constraints in radio propagation through ionized and non...

Indian Journal of Radio & Space PhysicsVot 16, February 1987,pp. 114-126

Constraints in Radio Propagation through Ionized andNon-Ionized Media: A Synoptic View

B M REDDYNational Physical Laboratory. New Delhi 110012

Received 9 January 1987

The ionosphere and troposphere in the terrestrial atmosphere are the ionized and non-ionized media, respectively, thatare relevant to radio communications from VLF to EHF bands. While the progress in sophisticated system electronics to aidradio communications has been exponentially increasing in recent years, our knowledge of media characteristics that im-pose limitations on system reliability has advanced very little. The paper describes briefly the present state of art in this areaof radio propagation with special emphasis on the Indian radio environment. The paper also gives examples of the variety ofbasic data that are presently available in India for radio system designers.

1 IntroductionOur radio environment is a precious natural re-

source that has to be skilfully exploited to optimize avariety of civil and defence communication systems.The progress of the radio wave between the transmit-ter and the receiver cannot be taken for granted; theterrestrial atmosphere imposes certain limitationsthough it also offers several advantages that can be uti-lized to promote radio communications. Some out-standing examples of such advantages for beyond-the-horizon communications are the existence of theionosphere that can reflect HF band radiowaves, tro-pospheric refraction that extends line-of-sight ranges,troposcatter caused by omnipresent turbulence in theboundary layer and refraction of long waves aroundthe earth's curvature. The increasing pressure on HFto EHF bands in terrestrial radio systems and satellitecommunications demands an a priori knowledge ofionospheric and tropospheric effects that contributeto system deterioration. It is essential to characterizethe average morphology of the radio atmosphere andto account for the variability in these environmentalfactors to design robust radio systems that can deliverplanned performance levels with some facility toadapt to changes in environmental factors. There areprimarily two reasons for advancing the state of art inpropagation research: firstly, the explosion in sophis-ticated electronics systems hardware with zero failurerate has made the medium uncertainty as the only li-miting factor in attaining super high performance. Sec-ondly, the high data transmission rates, facilitated byadvanced systems and necessitated by economics ofspectrum management require a much more accurateprediction of radio climatology than available cur-rently.

114

2 The Ionized MediumThe ionosphere, extending from 60 km to about

1000 km altitude, plays a major role in aiding long dis-tance HF communications as well as in deterioratingperformance of satellite radio systems in the VHF,EHF and perhaps SHF bands. Perhaps the most at-tractive aspect of the ionospheric medium is its abilityto support low bandwidth channels in the HF band forlong distance communications with very inexpensiveinputs. The reliability is usually limited to 90% thoughit can be stretched to 95% with proper planning in-cluding short-term predictions. The starting point inplanning HF communications is to make long-termsolar predictions, because ionospheric parametersare essentially controlled by the varying solar activity.The Radio Communications group at the NationalPhysical Laboratory, Delhi, has developed its owntechniques for long-term solar predictions; comparis-ons during the last 3 solar cycles show that these pre-dictions have been consistently better I than most ofthe predictions in the advanced countries. The rela-tionship between the sun-spot number and 10 F2 isusually not linear and it is advisable to fit second orthird degree curves for different local times and lati-tudes and generate a set of constants. Such constantshave been successfully generated by the group at NPLand are currently in use for routine predictions.

The most cardinal inputs required for planning HFcommunications are given in Table 1.Examples of on-ly the salient features in HF communication predic-tions will be given in this paper with supporting dia-grams. Fig. 1shows the monthly median values of10 F2(in MHz) for February 1987 predicted by NPL groupand issued to users in September 1986. A similar pre-diction is also issued for the distribution of

REDDY: RADIO PROPAGATION THROUGH IONIZED & NON-IONIZED MEDIA

Table 1-Inputs for HF Communication Predictions

D-Region Absorption E-RegionFrequency Dependence etc. 10 E (Dawn, Dusk

and Night)

Es-LayerBlockingMode Interference(Auroral, Mid andEquatorial Lats.)

