
the wcr 1d for-
the experimental
was
thorough
studies
were
understan­
in this
regard
techniques:
terrestrial
microwave
1 inks
nara el
Sen. & t.aL ; 1QH5 >.
2. Rain attenuation studies on
i inks cDr-u/uca,
arid.
Up pal . i
In
all
to
H ogg
J e n k ia s o r i, j 9 / 3 ;
and
c/vu» t y / £ > ;
these
techniques,
field
end
strength
either
the AGC output.
by
ana
sir e n g 1 .i
'
JB ’ w n e r
This charter describes
ea
the
-
n 1e ■_b n
technique
measurements a nd var io us ty pe s
o
measur emer ts
taking
Tec hn 1cues
the calibration of the ie o or d ing sy steins
fall
R a i na
,
output or by tapping
g ria i
microwave
its correlated studies using radiometers
performed at the receiver
si
satellite
1Q S 2 ; F s - d i 1QS1 ■ Pa.xa.bom , lQ S 2 ; Mac chi ar-& L ia.,
3. Rain attenuation and
CLr-atg
earth
:
u ue
in
the
1 an :’
tern:-
of
cste'-teb
2
-mo 1o /
received
:> the i— c o r u n g
adopted
for
w e re
s/
e ..
at tenust ion
f rain g a ug e s utilize d f or rain­
rate measurements along with
the raindrop
size
system and a fast response raingauge developed by
the
monitoring
group.
A
few more circuits and
instruments
fabricated for convenient
have been described.
that
are
also
designed
recording of the supporting
The details of these circuits
are available
in the last section of the chapter.
3.2 MICROWAVE
FIELD
and
parameters
and
systems
STRENGTH MEASUREMENTS:
3.2.1 MICROWAVE LINK CHARACTERISTICS:
Microwave field strength measurements are made over a clear' line
of sight(LOS)
at 11 GHz.
!a nodal
links!
link of Posts and Telegraphs department,
It is an intracity network,
receiver station
for
Calcutta,
with Panbazar over a path
illustrates
river Brahmaputra.
Jorhat
link • Both
link are s ituated on
tb 0
The neces s a ry circuitry o f th 6
made by Nippon electronics company
information of the inic rowave
link
Durgasarover
and
length of 3.2 kms.
the terrain prof i 1e of the
and transmitter of this
connecting
(NECj
j apan.
is given
operating
Shillong
Fig.
the
bank
3.2.1
receiver
of
network
Th e
the
were
technical
in ta b 1e 3.2.1.
T able 3.2.1
1.
Frequency
2.
T ra n sm ittin g
antenna
h e igh t
115
3.
T r a n s nil 11 i n g
ant enna
gai n
4 6 .5
4.
T ra n sm ittin g
power
S.
R e c e iv in g
antenna
h e igh t
248mC ASLI)
6.
R e c e iv in g
an tenna
g a in
4 6 . 5 dB
7.
R e c e iv e r
8.
Path
Q.
Feeder
10. B a s ic
11. F a d e
11 GHz
le n g th
6 7 . 5 dB
3. 2 kms
4. 45dBm
lo s s e s
m a rg in
dB
40 dBm
s e n s itiv ity
free
mC ASLD
space
lo ss e s
1 2 3 . 3dbm
45dB
* T he sig n a l levels g iven in this ta b le are with r espec t to 1n>w
page 42
chapter 3
<
m
Distance
f i u .3.2.1.
In
Km
t e r r a in p r o f ile o f th e m ic r o w a v e l in k .
chapter 3
aa
I I
nu->
IZ _
\ J \
page 43
c
t & r .5
44
oo.
3.2.2 EXPERIMENTAL
MEASURMENTS:
ORGANIZATION
FOR
MICROWAVE
ATTENUATION
The block diagram of the receiving setup is shown in
It constitutes of antenna, wave guide, RF section,
AGC port. The transmitted signals are
received
f ig.3.2.2.
IF stages and
by
a parabolic
horn feed antenna of gain 40 dB. The feed placed at the focus of
antenna picks up the signals focused by the antenna
and
the signals through a rectangular wave guide to the RF
The normal pressure inside the wave guide is 5
suitable amplification,
oscillator
and
are
guides
section.
kg/sq.cm.
After
the RF signals are mixed with the
local
down
converted
to
70
MHz
intermediate
frequency! IF 1. The local oscillator is made by using Gunn
which produces a frequency of 10,930 MHz.
amplified through two
stages
of
The
amplifiers
IF
diode
signals
and
are
are
finally
detected by a detector.
The AGC output is then tapped and fed
to
an
Ester line
Angus
chart recorder in the form of voltage. Since this chart recorder
is basically a current meter with full scale deflection of 1 mA,
the AGC voltage is first converted to corresponding current
fed to the recorder through a buffer circuit. Here,
the
and
buffer
protects the system from loading effects. The circuit diagrams of
the voltage to current converter and buffer are discussed in the
subsequent section, along with the systems
like
raindrop
size
measuring system and fast response rain gauge.
3.2.3 CALIBRATION OF THE SYSTEM:
The entire recording system is
help
of
generator,
a
precision
regularly
attenuator
and
calibrated
a
with
synthesized
in collaboration with the P&T, Guwahati,
for
signal
fixing
the dynamical range of the chart in terms of signal strength
dB. Fig.3.2.3a and Fig.3b illustrate the procedure
calibrating the system.
the
adopted
in
for
£}£Lp?^* 45
f ig .3.2.2 b l o c k
d ia g r a m
of
the
r e c e iv in g
setup
C
t.
e 1'
/V"; LV'
3
While calibrating with the precision attenuator,
the
procedure
shown in fig.3.2.3a is followed. The attenuator pad is introduced
in between the mixer and IF sections and
are
attenuated
at
different
levels
the
incoming
and
the
signals
AGC
output
corresponding to each attenuated level is recorded on the
paper,
chart
thus calibrating the paper in terms of dB.
Calibration with the signal generator has been done according to
the procedure given in fig.3.2.3b. First of all,
IF section
of
the receiver is disconnected from the preceeding RF section and
a signal of 70 MHz frequency was injected from
the
signal generator into
signal
the
IF
section.
This
synthesized
is
then
attenuated in steps of 2 dB, using the inbuilt attenuator of the
signal generator. The corresponding AGC output
the chart paper that fixes the dynamical
is
range
recorded
of
Fig.3.2.4 shows the AGC voltage at each RF signal
the
chart.
level.
The minimum signal the system can receive is 67.5 dB i.e
below the normal
level.
In
other words, this
sensitivity. Therefore the 45 dB
is
the
is
fade
the
45
level
margin
up
to
dB
receiver
for the
system. The dynamical range of the system is adjusted so
record the signature of the attenuated
on
as
the
to
fade
mar g in (45dB).
3.3 COLLECTION OF SUPPORTING DATA:
The observed microwave attenuation is then examined in
to rainfall rate and raindrop size distribution,
relation
because,
it
mentioned in the previous chapter,
that microwaves above 10
are attenuated due to
this
rain.
For
purpose,
measurements are taken in and around the
receiver
the
is
GHz
rainfall
site,
using
three different types of rain gauges,Viz. a) non recording
rain
gauge b)
Fast
self
recording
syphoning
rain
response rain gauge with analog and special
gauge
type
and
of
c)
recording
facilities. Fig. 3.3.1 presents the locations of the rain gauges
placed for rainfall measurements.
chapter 3
page 47
fig .3.2.3a c a l ib r a t io n with a t t e n u a t o r
fig .
3.2.3b .
c a l ib r a t io n with s in th e s iz e d s ig n a l g e n e r a t o r .
chapter
3
page
RF level Vs AGC vo ltag e
1 1 GHz m ic ro w a v e link
10
4-;
F .O
A
fig.3.2.4- AGC voltage at each RF level
^normal RF
level ;-22.5 dB and fade margin ;-62.5 dB)
48
chapter 3
page 49
3.3.1 NON RECORDING RAIN GAUGE:
This rain gauge collects the rain
water
through
201.08 sq.cm collecting area. The total
daily
rain can
a
be
measured
manually,
with
a
funnel
of
accumulation
of
standard
measuring
cylinder which gives the rainfall in mm. The rainfall
rate
be calculated by dividing the total
by
rainfall
in
mm
can
total
duration in hours. This rain gauge is placed at Ulubari. On
few
occasions when the rainfall is very intense, rain rate are
also
measured at 5 min. interval using this rain gauge.
3.3.2. S E LF RECORDING SYPHONING RAIN GAUGE:
In this rain gauge, the rain water collected through a funnel of
314.2 sq.cm, collection
area is
This cylinder is provided
with
accumulated
syphoning
in
a
cylinder.
mechanism.
A
light
weight float placed inside the cylinder moves up along with
the
accumulated rain water level causing a vertical spindle! mounted
on it) to lift up to a height in proportion to the water
level.
Fig. 3.3.3 shows complete arrangement of the rain gauge. A small
and light weight pen is
attached to
such a manner that its tip touches
the
the
vertical
paper
pressure to get an impression of the ink.
The
made to suit the system, can be fixed over
a
spindle
in
with
sufficient
chart
specially
steel
drum
which
rotates with a rate of one rotation per day. The x-axis
of
paper is calibrated in terms of time in hours,
minimum
with
a
the
resolution of 15 min. and the y-axis is calibrated in
terms
millimeters with a minimum resolution of 0.5 mm.
syphoning
The
of
level in the cylinder was adjusted so as to get one syphoning at
each 10 mm collection of rain water. This type of rain gauges are
placed at 5atribari and at University.
3 .3 .3 F A S T RESPONSE RAIN GAUGE:
The non recording
and
syphoning
types
of
inherent resolution limitations for measuring
rain
rain
gauge posses
parameters
N
A |R - P O R T ( ' i 0 K M S J
FIG
3 . 3.1 RAINFALL MEASUREMENT LOCATIONS
\.K tlO tL tV
VER)
£
•jo
r\
P*
P
r**
n»
1
QJ
ifa.
Art
ch a p ter 3
page 5i
FIG 3.3.2 SYPHONING RAIN GAUGE
for microwave propagation and its correlated studies.
all the non recording rain gauge
is
suitable
total rainfall measurements. On the other
only
hand,
for
the
rain gauge is also not found suitable due to two
response time and averaging or integration
First
daily
syphoning
factors.
Viz.
The
response
time defined as the time taken by the float to rise in
response
to fast fluctuations in rainfall,
time.
of
is considerably large. The in­
tegration time during which the slope of the rising
level can be sampled to compute the rainfall
rain
intensity,
much higher in comparison with the integration
time
water
is
in
also
which
microwave attenuation data were to be sampled. During the events
of very intense rainfall that lasts for a couple of seconds
the
syphoning rain gauge estimates lower rain rates because
the
of
above limitations.
In order to counteract with these
problems
tional rain gauges, a fast
response
under the present research
scheme
rain
faced
with
gauge
C B a rb a r a
is
< s t.a l
conven
developed
i QQ31>.
circuit details and working of this rain gauge are presented
The
in
section 3.5.
3.4 RAINDROP SIZE MEASUREMENTS:
3.4.1 REVIEW OF THE EARLIER TECHNIQUES:
In an attempt to correlate microwave as well as optical attenua­
tion with rainfall
intensity, a number of workers have conducted
experiments on raindrop size distribution (RSD).
has measured the
size
of
the
raindrops
by
In i 8 Q B t Itemsr
absorbent
paper
method, which consisted of exposing sheets of paper dusted
with
dye in rain and measuring the spots caused by raindrops. A known
relation was used to convert the size of
filter paper into
actual
investigators used the
size
flour
of
spots
raindrops.
method.
In
this
of raindrops
The
on
subsequent
method
a
pan
containing fine flour was exposed to rain and size ofthe pellets
produced by rain are measured >' Leras arid F a r s o n s . 1943:
arid F a l f i i s r > 194 7D.
M arsh a L
Recent measurements have been made with more
c h a p te r
page
3
sophisticated
instrumentation.
"electromechanical
sensor"
These
c Joes
may
at.at
include
19583,
on a diaphragm into
sensor" CL carmers,
electrical
19693,
pulses;
which
or
measures
electric charges on raindrops; or
measures the size of raindrops
3■->n-~^ » 1
; i tow e*t.at , 1uQi j .
an
an
the
as
falling
"electrostatic
size
"optical
crossing
an
known
distrometer, which transforms the momentum of raindrops
5a
dependent-
detector"
light
that
beamfStou
and.
3.4.2 TECHNIQUE DEVELOPED FOR RAINDROP SIZE MEASUREMENT:
In the present work the raindrop size measurements are made with
an optical distrometer which is
developed
under
the
scheme CBa.rba.ru. et.al 19933. The technical details
research
and
working
of optical distrometer are presented in the section 3.5.
3.5. CIRCUIT DEVELOPMENT:
3.5.1. VOLTAGE TO CURRENT CONVERTER:
The voltage to current converter is a simple
converting
the
feeding the
AGC
chart
voltage
recorder.
into
circuit
corresponding
Fig.3.5.1
presents
diagram of the voltage to current converter.
used
for
current
for
the
circuit
It is designed with
an operational amplifier. The voltage to current converter
the chart recorder as the
floating
load
(2k
ohms).
uses
The
AGC
voltage is applied to the noninverting input terminal through
buffer and the feedback voltage across R drives
terminal. This
circuit
works
as
a
the
current
across
output current and
the
is
in
series
with
inverting
series
feedback amplifier. The feedback voltage
R
negative
depends
input
a
on
difference
vo1tage V . .
Writing Kirchoff's voltage equation for the input loop, we have,
V
in
= V
id
+ V
f
....
....
....
....
....
(3.5.1)
p c tg e
chapL& r
FIG. 3.5 .1 V O L T A G E T O C U R R E N T C O N V E R T E R
no 352
charaterlstlcs of V/l converter
Output current
1 .2 ---------------------------------------------------------------------------------------
1.01-...............................................................
0.8
- .................................................................................................................................................................................................................................................................................
0.8
- ..............................................................................................................................................................................................................................................................................
0,4
-
........................................................................................................ ...........................................................................................................................................................................................................................................................
0.2 L ................ ............................................................................................................................
o.c------- :------- 1
------- 1
------- 1
------- 1
------0
9
-I
0
Input voltage (v)
——
i
Series A
8
10
12
54
page 55
chapter 3
The processed di gi ti zed output
FIG. 3.5.2b computer o u tp u t of the microwave a tten u a tio n
chapter
pa.g&
5&
since the voltage gain is very high V.^= 0
that implies, V . = V , or V. = RI ....
id
f
in
o
....
....(3.5.2)
where I is the output current.
The above equation shows that the output current varies with the
input voltage. Fig
3.5.2
shows
the
response
curve
for
the
voltage to current converter.
3.5.2. DEVELOPMENT OF FA S T RESPONSE RAIN GAUGE:
Basic principle: The basic principle of the
rain
gauge
is
to
count the number of raindrops electronically. The rain water
is
allowed to pass through a funnel of 75 sq.cm, and is
fed
to
standard nozzels so that the drops falling out of it
are
equal
pulse
while
in size. Each raindrop then produces an electrical
passing through an
These
electrical
pulses are recorded on a heat sensitive paper, after
processing
though a voltage
optoelectronic
to
frequency
system
.
a
converter
(VFC)
and
a
power
amp 1if ier.
The block diagram of the system which consists of the following
units,
is shown in fig.3.5.3 :
a) optoelectronic sensor
b) counter
c! recorder
d) automatic speed control unit.
aD Optoelectronic sensor: Fig 3.5.4 shows
the
sensor
that
is
arranged inside a light tight box. Each drop of water coming out
of the nozzle crosses the
field
of
view
of
the
produces an electric pulse. This pulse is then
by a monostable multivibrator
channeled
into
3
directions
(IC
for
1).
sensor
suitably
This
master
recording,
and
shaped
pulse
counting
is
and
automatic speed controlling of the chart drive.
b!) Counter: This unit is shown in fig. 3. 5.4.
The
decade
coun-
C t\Q . p t f T
jQCl £ &
FIG. 3. 5.3 B LO C K DIAGRAM OF F A S T R ESP O N SE R A IN O A U O E
‘l j f
ter (IC 7490) connected in 2x5 mode, counts the master pulses
arriving through an electronic DPDT switch made up
and gives a high output (with
time
every 10 master pulses. This
duration
output
is
of
used
2
to
recorder for 2 pulses. When the rainfall
is
DPDT switch disconnects the counter and
master
low
is
high,
>60mm/hr,
IC
4066
pulses,'for
inhibit
the
.<60m/hr.
the
pulses
the recorder directly making 10 dot marks on the
paper. But when rainfall
of
reach
thermal
the
chart
DPDT
switch
connects the master pulses to recorder through counter.
In
this
case for every 10 pulses we get 8 dot marks and two blanks. This
is
done
to
avoid
overlapping
of
individual
drop
into
a
continuous line during high rainfall.
cl) Recorder: This unit is shown in fig. 3.5.4.
VFC and a power amplifier. Here,
pulse
into
frequency
1200
the
VFC
KHz)
converts
which
transformer of 1 : 2000 turn ratio,
It consists
is
and
2N
power of this frequency to drive
the
coil.
black
dot
mark
on
corresponding to every raindrop.
2218,
timer
the
recorder. It puts
a
dot
to
thermally
amplifies
The
high
a
which
paper,
In order to mark time on
connected to
mark at regu Iar
is
minutes. This timing interval can be
var ied
the
tension
recorder,
sensitive
paper, a timer circuit is also provided in the
The output of
master
connected
output of coil is connected to stylus 1 of the
a
a
through a power amplifier. A
darlington pair, made of 2N 3055
gives
the
of
chart
recording unit.
stylus 2 of the
intervs!
ls
of
1.5
accord ing
to
the
requirement of the user.
dl) Automatic speed control: This unit is provided in the
for economic use of chart paper. When
chart moves at low speed and when rain
speed,
through
this
unit'fig.3.5.5>.
there
starts
For
is
no
it
this
system
rain,
the
changes
the
purpose,
a
programmable stepper motor is used to drive the chart paper. Fig
3.5.6 shows the circuit that generates necessary logic sequences
for stepper
motor.
It
consists
of,
clock
divider, shift register and Motor drive unit.
pulse
generator,
f ig
.
.
.
-
35 4
C IR C U IT
FO R F A S T
s e n s o r unit
RESPONSE
R A IN G A U G E
r e c o d e r u n it
c ounter
C fXCLf?L
p a g e 60
3
The speed of the motor is determined by frequency of clock pulse
generator. To produce proper sequence of pulses,
the clock frequ­
ency is divided by 8 with the help of JK flip flops
( IC 7473)
and afterwards shifted by 2 with shift register!
designed with
1C 7473).
points of the
The four outputs obtained at
final JK flip flop (fig. 3.5.6) are amplified for power,
using
darlington pair and then are applied to four windings of stepper
motor. Table 3.5.1 shows the required sequence of the logic for
driving the motor in one direction.
T a b le 3.5.1
SWITCHING LOGIC FOR STEPPER MOT O R TO DRIVE
SWITCHING
SEQUENCE
STEP
SW-1
IN
ONE DIRECTION:
4 STEP INPUT SEQUENCE
SW -2
PH-1
P H -2
1
2
1
2
0
1
0
1
1
4
1
0
0
1
3
4
3
4
1
0
1
0
3
O
0
1
1
0
5
1
D
0
1
0
1
When
there
is
of
pulses
train
pulses
drive
no
rain,
with
the
clock
frequency
the motor
triggers
changing
2KHz
generator
the clock
produces
(adjustable)
to pul l the chart
lmm/min. But when rain starts,
sensor,
pulse
paper
and
a
these
at a rate
of
the master pulse produced by the
to a higher
the speed of motor which
pulls
frequency
and
the paper at
thereby
lOcm/min
rate.