External Noise(Atmospheric, Galactic,and Man-made)

Polarization Coupling Loss(Equatorial East-West)

Jungle-Mountains

F-Layer-/o F2, M(3000)F2Horizontal-Gradient'sResponse to Events;Day-to-Day Variability

+Frequency '"Power

Time and Location Dependence

'"Antenna Elevatio~ Angle

v-"2 / / ,\ "---V --- / / fJ- r----. 2 r--..

0I\.

3' .I r-, '-r-;-51- / / .: ......... I'---. r-, .......3,

0 - /"' I--r",

r- <, V / VV I-- -7 -t-.. <,r-r-../

" ( V -: V/ v .,--9 ~ r-:::::- --::: I'---.')

J '"\ -, \ 1'\ r-;r--.. -8:./ ../ 1\ <, <,~ r-- "-

31' I \\ ) ....- 8.....-, r'\ I/'--' / ( C ~

1,.1 ,/ V'<, .I ,/ Y") 1

/ { '\ r-, --r-- ~--1-r- /"' //r-- 8 --/ [ \ <,r-, <, 7 1- -....- ,./ /6

V,/ \ r-, <, '-........ 5 .-- / ...•.-.--

2-"\ 1\ <,<, /" {

/"

r-, ,..-- 4 /I \ '\ 1"'- ./ /

\ V

FEBRUARY 198780~

:z:!;; 1>0o:z:

40

OZ 04 08

ZONE E

10 16 18 Z4ZO ZZ1Z

Fig. I-Contour maps of predicted monthly median values oflo F2 in MHz for February 1987 for the Eastern zone (500E-1700E)in local mean time vs latitude frame

MUF( 4000 )F2 parameter which contains informa-tion about the F2 region altitude. These parametersare enough to make a prediction of the median MUFfor the month of February 1987 between any twopoints in this longitude zone. The elevation angles forantenna optimization can also be calculated from the Fregion height parameter. Fig. 2 shows the distributionof fa E for the month of January and for low solar activ-ity period such as the year 1987. The height parameterfot the E region can be taken to be constant for

105 km. The fa E morphology is necessary for plann-ing short and medium distance links as well as to takeprecautions regarding obscuration of frequenciesmeant for F region communications.

2.1 Day-to-Day Variability in F Region ParametersThe state of art allows credible predictions as far a

the median MUF parameter is concerned. Such medi-an predictions by definition hold good only for 50% ofthe time. The F region is subject to a large day-to-day

115

INDIAN J RADIO & SPACE PHYS, VOL 16, FEBRUARY 1987

JANUARY (LOW SOLAR ACTIVITY)

35

30

0>:3 L5

wo::J'::::20~<[...J

150.4

Fig. 2 -Contour maps of monthly median values of fo E in MHz over the Indian sub-continent in'local mean tittle vs latitudeframe for January for low solar activity period [These values are strongly dependent on solar zenith angle which is a function

oflocal time.]

APR

MAV E"=::=:=::::=:t.0 J~Er-,<:' JULY

AUG'SEPT

OCT

08 10 12 14 1& 18LOCAL MEAN TIME (HRS)

20 22

Fig. 3 - Monthly and diurnal variation of coefficient of variation( V) for fo F2 for Kodaikanal during the low solar activity year 1975

variability which is apparently unrelated to any specif-ic solar or magnetic event. It will be necessary to scaledown the working frequencies below the predictedMUF by various degrees to attain increased reliabilitylevels. The extent of this variability is dependent ongeographical location, local time, season and solar ac-tivity; to optimize the frequency usage, it is imperativeto know the morphology of this day-to-day variability.Rush et al? have studied the day-to-day variability offa F2 and hm F2 at mid-latitudes and have concludedthat fa F2 variations are more important than hm F2variations in affecting HF communications. Somestudies=' were reported from India in recent years us-ing the long series of Indian ionospheric data. Fig. 3shows the coefficient of variation in fo F2 for Kodai-kanal for the low solar activity year of 1975. Coeffi-cient of variation is defined as ratio of standard devia-tion to its average value in per cent. The effect of thislarge day-to-day variability is two-fold: since all the us-

116

600(I)