This speed of the chart continues till the rain ceases to
fall.
If rain stops,
the speed of the chart will
come down to
the original rate(lmm/min) after an interval of 2min.
C F i.C L p t& r
fig .3.5.5.
pa^e o i
-3
A U T O M A T IC
SPEED
CONTROL
FIG.
3 .5.6
LO G IC C IR C U IT FOR S TEP P ER M O TO R S P E E D C O N T R O L
Cl t LCLtj L
Cl>Cl iY ?Z?
_I>
CALIBRATION:
To find the rainfall
intensity in mm/hr ,
it
is
necessary
know the number of drops collected by the funnel and the
to
volume
of each drop. The product of these two i.e number of drops and the
volume of each drop, will give total volume of water
The following relation is used to calculate
the
collected.
rainfall
rate:
NxVx 10
where, N is the total number of drops,
V is volume of each drop in cc,
A is collecting area in sq.cm,
t is time duration in hours.
To find the volume of each drop, following procedure is adopted:
Water is sprayed over the collecting funnel and the
drops
from
the nozzle are recorded. The drops coming out of the nozzle
are
allowed to accumulate in
a
cylinder.
The
volume of
drops
thereby
the
water for 100
standard
is
measuring
measured
and
volume of a single drop is calculated. This process is
several times and the volume of a single drop is
each case. From this experiment,
the drop coming out of nozzle is
repeated
calculated
at
it is found that the volume of
0.12
cc
with
a
correction
factor of ±0.03.
To compute rainfall rate, the number of dot marks on the
chart
in every 1 sec or 10 sec are to be counted and this value has to
be substituted in formula 3.5.3 along with
the
volume
of
drop. Fig. 3.5.6 shows the calibration chart to calculate
fall rate directly from the total number of drops in 1
the
rain­
sec
and
RESPONSE
RAIN
in 10 secs.
3.5.3 ANALOG RECORDING TECHNIQUE
FOR
THE
FAST
GAUGE:
In addition to the recording technique that is described in pre­
vious section, an analog recording facility
in fast
response
2
f-h ,
1
^
00.& &
—r
Calibration curve
fast response rain gauge
r i r t s ii rst» irr1—
550 r
•
.............................. ..........................................
500 - ■
1 0 0 - ■■
30 -
................
......................
...................
■■
------------------1G
J
_,
2t'
30
40
M u -n o e r g r d r e s s ?'n 10 s e c
—
:
00
oc
i
Sarisa *
calibration curve
fast response rain gauge
'in fa i. rs » ( r m / v :
3CC i-----------23C - ........
'
5 0 0 -........ .............................................................y
* * -
........................................
>
I K - .........
i
<r.£, l - ........
30 !“
1
*
3
4
N u m b e r g f g r ^ e g ( :n 1 g e e ;
‘
r *vj.^>,^.^>,
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m irit #
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r~
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o
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r- ▲ »
~
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"r rir»‘»rt#
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iir ir
r rtC
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n a11. i »
*i ai u*^r
KM
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j. .
,
£><i
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chja.pt&i- 3
p a .g &
66
rain gauge is also provided for simultaneous recording of rainfall
rate and rain drop size on a single recorder. This is done by a
digital to analog converter (DAC).
Fig. 3.5.7 shows the circuit diagram of the DAC. The master
pulses produced at the sensor unit are fed to a 4 bit binary
coded decimal (BCD) counter. The outputs of this BCD counter are
then connected to an operational amplifier, through a resistive
ladder network as shown in figure. The final output of the
operational amplifier is given by,
Vo= - V ( 1/2 + 1/4 + 1/8 + 1/16) ___
....
...(3.5.4)
where, V is supply voltage.
This output rises by one step for each increment in the decimal
count of BCD counter and attains maximum (Vcc ) for sixteen (hexdecimal) counts. The BCD counter resets after each sixteen
counts. Fig 3.5.8 shows the sample records of the rainfall
recorded in analog form and also as a black dot received on a
heat sensitive paper.
MERITS OF THE SYSTEM:
The rainfall rates measured by the fast response rain gauge are
compared with rainfall rates
simultaneously
measured
by
syphoning rain gauge. It is found that fast response rain gauge
gives a higher rainfal1 rate than that given by a syphoning
gauge for the rainfall rates below 30mm/hr and above 60mm/hr.
And for rainfal1 rates between 30-60mm/hr, both the gauges give
almost same readings. Sample records of rainfall rate taken by
both the type of rain gauges are shown in fig. 3.5.8b, It can be
seen form this figure that the fast fluctuations rainfall rate
could be recorded by the fast response rain gauge where the
rainfall rate for 1 sec. can easily resolved. Where as in
syphoning rain gauge these fluctuations cannot be seen at all
' h, sy
t o r '
5
p a g e 67
R A IN FA LL R A T E RECOR DED O N TH ER M A L SENSITIVE P A P ER
R A IN FA LL R A T E RECORDED O N S TO R A G E
O S C ILLO S C O P E IN A N A L O G FORM
FIG .3 .5 .8 SA M P LE RECO R DS OF R A IN FA LL R A T E M EA S U R EM E N T
chapter 3
page
SIXTEEN EQUlVOLUME O W U o M m r t
R A IN F A L L R A T E R EC O R D E D O N S Y P H O N IN G R A IN G A U G E
R A IN F A L L
RATE
R EC O R D E D WITH F A S T
R E S P O N S E R A IN G A U G E
F IG .3 5 .8 B S A M P L E R E C O R D S OF R A IN F A L L R A T E M E A S U R E M E N T
£>£?
go
f /—>£i02’s .3
and the minimum resolution time is 15 min.
that the high rainfall rates (>
couple of minutes or even
100
less,
It has been
mm/hr)
which
can
accurately with the fast response rain gauge,
last
only
only
be
rates.
Fig.3.5.9
for
a
recorded
therefore the fast
response rain gauge is more suitable for measuring
well as high rainfall
observed
shows
the
the
low
as
correction
factor to convert the syphoning rain gauge readings to those
of
fast response rain gauge.
3.5.4
DEVELOPEMENT OF RAINDROP SIZE & SHAPE MEASURING INSTRUMENT
The schematic diagram of the distrometer is shown in fig.3.5.10.
This instrument mainly consists of
a) 1 mW He-Ne laser
b) Detector
c ) Am p 1if ie r
d) X/Y recorder and computer based data logger.
Fig.3.5.11 shows the arrangement of optical system. A laser beam
illuminates the detector
wide. The detector is
in the form of a pencil beam
kept
inside
a
black
hollow
of
6
b o x ’ like
enclosure to minimise the noise due to ambient light.
To
error arisen due
the
simultaneously,
to
but
more
at
than
one
different
drop
cross ing
points,
only
a
avoid
beam
specified
portion of the laser beam is exposed to the rain. Two fine
sheets, placed
oneither
side
of
the
exposed
mm
beam,
foam
prevent
formation of secondary droplets, which may otherwise fall though
the illumination in the form of mist leading to
degradation
of
S/N ratio.
Raindrops, while passing through the beam,
intercept some amount
of light falling on the detector and give rise to a pulse . The
height and width of this pulse are proportional to the
amount of time taken by raindrop to cross the beam. This
size and
pulse
is processed in various stages for determination of size of
raindrop.
the
correction factor
20
td
CO
cz
O')
o
- I-. r - , r- » “« T
I »*» t
o u R R t c . I IU IN
I
r~ *
r
40
60
80
rain fall rate (m m /hr)
r~t ~T* r~% I~ «
I U R
100
120
chapter d
page 71
chapter- 3
p age 72
BLACK ENCLOSURE
EXPOSED PORTION OF LASER BEAM
-- --------------- FOAM SHEETS —
FIO .3.5.11
ARRAGEMENT OF THE O P T IC A L SYSTEM
chapter 3
page 73
The detector forms one of the arms of a
DC
Fig. 3. 5.14
bridge.
shows the DC bridge along with an instrumentation amplifier.
variable resistance provided
in the DC bridge
is
usedfor
adjustment at various ambient
light conditions,1 ike cloudy,
night,etc,.
DC
The
output
of
bridge
is
applied
The
null
dark
to
an
instrumentation amplifier the output of which is given by,
R..6R, V ,
f
dc
V
out
(3.5.5)
R .. 4R
l
where,
£R is a small change in resistance of the detector due to
obstruction in light beam produced by raindrops.
R,
is equal
to resistance in each arm of DC bridge,
R. and R^are input and feedback resistances
in the amplifier.
Vd c is voltage applied to the DC bridge.
Reco nstr u ctio n of raindrop sizes from the signature obtained
RECORDER/DISPLAV:
Fig.3.5.13 shows the shape of a pulse as it appears on
when a raindrop crosses the light beam.
X-
axis
gives the time taken by raindrop in crossing
time in turn is equal
the pulse represents
suitable
the
X-
of
dimensions of raindrop,
axis
both
actual
Fig.3 . 5 . 13b shows typical
crosses the field view of
of
axes
in
terms
the
detector.
After
beam,
the
on
the
sensor
is seen as a pulse in the oscilloscope
in computer based
beam.
This
Y-axis
Thus
of
the
by
data
of
a
the
axes
of
interruption
of
the beam.
is
logger.
Fig
it
point
When
restored.
(fig).
signature of the raindrop is recorded on X/Y recorder
stored
pulse
sequences of raindrop positions as
the entire drop crosses
intensity of light falling
the
The
raindrop.
display,
size of raindrop can be computed.
contact of raindrop with the light
signal
laser
to Y-axis of the raindrop.
calibration
light continues till
of
on
3.8.14
The
and
shows
the
This
final
also
the
output pulses and corresponding shapes of the raindrops recorded
d r
Q
FIG.3.J.12 DC BRIDGE AND INSTRUMENTATION AMPLIFIER
.O ■<£>
10mV
L- 1- 1__ I__ l l i i
1__I__I__1__ 1
0 50 100 175 msec 0
125 msec
0
1mm
fig .3.514 o u t p u t
i__ i__i
0
0
pulse
for
c o r r e s p o n d in g
i
j
120 msec
1 mm
r a in d r o p
s iz e
chapt&r 3
pa.£& 70
C A L IB R A TIO N :
The
system
has
been
calibrated
with
nozzles
of
different
diameters, as described below.
First of all, the detector is fully illuminated with laser
and the dc bridge is adjusted for null output at
ambient
beam
light
conditions and for the intensity of the light of laser beam. The
water drops from nozzles are then allowed to
fall
through
beam terminal velocity. The signatures of drops
from
nozzles are recorded and
signatures
dimensions
noted down. These dimensions
are
of
this
finally
used
different
for
calibration
Here, curve 1 and curve 2 gives the calibration for
are
dynamical
calibration of chart or display in terms of axial dimensions
the water drop. Fig. 3.5.15 presents the
the
of
curves
x-axis
and
y-axis respectively.
A FEW C H A R A C TE R IS TIC F E A T U R E S O F TH E SY STEM :
1.
=8K/LUK
Sensitivity of the detector
2. Response time
=1 microsecond
3. Amplifier sensitivity
4. ADC conversion:
=10mV/ohm
a) conversion type tsuccessive approximations
b) conversion speed:100KHz
5. Minimum drop diameter that can be measured=0.3mm
6. Error in measurement of drop size :
a) in measuring x-axis
=+/- 0.05
b) in measuring y-axis
=+/- 0.01
Sample records of the raindrop size measurements
are
shown
in
fig. 3.5.16.
CONCLUSIONS:
The experimental techniques adopted and developed for convenient
recording
rainfall
of
rate
the
and
parameters,
raindrop
interfacing circuits have
been
namely
size,
microwave
are
developed
attenuation,
described.
A
few
as
of
the
a
part
Pulse height (mv)
Pulse width (msec)
Drop diameter (mm)
fio .3.5.15 c a l ib r a t io n
1. X-AXIS
curves
f o r o p t ic a l
2. v - axis
d is tr o m e te r
c h a p te r 3
fig .3.5.16 s a m p l e
p a g e 7&
records of r a in d r o p size m e a s u r e m e n t
r- h
-
'
t ^ v*
w w *
C?
p < 2 £*?•*=*
-*
research work. The circuit details and
discussed.
To
overcome
the
working
limitation
put
principles
forth
by
7Q
are
the
conventional rain gauges a fast response rain gauge is developed
by the group with two types of recording techniques,
and
dot
recording.
The
development
of
the
viz.analog
raindrop
measuring instrument is a major and significant fallout
research scheme. The detailed description
of
the
size
in
the
circuits and
system specifications of this instrument are presented.
chapter 3
page SO
REFERENCES
Re f e re n ce s :
1. Barbara .AK, Devi. M, Timothy. KI# S. Sharma C19933 " RAin drop
size
measurements"
Technical
report
4-93,
submitted
to
Department of Electronics, Govt.of India.
2. BAr-bar-a. AK, Devi. M? Timothy. JCI, £. Sharma C19033 " Fast
response rain gauge" Technical report No. 5-
93,
Submitted
to
Department of electronics, Govt, of India.
3. Craig. EE and GF. JenkinsonC19733 "
the
study
of
tropical
rain attenuation at 11 GHz using a solar radoimeter",
Australia
telecommunication research , Vol.7, pp3-10.
4. Dr-ufuea. G and G. maccitiarel 1a
£19833
"A
rain
model for the SIRIO-SHF experiment” Atmospheric
attenuation
ocean,
Vol,20,
No. 1.
5.
Fedi.
F
£19813
"Normalized
procedures
rain
attenuation
for
techniques for
to-satellite
radio
links”
IEE
and
prediction
terrestrial
conference
on
and
earth-
Antenna
and
propagation, Conf. proceedings No.5.
6.
Hogg. DC and TA. Chu £19733 "The role of
rain
in
satellite
communication" Proceedings of IEEE, Vol.63, pp 1308-1331.
7.
Ibara. T*
Faruhama. ¥
and
T. Manabe
C19S43
"Inference
raindrop size distribution from rain attenuation
of
statistics
12, 35 and 82 GHz, The transactions of IECE of Japan,
Vol.
at
67,
pp 211-217.
8. Joss.J* Thams. JC and A. Waldvogel £19683
of raindrop
size
distribution
at
"The
Locarno",
variation
Proceedings
of International conference on Cloud physics, pp 369-373.
9. Lanmiers.
UHW
£19693
"Electrostatic
analyses
of
raindrop
distribution " J. of Applied meteorrology, Vol.8, pp 330-340.
10. Laws. JO and DA Parsons £19433 * The relationship of raindrop
size to intensity' Transactions of American
Geophysical
Union,
Vol. 24, pp 452-460.
11. Macchiarella. G and M. Mauri C19823 "Attenuation statistics
11.6 Ghz in
satellite
an
S1RI0
earth-to-space
after
four
obtained
years
international conference on Antennas
and
through
of
the
activity"
propagation
at
Italian
Third
ICAP'83.
chapter 3
page Si
Part 2. pp 69-71.
12. Marshal.JS
and
WM.
raindrops with size’
J.
Palmer
of
Cl 948}
"the
Atmospheric
distribution
science,
of
Vol.32,
pp
1401-1411.
13. Paraboni. A C1982} "Charaterization of
second
order
approximation
attenuation beyound 10 Ghz,
propagation , May 1982.
theory
1EE
rain
and
profile
its
Transactions
in
influence
on
a
on
antennas
and
14. Raima. MK and GS. Uppal C1984J " Frequency dependent of rain
attenaution
at
microwave
frequencies"
IEEE
transactions
on
Antenna and Propagation, Vol.AP-32, pp 185-187.
15. Ryde. JW and D. Ryde t'19451) " Attenuation of centimeter waves
by rain, cloud." Rep. No. 8516, General electrical Co., research
labs, Wembly, England.
16. Sarkar. SK, Ravindran. VR, Ramakrishna. M, Beiier jee.
HN. Dutta (.1980} "Rain rate measurements
studies at 7GHz in northern India.
and
rain
S
and
attenuation
Indian Journal of radio and
space physics, Vol.19, pp 47-51.
17.
Sen AK, Mitra. A, Mazumdar.KK,
Ghosh. A, Ghosh SN and JS Sehra
Proceedinds
of
international
C
Tarafdar.
PK,
Tarafdar. G,
1985!) *'
sympossium
on
antenna
and
propagation, Held during 20-22August, at Kyoto, Japan.
18. Stow CD and K.Jones C 1981J ’ a self
evaluating
for the measurement of raindrop size and charge at
disdrometer
the
ground"
J. of Applied meteorology, Vol, 20, pp 1160-1176.
19. Stow. CD
f
Bradley.SG, Paulson. K and L.
simultaneous measurements of rainfall
distribution and scattring
of
Couper
intensity
visible
light'
and
J.
C1991J
"The
drop
size
of
Applied
Meteorology, Vol. 30, pp 1422-1435.
20. Tewari RK, Kumar. KS and VC Bahuguna C1986)
"
Experimental
studies on rain attenuation charateristics of centimeter waves"
J. of IETE, pp 130-134.
21
Uratsuka. S,
Ibar a. T,
Kitamura. K,
Manabe. T,
Y. FuruhaiaaC 1985!)" W or s t month statistics of rain
Imai.Y
and
attenuation
at
34.5 and 31.8 GHz " IECE Transactions, Japan V0 I.6 8 E. No.12.
pp
785-787.
page 82
CHAPTER 4
OBSERVATIONS AND ANALYSES
4.1. INTRODUCTION
A precise survey of the documented reports and
research
papers
on rain attenuation and its associated studies, reveals that for
proper understanding of the subject, and to meet the recommenda­
tions formulated by the CCIR. €19321}*
procedure is to be adopted.
a
common
( Dru/uca, 1874; Wawim, 1978;
Sarhar et.ai, 1979; Uratsuha e£. al, 1985;
1974; Ippelito*1981;
observational
Marita.*
1971;
1985;
Oyinloye, 1987
Olsen
and
Crane*
Khar adly *1971;
Harden et. al,1978y. This procedure is illustrated in
the
chart
4.1. According to the CCIR recommendations, the reliability of a
communication network is stated in terms of percentage of time in
a year the network is available or percent of time the
cutoff.
In connection with this aspect,
budget or allowances are studied
probability
distribution
of
with
the
the
aid
parameters
rainfall rate etc. Besides that, the rain
analysed in terms of its seasonal
link
link
availability
of
like
cumulative
attenuation,
attenuation
occurrence
is
and
data are
types
or
patterns of attenuation. The rainfall data are analysed to study
a) total annual rainfall over the area
concerned,
number of rainy days in a year, c) worst
rainfall rate that exceeded
for
0.01
month
for
finding
the
percent
correlation
total
statistics,
of
reliability factor).The statistics of attenuation
are then used
b)
time
and
existing
d)
(CCIR
rainfall
between
them. The raindrop size data are analysed to find raindrop size
distribution (RSD), its variation from one type to another
type
of rainfall and its contributions towards rain attenuation.
This chapter describes the observational study
on
attenuation,
rainfall and raindrop size, that is organized in accordance with
the standard procedure as explained above.
chaptsi 4
pcif-f iS3
Chart 4-1 OBSERVATIONAL PROCEDURE:
<
1. At tenuat i on
•t
St tip 1.