!:I00

LOW SOLAR ACTIVITY-WINTERJANUARY 196!:1 LATITUDE

Ge09" Ge0""'!l.(I ) KODAIKANAL IQ2°N O.lIoN(2) AHMEDABAD 23.o°N 14.0

oN

(3) DELHI 28.6°N 19.2°N

24

~!... 400QI

Z300Z

~ 200Z<[J: 100U

(2)

Fig. 4 - Diurnal plots of percentage changes in electron densities(Ne) for three stations in India for a low solar activity winter month(January 1965 )[The dramatic increase in N; following local sunrise

can be appreciated forthe Indian zone.]

ers have to choose frequencies much lesser than pre-dicted MUF values, the available frequency spectrumis reduced; at frequencies far less than the MUF va-lues, the power requirements steeply go up resulting inexpensive equipment and in avoidable pollution.Thus the necessity for accurate evaluation of the day-to-day variability cannot be over-emphasized.

2.2 HF Communication Problems due to Large Spatial andTemporal Electron Density Gradients

The rapid variations ofF region critical frequenciesduring sunrise hours and the large horizontallatitudi-nal gradients in the F region electron densities asso-ciated with geomagnetic anomaly cause serious prob-lems to HF communications at low latitudes", Fig. 4


shows the percentage change in electron density com-pared to the previous hour for winter during the lowsolar activity period of 1965. As can be expected, thechanges are spectacularly large at Kodaikanal and be-come modest as the latitude increases. The problemsposed due to such rapid dawn transition are multi-fa-ceted. Typically HF link operators employ one day-time frequency and one nighttime frequency. The useof nighttime frequency during sunrise will requiremuch larger power than is normally permitted whilethe frequency aliocated for the daytime will not besupported during such transition by the ionosphere.Also, point-to-point links normally use inexpensivetuned directional antennas and frequent change of op-erational frequency is not possible. The obvious rem-edy of course is to have a third frequency allocated forthe transition period. This third frequency has to bejudiciously selected from a study of long series of ob-servations during transition periods. The problemsposed by large spatial gradients are particularly seriousin the equatorial zone though similar problems do existin the mid-latitude trough region. For example, if weconsider the equatorial anomaly peak in the northernhemisphere to be at 15° north geomagnetic latitudeand if the north-south HF circuit is operating such thatthe reflection point is on either side of the peak, a pecu-liar situation arises. If the point of reflection is equator-ward of this anomaly peak, the radiowave incident onthe ionosphere for the northern circuit will continu-ously come across increasing levels of electron densityon two counts, namely, the one due to the vertical gra-dient as the radiowave penetrates higher into the ion-osphere and the other due to horizontal gradient as thewave progresses towards the direction of increasingelectron density. On the other hand, for the same linkin the north-to-south direction the horizontal gradientis reversed while the vertical gradient still continues tobe positive. Fig. 5 shows how the maximum usable fre-quency changes as the horizontal gradient transcendsfrom negative to positive values for three differentangles of incidence at the ionosphere. It was observedfrom actual observations from top-side sounder satel-lite that horizontal gradients of three electrons per ccper metre do exist in the equatorial ionosphere whichmay yield MUF values ranging between 20 MHz and39 MHz for a predicted MUF of 28 MHz. Thus HFcommunication in one direction only is possible if afrequency predicted assuming zero gradient is used.

2.3 Magnetic Storm Effects on HF Communication

Magnetic storms are known to be a consequence ofinteraction between the terrestrial upper atmosphereand enhanced solar wind. While the ionosphericstorm behaviour" - 8 is complex and defies a uniquedescription, the following simplified picture is useful

in modelling HF communications. During the mainphase, the electron densities are depressed at mid andhigh latitudes while they increase to a lesser extent atlow latitudes. The F region height however, increasesat all latitudes. The increased height and enhancedfo F2 at low latitudes partly compensate for each otherand the MUF values undergo but marginal changescompared to the predicted values. This is one reasonthat continues to retain HF communications at lowlatitudes as attractive. While the predictability of thestorm-inducing solar event itself remains elusive, it isnow possible to predict the storms after seeing the so-