Procuring the data of —
> <
Rainfall
2.
i
3. Raindrop size
<
Step 2. Analytical study:
4
Attenuation
i
<
1. Obtain CPD
<
2. Oblain A0 0 1
<
1. Obtain CPD and its year to
year variability
/\
3. % of time fm is exceeded
V V V V V V
1
1
1
1
4 Rainfall
<
V V
<
i
2. Defi ne warst mont t h
3. Obtain E0 0 1
4
.
findout seasonal variation.
number of rainy days and total
annual rainfall.
< 1. study RSD for
<
t l i u n d r s t roms
<
★ Raindrop size distribution - ->c 2. study RSD for
<
wide spread r
<
<
<
step 3. C o r r e l a t i v e s t u d y :
3. study RSD for
drizzle
the correlat ion
bet ween r ai nf all
at tenuat i on
this relation is
A *
a R b
* Find the suitable values of
c oef flei ent a, b
» Observe the dependence of rain
-a f 4 a r i n a t
i #*« ri
page &4
ch a p ter d
4.2 DATA BASE:
1. Microw ave
2. Ra in fa l l
a t t e n u a t io n d a t a : m ea su r ed o v e r th e
d a ta :
P&T
link .
1. c o l l e c te d from iMD fo r 8 y ea r s Cd a ily t o t a l )
2. COLLECTED FROM FLOOD CONRTOL DEPARTMENT,
GUWAHATL FOR 10 YEARS CDAILY TOTAL)
3. Mea s u r e d a t s a tr ib a r i 3 y ea r s
3. Ra in fa l l r a t e : 1. Mea s u r ed a t s a tr ib a r i
CCOLLECTED
CSYPHONINO RAIN OAUOE)
DURING OBSER- 2. MEASURED AT UNIVERSITY
VATIONAL
CFAST RESPONSE RAIN GAUGE)
PERIOD)
3. Mea s u r ed a t receiver site
( f a s t r es p o n s e rain g a u g e )
4. Co l l e c te d from IMD
Csypho n in g rain g a u g e )
4. T hunderstorm d a ta : 1. collected from IMD
5. Rain d r o p
size d a t a :
1. Mea s u r e d
at
Un iv er sity .
SEASONS OVER GUWAHATI:
The
rain
attenuation
is
highly
sensitive
meteorological variabilities. So it is
to
essential
weather
to
study
and
the
rain attenuation features of a station at different seasons of a
year. Therefore the
follows^by
taking
seasons
in
to
over
account
Guwahati
of
1. WINTER----------------------------- JAN- FEB
2. prem onsoon ---------MAR-APR-MAY
ruoin
u
iN
ou
u
N
__ C E T Q -
uu
. ll ll . .«uv?
hi in
OCT
n v »
l / i_
classified
temperature,
humidity at different months.
II INL
is
U
rain
as
and
chapter 4
page He
4,3 OBSERVATIONS ON MICROWAVE ATTENUATION OVER THE LINK UNDER
TEST:
The microwave attenuation data collected over the 11GHz LOS link
by the technique discussed in the previous chapter are
analysed
to study the following factors:
1) Various patterns of attenuation
2) seasonal variation of attenuation
3) cumulative probability distribution of
attenuation
and
its year to year variability.
The detailed description of these factors is given below.
4,3.2 VARIOUS PATTERNS OF ATTENUATION
During
the
observational
period,
the
received
attenuation
patterns can broadly be classified into two groups.
Samples
of
these patterns are presented in the fig.A.3.1.
In pattern 1, signal
fade margin (45dB).
level fluctuates very rapidly touching
the
In this pattern, the fadings are distributed
as shown in the figure A.3.2.
It can be seen from
figure
that
the fadings in this kind of pattern follows the Rayleigh distri­
bution. The probability of
from 5 dB to 20 dB
time
exceedance
increases
sharply
and falls exponentially afterwards.
Fadings
with A5 dB depth are present in this pattern. The mean value
fade depth is around 20 dB.
In pattern 2, the signal
down suddenly and lingers at that level
for
some
level
time
reaching the normal level. The recovery may be rapid or
It indicates that the
signal
to follows exponential distribution in this case.
It
goes
before
it
occur in steps. Fig A.3.3 shows the distribution of signal
followed in this pattern.
of
may
level
prefers
has
been
observed that signal fluctuate rapidly when the attenuated level
is >30 dB.
4 .0j .0u
OrAO/MIAI
U L M w v n m .
The seasonal
\/ADIATir\M
ym i \i« ii\wiy
variation
r\u
V/l
of
A TTtTKII lA T irv M .
m
i i
i iv/iy.
microwave
attenuation
is
studied
c h a p te r
4.
P A TTE R N 1
P A TTE R N 2
FIG.4-.31 P A TTE R N S OF RAIN A TTE N U A TIO N
p a g e 86
pa ge &7
ch a p ter 4
FIG.
4 3.2
.
PROBABILITY DISTRIBUTION OF FADES IN PATTERN
1
ch a p ter 4
page S3
Probability distribution for pattern
Guwahati, 11 GHz, 3.2 kms
% of time absolasa e*o&eded
FIG. 4 3.3 PROBABILITY DISTRIBUTION OF FADES IN PATTERN 2
.
eh p a t er
4
pa.#& 8Q
during the years 1991,92 and 93. This information is graphically
presented in figures 4.3.4a,4b and
4c
plot the attenuation is classified in
respectively.
groups
of
starting from 1 dB to 45 dB. The percentage of
For
5dB
time
is
plotted
against
the
seasons.
A
brief
interval
that
group of attenuation level is exceeding in a year is
and
account
each
In 1991, the signal suffered attenuation in
months
May and June only. No attenuation has been
each
calculated
character istic features of seasona 1 variation in
separately given below.
the
this
on
year
experienced
is
April,
in
the
months other than these months. The link cutoff(cor responding to
4 J ^® attenuation! events are detected
only
during
April
and
May. Attenuation of 45 dB is exceeded for 0.0007 percent of time
in April and 0.0010 percent of time
in
May. The
attenuation levels in the month of April,
5-10dB, 10-15dB and 0-5dB respectively.
May
most probable
and
June
are
In 1992, unlike the year 1992,signal suffered attenuation during
premonsoon and monsoon periods (Mar-Aug).
But
level exhibited month to month variations.
are seen in April and July. The percents
exceeded,
the
Link
of
time
in these months are 0.00018 and 0 . 0 0 0 0 1
attenuation
cutoff
events
link
cutoff
respectively.
Approximately 10-15dB is the most probable attenuation level
in
all the months. Though the average attenuation is higher in this
year the link cutoff events are fewer in number.
In the year 1993 signal suffered attenuation from Feb. to
The months March and June register 1ink
percent of 0.0001 and
0.002
Sept.
cutoff with exceedence
respectively.
The
most
probable
depth of attenuation in all the months lies in the range of 5 , - 1 5
dB.
This study indicate that so far the reliability of
link is concerned,
communicaton
the premonsoon and early monsoon periods
are
unfavorable. The probable reason for getting attenuation as high
0- 5dB
2.
5 -1 0 dB
3.
1 0 - 15 d B
A.
15-2 0 dB
cr
20 - 25dB
6.
25-3 0 dB
—>
30 - 40 dB
8.
A0
Percent of tim e specified
le ve l of attenuation exceeded
1,
f ig
.4 .3 .4 a
s e a s o n a l v a r ia t io n
or
a t t e n u a t i o n (1 9 9 1 )
-
A5
dB
1
X
3
p,
0
-
5
d
1
0
-
2
0
-
2
5
3
0
-
4
0
1
5
B
5
d
d
d
4
B
,
-
1
0
d
B
1
5
-
2
0
d
B
6
.
2
5
-
3
0
d
B
5
,
4
n
-
4
5
d
Percent of time specified
level of attenuation exceeded
~7
r>
•
F1G.4.3.4R
SFASONAI
V A R IA T IO N
OF
A T I T N I JA I IO N
(1 9 9 2 )
B
B
B
C _ < hr
15 ~ 2 0 d B
2 5 - 30dB
4- O ~ 4 5 d B
o f
a t t e n u a t io n
e x c e e d e d
2•
4.
6.
3.
le v e l
P e r c e n t
o f
t im
e
s p e c if ie d
1. 0 - 5dB
, 10 - 13 3 B
cv
- 25dB
7 . 30 - 4 0 d B
M
4.3.4c
:> E A S O N A L
A
V A R I A T I O N
M
O F
0
A T
E N U A T iO N
(1993)
N
ch a p ter
p a g e 93
4
as 45 dB is
to
be
examined
in
relation
torainfall
rate,
raindrop sizes and their shapes.
With respect to the reliability of the communication
pre monsoon months (March, April, and May )
and
link,
early
the
monsoon
periods (June) are unfavorable. The probable reason for getting
attenuation as high as 45 dB, in this period only is either
rainfall rate is high or drop
size
distribution
the
different
these months from that in other months. These points
have
in
been
examined more clearly in the final section of this chapter.
4.3.4 CUMULATIVE PROBABILITY DISTRIBUTION CCPD)
The
cumulative
probability
attenuation will
distributions
of
be studied separately for each
the the presence of year to year variability
the observed
year to
examine
if any, in the CPD.
Fig 4.3.5a,5b and 5c.shows the CPDs for the years- 91,92
respectively.
In
these
plots
the
x-axis
time
specified level of attenuation is exceeded in a year,
from
the
figure
that
there
exists
93
represents
attenuation in dB and Y-axis gives the percent of
seen
and
a
the
that
it can
year
to
variability in the CPD. Attenuation exceeded for 0.01%
be
year
of
(reliability factor as required by the CCIR) in the years
a
time
91,92
and 93 are 16, 15, 18 dB respectively. However these figures may
not
be
confused
with
most
probable
attenuation
mentioned in the above section. The year 1991
experienced
cutoff for 0.0017 percent of time, where as 92 and
the
link
cutoff for
0.0002
and
respectively. Therefore, so far the
0.0021
level
93
percent
reliability
link
suffered
of
is
as
time
concerned,
the link is reliable for 99.998 percent of time in a year.
4.4 OBSERVATIONS ON RAINFALL OVER GUWAHATI:
The rainfall data collected and measured at various
places,
mentioned in the chapter 3, are analysed to study the
as
following
aspects.
a) seasonal variation of rainfall.
b) preferential time of occurrence of rainfall and its
seasonal
chapter 4
page
__
__
__
_____________
Q4
OBSERVED CPD €1991)
Be s t f it c u r v e
( A = 3 .5 9 P ~ ° '33}
O
O
d
o
o
d
CM
attenuation (dB)
O
o
O
CD
a
"D
35
70
03
C
H
O
C3
<
pa
03
03
>
m
Q o
£
<
Lu
o
m
71 <
C
> ~Q
H
c:
O
Cl
3
r*
>
>
Skk
lo
CJ1
55
<D
~U
od O
o
o
OBSERVED CPD C199L9
Best fit curve
r* i/-<
r ti.T
^
^
v_/|ri
,K '* 1 "
* TH
tr
V U l 'I U L M M V L
*“»F‘
ur
r-M -w-*r-» A r»M m
/
r h iU d M D IL I I Y
A TTf*. M l *
I
M I I C.IN KJf-\ * IV_yiN
r .i^ r m n i
i t i /m
. i
I K lt * V > I IV_>|N
/ T 'D r S ' N
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__
__
_
OBSERVED CPD (1993)
___________
Best fit c u r v e
-0.41.
(
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r iu
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rMorrur**
l /io i
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t x io u i iv jin
m o r n 's
w i
u/y
chapter 4
page 97
dependence.
c) percentage of occurrence of thunderstorms.
d) cumulative probability distribution(CPD) of rainfall.
These analyses are used for defining the worst month,
respect to the microwave propagation studies.
with
4.4.1 SEASONAL VARIATION OF RAINFALL:
Fig.4.4.1 shows the seasonal variation of rainfall, in which the
monthly total rainfall is studied. For this plot, 7 years (198693) of data have been analysed. On the average Guwahati, experi­
ences maximum amount of rainfall in month of July (450mm). The
total annual rainfall varies from 1900mm to 2400mm
with the
average number of 90 rainy days per year.
4.4.2 PREFERENTIAL TIME
SEASONAL DEPENDENCE:
OF
OCCURRENCE
OF
RAINFALL
AND
ITS
The preferential time of occurrence of rain fal1 is seen at
different seasons will be studied here. fig.4.4.2 shows the
contour plot of the preferential time of occurrence of rainfall.
For this plot three years (91-91) average preferential time is
taken. Here, a full day is divided in to four parts of 6 hours
of interval i.e 0000-0600, 0600-1200, 1200-1800 and 1200- 1800
hrs. It is seen from the figure that during the months of April
to June rainfall pattern shows a preferential time at 0000-0600
hrs. For the other seasons it does not display any preferential
time of occurrence. More data is necessary to assign a definite
pattern.
4.4.3 PERCENTAGE OF OCCURRENCE O F THUNDERSTORM:
Precipitation associated with a thunderstorm is often torrential
and is of short duration. The release of rain is very sudden and
raindrops are relatively large in size. Vigorous convectional
currents and high specific humidity are the, two factors that
cha.ot&i
4
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if 1.
=*
KV S I
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Minimum contour : 4
Maximum contour : 44
contour interval: 4
(contour levels represent the probability of occurrence)
T
I
M
E
1
S
T
months
A A
‘1
riME o
RAINFALL
J~-'£1C?t ■<E?T' 4
produce
C 'B r u n t ,i Q £>£;
thunderstorms
Ratri£ig& > 1 9 7 i 3 Since
the RSD
(like
showers
drizzling,
significance
contro l l i n g
Fig 4.4.3
in
of
shows
premonsoon
activity.
etc,.),
mi c r o w a v e
The
part
the
in
of
are also
in the figure.
shown
the height
of
of
freezing
about 3 0 0c and
kms when
that
high
the favorable
4 .4 .4
CUMULATIVE
period
PROBABILITY
towards
thunderstorms
more
t h u nderstorm
level
and
surface
that
there
and
it goes
activity,
surface
up to
some
temperature
months
the
temperature
the height
31°c.
low
by
exists
surface
about
maximum
t empe r a t ui~'e
premonsoon
kms with
is
during
contributed
with
temper a t u r e and
for
of
rainfall
During
temperature
situations
types
enough
experiences
activity
point.
surface
bear
observed
May
It shows
is 3-3.5
in monsoon
the surface
indicates
are
point
is
total
t h u nderstorm
freezing
in other
studies
occurrence
t he 0 "c isotherm
cori"elation between
height
of
is about 6 % . Along
the variations
that
by rain.
month
the
N i& u w o 1 1 ,i y 7 ? ;
thunderstorms
T h u n d e r s t o r m activity
and
from
p ropagation
the percentage
months
thunderstorms
cole. 1 975;
is d i f ferent
the attenu a t i o n
over Guwahati.
and
)C
xS
of 5-6
This
study
freezing
level
the thunderstorms.
DISTRIBUTION
OF
RAINFALL
RATE
CCPD>.
One of
the most
a reliable
microwave
c u m u lative
concerned.
rate
relevant
requirements
link
of
rainfall
this d i s t r i b u t i o n
that exceeded
planning
, at frequencies
distr i b u t i o n
From
for
the percent
above
rate
time
over
for which
designing
10 GHz,
one can e s timate
of
and
the
the
the
is the
area
rainfall
reliability
is recommended.
The
rainfall
rate data
fast
response
rate
is studied
shows
period
rain gauges
separately
the cumula t i v e
of
exceeded
collected
1991,92
for 0.01
during
percent
by syphoning
have
been used.
The
for
each year.
F i g . 4 . 4.4a,
distribution
and 93
91-93,
of
respectively.
of
time
CPD
rainfall
The
varies
of
rate
rainfall
c o n siderably
and
rainfal1
4b and
for
4c
the
rate
that
from
year
FIG .4 .4 .3
PERCENTAGE
OF
OCCURRENCE
OF
TH UNDERSTORM S
surface temperature (°c)
occurrence percentage of thunderstorm
rainfall rate in
m m /h r
chaoi
T~t
M V?
%
A A A
of time ordinate exceeded
a
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/r ' j O O ^
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page 103
chapter 4
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CL
Cl
rainfall
rate in mm
J
-n
c h a p t e r 4-
page 104
chapter 4
to year.
page 105
In 1991 Rq q
is 90 mm/hr whereas in the years
93 it is 85, 100 mm/hr respectively.
92
and
In order to define the worst
month with respect to microwave propagation, the monthly CPDs of
rainfall rate are studied. Here,
the
definition
given
by
the
CCIR for worst month is adopted (CCIR, 1982):
‘‘Let P (z > be the time percentage exceeding a threshold level z
in the i-th month of a year. Then the month for which the
is highest among the 12 months is the worst
month
P^tz)
Accordin­
gly, the month May is found to be the worst month over Guwahati.
Fig. 4.4.4d shows the CPD of rainfall rate for worst month along
with CPD of other months.
4,4,5 CORRELATIVE STUDY OF ATTENUATION AND RAINFALL RATE:
The observed characteristic features of attenuation and rainfall
are studied in association with each other to examine the
tion existing between them.For this purpose CPD
of
rela­
attenuation
is first calculated by measuring the duration for each level
of
attenuation ranging from 1 to 45 dB. The time resolution is kept
at lmin. for this study. The CPDs of attenuation and
rate for the years 91, 92 and 93 are shown in
and 5c respectively.
It was found that there
correspondence between the attenuation
and
variation pattern follows the relation
A
rain
figs
4.4.5a,
exists
a
5b
one-to-one
rainfall
=
fal1
rate
Rb,
given
The
by
Olsen et.alCtQTSy with different values of coefficients a and b.
Next,
to
rainfall,
study
the
attenuation
dependence
during
of
the
attenuation
on
thunderstorms
is
type
of
studied
separately. Fig.4.4.6 shows the attenuation verses rainfall rate
for thunder storms and for other types of rainfall, like
and drizzle. For this plot, 24 thunderstorms
events
simulteneous rainfall rate and attenuation data
and suitable number of other type of rainfall
are
(when
the
available)
events
observed during 91-93 are taken into consideration.
showers
that
can
be
seen from this plot that thunderstorms produce higher levels
of
attenuation .
to
The
values
of
a,
b
for
It
are
attenuation
thunderstorms are found to be equal to 0.035 and
1.109
due
whereas
C }\C Lp t& Y '
4
p C L tfr ?
|— t— f Apri 1
— ---- May
-*--- *— June
B
—t
August
rate
rainfall
T * T*
A
M V?.
t . t . W
A
A
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rr»r»
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'lO O H
ic / v i/ 1.
n iP T m n i
it ia i. i
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July
__ September
(m m /h r)
-1
t O S
y
-\i—
V?r
r>
a
itk ir~
a
« i
f\ M IIN r A L L
attenuation (db)
rainfall rate ( m m /h r )
F IO .4 4 .f3 A
C U M U L A T IV E
D IS T R IB U T IO N S
R A IN F A L L R A T E C 1 9 9 D
OF
A T T E N U A T IO N
AND
rst
00
o
attenuation (db)
rainfall rate(mm/hr)
NJ
<Oy>
o
O
o
rsi
o
-T
#
103
102 101 10° 10'102
percent of time ordinate exceeded
11( 1.4 4
flH
(jlJ M U L A IIV E
R A IN FA LL
D IS I R I B U I I O N S
RA
1E
(
1992)
OF
A I T ENV JA I IO N
AND
40
30
20
attenuation (db)
rainfall rate (mm/hr)
50
10
104
103
102 101
ioPio^ o2
percent of time ordinate exceeded
H O .4 - . 4 5 c
CUM ULATIVE DISIRIBUFION OF
R A IN F A L L R A T E
C1993)
AI TENAUI ION AN D
h,
f ^ Y"*
1 1O
d.