45r----------,-----------,

40

X:::E 35ruzlJ.J 30::lClJ.J0::u,

lJ.J 25--'CD<tCfl 20 t::l:::E::l 20.4 MHz:::EX 15 15MHz<[

:::E

10

0L.......L...JL...JL....IL....I----1.----1.----1.----1.----1.----1.---.J-6 -4 -2 0 2 " 6

HOR IZONTAl GRADIENT,electrons/cm3 / m

Fig. 5- The changes in MUFs caused by varying values of horizon-tal gradients for different angles of incidence

DELHIGEOMAG. LAl 19.2••••

SU •••• ERAp >30

,60

IA. "0::l~~ 20

wCI -40

~::l-600:

~ -~~~~----1. __ ----1.__ -L__~ __~ __~

o " 8 12 16 20 2<t

LOCAL TIME (HRS 1ST)

Fig. 6-Percentage deviations in MUF (4000)F2 from monthlymedian values for several disturbed days duringsummerfor Delhi

117


lar events through optical and radio observations. Pre-dictions? of ionospheric departures are best donefrom statistical patterns developed from mass plots ofpercentage deviations in MUF as shown in Fig. 6.Del-hi obviously shows the mid-latitude behaviour of cleardepression except in the predawn hours where the si-tuation is rather mixed. It is only appropriate to men-tion here the services available at NPL (Delhi) throughits Associate Regional Warning Centre (ARWC) ofthe International Union of World Days Service(IUWDS). Daily messages regarding nascent solarand magnetic conditions pour in from around theworld every day as shown in Fig. 7 which are interpret-ed in terms of disturbances to communications andare supplied to users.

2.4 Trans-Ionospheric Propagation Effects

Some of the serious limitations 10-12 imposed by theionosphere on the performance of trans-ionospheric

radio systems are due to time delay, refraction andscintillation fading (inter-pulse interference). Whilethe predictive capability for quantifying such deterior-ation does not exist, it is possible to develop morpho-logical models so that appropriate corrections can beapplied to mitigate the problems. Fig. 8 shows the per-centage of time that reception is unacceptable for dif-ferent data transmission rates at 1.4 GHz. Such mod-els help in cutting down data transmission rates to ac-ceptable levels of reception for different solar epochsand latitudes. Fig. 9 shows elevation angle error in mil-liradians at Kodaikanal for different elevation angles.Fig. 10 shows a summary of scintillation data at4 GHz recorded from INSAT-1B signals during1984-85. The scintillation indices were rather low,partly because of the low solar activity and partly be-cause Delhi is at the fag end of the scintillation belt. It isknown that very intense scintillations do occur even at4 and 6 GHz at low latitudes during higher solarepochs.

ARWC-NEWDELHI

Fig. 7 -A schematic diagram showing the various channels through which ARWC (New Delhi) exchanges solar-geophysi-caldata

118


CONSTRAINTS ON DIGITAL COMMN.

W...J

~ 4.0>-0..

~ 3.5u<!Z::> 3.0

I.4GHz± 6" G.M LATITUDE

MEDIUM SOLAR ACTIVITY -HIGH SOLAR ACTIVITY--- ,

IIII

w I

i2.5 //

~ Io /~2~ /w /w /~ 1.5 />- "u, "a 1.0 ,'"w ,,-~ ,/g 0.5 ,,"w "u __,

~ °0~~5~0~~10~0===1~50~~2~OO~~25~0~~3~0~0--~3~50~~4~0~0-

DATA TRANSMISSION RATE (rnillions j s )

Fig. 8 - Deterioration in digital communication systems in terms ofpercentage of time for medium and high solar activity epochs at afrequency of 1.4 GHz at a low latitude station [These are derived

from a modified global model of scintillation using Indian data.]