Rain attenuation at 11 GHz.
2>ttsnuat!on ;aS;
O'-'
u L. .
_-r»’
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a .t
Series 2
r' Vr .»Sn-K » •i
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series C D ^ i i L E
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f—\r~\ A
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r io . * t - . * t - . o
n
a h
ir- a t i
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4 i f " r » r » i i^%
V C .K O U O
A T
M I
T k
II I A T » / M . I
L I N U A « IW IN
chapter
4
page
for drizze and showers the appropriate
figures
are
0.013
111
and
1.24 respectively. This signifies that attenuation non-linearity
with rainfall rate is more
during
thunderstorms.
probable answers for this fact could be the RSD,
from one type of rainfall to other
One
of
which
the
differs
lQQUJ.
In
rainfall
is
The raindrop size distribution for various types of rainfall
is
next section the RSD
described indetail.
for
CTiiivDth.y
different
el.al,
types
of
4,5 OBSERVATIONS ON RAINDROP SIZE:
studied from the measurements of raindrop size made with instru­
ments described in chapter 3.This section presents the study
on
RSD observed at different seasons and
of
for
different
types
ra inf a 1Is.
4,5.1 RSD FOR THUNDERSTORMS:
The
RSD
for
thunderstorms
size events recorded
during
is
studied
1993.Here
with
three
help
raindrop
typical
RSD are
presented .
a ) 29 th April 1993:
This event falls during the premonsoon period.
0615 hrs.
It was a short lived (2 min)
It
rainfall
occurred
with
at
frequent
thunders and 1ightning.The average intensity of rainfall for the
period of 120
sec.is
61.2
mm/hr.
Temperature,
pressure
humidity measured at the surface level before, during and
the event are presented in table 4.5.1. Temperature
down
by
4°c
reaching
18°c
and
humidity
had
increased
and
after
fallen
by
33%
attaining 98% during the event. The rain drops in this event are
distributed as shown in the fig.4.5.1
represented by continuous
line. The horizontal extent of the rainfall
is less than
The mean drop diameter in this event is 1.6 mm.
5
kms
pcxg<& i t £
Number density
ch apter- 4
FIG
4.5.1 RSD FOR THUNDERSTORMS
_________
29th A p r i l
1993
.............
14th May 1993
- - - -
9th June 1993
page
chapter 4
113
T a b l e 4 . 5.1
AFTER
DURING
BEFORE
PARAMETER
Temperature (° C )
22
18
20
Pressure Smb )
980
988
985
Humidity
65
98
90
(%)
b_>_ 14. th. Mail 1993 ;
This event was observed at 1600 hrs, and it was also accompanied
with thunders and lightning.
The 8 min.
average
rainfall
was 80 mm/hr. The horizontal extent of the rainfall
than
5kms.
The
RSD
for
(represented by dotted
line).
humidity recorded before,
in table 4.5.2.
event
is
The
shown
in
is
the
temperature
rate
greater
fig.4.5.1
.pressure
and
after and during the event
are
shown
The mean drop diameter in this event
was
about
2mm.
T a b l e 4. 5.2
PARAMETER
DURING
BEFORE
AFTER
T emperature(°C )
22
20
19
Pressure (m b )
988
989
988
Humidity
70
85
85
(%)
c ) 9 th June 1993 :
This event is during the onset of monsoon
1140 hrs. The average rainfall
is
60
mm/hr
The
are
shown
meteoro1og ica1
in
occurred
rate during the period of 5
pressure and humidity recorded
event
period,
tab 1e
parameters,
before,
4 .5.3.
during
and
During
at
min.
temperature,
after
the
the
event
page 114
chapter 4
temperature
is reduced
by 3°c
and
pressure decreased
whereas the humidity increased by 10
%
by 6 mb
. The RSD for the event is
shown in fig. 4. 5.1 (represented by dashed)
line.
The mean drop
diameter in this case is 1.7 mm.
The RSD for thunderstorm therefore show significant presence of
raindrops with diameter lying between 1.5-2 mm.
It is also to be
noted the absence of drops of diameter less than 0.5 mm and the
presence
of
drops
with
diameter
up
to
4.5
mm
in
all
these
events.
T a b l e 4.5.3
PARAMETER
BEFORE
DURING
AFTER
Temerature ( C)
31
29
24
Pressure (mb)
980
980
981
Humidity
70
80
80
( % )
4.5.2 RSD FOR DRIZZLE:
RSD for drizzle is studied for the events recorded during 1993.
Here three typical events are studied.
a ) 6. May 1993 ;
This event occurred at 1010 hrs with rainfall rate of 10 mm/hr.
Horizontal
extent
kms.Temperature,
of
the
rainfal1
was
greater
pressure and humidity recorded before,
and after the event are shown in table 4.5.4.
and characteristics
under
the drizzle
of
the rainfall,
type of
rainfall.
table that there is no change
this
From
event
It can
be
is presented
5
during
the nature
is classified
seen
from
the
in the temperature and pressure
during the event, but the humidity is increased by 5
for the event
than
%
. The RSD
in fig.4.5.2 with continuous
line.
Raindrops of diameter 0.5 mm are much abundant and the number
chapter
4
page
6th May
17th May
1993
1993
28th May 1993
E
MV.1
A FZ O
Don
l\ J U
l
-
r V JK
r
-
L » rM ^ L Z .L t.
1 1 £>
pa.g& 1 i &
ch a p t& r- 4
density
falls
linearly
with
drop
diameter.
Larger
drops
with
diameter up to 1.5 mm only are detected.
T
a b l e
4 .5 .4
PARAMETER
BEFORE
Temperature (°C1
18
18
18
Pressure (mb 5
989
989
989
Humidity
85
90
85
( %)
DURING
AFTER
fei. 1 1 May. 1993:
Occurrence time of the event is 1100 hrs, with constant rainfall
rate of 7 m/hr . The horizontal
extent of the rain is greater
than 5 Kms. The three parameters Viz.
temperature,
pressure and
humidity for the event are shown in table 4.5.5. Fig 4.5.2 shows
the RSd for the event represented with dotted line.
Table
PARAMETER
4 .5 .5
BEFORE
DURING
AFTER
Temperature (°C)
19
17
18
Pressure (mb)
986
986
986
Humidity (%)
70
90
90
c ) 28 th May 1993:
On this day at 0930
From
its nature,
hrs,
this
a rainfal 1 of
event
11 mm/hr
is classified
under
is recorded.
the drizzle
type of rainfal1.Table 4.5.8 presents the temperature,
and humidity recorded for this event.
event
are
distributed
as
shown
in
pressure
The raindrops during the
fig.4.5.2
represented
by
chapter 4
page ill
dashed 1ine.
T a b l e 4 .5 .6
DURING
BEFORE
P A R A M E TE R
AFTER
Temperature(°C)
20
19
19
Pressure (mb)
Humidity 1%)
980
90
980
980
90
90
The above observational study on RSD indicates that the thunder­
storm distributes
the raindrops
in a fashion that
is entirely
different from that for drizzle. When drops of diameter about 2
mm are most abundant
in thunderstorms,
number of drops of 0.5 mm diameter.
drizzle contains
large
In next section the RSD for
showers is presented in a similar observational format.
4.5.3 RSD FOR SHOWERS:
Here also the RSD for showers is studied with events recorded in
1993 and a typical RSD is presented below.
11 th June 1993;
On this day at 1815 hrs a heavy rainfall
without
thunders was
observed The average rainfal1 for the period of 200 sec. was 48
mm/hr,
this
RSD for this rain event is presented
case
too,
the
temperature,
pressure
in fig.
and
4.5.3.
In
humidity
is
recorded before,during and after the event. Table 4.5.7 presents
values of these parameters. Here,
range
of
1 to
1,5
are
most
the drops of diameter
abundant
and
the
number
in the
density
decreases with drop diameter like that in the case of drizzle,
but unlike in the drizzle,
detected during the showers.
the drops of diameter upto 4 mm are
chapter
p a g e 11 &
4
17th June 1993
<E
FIG
4- 5.3 RSD
FOR SHOWERS
chapter- 4
page 1 IQ
T able 4.5.7
PARAMETER
DURING
BEFORE
Humidity (%)
16
979
85
16
979
85
18
980
80
Temperature
Pressure (mb)
AFTER
4.6 RAINFALL RATE AND RSD: A CORRELATIVE STUDY:
It is well
known
that
the
rainfall
rate
is
related
with
the
drops size distribution by an integral equation
R = 6X104 n
11^ °o° D 3 V ( D ) N ( D ) d D . . . .
....
....(4.5.1)
where, N(D) is the number density ,mm -1 .m-3
V(D) is the terminal ve1ocity,m/sec,
and
Here,
D is the drop diameter, mm.
the number density N(D)
is described by three parameters
namely the total number of drops, the mean drop diameter and the
standard deviation of drop diameter, as discussed in chapter 2 .
In this section a case study on rainfall
simultaneously on 9th June 1993,
2min.
between
duration
with
10-60
mm/hr,
very
is described.
intense
associated
rate and RSD,
rainfall
with
recorded
It is an event of
that
frequent
is
varying
thunders.
The
three RSD parameters (total no. of drops, mean drop diameter and
standard deviation) are studied to examine their variation within
the rain event.
Fig.4.6.1 shows the rainfall rate, drop sizes versus time. Dotted
line in the figure corresponds to the mean drop diameter varia-
Drop diameter (mm)
Rainfall rate (mm)
H0.4.(x 1
R A IN F A L L
RATE
AND
DROP
S IZ E S
l 21
C }\U-p t €r~f
tion.
The
total
number
of
drops
(N^),
mean
and standard deviation in drop diameter
the event.
drop diameter
1 )
are calculated
ic)
for
There is no point to point correlation between these
parameters and rainfall
rate. But on average,
the best fitted
equations for these parameters can be presented as:
D„ : 1 , 1 2
M
....
0 39
= 180 R '
N
a
t 0.18 In R
=
The
exp (0.48 - 0.03R)
equation
4.6.1
....
....(4.6.1)
....
....
....
....(4.6.2)
....
....
....
....
shows
1ogar ithem ica 1 1y with
....
that
the
the
rainfall
median
rate
drop
and
it
(4..6.3)
size
is
varies
found
that
the median drop size is always greater than 1.1 mm. The value of
, extrapolated for 250 mm/hr rainfal1 is around
1.7 mm.
which
indicate that in any rain event the most probable drop size will
not exceed 2mm as the maximum limit of rainfall
over
is
the link path.
found
to
(eq.4.6.3)
be
varying
giving
indicate that,
diameter
The standard deviation
less
with
with
deviation
increase
decreases,
increases
exponentially
or
the
at
high
in rainfall
in other
rainfall
words
rate.
Q-factorof RSD towards controlling
rate is 300mm/hr
in the drop diameter
with
rainfal1
rainfall
rates.
rate
This
rate the range of drop
the Q-factor
The
of
significance
the RSD
of
the
the microwave attenuation
is
described in the following section.
4.7
RSD AND ATTENUATION:
We present
drop
here a typical
size
on
case
microwave
study,
in which
attenuation
the control
is studied
from
of
the
simultaneous measurements of attenuation and raindrop size, when
the average rainfall
rate was 68 mm/hr for the period of 25 min.
Fig.4.7.1
presents
the
diameter.
In
minute
this
interval
is
plot
variation
the
of
attenuation
maximum drop
plotted against
observed during the same interval.
the
diameter
maximum
with
drop
seen in
attenuation
It can be seen from the plot
1
chapter
page
4
attenuation Vs dropsize
Guwahati 11 GHz, 3 .2 Kms
attenuation (dB)
10
to r
s -
a-
drop diameter (mm)
'
FIG.
Series
A
4 .7.1 VARIATION OF ATTENUATION WITH DROP DIAMETER
page 123
chapter- 4
that the drop diameter less than 2mm does not have much control
towards
with
attenuation.
the
drop
But
diameter
attenuation
for
the
increases
drops
bigger
exponentially
than
3
mm
in
diameter CDevi et.al, 19933. From the simultaneous measurements
of
microwave
attenuation
and
drop
size,
towards microwave attenuation is studied.
is a correlation
attenuation A
following
between
(dB)
the
. This
slope
the
control
of
RSD
It is found that there
(negative)of
relation can be
the
represented
RSD
and
by
the
equation.
A = R log(s)/(i5+log(R) ___
___
___
(4.7.1)
where, A is attenuation, dB
R is rainfall rate, mm/hr
and
s is the negative slope of RSD , mm
m
-3
Fig.4.7.2 presents the variation of attenuation with the slope
of RSD. From this study it is found that, microwave attenuation
follows the magnitude of the negative slope 1ogarithemical1y .
In the next chapter the control on attenuation by size and shape
of raindrops is described and probable reason for getting large
attenuation
during
the
thunderstorms
is
discussed.
-
h
,-v
24
FIG. 4.7.2 RSD S L O P E . R A IN FA LL R A T E A N D A T T E N U A T IO N
chapter- 4'
page i25
4.8 SUMMARY:
The observational study on rain attenuation, rainfal1 rate and
raindrop size distribution is presented in this chapter. Rain
attenuation data is collected over a LOS link oprating at 11
GHz and with the path lenght of 3.2 Kms during the period
1991-93. This data is analysed for studying different patterns
of attenuation, seasonal variation in the depth of attenuation
and cumulative probabilty distribution (CPD). Two distinct
patterns of attenuation are received during the study period.
The charateristic features of these patterns are presented. A
well defined seasonal dependence of depth of attenuation is
observed during the study period. Signal suffered maxi mum
attenuation C4SdBQ during the premonsoon months and early
monsoon periods. In 1991, siganal suffered attenuation during
the months April, May and June and April and May months
registered the link cutoff events. In 1992, signal suffered
attenuation from March to August. The link cutoff events are
present in the months of April and July. In 1993 , signal
suffered attenuation right from February to September. The link
cutoff events are present only in the months of March and June.
The percents of time that the attenuation of 4.5dS Clink cutoff3
exceeded in 1991,92 and 93 are 0.0017, 0.0002 and 0.0021
respectively.
The cumulative probability distribution of
attenuation projects a significant year to year variability. The
attenuation that exceeded for 0.01 % of time in a year for 91,92
and 93 are 16dB, 15dB and 18dB respectively.
The total daily rainfall data collected over seven years at
various places in Guwahati are analysed to study the seasonal
variation, total annual rainfall. From this analyses it was seen
that Ouwahati experiences 1900*=2400mm of annual rainfall and the
month June receives maximum amount of rainfall Cabout 450mmD.
During the study period, it is observed that the thunder
a c t i v i t y over G uw ahati is more during the premorxsoon months with
a peak 3t May. The rainfall rate data recorded with syphoning
chapter 4
page 1£6
and fast response raingauges are analysed to study the
cumulative probability distribution(CPD> and to find the
correlation between microwave attenuation and rainfall rate. The
rainfall rates exceeded for- 0.01% at time in a year- are 9G, S3
and iOO mm/hr respectively for the years 1931,92 and 93.
On the average there exists one to one correspondence between
the rainfall rate and attenuation. Attenuation follows rainfall
rate through the relation A » a R**, Thunderstorms produce more
attenuation than other types of rainfall. The values of
eoeffeicients a ,b are 0.033 , 1.109 respectively for
thunderstorms and for other types of rainfall the appropriate
figures are 0.013, 1.24 respectively. During thundershowers
attenuation non-linearity with rainfall rate is more.
The raindrop size data are analysed to study the raindrop size
distribution <RSD) for different types of rainfall. It is
observed that RSD is different for different types of rainfall.
For thunderstorms, RSD follows a lognormal pattern where as for
other types of rainfall it follows a negative exponential
distribution. The appropriate values of the parameters of the
lognormal distribution for RSD of thunderstorms over Guwahati
are
DM CMedian drop diameter!) e 1.12+0.18 In R
N^, Ctotal number of drops 3 m 180 R^'
s tstandard deviation of drop diameter! ■
0.03R)
chapter 4
page isrr
REFERENCES
Re f e r e n c e s
1. Brunt. D C19S23 "Physical and dynamical
meteorology, Cambri­
dge university press.
2. CCIE report 731-1 C1978-8203
"Attenuation by hydrometeors in
perticular precipitation and other atmospheric particles"
3. Cole. FW C19753 "introduction to meteorology" Wiley, 1975.
4.
Crane.
EJC C19743
"The
rain
range
through simulated rain environment"
experiment
-propgation
IEEE transactions on Antenna
and propagation, Vol.AP-22, pp 321-328.
5. Devi. M, Timothy. ICE, Sharma. S and AK. Bar bar aC 19933
drop,
rain rate and microwave attenuation"
international
and
6.
conference
on
advances
in
proceedings
pattern
"rain
of 3rd
recognition
digital techniques, Calcutta, 28-31 Dec.pp 539-544.
Drufuca.
frquencies
G
Cl9743
"
10 GHz
from
above
Rain
attenuation
rain
gauge
statistics
observations"
for
J.
Of
research. Atmosphere, Vol.8, pp 339-411.
7. Harden. BW» Norburay. JR and WJK. White C1978a3 "Attenuation
and rainrate relationship on terrestrial microwave links for the
frequency range 10-100Ghz" Electronic Letters,Vol,14, ppl54-155.
8.
Ippolito.
LJ
C19813
"
Radio
propagation
for
space
communication systems* Proceedings of IEEE, Vol.89 pp 897-727.
9. Mawira.
A C19783
" statistics
on rainrate some worst month
consideration. Ann. Telecommunication, Vol.35, pp 423-428.
10.
wave
Mori ta.
K
Cl9713
attenuation
due
"Statistical
to
rain
studies
on
e 1ectromagenetic
, Rev.E 1ectronic
communications
labs, Japan, Vol 19.
11. Nleuwolt. S C19773 * Tropical climatology" Wiley,1977.
12.
Olsen.
RL
and
MM2.
Kharadly
Cl9763
"
Experimental
investigation of the scattering of electromagbetic waves from a
model random medium of discrete scatters* Radio science, Vol.11,
pp39-48.
13.
Sassage,
CS
Cl 9713
"
Monsoon
meteorology"
Academic
press,1977.
14. Sarkar. £K» Ravlndran VR» Ramakr-ishna. M. Benerjeee.
HN.
Dutta €19793
" Rain
PK and
rate measurement and rain attenuation
studies at 7 GHz in Northern India"
IJRSP, Vol. 9, pp47-51.
p€L£e
c hap te r- 4
15
C l 9933
" rain
attenuation characteristics over a clear LOS microwave
link at
T im o t h y .
11 GHz.
16.
S.
1 2S
K I,
U ratsu k a.
and
785-787.
M. D e v i ,
AK . b a r b a r a
Indian journal of radio and space physics (in press)
S,
F a r u h a m a . Y C198Q3
34.5
S h a rm a ,
81.8
I h a r a . T,
K it u m u r - a . K,
M an abe. T,
Im a i. Y
and
"worst month statistics of rain attenuation at
Ghz."
IECE
Transactions
Japan,
V 0 I.E6 8 ,
pp
M m i E M i a i K i i H W M i i i i n i H a i i i w i an mki mm mm mm mmi mm mm mm mm mm mb m
COMPARISON OF OBSERVED
RESULTS WITH STANDARD
MODELS AND MODELLING OF
RAIN ATTENUATION AND
RAINFALL RATE
M M i M i a n i t t i H M B i M a i i i i i a n i M i i i a i M t r i M M t w M i M u
CHAPTER 5
PAGE 129
COMPARISON OF OBSERVED RESULTS WITH STANDARD MODELS:
5.1 INTRODUCTION:
For
a
meaningful
observations,
it
interpretation
is
worthwhile
to
of
the
examine
the
existing models and to bring about a new model
In
this
regard,
the
on
with
when necessary.
attenuation,
rainfall
rate and raindrop size are compared with
the existing
models,
that
global
results
results
rain
claim
observed
experimental
applicability
and
also
with
the
documented results reported from India as well as from out side
India.
There are a number of models defining rain attenuation
of rainfall
in terms
rate and dropsize distribution etc,.The CCIR model
published in 1982 C CClRt 19829 and lognormal model
reported
in
1982 by AJoyi et. al, and in 1985 by Muller and Si/ris. are some of
the notable ones. This chapter deals with the comparison of our
rain attenuation with these models. The results on rainfall rate
will
be
compared
with
(Crana,
19303 and
results
on
RSD
two
existing
the CCIR
are
{Marshal arid Palmar,
19 £‘59 .
model,
mode IsOCC IR,
compared
with
19459 and
2983c 9.
negative
lognormal
namely
the
Global
Similarly
exponential
model
the
model
CFarig and Chen
Many experimental results on the subject have been reported from
all over the world during past two decades cCrana, 1977,
1982;
Madhars t,1965;
Ajoy i ,1985, 1990;
a t.a l, 1984; £ark.ax-, 1978;
The
observed
results
are
Mac ia l at.a l ,1 990 ■ Iha ra
Raima at. al 1984;
studied
1980,
son at. al , 19859.
in comparison
with
reported
results from temperate and tropical zones.
5 .2 COMPARISON OF OBSERVED ATTENUATION RESULTS WITH MODELS:
K O A IWITU T U r
o . i __i m i l l
i ■ ii __
f'f'ID
w w ii\
MrvrMri .
i ' i v / w u __
A global model for estimating specific attenuation
(dB/km) from
the rainfal 1 rate as a function of
polarisation
frequency and
page i30
chapter 5
state,
was proposed by CC1R,
with recent
b
In this model the empirical relation A = aR was
modifications.
taken
as
computed
the
basis
from
the
and
as early as in i983,
values
negative
of
coefficients
exponential
a
and
distribution
b
for
are
RSD.
These values are tabulated against frequency and horizontal
or
vertical
polarisation
11 GHz horizontal
(Here,
states.
The
polarisation are,
only horizontal
polarisation
representative
values
at
a = 0.0125 and b = 1.232.
is considered
because
the
present study deals with the terrestrial microwave communication
link that uses horizontal polarisation).
The path averaged attenuation CdB) at 11 GHz is given by
n.oe o
a = ,L x 0.0125
R 1*232 ....
....
(5.2.1)
where, L is the path length in km.
But rainfal1 may not be homogeneous over the entire path and may
give
erroneous
results
if
specific
attenuation
is
multiplied
with total path length to obtain path averaged attenuation.
It
was then suggested that the actual path length is to be reduced
to an effective path length depending on the rate of rainfall
and horizontal dimensions of the raincell.
an empirical
relation
for
In tQ7Q Lin proposed
the effective path
length which
is
given by,
L
L “
e 1+L(R-6.2)/2636
....
«...
«...
....
(5,2.2)
Where, Le is the effective path length in km
L Is actual path length in km
R is the rainfal1 rate in mm/hr.
The CCIR also proposed an empirical
formula for the effective
path length, based on rainfall measurements made at various
places CCCIR.t 1983j , and is given by
I. - r L
e
«...
....
«...
....
«...
(5,2.3)
chapters- 3
where
page 131
r= 1/(1+0.045L) , is the reduction factor.
Fig. 5.2.i shows the comparison of the observed attenuation with
CCIR predictions. The attenuation values are calculated as
described below. First of all the microwave field strength data
are divided into two parts, in which part 1 represents the data
during rainy period
and part 2 gives the data
for no rain
period. The attenuation observed during the rainy period is
caused only due to rain. In all the cases signals are attenuated
and enhancement in the signal strength is not at all seen during
the rainy period. The signal strength valueslabove free space
loss) obtained during the rainy period are divided by the total
path length to obtain the specific attenuation (dB/km). Since
the rainfall rate may not be uniform over the entire path the
path length is reduced to the effective path
length. The
reduction factor proposed by CCIR theis taken into consideration
for the purpose. The attenuation caused by thunderstorms is
studied separately from those caused by other types of
rainfalls. This comparative study of observed attenuation with
CCIR values shows that the observed attenuation is higher than
CCIR
predictions,
in case
of
attenuation
caused by
thunderstorms.
But
for drizzle and showersthe observed
attenuation follows the CCIR values.
5.2.2 WITH LOGNORMAL MODEL:
Since the CCIR model gives the best agreement for attenuation
produced by rain other than thunderstorms the attenuation caused
by thunderstorms only will be studied for comparison with the
b
Lognormal model. In this model, the emperical relation A = aR
with coefficients a and b calculated from Lognormal distribution
for R5D,
is used
for estimating
the attenuation.
The
representative values of a and b are 0.034 and 1.113
respectively at 11 GHz and for horizontal polarisation. The path
averaged attenuation is then given by,
1 i13
a = 0.034 R *
x L ....
___
___
___
(5.2.4)
e
ch a oter 5
y N / V K ^ ri a n ir tz - t k . t
p ag e
«m ~
M U frfF^iKte*UfM Ml-
*%r»Arr«i ^rr»
U tte»tK ¥fc»M
AT T T i II I AT lA i I
UIITI *
I I L I W A I IMW WI I H
r '/ 'ID
WMIh
iui*%nri
i'iM M L L
i
32
chapt&r 5
page 1a'3
Fig.5.2.2
shows
lognormal
the
model
comparision
predictions.
of
observed
The
solid
attenuation
line
in
with
figure
corresponds to attenuation values calculated from the lognormal
model
using
the
equation
observed over Guwahati.
attenuation
at
11
5.2.4
and
the
"+"
marks
are
those
It can be seen from the figure that the
GHz,
for thunderstorms
over
Guwahati
is
following the Lognormal model.
Therefore
GHz,
for
CCIR
produced
the
model
observed attenuation over
gives
the
by drizzle and showers
best
approximation
case
the CCIR model
gives
this
discrepancy
the
model
the
is
The
effect of
larger
in
the
fit
and
in
case
lognormal
lower
values
raindrop
size
. The
negative
a
and
b
model
exponential
present
subsequent
In the
main
model
On
later
The
CCIR
types
which
other
for
mode i to
underestimates
the
11
the
reason
different
in thunderstorms!
sections).
gives
exponential
for
at
attenuation
distribution.
RSD given by negative
drops
of
thunderstorm attenuation.
'coefficients
rainfall.
seen
for
makes use of
compute
best
Guwahati,
of
the
can be
hand,
the
lognormal model gives the best fit for the RSD in thunderstorms
and therefore,
utilization
the values of coefficients a, b , calculated by
of
lognormal
model
gives
the
best
fit
for
the
observed attenuation by thunderstorms.
5.2.3 WITH REPORTED RESULTS:
The observed attenuation is compared with that reported by the
Radio science group (NPL.T^elhi, at 11GHz,
19783, Propagation
study
group
(DEAL,
skypath)C’Sor/tar et.a.1
Dehra
Dun
at
11GHz,
15kms )CT&wari <et.a.1,iQSSJ and microwave study group (IRPEL, Cal­
cutta at 11 GHz 35 kmsICSen. et.ct.1 .fyS'SD. All these results are
obtained at 11GHz .The study periods of NPL, DEAL,
IRPEL are 1,
1 and 2 years respectively. The results reported by NPL are over
a sky
path
link
5.2.3 presents
and
others
are
over
this comparative study.
about the type of rainfall
terrestrial
As
in the results
there
links.
Fig.
is no mention
reported
from above
ch a p ter 5
page 13$
Rain attenuation at 11 GHz.
comparlslon with models
Attenuation (dB)
aerie? A LOGNORMAL
^
Series B THUN.3TORM
study period 1001-93
FIO.5.2.2. COMPARISON
MODEL
OF
OBSERVED
ATTENUATION
WITH LOO
NORMAL
t -'
Rain attenuation at 11 GHz.
comparision with reported results
T
able
5 .2 .3
i
|R A IN F A L L R A T E
A T T E N U A T IO N Cd B / k m )
| Cm m / h r )
c c ir
NPL
DEAL
110
0. 21
0. ^
0. 5
0.3 5
j 50
1. 5
1. 9
1. 4
1. 2
| 100
3. 6
3. 9
3. 9
3. 9
4. 9
| 150
5.9
-
6.2
-
9. 06
1
“
_
-
12. 4
1
-
|200
C ALC U TTA
G U W A H A TI
1
1
1
I
1
0.4
1
f i o .5.2 .3 .
M/>r>ri
c o m p a r is
ON
OF
OBSERVED
A T T E N U A T IO N
WITH
LOO
NORM AL
pag& f 3&
chapt&r B
stations,
we
compare
our
attenuation
values
types of rainfall with the reported results.
observed
links which supports
all
It can be seen from
the plot that attenuation over a sky path is
the terrestrial
in
less compared
the fact
that
to
vertically
polarised wave is less attenuated than horizontal polarised wave
and attenuation
increases with path
length.
Table 5.2.3
the specific attenuation values as calculated
along with those observed at NPL, DEAL,
This
table
indicates
that
the
shows
from CC1R model
IRPEL and at Guwahati.
attenuation
values
observed
at
Guwahati are higher than those seen at other stations for same
rainfall rates.lt was reported by NPL, DEAL and IRPEL that their
observed
attenuation
predictions.
than
CCIR
showers,
5 .3
is
wel 1
But attenuation
values
for
in
agreement
observed
with
over Guwahati
thunderstorms
while
for
the
CCIR
is higher
drizzle
and
it follows the CCIR predictions.
COMPARISON O F RAINFALL RATE R ESU LTS WITH THE MODELS:
5.3.1 WITH GLOBAL MODEL:
Crams in ±980, has proposed a model, known as global model,
for
prediction of inter-annual and inter-1ocation variability of the
rainfall rate. The model was recommended by him for use, when no
other
data
on
rainfall
are
available.
In
this
section,
this
model is assessed with the observed rainfall.
This model divides the Globe into 10 rain climatic zones.
5.3.1
presents
Classification
temperate,
the
of
CPD
rain
of
rainfall
climatic
rates
zones
subtropical and tropical
Is
Table
for
these
zones.
made
under
polar,
regions,
by taking average
annual rainfall. According to this model Guwahati falls in zone
G, with Rq
67 mm/hr (fig.5.3.1). But rainfall data collected
by us with fast response raingauge give higher rainfall
The
measured
average
R„
over
Guwahati
mm/hr, with 15% year to year variability.
is
found
rates.
to be
100
K \ r 1 C ftC i
v - r y r y r s f_ e=> '£■
polar
HjA dry
temperate
subtropical
C maritime S3 E wet
tropical
0Gmaderate
|H wet
LATITUDE (deg)
POLAR.TEMPAR'TROPI «TEMPARATE • POLAR
ATE
CAL
B moderate^ Dcohtinent’H F arid
180
1?0
*20
90
60
30
o
30
6o
9O
120
150
LONGITUDE (deg)
(Ki.n.s.i
I : AI M' A l I
I ’A ! I
D l:-. I l : t i : ' I f l M t ( ■: G t I
o
HAL
Mo I H L )
1B0
page 138
chapter 5
5.3.2 WITH CCIR MODEL:
Based on rainfall
rate measured
i£>83 has formulated a model
14
rain
climatic
at different
in which the world
zones(Fig.5.3.2a).
Table
rainfall
distribution for these zones.
rainfall
rate
rainfall
distribution
regions
of
distribution
CCIR
with
this
over Guwahati
model.
model CPD of rainfall
places,
Fig.5.3.2b
is divided
5,3.2
presents
Comparision
model
falls
CCIR., in
that
in between
the
the
of observed
indicate
presents
into
the
N and
measured
P
and
rate, where, CPDs for N and P regions are
compared with the observed CPD.
TABLE
5.3.1
R A IN F A L L R A T E D IS TR IB U TIO N V A L U E S (M M / H r ) V E R S U S P E R C E N T O F Y E A R
R A IN R A T E IS EXCEEDED :
percen t
GLOBAL MODEL
of
ra in
year
a
c lim a tic
re g io n
b
c
dl
d2
d3
e
la
f
h
0, 001
28
54
80
90
102
127
164 65
i 129 251
0. 01
15
19
23
37
49
63
93
23
i 67
147
0 .1
e
6
7
11
15
22
35
5
i 22
51
1
l
1
1
2
3
4
4
1
j3
6
GUW AHATI
T a b l e 5. 3. 2
R a in fa ll
in te n sity
e x c e e d e d Cmm^HriCCIR model
percen t
of
a
b
1
-
1
0.1
2
3
8
12 15 19 22
n
tim e
n
i
0. 001
d
e
f
g
h
j
k
1
m
n
P
-
3
1
2
-
-
-
2
-
4
5
j 12
5
8
6
S
1210
20
12
15
22
35
;85
28 30 32 35
42
82
63
95
i 145
78 85 83 55 100 150 120 180
;250
22 32 42
A
" ■1
l 70
Gu w a h a t I
30
0
30
LATITUDE
60
page 139
o
U >
i
eo
1
I
so
I
I
n
I
o
I
leo
I
) ________
iso
LONGITUDE
RAIN CLIMATIC ZONES(CCIR 1983)
fig .5.3.2a r a in c l im a tic z o n e s
CCCIR
m odel)
14Q
lint all rata in mm
chapter
af
I
FK3.53.2b
T i rv- PL
1
•
>"
^ H a rc i
c o m p a r is o n o f o b s e r v e d
WITH CCIR MODEL
!
y—
CPD
A
w_ 's_x'
o y
v_ X
' '••
'—
'
V_y
o f r a in f a l l r a t e
v_y
v_y
page i41
chapter 5
5.3.3
WITH REPORTED CPD FROM TROPICAL COUNTRIES:
Fig 5.3.3 shows the CPD of rainfall rate reported from Malaysia,
CZainal
et.pl,
f£?9£?3Brez i 1
CMacial
et.al,
iQQOy and
Nigeria
CIppelito, tQS7'J along with that for Guwahati and Calcutta.
R
The
for these places are 125, 120, 70-100 , 90-120 and 80mm/hr
U •v X
respectively.
It is wise to point out that the appearence of a
range of variation in rainfall
rate at Guwahati
is because of
the type of rain gauge used for measuring the rainfal1 rates.
In Malaysia,
et.at
with
rainfall
tipping
rate
measurements
bucket
type
of
were
rain
made
gauge
by
of
Zainal
1
min.
integration time,for the period of one year. Their observations
show that the highest amount of rainfall occurs in the month of
November with total
annual
rainfall
of 2400-3200mm.
In Brezil
the rainfall rate measurements were also made with similar type
of
rain
gauge.
highest
Here,
rainfall
the
month
and the total
of
September
annual
experiences
rainfall
the
is around 2500
mm. The total annual rainfall over Nigeria is 2200-2600 mm where
month of April experiences the highest amount of rainfall. These
data
are
from
rainfall
rate
measurements
made
over
Ile-Ife,
Nigeria for the period of 3 years with fast response rain gauge
of
10
ofJune
sec.
integration
experiences
time.
maximum
Whereas
amount
of
in
Guwahati
rainfall
and
the
month
the
total
annual rainfall varies from 1900-2400.
From the above comparative study of rainfall over Guwahati with
that reported
from tropical
countries
shows
that Guwahati
can
also be grouped under the tropical zones.
5 .4 COMPARISON OF RSD WITH THE MODELS:
5.4.1 WITH NEGATIVE EXPONENTIAL MODEL:
The
negative
exponential
distribution CMP)model
for
RSD
is
described in chapter 2. This model gives the RSD in the form
-\D
N (D) = N e
___
....
....
--..(5.4.1)
o
p a g e i4 a
'.h a o tr B i-
b
b
Cj
O
CJi
O
O
j
o
o
o
ClJ
i.N
O
O
cj
UJ
rainfall rate in m m
O
o
r'J
in
c si
p.~\ 1
>6
FIG.
7 !rn
0 . ■U'U I
0
:
5.3.3
o
m
CO M P A R ISO N OF OE«8ERVED
T H A T OF TR O P IC A L C O U N TR IE S.
■
y
•\
o
CPD
n ^vy
p
■
''1 —
b_/ V_J ^—/
O F R A IN FA LL R A T E WITH
c h a p te r
page
5
14.3
o
where, N is constant and X is a function of rainfall rate,
The observed RSD
is studied
in comparison
with
this
model.
A
representative plot (in which the RSD for an event of drizzle is
studied)
event
is
estimated
is presented
around
from
10
in fig. 5.4.1.
mm/'hr
lognormal
The rainfall
occurred
model
on
6
is also
which will be used in the next section.
may
rate for
1993.
shown
in
the
The
the
RSD
figure
,
It has been noticed that
for drizzle the observed RSD is in fair agreement with RSD given
by
the
Negative
thunderstorm
exponential
model.
is also compared with
Similarly
the model.
comparison of the RSD for thunderstorm.
as
is
noticed
in
most
of
the
the
RSD
Fig 5.4.2
for
shows
This type of variations
thunderstorm
cases,
stands as a representative of thunderstorm RSD.
this
case
It can be seen
from this graph that observed RSD for thunderstorm
is entirely
different from that given by the model. The event occurred
the month of April with an average rainfall rate of 60mm/hr.
indicate
that
this
model
cannot
thunderstorm drop size distribution,
describe
the
in
It
observed
but makes a good agreement
for the RSD of drizzle,
5.4.2
WITH THE
LOGNORMAL MODEL:
A detailed account of this model is presented in chapter 2. This
model
gives
the RSD
in terms
of
total
number
of
drops
(N^),
geometric mean drop diameter (D^ ) and the standard deviation of
the drop
diameter
(<?).
The observed
attenuationis studied
comparison with the RSD given by this model. Fig 5.4.1,
give
the
RSD
for
drizzle,
and
thunderstorm
in
5.4.2
respectively.
In
fig.5.4.3 the observed RSD for widespread rain has been compared
with
lognormal
Lognormal
model
thunderstorms.
lognormal
for
the
model.
It
gives
the
However,
model
can
best
probably
parameters
N , D^,
1ognorma1 mode 1.
for
seen
fit
from
for
For widespread
can
rainfal 1 rate,
be
the
rain,
be accepted.
a
are
thunderstorms,
The
given
along
the
figure
observed
RSD
The RSD
given by
suitable
below in
with
that
those
for
values
terms of
given by
/ a r>
^
page
144
Observed RSD
Negative exponential model
Lognormal model
r£
fig.5.4.1 c o n p a r s io n of observed
EXPONENTIAL MODEL
RSD
for d r iz z l e w it h negative
i-
0 3
O
L£
h-
0 3
D
a
u
a
z
X
H
CiC
o
Li.
0 3
o
II
o
:? 03
a
o
LU
.J
C
H
a
D'
a
LlI
>
a
LJ
.j
*<
—
c*
o
7 Z
6
»
C
D
CO
etc j
< X
CL t
o
3
O
aID
•r*’
01
O
3 3
a
il)
<
z a
<
d cr
an Bl
oj «
It'
"X\
fl
tJl
‘ ■t
mrn1)
n
;y
Q
ts
(rn
-*
d e n s ity
4
C
T 5
Q)
C
tt
O
a 0
X E
a?
fO
B
ou
c
TU?
o
J
o
Z :i
a
o
0J
iZ
iriCD
number
145
page
Observed RSD
Lognormal
FIG.5.4.^ CONPARSiON OF OBSERVED
iim
t i
W im
i
i
a
i
M
A
r t n
L U U IN U re riM L 1‘ IU U L L
RSD FOR SHOWERS
model
1do
page l47
chapter 5
Pa r a m eter
Ob s er v ed
Lognorm al
F a n g &Ch en
180 R0 3 9
nt
1.12+0.18
dm
sy
5.5
m odel
4 .6 R ° 5 5
ln R
0 .2 2 + 0 .3 9
e x p C 0 .4 8 -0 .0 3 R )
A j o y K 1985 >
11982)
108 R ° 3 6 3
ln R
-0.19+0.19
e x p C 0 .5 -0 .0 0 3 R )
ln
0 .1 3 7 -0 .0 1 3
R
ln R
RAINDROP SIZE, SHAPE AND MICROWAVE ATTENUATION: A MODEL
Water
is
chemical
physical
a
complicated
properties.It
structure
material
has
which
a
with
distinct
definite,
depends
on
the
molecules with respect to one another
and
though
and
varied
changing,
orientation
of
to the molecules
its
of
the dissolved substances.
Water consists of one Oxygen atom and
d istance between
the centre
each of Hydrogen atoms
two Hydrogen
of Oxygen
is 0. 9 A.
atom and
atoms. The
the centre
The ang 1e formed
of
by the two
Hydrogen atoms is 105°>
The arrangement
of water molecules
in its
liquid state
shifts
continuously. The angle between two Hydrogen atoms do not remain
fixed at near right angles, but is variable i.e, the molecule is
flexible.
A Hydrogen atom does
with the Oxygen atom to which
not share
its electron
it is attached,
equally
the electron
is
closer to the Oxygen atom than to the Hydrogen atom. As a result
the
Hydrogen
negatively
atom
charged.
is
positively
Therefore,
charged
in
an
and
the
aggregation
Oxygen
of
is
water
molecules, we may find an Oxygen atom surrounded by five or six
Hydrogen atoms and a Hydrogen atom is surrounded by as many as
three
Oxygens.
In
this
closely
knit
flexible
structure,
the
hydrogen atoms constantly shift their positions and displace one
chapter- 5
page 148
another.
zipper
Each
such
fashion
consequence
of water.
Because
displacement
throughout
in viscosity,
of
the
above
is
the
propagated
liquid.
in
This
a
chain
effect
has
or
a
dielectric constant and conductivity
features
of
the
water
molecule
the
attenuation of microwave signal by rain is a complex phenomenon
and therefore the attenuation depends on shape, size and also on
vibrations and polarisation developed inside the drops.
Taking this background into consideration, many workers tried to
explain the microwave attenuation in terms of raindrop size and
shape.
The
first
attempt
made
with
the
flour
electromechanical
sensor
adopted.
Recently,
for
measuring
method.
the
Then
, electrostatic
more
raindrop
sophisticated
in
size
late
sensor
sixties
methods
techniques
of
was
were
measuring
the raindrops with optical detection method are developed C S fOW
and
1991■
Barbara
et.al
19930,
A
detailed
account
of
these
techniques is given chapter 3. The photographic method for
measuring
raindrop
shape
are
used
by
many
workers
CBlanchard,1950; Magano,1954; Jones >1959; Pruppacher and Beard,
1970; j. These measurements show that the raindrops smaller than
2mm in diameter are spherical
2mm <D <5mm range,
spheroidal
cardoids.
one
of
oblate
Along
with
of
such
terminal
the shape of raindrop evolves through oblate
to
calculations
in shape. For larger drops i.e in
spheroids
the
velocity
the
is
flattened
experimental
the shape of
theories
of
techniques,
raindrop are also
shape
of
calculated
the
by
solving
of
a
drop.
It
is
almost
impossible
to
almost
theoretical
progressed.
raindrop
describing the balance of internal and external
surface
base
falling
the
In
at
equation
pressure at the
to
solve
this
equation analytically. Assuming that deformed drops were oblate
spheroids,
Sphilha-us C19480 calculated the axial
large drops.
Irani
C195Q3
calculated
the
shape
ratio of fairly
of
small
drops
with the assumption of potential flow around the drops and found
that
the
deformed
drops
were
wel1
approximated
by
oblate
chapter 5
page 149
spheroids,
Pruppa.ch.er
conditions
that
arid. Fitter
satisfy
the
C19713
internal
have
and
formulated
external
the
pressure
balances at the surface of a raindrop. Oguchi C19773 has further
represented
the
cross
sectional
contour
of
raindrops
by
the
F our ier series.
The influence of shape of the raindrop on microwave
propagation
was
Oguchi
investigated
Mosoya,
1974■
that,
the
by many
Morrison
forward
scattered
signal
or
in
researchers
and
Cross.
backward
very
COguchi
19747.
These
scattering
much
i973;
sensitive
to
(cf.chapter
2
and.
studies
tell
amplitude
of
the
the
of
the
2.3),
the
shape
raind rops.
As
we
have
seen
in chapter
2
scattering cross section of spherical
Q
=
But
raindrops may be given by
80/3 x k4 |(£-1 )/ <i'+2).|2 x a6
for
the deformed
drops
scattering
cross
section
cannot
calculated
expressions
be
like
less
deformed
Watson
and
applicable.
various
drops,
Arbabi
For
section
Rayleigh
however exist for small
to give
Morrison
or
results
2
For
cross
1953
19733
we
is
have
section
forthedrops
a3
larger but
COguchi,
These numerical
least
the
Approximate
CStevenson,
range
(cf.chapter
at
also.
Chu,
millimeter
give
scattering
method
and
(5.5.1)
cannot
raindrops.
perturbation
techniques
good
the
analytically
calculate the scattering properties.
seem
relation
approximation
the
1973;
....
and
and spherical
microwave
numerical
....
the above
correct
section
1973;
mostly
to
2.1.4!
use
to
techniques
with
aQ<3mm
where a^ is the equivo1umetric diameter of the raindrop and with
axial
The
ratio a/b < 0.7 in the 4-35 GHz range.
two
scattering
meteorology
are
the
backward scattering
with
minor
and
properties
forward
communication
scattering
amplitude f(K
major
in
semi-axes
amplitude
and
radar
f iK ,k;^ i and
,-K^). For an oblate spheroid
a and
b respectively,
the
chapter 5
page t50
forward and backward scattering amplitudes are given by
k2 ab2
is-1)
(*-l)
2
--------------- [ -------------------- cos ot + -------------------- s in 2os]
«*
fv
3
fh =
.
1+(£~1)A,
1+ C^-1)A0
k2ab2/3 [
where, f^, f
....
....
(5.1.2)
are forward scattering amplitudes for vertical and
horizontal
polarisation
constant,
k,
is
respectively
propagation
and
constant,
s
A^
is
the
and
dielectric
A^
are
the
geometric constants that are given by
A a = [Cm2/ y(m2-l)3.tan~1 (ym2-l)-l]
A2 = l-2Al
---
---
---
---
... (5.2.3)
m = a/b
---
---
---
...
... (5.2.4)
The real and imaginary parts of the forward scattering amplitude
are related to the phase shift and attenuation of a wave when it
passes through a rain slab .
Fig.5.5.la presents the forward scattering amplitude at various
drop size at 11 GHz and for horizontal polarisation by adopting
the axial ratio a/b given by Jones, 1959 (fig.5.5,lb).
The measurement of rain attenuation on terrestrial
or satellite
microwave links as well as on radars and interpretation in terms
of scattering properties of raindrops has renewed the interest
on
possible
raindrop
oscillations.
JaruesC i9592
and
Beard
et.a.lCiQS£2 have emphasised the existance of oscillations in the
raindrops
while
falling
under
gravity.
The
presence
of
oscillations would make the scattering phenomenon different
view of the
asyemmetric,
in
non-linear and non-hormonic nature of
these oscillations and the shape of the oscillating drop will be
different
from
its
non-oscillating
equilibrium
shape
i.e.
c h .a o t
CCLg&
DROP DIAMET
15!
i V I IV I /
FIG.55.1A. FORWARD SCATTERING AMPLITUDE VERSUS DROP DIAMETRER
DROP DIAMETER. ( MM )
/
F IO .5 5 .lB .
A X IA L
R A T IO
Ca/b)
VERSUS
DROP
D IA M E T R E R
chapter 5
page 153
Pruppacher-Pitter shape.
Each
of
the above
mentioned
properties
of
water
molecule
has
different significance at different frequencies. The rotation of
the*dipole of the water molecule produce rotational
spectra at 22,
developed
183.3 and 324
inside
reflectivity
the
raindrop
parameter
depolarization
GHz.
at
effects
are
reported
millimeter
are
millimeter wave lengths.
Similarly the
found
However,
absorption
oscillations
to
affect
the
and
the
in centimeter
and
wave range
both
our study at 11 GHz
(X = 2.7
cm) the wavelength is too far away from above frequency range to
observe
the effects
properties.
due
Therefore,
to raindrop
oscillation
we
explain
try
to
or
rotational
relatively
large
attenuation at 11 GHz in terms of dielectric polarization
raindrop
(which
is
likely
to
depend on shape and
size
in a
of
the
drops).
We have measured the shape and size of the raindrops with the
instrument developed for the purpose.
system are given
The
observed
in chapter 3
shapes
of
the
(cf.
Technical
chapter 3,
raindrops
are
details of the
section 3.5.4).
presented
in
next
section and the results are compared with those given by above
mentioned workers.
levels
of
Finally,
attenuation
in
the explanation
rain
for getting
containing
large
higher
drops
is
d iscussed.
OUR OBSERVATIONS ON RAINDROP SHAPE:
Fig.3.5.14 of chapter 3 shows
response
to
various
drop
the output
sizes.
The
of our
shapes
instrument
of
in
raindrops,
reconstructed from the signatures recorded by the instrument are
also shown in figure 5.5.2.
The raindrops of less than 2mm are
observed to be spherical.
But,
shown
in type 2
present
equal
to 1mm. As the drop size increases the x-axis of the drop
is elongated.
is also
on few occasions even the shape
for a diameter
We have seen an ellipsoidal
less
shape for
than
or
the drops
/— 2-i /-r
i* £=> •
/ - x T .O ils
0
2)
A
0
1rnm
—
►I
1mm
3)
A
0 ^Imm-
±
2 mm
0
T
1mm
1mm
4)
A
A
Q tS m m
( ^ ) 2 mm
— H
H—
2m m
HO.
)
6
___ A
C
)
2m m
T "
3m m
OBSERVED
A
h-
-H
3-5 mm
HARES UF RAINDROPS
page 155
chapter 5
greater than 3mm in diameter.
However,
it was
not possible
to
receive the shape of the third axis of raindrop. So, we confine
our discussion to 2 dimensions only.
COMPARISON WITH THE THEORETICAL SHAPES OF RAINDROPS:
A raindrop falling down in air under gravity takes a form such
that the internal and external pressures balance with each other
at
surface
of
formulated
the
drop.
Fnippacher
theseconditions
and
arid. Fitter
havegiven
C iQ7 iJ
the
shapes
have
of
raindrops. Ogichi CiQ77'J further represented the cross sectional
contour on a vertical plane in the symmetry axis of drop by the
Fourier series,
r = a o ( l + F“ n C n . CosnS),
n =0
with
C 1= 0
in the polar coordinate system shown
....
....(5.4.1)
in fig.5.5.3.
His
results
show that smaller raindrops are close to spherical and that with
increasing
diameter,
the
shapes
turn
into
asymmetric
oblate
spheroids with flattened bottom. Fig.5.5.4 presents our observed
raindrop shapes along with those calculated from the theory. The
shaded shapes in the fig. are the ones we observed and the whole
drop outline is the theoretical
shape.
Therefore,
the observed
and theoretically predicted drop shapes agree fairly well.
In
addition
to
above
mentioned
factors,
depolarisation
of
incident radiowave is one of the important effects that raindrop
cause on microwaves.
nature
of
the
This affect arises due to the nonspherics 1
raindrops
electromagnetic wave
<Oguchi
and
Mas-oya,
1Q74i>.
is incident on a oblate shaped
raindrops,
the polarisation direction of the incident wave rotates
an angle & . as shown
in fig.5.5.5,
and changing
When
through
polarisation
state of the wave. E^ ,E^ in the figure represents the reference
polarisation states and E , E, denotes rotated polarisation.
v
h
/-- r : _<-** <•-) t tZi -r-
y
The x anti y axes are placed on a
vertical plane through the gravity
center.
FIG.
5.5.3 SHAPE OF THE RAINDROP (O g u c A i ,
IQ ? /')
LUUU7
1
H—
LUUJl,
\
-^ -r
h— — 1
T H E O R E T IC A L S H A P E S
FIG. 5 .5 .4 C O M P A R IS O N O F O B S E R V E D R A IN D R O P SH A P ES WITH
«-
rH —zrm -r^
-J-
—
r-’ I'D
*0
r~Y
ft
H
FiO. 5 5.5 D EPOLAR ISATION BY RAINDROPS
chapter
page
5
The result of depolarisation of radiowave manifests
of energy
from one
polarisation
state
to another
15Q
in transfer
or
in other
words, there will be a net loss of the energy in the process of
depolarisation.
It is considered
the interaction of
that depolarisation
is due to
incident radiowave with the electric dipole
of the raindrop. Therefore, considering these factors we now try
to
explain
the
microwave
attenuation
by
interrelating
effects with possible dimensions of the dipoles
various
shapes
resonance
and
sizes.
conditions
at
Our
first
which
aim
maximum
is
the
in raindrops of
to
calculate
energy
tranfer
the
takes
place.
Now,
we
examine
the
association
of
the
shape
and
size
of
raindrop with the observed attenuation. As we know that when the
electromagnetic
wave
is
incident
dipoles
in raindrop try to align
field.
Therefore,
polarisation
than
the
if
a
drop
on
raindrop,
in the direction
size
of
rain
electormagnetic
field!
parmanent
of
incident
increases,
that might have been acquired by
incident
the
the field
for
example
any
other
, field
between the charged thunderstorm cloud and the earthlmay develop
in
drop
would
raindrop
shape
size
may
not
show
and
up
as
shape.
affect
a
dipole
The
dipole
the
microwave
deviates from spherical shape,
will
make an
length
moment
signal
depending
in
the
but
as
on
spherical
its
shape
the dipole moment of the droplets
interaction with
frequency on the drop is equal
dipoles,
of
incoming
signal.
If the applied
to the oscillations made by the
then we may expect a resonance condition where maximum
scattering will takes place.
D E T E R M IN A T IO N O F T H E R E L A T IO N B E T W E E N D R O P D IA M E T E R A ND R E S O N A N T
FR E Q U E N C Y :
Let
us
consider
that
a force
equal
to
eE
is applied
on
the
raindrop. Because of this force the dipoles try to align in the
direction of applied field. The restoring force developed inside
the drop by the dipoles is given by
R = -Kx ....
....
....
where x is the dipole displacement.
....
....
<5.4.2)
ch a p ter
p a g e 160
5
From the coulombs law and Debey's treatment, it can be shown that
2
e
__________
K =
4n £■
....
....
....
....
(5.4.3)
d3
where, e is charge of electron,
is the free space permittivity,
£
D is the drop diameter or dipole length.
The frequency produced by the dipoles can then be calculated by
dividing K by the mass of the electron.
Therefore
= K /m .. . .
....
In the real 3D situation,
....
....
(5.4.4)
the drop size will have dipole axes of
different lengths along the x,y and z directions and accordingly
the
resonant condition
frequencies.
A
few
is expected
samples
of
to spread
orientation
over
of
a range
dipoles
in
of
the
observed relevant diameters are shown in fig.5.5.6. The dropsize
and
shapes
11
GHz.
coded as 4,5,6 in the figure
Therefore
considerable
number
any
of
rain
these
can produce resonanceat
event which
type
of
can
raindrops,
produce
will
a
cause
a
large attenuation at 11 GHz. The shape and size of raindrops has
been studied in different types of rainfall.
number
density
of
drops
of
different
Fig.5.5.7 show the
shapes.
The
numbers
(1,2...4) on X-axis corresponds to the drop shapes as coded
figure 5.5.6.
in
It can be seen from the figure that thunderstorms
possess a considerable number of drops of shape coded as 4 in
the
figure.
Showers
also
posses
these
type
of
drops
but
the
drops of this shape are few in number. Where as drizzle contains
the
drops of
shapes
coded
as
1,2
and 3.
Therefore
from
this
study we may say that the dielectric displacement is one of the
factors that control the attenuation and
large
deformed
drops
attenuationresonance
at
which
11 GHz.
attenuation as high as 45 dB
due to the presence of
can
The
produce
thunderstorms
a
large
which
gives
leading to link blackout
goes
to
support this view.
fi c :
U .U
unnn
i iki/2
l ’ IV L / L .I_ L .IM U
f\tr o
VSI
a ikic a i i
IX M IIII H U . L
Based on the observational
D A T IT
I \ m u
n tC T D ID I ITir\M nV/lTD
U IO
I l\ IU U ! IV I1
results,
W ¥ I__I\
A C O AM
\/A I I CV.
r t O O M I ' .............................
modelling on rainfall
rate
1
R A IN D R O P SHAPED
R E S O N A N T FR E Q U E N C IES
193 GHz
©
I
a
i
N
2 mm
C£
CO
N
X
o
CJ
ro
2 mm
|
OJ
ro
€ £ >
1 mm
8 G - 11, 7G GHz
3 mm
8G - 11- 32 GHz
3 mm
FIG.
5.5.6
R A IN D R O P
SH AR ES A N D C O R R E S P O N D IN G R E S O M A N l
i
FR E Q U E N C IES
r - } \ r~f r-y f
•- *
v. w
*
t-
page - 1 6 2
—
- — — Thunderstorms
------ Drizzle
n u m b e r density
--- * ■ Showers
1 2
5
V Z
v
j .
j
.
(%
j
” 7
1. /
a i* <
/ A O
k
i N
it
I*. j r " . r * r " t
u
r i D
# - . » * » r » r r »
L . U
U
L U
t i K
i /* t i T w
u
u
i
I I N
l
r
i n
o
i
I V
R
J . U
i
F I
. U
t
f ?
. U
/• • r ”
r s
u
R
r
a
A
ik
H
r » i r T T " r * « n f c
N
U
R
u
r
o
u
r
u
i r
r
l
k
l
i n
a
it
i
o
n
A
r * i r * tr %
r
t
u
attenuation
X)
0
n
m
chapter 5
page 1G3
distribution is done which can provide necessary information on
rainfall rate distribution to the system designers to install
future communication networks in North Eastern India. This model
divides the Assam state into 5 climatic zones and each climatic
zone is defined in terms of R_ _,and total annual rainfall.
0 . 01
This model make use of the following parameters:
1. The total annual rainfall in mm ,
2.
Total
rainfal1
during
premonsoon
thunderstorm activity is present.
months
when
high
The second parameter is used to estimate the portion of rainfall
contributed
by
the
thunderstorm.
Since
observational
results
show that almost all rainfalls in premonsoon are associated with
thunderstorm,
the
premonsoon
months
contributed
by
portion
gives
of
the
rainfall
contributed
approximate
the thunderstorms.
The
share
following
of
by
the
rainfal1
formula
gives
the thunderstorm ratio.
M
ft =
P
-- x 100 ....
M
---
....
---
..(5.6.1)
a
The thunderstorm ratio calculated from this formula is compared
with those observed
during
91-93 and
method gives the share of rainfall
it
is noticed
that this
due to thunderstom with an
accuracy of 80% (the 20% inaccurecy arises due to year to year
variability. Table 5.6.1 shows the estimated thunderstorm along
with that observed during 1990-1993.
The cumulative probability distribution of rainfall
is then can
be estimated as follows.
p(R> = 2000x(p+q)/90
where,
p(R>
is
the
a specified rainfall
---
....
probability
--of
percentage
of
time
rate exceeds in a year.
p = 0.49 x ft x exp(-0.09R) ....
and
a
....(6.6.2)
....
..(6.6.3)
q = 1.4 ( exp(-0.25R) + 1.8 (exp (-1.63 R)) ..,.(6.6.4)
page t64
chapter 5
Fig.5.6.1 shows the CPD of rainfall
model
and also observed
during
rate as determined by the
1991-93.
The assessment of the
model with the observed rainfall rate shows that this model can
give a CPD of rainfall
rate with a 20 percent of year to year
variabi1ity.
Now, adopting this model an attempt is made to classify rainfall
zones
of
Assam
Table
5.6.2
shows
the
five
climatic
Assam with CPD of rainfall rate over those zones.
zones
of
It can be seen
from the table that most of- the districts of Assam come under
zone
D
where
^is
85-90
mm/hr.
The
districts
Karimganj,
Kachar, Guwahati, Lakhimpur and kokrazar receives highest amount
of rain fall with R0_01- 10 0 - 1 2 C W h r .
Therefore,
designing of
microwave links over these regions must be made with proper care
in order to maintain 0.01 % raliability.
link
is
to
be
designed
over
these
If a 11 GHz microwave
regions,
the
path
length
shound not exceed 10 kms to have 0.01 percent reliability, since
the
attenuation
that
exceeds
0,01
percent
of
time
approximately 5 dB/km corresponding to the rainfall
mm/hr which may some
times
cover
more
than
10 Kms
would
be
rate of 100
horizontal
distance and if higher frequency links are to be installed the
path length must still be shorter.
T able 5.6.1
OBSERVED ft
1991
1991
1993
8.2%
6%
7,9%
MODEL ft
8%
.
- w
-
page
w
rate
in
w
J
. O
rv
fig 5.6.1
O
C
1
')
\j
!
n;
ti m e
Observed and model
■) r (r '* i
CPD
165
CRq
239
230
a)
Z
242
165
26 0
175
95
30
20
10
(R q q ^«95)
D
Assam
275
180
100
35
20
10
CRq 01-100)
a)
#■*t.*r~ C"
/.U I N L l_
3.
t
FI
f A J
a
u A P v iniki..nir>
r iu*
rr;»
Ka r im g a n j ,
Dib r u g h a r .
*\ I
a)
C
zone
Q 3NOZ
l) J O R H A T , J )
b)
b)
Ca c h a r .
T e jp u r ,
)
d)
c
Gu w a h a ti
S o n it p u r
S lV A S A G A R . K ) U T T A R C A C H A R
Du b r l b ) T e z u , c ) Na g a o n , d ) Ka r b ia n g l o n g
e ) Ba r p e t a , f > Na l b a r l g ) D a r r a n g , h ) Go l a g h a t
a)
Go l p a r a
160
155
90
26
20
85
15
a 3NOZ
l
80
18
15
10
q ^®85)
10
CRq
G
CRq q ,j"90>
10
0
B
o f r a in f a l l z o n e s o f
T a b l e 5.6.2
5
q ,j« 8
A
3NOZ
D IS T R IC T S U N D E R
1 0
0 1
0 01
0 001
0 0001
10
EXCEEDED
% O F TIME
Cl a s s if ic a t io n
v
page 167
chapter 5
5.7 CONCLUSIONS:
The
observed
results
on
rain
attenuation,
rainfall
rate
and
raindrop size distribution are studied in comparison with the
standard models and with the results reported by other workers
from various places.
The observed rain attenuation
is compared with the CCIR model,
Lognormal model and with the results reported by the NPL, Delhi,
propagation
group
at
study
IRPEL,
thunderstorms,
the
CCIR
group at DEAL,
Calcutta.
In
Dehra Dun
case
of
and
radio
attenuation
physics
caused
by
the observed attenuation values are higher than
values
and
for
attenuation
caused
by
drizzle
and
showers CCIR values are in agreement with the observed values.
The attenuation over Delhi, Calcutta and Dehra Dun are well
in
agreement with the CCIR values.
The CPD of rainfall rate observed over Guwahati is compared with
the
Global
model
and
the
CCIR
model.
According
to
classification of rain climatic zone made in the Global
Guwahati
through
is
the
put
fast
in
G-zone.
response
But
rain
the
gauge
observed
are
rainfall
higher
than
the
model,
rates
those
predicted by the Global model. The observed rainfall distribution
is somewhere
in between N-region and P-region distributions of
CCIR model. Comparison of CPDs of rainfall
rate over Guwahati
with those reported from Nigeria, Brezil and Malaysia shows that
all these places can be grouped under one category.
A simplified model is presented in which the probable reason for
receiving large attenuation during thunderstorms is explained in
terms of the dielectric and acquired polarisation properties of a
water molecule.
REFERENCES
chapter 5
page 168
Re f e r e n c e s :
1.
Ajoyi.
Go
implication
C1982D
to
"
Trashorizon
radiowave
propagation
propagation
of
UHF
frequencies in Nigeria" The Nigerian Engineer
2.
Ajoyi.
GO
C1985J
Characteristics
attenuation and phase shift"
and
and
its
higher
Vol.17, pp 72-75.
of
rain
induced
Internation Journal of Infrared and
mm waves , Vol.6, 771-806.
3.
Ajoyi.
GO C1990J
" Some
aspects
of
their effect on microwave propagation"
tropical
rainfall
International
and
Journal
of
satellite communication, Vol5, ppl63-172.
4. Beard.
KV, Johnson.
DB and AR.
Jamesson C1982J
"collisional
forcing of raindrop oscillations" Journal of Atmospheric science
5.
Blanchard.
terminal
DC
C19503
velocity
in
" The
air
"
behaviour
of
Transactions
water
of
AGU,
drops
at
Vol.31,
pp
836-842.
6. CCIR C1982J " Propagation in non-ionized media.
Rep.
721-1,
In recommendations and reports pp 167-181.
7. CCIR C1983e-3 " Prediction of attenuation due to rain on an
earth-space paths, ANNEX.3, ITU, Geneva.
8.
CCIR C19883
"Conclusions
of
the
interium
meeting
of
study
group 5- Propagation in non-ionized media, DOC 5/204-E.
9. Chandra.
the
M,
influence
Jain.
of
YM
and PA. Watson C1983J " Asssessment of
raindrop
radar measurements. 3rd.
oscillations
on
dual
polarization
International conference on antenna and
propagation, proceeddings No.219, pp 34-37.
10. Crane RK C19773 " Prediction of effects of rain on satellite
communication systems' Proceedings of IEEE, Vol. 65 pp 456-474.
11. Crane RK C1980J
" Prediction
of attenaution
transactions on communications Vol.COM-28,
12. Crane.
by
rain
" IEEE
pp 1717-1733.
RK C19823 " A two component model
for prediction of
attenuation statistics" Radio science, Vol.17 ppl371-87.
13
Fang. DJ
and
centimeter/millimeter
CH.
Chen
waves
along
<11982
"Propagation
')
a
slant
path
through
of
rain"
Radio science , Vol.17, pp 989-1005.
l/ i
i H
»
T j_
h|
*A
jC
rA
i ♦
T IT o r n4
hy o m o
4U
4
V
aC
n
rl
l4
X
M o n -a K o
«
»
»
1
4
4
II
K
4
»
f 1 Of-? A 1
* I n•f & r & n J
C'IC
04
o
fO
C
4
4
4
V
1
4
4
4
C
1
M
V
.
4
J
.1
—
»
pag& 169
chjapter 5
raindrop
size
distribution
from
rain
attenaution
statistics*
IECE(Japan), Voi.E67, pp219~226.
15.
Imai.
I
C19SQ3
"on
the
velocity
of
raindrops"
ffalling
Geophysical magazine (Tokyo) vol.21 pp 244-249.
16.
Ippolito
LJ
C19813
'Radio
propagation
for
space
communication systems" Proceedings of IEEE, Vol.69.
17.
Jones,
DMA C19593
" the
shapes
of
raindrops” Journal
of
meteorology, Vol. 16 pp 504-510.
18.
Lin. SH C19793
"A method
for
calculating
rain
attenuation
distribution on microwave paths" BSTJ, pp 1051-1056.
19.
Macial. LR„
SA,
distribution*
Maura C19903
Internation
journal
"Tropical
of
rainfall
satellite
drop
size
communication,
Vol. 18 ppl87-186.
20. Magano,
C C1954.3 " On the shape of water drops falling
in
stagnent air" Journal of Meteorology, Vol.11, pp 77-79.
21.
Marshal
JS
and
WM
Palmer
Cl9483
"The
distribution
of
raindrops with size" Journal of meteorology, Vol.5, pp 165-167.
22.
Medhurst
comparision
EG
of
C19653
theory
"Attenuation
and
of
measurements*
centimeter
IEE
waves
Transactions
on
antennas and propagation Vol. AP-32, pp550~564.
23. Morrison. JA and TS. Chu C19733
" perturbation calculation
of rain induced differential attenuation and differential
phase
shift* BSTJ, Vol.52, pp 1907-13.
24.
Morris!on.
JA and
MJ.
Cross
C19743
"scattering
electromagnetic wave by assymmetric raindrops. BSTJ,
of
plane
Vol.53, pp
955-1079.
25.
Moupfouma
prediction
F
for
C19873
radio
*
More
systems
about
rainfall
engineerinh"
rate
Proceedings
their
of
IEE
part-H Vol.134, pp 527-537.
26.
Muller.
EA
and
AL.
Sims
Cl9683
"
Investigation
of
quantitative determination of areal precipitation by radar echo
measurement*
Tech.
Rep.
ECOM-001,
32FI 1 I, State water
survey,
U rbane.
27.
Oguehi.
T C19733
" attenuation
and
polarisation
of
radio
waves due to rain: calculation’at 19.3, 34.8 GHz" Radio science,
Vol.8, pp 31-38.
chapter 5
28. Oguehi.
oblate
page 170
T and Y. Hosoya C19743
raindrops
and
cross
" scattering
polarization
of
properties
radiowaves
due
of
to
rain" Journal of RRLIJapan ), Vol.21, pp 191-259.
29. Oguchi. T C19773
Pitter
form
" scattering properties of Pruppacher and
raindrops
and
cross
polarization
due
to
rain
Calculation at 11,13,19.3 and 34.8 GHz." Radio science,
:
Vol.12
No. 1, pp 41-51.
30.
Pruppacher.
investigation
drops
Hr
of
falling
the
at
and
KV. Beard
internal
terminal
C197G3
circulation
velocity
in
"
a
and
air"
wind
tunnel
shape
of
water
Quart.
J.
Roy.
Meteor.Soc. Vol.96, pp 247-256.
31.
Pruppacher.
Hr and
RL.
Pitter
C19713
" A
semiemper ical
determination of the shape of cloud and rain drops"
Journal
of
Atmos. Sciences. Vol.28, No.1 pp 88-94.
32. Raina MK and OS. lippal C19843
"Frequency depedance of rain
attenuation at microwave frequencies"
IEEE Transactions
Antennas and propagation, Vol. AP-32, pp 185-187.
33. Sarkar.
SK,
Ravindran.
VR,
Ramakrishna, M, Benerjee.
on
PK and
HN. Dutta C19783 "Rain rate measurements ane rain attenaution at
7 GHz in Northern India"
34.
Sarkar-.
SK
IJRSP, Vol.80 pp 47-51.
C19783
“
Rad ioc 1imato 1og ica 1
effects
om
tropospheric radiowave propagation over the Indian subcontinent,
Ph.D thesis , Delhi university, Delhi.
35. Sen. AK, Ultra. A, Mazumdar. KK„ Tarafdar. G. Ghosh,
Ghosh.
SN and JS SehraC198S3 Proceedings of international sympossium on
antenna
and
propagation,
held
during
20-22
August,at
Kyoto,
Japan.
36.
Sphl1haus
. AFC194S3
"
raindrop
size,
shape
and
falling
speed" Journal of meteorology , Vol.5 pp 108-110
37. Stevenson. AF C19533 "solution of electromagnetic scattering
problems as power series in the ratio dimensions of scater wave
length" Journal of Applied Physics, Vol.24 PP 1134-1142.
38. Stow. CD,
Bradley.
SG,
simulteneous
measurement
Paulson. K and L. Couper Cl9913
of
rainfall
intensity,
raindrop
and scattering of light' Journal of Applied meteorology,
ppl422-1434.
” The
size
Vol.30
page i 7 t
chapter 5
39. Tewari.
Kumar. KS amd C. Bahuguna C198S3
" Experimental
studies on rain attenaution characteristics of centimeter waves,
IETE, pp 130-133.
40. Watson.
PA and M. Arbabi C19733
"rainfall
loss polarisation
at microwave frequencies” Proc. of IEE vol,120, pp 78-118.
41.
rate
Zainal.AR,
and
drop
81 over. IA
size
and
PA.
distribution
Watson
Cl 9933
measurements
IGRASS'93 Proceedings, Vol. 2 pp 309-311.
in
■
Rain
Malaysia*
RESULTS, DISCUSSIONS
AND CONCLUSIONS
CHAPTER 6
-O fd
.P
i
RESULTS, DISCUSSION AND CONCLUSIONS
6.1 INTRODUCTION:
It
Is
well
established
that
the
microwaves
are
attenuated
rain due to absorption and scattering of the signal
as
well
whether
as
by
the
polarisation
chracteristics
one climatic condition
understood.
Recent
attenuation
(C r a n e
Ajcj
i 990;
of
effect
rain
of
developments
1 QQQj
Din
;
in
show
that
the
the
92;
t r u e , one
at tenuation
conditions
is
oath
predicting
The
w ith in
would
necessary
link
may
a
a
constant
rain
relatively
long
period
the
experimental
study
the
pool
of
knowledge.
(R S D ).
would
ia i n
of
c I 'ina t io
character
which
So
it
over
help
a
ir.
rain situations.
on
rain
This
with the results obtained on rain attenuation,
drop size distribution
If th is
the year.
has yielded a few noteworthy
common
~ L . <il ,
to other.
throughout
attenuation
rain
attenuation
be cause,
the rain attenuation at various
to
of
seasons 1 d e p e n d e n c e
1o c ation
the
three y e a r (1991-93)
added
given
examine
at Guwahati, Assam,
be
a
expect
no*: remain
to
for
a.1so
from
to be well
Ma.c l a i
rain
But,
change
subject
character is tics change from one climatic zones
were
do
is a question
Thar&k., 19
a .nd
by raindrops
molecules.
attenuation
to other or not.
,1 9 9 0
water
by
attenuation
resuIts
thg t ca
chapter
rainfall
and
deal s
rain
ch a p te x
page
&
173
6.2 OBSERVATIONAL RESULTS:
* R a i n a t t e n u a t i o n is c l e a r l y s e e n over
link
of
o p e r a t i n g at 11
GHz and a well
a 3.2 k m long
defined
seasonal
mi c r o w a v e
dependance
r a i n a t t e n u a t i o n is detected.
In
1991,
signal
suffered
attenuation
during
premonsoon
early monsoon months only. The black out (link cut-off)
corresponding
to 45 dB attenuation
are seen
only
and
events
during
pre­
monsoon months.
In 1992, though the signal suffered attenuation right from March
to August,
the large attenuation >30 dB are received only during
premonsoon and early monsoon months and most of the link cutoff
events are present during premonsoon period. A few cases of link
cut-off are also seen during the peak monsoon month of July.
Similarly,
in 1993, rain attenuation is seen right from February
to September. But attenuation >30 dB and link cut-off cases are
seen in premonsoon and early monsoon months.
I ti
s runiifie?x-y
we?
can
say
th a t
d u x - ir ig pxe?nucnsc>on a n d e a r L y
ts
h ig h
number.
this
*
a t t e ? r iu a t i o r i
siorii-uoi'i p e ? x lo d s a n d
dux
tn g nuonsoori m o n t h s
Linh
In
ch a p te r
4.3.3.
x e s u lt
3
s e c t io n s
cutoff
is
th o u g h
o von t s
4. 3 . 4
arid
s-eiere
mast
axe?
the?
xainfalL
feooi
ln
details
of
axe? g i wen
Simultaneous
average,
xam
observations
on
rainfall
indicate
that
on
the
there exists o n e - t o - o n e c o r r e s p o n d e n c e b e t w e e n rainfall
r ate
and
well
k n o w n empirical
individual
attenuation.
cases,
Attenuation
relation
Am
follows
aR^.
This
rainfall
m a y not
rate
by
a
be true for
where s u d d e n a p p e a r e n c e of l a rge a t t e n u a t i o n is
d e t e c t e d d u r i n g l o w rainfall
rates of thunderstorms.
The coefficients a and b are found to be sensitive to the type
of rainfall. The appropriate values for a and b for thunderstorm
are 0.035,
1.109 and for showers and the figures are 0.012 and
1.24
page
6
c h a p te r
The
respectively,
thunderstorms
produce
higher
1/4
levels
of
attenuat ion.
T h e re f o r e ,
U/i t h
oe
rO S peC t
a tte n u a tio n .
£ flic!?X~&
A
The
'L'\P%
stu d y
re v e a ls
d u rin g
to
on
th at
t
th a t
sclsj
t h u n d e r ' s t o r tris
r / iic r o w a i> e
tO
4 .4 .3 ’
se c t i o n
*
c an
n c ■n
a re
p r o p a g a t to n .
U n e a r i ty
o f
Is
d n
s i g n i f i c a n t
to w a rd s
m
a t Le n a c t t i o n
f Jn ■.
'uTusl&X' ^ t .orftvS
m o re
SS&CL
C o n t r o l l i n g
r a i n f a l l
th
l ’l
r a t e
^
Cj u 7"u C 2
4 .4 .5 .
th u n d ersto rm
G u w ah ati
prem on soon
a c t iv it y
e x p e rie n c e s
m on th s
w ith
a
d u rin g
h ig h e s t
peak
at
th e
p e rio d
th u n d erstorm
1 9 9 1 -9 3
a c t iv it y
M ay.
Precipitation associated with thunderstorm is very heavy and is
of
very
like
short
vigorous
surface
the
duration.
The
convectional
temperature and
formation
of
season of a year.
favorable
currents,
high
the height of
thunderstorm
are
atmospheric
conditions
specific
the freezing
abundant
humidity,
level,
during
for
a specific
It has been observed that these condition are
mostly developed during the premonsoon and early monsoon months,
over
Guwahati. In
chapter
4
this
feature
is
graphical !y
explained.
*
O b s e rv a tio n s
th u n d erstorm s
fa s h io n
d rops
is
C £mmi
c o u p le d
RSD
lo r
d is t r ib u t e
fro m
a tte n u a tio n
on
th a t
fo r
a re
a ls o
re c e iv e d
to
th e
d iffe r e n t
th e
o th er
ra in d ro p s
typ es
m or e
d u rin g
typ es
o f
in
a
r a in f a ll
c o m p le te
r a in f a ll.
abundant
in
prem onsoon
th u n d erstorm
o f
and
show
d iffe r e n t
M oreover,
la rg e
th u n d erstorm s.
e a r ly
th at
m onsoon
Lar ge
m onths
a c t iv it y .
In drizzle or showers smaller drops (<1 mm) are more and RSD
follow
a
negative
exponential
distribution
and
the
number
drops reduces with drop diameter. Where as in thunderstorms,
of
the
smaller drops are fewer in number. Drop size distribution follow
a lognormal
is
much
pattern giving rise to a median drop diameter
abundant.
Even
4
mm
drops
that
control
significantly, are also recorded in thunderstorms.
that
attenuation
page 175
c hapter 6
In showers,
the
large drops are present but fewer
in number
and
number density decreases with drop diameter.
In su.wfterv we can say that
fallow
negatiue
respectively
the P.SD far art sale and thanders tar tins
exponential
and
for
and
showers
lognortrunl
P:hD
is
in
d i s t n bu t ton
between
these
two
distributions as elaborated in chapter 3 section 4.5.
♦ Due to the presence of large drops >2 mm , the thunderstorms
produce higher levels of attenuation.
As
the
drop
size
increases,
section
increases.
directly
as
cm).
The
varies
the
sixth
directly
section
cross
the
drop
section
drop
higher
scattering
section
than
(measured
increases
but
the
that
of
cross
increases
diameter
also
diameter,
is
and
in
.since
it
increase
in
in
absorption
Fig.6.2.1 shows variation the ratio of absorption
section
diameter.
of
cross
with
cross
cross section.
scattering
power
absorption
scattering
cross
The
absorption
to the scattering
It
can
be
seen
than
1,
from
cross
the
plot
indicating
that
ratio
is greater
drops
< 1m m is mainly due to absorption
sizes
the
ratio
falls
section
down
rapidly
(a /a ) with dr on
a s
that below 1mm , the
the attenuation
. But
and
for
reaches
by
the higher
0.01
at
the
dr on
0.55
cm
d iameter. This
is
ind ioate tha t the attenuation by the large c : g r.--■
due to scatteri;-g of the signs I. ^ - - p'- ^ men or
mainly
oc c-urs more
)-e picture
if
i.
r itu a ‘ io n
c rcss
d ipoie
thunderstorm jz-« w h t r t
1cw 3 ::d s
may
not
sections
be
also
p o 1a r i5at ion
attenuation
during
explained.
This
*
p r op e r t i e s
Vari ou s
dipole
formed
produced
inside
cg ,
”it r g 1 1 i,Ig
t ru e
varies
for
a
particular
the
ihe dr ops
signal
other
a 1s o a nother
Oxygen
1s rg e dr ops
come
frequencies,
and
fa ctor
type
in -o
■
attenuation.
this,,
b y whie h
of
7 h is
th c.
since
w it h w a v ele n g t h . Besides
is summer ised
by
of
is
of
t;.b
the
rainfall
he
iar a
p
may
be
of
the
in the following results.
water
drop
Hydrogen
l i ke
atoms
wisiie i3iiin(j 3i
rotation
and
oscillation
iei'iuinsi
control the propagation at different frequencies.
velocity
The observed
H O . 6 .LM D R O P D IA M E TE R V E R S U S
Uv /,>
)
c h a .p t
er 6
1 ~7~7
large attenuation during thunderstorms is explained in terms of
the dipole polarisation in the water
molecule as well
as the
dipoles developed in side the drops. A simple model in
this aspect is presented Ccf. chapter 5, section 5.3.j
It
is
well
molecule
known
that
produces
oscillations
an
the
rotational
absorption
developed
inside
band
the
at
drop
properties
of
22
also
GHz
and
while
water
falling
the
under
gravity affect the communication at millimeter wave lengths. The
large attenuation at 11 GHz during the thunderstorms,
which can
not be explained in terms of rainfall rate alone, might be due to
the properties other than rotational,
the raindrops are polarised
oscillatory behaviour.
If
(charge separation takes place) due
to the fields like the field between thunder cloud and the earth
surface
or
between
two
oppositely
charged
clouds,
there
wiil
formation of dipole whose direction depends on the direction of
the
field
coming
wave
that
causes
it.
This
electromagnetic wave.
is equal
to
dipole
if the
the oscillations
will
interact, with
frequency
made
by
of
the
the
in
incident
dipole
we
may
expect resonance condition at which large loss in energy of the
electromagnetic wave.
6 3 RESULTS OF COMPARATIVE STUDY:
*
Comparison
of
attenuation for
and
for
rain
attenuation
thunderstorms
drizzle
the
with
is higher
attenuation
models
shows
than CCIR
follows
the
that
,
predictions
CCIR
values.
Lognormal model gives a best fit for the attenuation produced by
thunder s tor ms.
* The reported Rain attenuation at 11 GHz,
by the NPL,
Delhi;
DEAL, Dehra Dun; IRPEL, Calcutta, is well in agreement with the
CCIR predictions, irrespective of the type of rainfall.
* Comparison of the CPD of rainfall
from Malasiya,
Brezil and Nigeria,
rate with that
reported
indicate that Guwahati also
can be grouped under tropical climatic regions with respect to
page tlB
chapter- 3
microwave propagation.
=* Comparison
lognormal
of
observed
model
gives
RSD
best
with
models
agreement
shows
for
that
the
the
RSD
of
thunder-storms and negative exponential model gives best fit for
the RSD of drizzle.
The numerical values of the parameters of RSD for thunderstorm
given below.
dm
= 1.12 + 0.18 InR
_0.39
N t = 180 R
a = exp 10.48-0. 03R)
^Comparison
of
observed
shapes
of
raindrops
with
those
calculated by theoretical models indicate that there is a good
agreement
between
the
observed
and
theoretically
calculated
shapes of the raindrops.
4fiased on the observation of rainfall, rainfall rate the CPD of
rainfall rate for Assam state is modeled using the total annual
rainfall and thunderstorms ratio by which the rainfall rate that
probably exceeds 0.01
3£ of
time
over
different
districts
of
Assam state can be estimated.
6.4 A SSESSM EN T O F T H E MICROWAVE LINK UNDER S TU D Y:
As far the performance of the microwave link is concerned it is
found that the link is well within the reliability recommended
by CCIR,
i.e the link is available for more than 99.99 percent
of time in a year. The following table presents the important
characteristic features observed over the microwave link.
page 179
chapter 6
PARAMETER
A 0.01
1991
1992
1993
AVERAGE
16 dB
15 dB
18 dB
1S.3dB
0.0G17
0.0012
0.0021
% OF TIME
L ink c u t -o f f
EXCEEDED
R0G1
WORST PERIOD
0.0016
90
85
120
PREMON­
PRE 8c
PRE &
PRE Sc
SOON
EARLY
EARLY
EARLY
MONSOON
98
MONSOON MONSOON
6.5 PREDICTION OF MICROWAVE ATTENUATION AT VARIOUS FREQUENCES :
Basing on the observed results on rain attenuation over Guwahati
at
11 GHz
,the
characteristics
of
rain
attenuation
frequencies are predicted for the same hop
at
other
length and for the
same climatic conditions. For this purpose the frequency scaling
method given by CCIR (1988) is adopted. According to this method
the attenuation at other
foilwing
formula
if the
frequencies
long
can
be
term statistics
estimated
by
the
are available at
one frequency.
A
where,
/ A2
A^and
frequencies
= G ( f A ) /G ( f 2 )
A^
are
and
the
f0
values
------
of
respectively
------
------ ( 8 . 5 . 1 )
attenuation
and
G(f)
is
in
dB
the
/km
at
function
which is defined as
G(f) = f
1 T>
-7 3 44
“ / (1/1+3x10 f
> ....
___
....(8,5.2)
Table 6.5.1 presents the probable attenuation values at 13, 18,
30 GHz.
ch a p ter
pa ge lSO
6
T a b l e 6.6.1
r
.... 1......... In...rr ■
11GHz
\ i
j 0. 1
5
7
1 0,01
J 0.001
1___________
!
:
13 GHz
:
............. r,l"n ..... ...............i
18 GHz
CD
1f
to
| A (p)
■
30 GHz
|
___________ 1
11.5
27
1
9.3
16.5
37
1
18
23. 9
41.6
97
1
48
63
111.5
260
1
.... i
The above table shows attenuation values (Alp)) that exceeds a
specified percent of time in year . 11 can be seen from this
table that if the attenuation 18 dB is exceeding 0.01 percent of
time in a year at 11 GHz , we may expect 23.9, 41.6 and 97 dB to
exceed for same percent of time at 13,18 and 30 GHz respectively
{ for the same
link characteristics).
Therefore if a microwave
link as to be installed at these frequencies over Guwahati,
the
path length might not exceed 3 kms for the frequencies above 13
GHz with the same kind of transmitter and receiver as
for 11 GHz having 45 dB as fade margin,
reliability.
However,
more
path
length
is used
to have a 0.01 percent
can
be
maintained
at
these frequencies with suitably modified system having suitable
fade margin allowances,
6.6 R E S U L T S O N CIRCUIT DEVELOPMENT:
* Raindrop size measuring instrument
lias been developed
as a
part of the research work* which gives a satisfactory results on
RSD measurements.
* The fast response rain gauge which is also developed as a part
of the research work, is found to be more suitable for microwave
propagation
studies.
Merits
of
this
instrument
are
given
in
chapter 3.
£ Besides this,
a few circuits like digital logic ci
speed controlling
of the stepper
motor,
*
W k A - e . v*
a, Vrfi
voltage to frequency
chapter
O
converters,
amplifiers,
power
s u pplies
are
designed
and
fabricated.
* A w i reless an e m o m e t e r
in a p p e n d i x Aj
is
designed
and
fabricated
<.' d e s c r i b e d
F U T U R E GUIDE LINES:
t.
The
fast
rainfall
response
rats
attenuation
x-ain
in m s e c . mus t
with
rainfall
gauge
be
rate
capabl&
used
fox
where
of
x-ecox-ding
associating
rainfall
the
microwave
rate
exceeds
a
certaia level.
2.
Global
model
needs
rote dietruhution o-ver
to
be
modified
tropi cal
so
far
regions a r e
as
the
rainfall
concerned and rfiore
do.to ir\pu t ore? neoessory f ox- reo list rig o px-ac it col floods l.
b .
CL IP. fi’lOdO l
p ro v id e d
4.
fOX ' X’O l ri O t t OXiOOt lOTl O U6?X'
w t t h . /Tioxe d o t o
Me a s ° i r e m sn t s
c o r x 'te d o x it
of
when x -a in
tX O p tC d l
X’OtflO TlS
IS'
also
in p o t.
ret i n d r o p
s i 2e
at te n o o tio n
and
shape
rT ie a s H x e frie n ts
sh ou Id
axe
i n va r i oh l
n io d e .
y
A p p e n d ix
A
o c l
*
I S id
■•«=»
A P P EN D IX A
Radio anemometer :
A wireless
wind
speed
and
monitoring
developed hy the group in the present
system
is
designed
research scheme.
and
Working
principle and details of the system are described below.
Working principle:
The design phylosophy of the system
mechanical
rotations,
driven
by
is based, on conversion
wind,
to
of
corresponding
electronic pulses through opto-eiectronic device.
The number of
pulses so obtained are calibrated to give the wind velocity at
that ins tan t.
The block diagram of the system is shown in fig.l.
It consists
of :
1) Cup and cone wind vane
2) Transmitter (range 150-200m!
3) Receiver with recording fecility
The cup and cone wind vane has three cups and cones of diameter
10cm
and arm length 22cm each. This vane system rotates with a
speed that is in proportion to speed of the wind and produces an
electric
pulse at each
transmitted
by
a FM
operating at 92MHz.
ten
pulses
rotation.
These pulses
transmitter
The
(rotations)
of
transmitter
which
range
are counted and
150-200
is triggered
transmits
meters
once
signals
and
in each
for
the
duration of two rotation and remain quescient for the nest eight
pulses.
This part of the system requires a power unit of
12V,
which is provided by a solar cell. The transmitter and the wind
vane can be placed at any place or at any height to measure wind
speed,
without
using
connecting
wires
or
battery
charging
prob1ems.
The transmitted signal
is received through a FM receiver
. The
,4
detected part of
the signal
r ecor dec
on a heat
is made.
The
the
time
s ens
is converted to a 1K H z frequency and
i t 1V 8
oia d e r after
1e n g th o f th 8 mar k
taken
by
w ind
o
vane
s ui ta b !e am p iif icstion
n th e chart
xe c u t in g
e:
for
represents
paper
c o m p 1e te
tw o
r o t at ions.
The
wind
speed
from the time
standard
the
can
be
interval
measured
between
from
length
the two marks.
nonrecording wind vane of Case 1 Ia Co,
, length
of
mark
and
c a 1 ibra ted.F i g .A.2 shows
time
duration
the calibration
and curve 2 gives
the calibration
tw o
c onsecut i ve
measurements are presented
marks.
of
the
Sample
the
re c ords
marks
Curve
in terms of the
of
mark
or
ltd., London make,
between
in terms
the
With the help of a
the calibration curves.
figure represents
between
the
1 in
are
the
length of marl-:
time du ra ti o n
of
wind
in f ig .A .3 ,
Fig A ,1 Block diagram of the radio anemometer.
s oe e d
2. y? ,4
£ q
p'izft't*
wind calibration chart
1 - m a r k length ; 2 - t i m e interval
v¥rHcj
speed {— n e e / ^
!" t f » r v a !
A O
M.L.
i >?4
pa.gi
A ppendix A
FiG .A .3
SAMPLE RECORDS OF WIND SPEED MEASUREMENT;
1SB
page l 86
Appendix B
APPENDIX B
T h e a t m o s p h e r e a f t e r r a in a s s e e n t h r o u g h SODAR
In t r o d u c t i o n :
A
monostatic
echosonde
pointing
vertically
upwards
capability to provide round the clock observations
thermal
structure
and
velocity
atmosphere upto a height
meters.
thermal
1975;
of
ground
plumes
and
based
other
Neff and Hall,
visually
of
the height
inversions,
associated
the
lower
of about
and
10
intensity
elevated
phenomena
the
regarding the
of 2 kms with a resolution
The echogram display
variation
parameters
has
inversions,
CHandies
et.al,
1976; Edinger-, 19759.
The Electronics and Radi ophy s ics group at Ga.uha t i university has
developed
Block
such
diagram
a
SODAR,
full
the system
is shown
this
instrument
is mainly
by
studies
of
microwave
Besides
the
echograms
inversions,
plumes
interesting
structures
typical
in
of
obtained
a
which
case
fading
of
and
in
humidity
after
which
in fig.B.l.
used
with
surface
for
since
The
the
atmospheric
based
we
rain
event.
atmosphere
have
1959.
echograms
correlative
parameters.
inversions,
fronts,
the
the
operation
elevated
received
Here
after
we
the
some
describe
rain
is
studied.
CASE STUDY: Fig.B.2.
June
1993.
hours,
presents
There were
two rain events
echogram
received
in the time
on 24
interval
of 5
a drizzle event at 2020hrs followed by a heavy showers at
0420 hrs of 25 June
immediately
reduced
after
giving
1993.
the
It can be seen from this echogram
rain,
rise to almost
for an hour and afterwards
slowly.
spell
a typical
The convection
of
heavy
the
thickness
no-echo
of
situation
the inversion
the
which
that
layer
is
continued
layer starts developing
took place once again to produce a second
showers.
thickness again reduced.
After
this
rain
The portion of
event
also
the
layer
page
parabolic dish
FIG. B.1 BLOCK DIAGRAM OF THE SODAR
lS T
JS * ui
Ml lift
pr ,ii
,«*
page
A p p e n d ix B
the echogram that is in between the rain events
that
of
during
the
transition period.
formation
The diurnal
of
1 SQ
resembles with
nocturnal
inversion
at
pattern of SODAR echograms show
that the no-echo situation is formed during the transition hours
that
is
during
the
time
at
which
process withdraws and nocturnal
no-echo
situation
tell
us
the
day
time
convection
inversion starts developing. The
that
immediately
after
rain
the
atmosphere becomes homogeneous by destroying the layer formation
and clearing out all the pollutents and atmospheric particulates
Effect
of
studied.
this
type of
Fig.b.3
situation
presents
the
on
microwave
simulteneous
propagation
measurements
microwave field strength and SODAR echograms
is
of
recorded on 3June
1993. The microwave field stength measurements were made over a
LOS
link
(Mi 1mi 1ia-Durgasarovar>
operating
at
6GHz.
The
technical details of the link are presented in table B.l.
E F F E C T O F THE A F T E R R A IN A T M O S P E H R E O N M IC R O W A V E P R O P A G A T IO N :
In a. homogeneous atmosphere,
the refractive index might be same
at all heights upto atleast lKm. This makes the radio refractive
gradient!
RRI
AN/AH)
equal
to zero.
(Raddio Refractive
microwave
signal.
if there
Index) we may not
However,
due
to
seen
in fig.
study during the time of no-echo
event.
B .3 that
i.e
just
after
the
fluctuating,
link CSarijery SKar-ma., i Q Q 3 j .
which
of
rain.
fading
regular
signal
remain
like situation after
When
inversion layer has come to its original
starts
any
in
of
ray
level to change.
There are slow fading of depth around
transition
also
expect
the absence
bending, one may expect the median signal
It can be clearly
is no fluctuations
almost
the rain
10 dB at time of
surface
based
level the signal
level
is a normal
the
feature
over
this
A
p part'd. zlx B
P &
Table
Mic r o w
ave
l in k
l
* :;* e?
I y O
B.1.Cm
in f o r m a t io n
il m il ia
-
d u r o a sa r o va r
1.
Transmitting antenna height
:105 meters
*■?
Transmitting antenna gain
:43.3 dB
3. Receiving antenna heaight
:228 meters
5. Operating frequency
:6 GHz
6.
:41 Kras
Fath length
7 , Terrain type
(H ASL )
a
CO
4. Receiving antenna gain
(HASL )
CD
:40dBm
CO
3. Transmitting power
)
:Marshy
REFERENCES:
1. Ednger.
half
km
JG Cl9753 " Acoustic sounding of the lowest one and a
over
meteorology
Boston,
2.
LOS
angles
,
conferrence,
In
preprints
Americal
PA,
gate,
Rep.
14,
World meteorological
Neff.
WD
measurements
the
meteorological
HAllCJrO.FF adn Owens.
results of acoustic echo sounding
3.
sixteenth
radar
society,
MAssachusetts, PP 253.
Mandies.
during
of
and
preliminary
of
scintific
organization, Geneva,
FF.
Hall
CJrD
radar
C197G3
meteorology
meteorological society, Boston,
4. SanJay shar-ma C19933"
"Preliminary
research,
layer
Vol-11,
PP 225
"
layer
Acoustic
sounder
, in preprints
conferrence.
Massachusetts,
of
American
pp 297.
Study of scintillation and fading with
respect to tropospheric irregularities,
the Gauhati University.
C19753
the marine boundary
of the southpole boundary
seventeenth
EJ
Ph,E> Thesis submitted to
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