0.7ELEVATION ERROR

KODAIKANAL1000kmWINTER1980-81

0.6

0.5

0.4

cQuE'E 0.3

~a::o~ 0.2w

0.1

OL-__~ __-L___L__~ __~L---~--~o 4 8 12 16 20 24

LOCAL TIME IN HRS

Fig. 9 -Ionospheric refraction errors for different elevation anglesat Kodaikanal at a frequency of 500 MHz using ray tracing tech-

niques on model profiles calibrated by ionosonde data

3 The Non-Ionized MediumThe terrestrial atmosphere below 60 km can be

considered to be non-ionized for practical purposes ofradio communications. However, the neutral atmos-phere above about 10 km is of little interest to radiocommunication systems. Thus the neutral atmos-phere below 10 km, known as the troposphere, is theonly non-ionized medium that we will consider here.

SUMMER 0

WINTER.(0) AUTUMNAL EQUINOX

--1984- -- 1985

zo~ 10...J...J

~ 5uU)

I.L 0~+--'-+-'--+-'--+-"=!-'-+----lo

z 1984-1985of=:320...J

f=~ 15U)

I.L

010wuzwg: 5~ugo~~~~~

18 20 22 00 0204 06LOCAL TIME (HRS)

(b) VERNAL EQUINOX--- 1984--- 1985

w~ 10w0:tr~ 5uuo

OULr-~++~+-'--~18 20 22 00 02 04 06

LOCAL TIME(HRS)

Fig. lO-Seasonal distribution of nighttime occurrence of scintill-ations (in percentage) at New Delhi during 1984and 1985

The neutral atmosphere in this region consisting of ni-trogen and oxygen with several other minor constitu-ents iswell mixed so that the percentage concentrationof each constituent does not change with altitude ex-cept for water vapour. The tropospheric propagationmechanisms arise due to the fact that the concentra-tion of the gases, especially water vapour change withaltitude and also due to the fact that the temperaturelapse rate in the troposphere sometimes can even re-verse. Refraction, scatter and attenuation are the threeprincipal features of the troposphere which exerciseprofound influence on the propagation mechanisms.To summarize, the following characteristics of the tro-posphere are most relevant to radio communications:

a Pressure decreases exponentially with altitude.

b Temperature and water vapour usually decreasewith altitude but, sometimes may not.

c Hydrometeors and water vapour absorb radiowaveenergy above several GHz.

d Deploarization due to non-spherical rain drops li-mits performance of frequency reuse systems.

e Tropospheric turbulence aids long distance scattercommunications but, also causes interference fromunwanted transmissions.

f Super-refraction and ducting cause anomalouspropagation conditions for radar and microwave ra-dio systems.

g Most of the above characteristics display wild var-iations in space and time especially at tropicallati-tudes.

Our present knowledge of the radio climatology ofthe troposphere is rather meagre for Indian condi-tions. It has been observed 13 that very large errorswould be committed if we use tropo path loss tech-niques developed in more advanced countries. For ex-

119

INDIAN J RADIO & SPACE PHYS, VOL 16, FEBRUARY 1987 .

ample, it is a weU-known observation throughout Eu-rope that summer field strengths are much higher thanwinter field strengths. This we know is almost nevertrue over the entire Indian sub-continent. The inter-face of land and sea causes very serious problems inthe Indian coastal regions. The warm ocean surfacesaround India abound with perennial ocean ducts ofvarying thicknesses increasing radar ranges, but alsocausing severe radar blindholes. The total refractivityof the neutral and ionized media is given by equation

No..5) = (n - 1 ) X 106 = NT + N)

= 77.6(..1 )+ 481Oe(S)) _ 40.28 x 1O-6N ( )1(s) Yls 1(s) l e s

where N( s) is the refractivity, nis the refractive index,NT is the tropospheric component of refractivity, Misthe ionospheric component of refractivity, 1( s) is theair temperature (in K), p{s) is the atmospheric pres-sure (in mbar), e( s) is the partial vapour pressure (inmbar),f is the radio frequency (in MHz), N; is the elec-tron density (m - 3), and s is the distance parameter.Fig. 11 shows (after Goodman 14) a typical refractivityprofile from ground level onwards past the ionos-phere. The tropospheric part isfor Bombay, from Sar-kar et al.15• A considerable amount of work has beenpublished on refractivity variability both in India andabroad. A cc.nprehensive radio refractivity atlas waspublished as early as in 1977 by Majumdar et al. 16 and

1000r::::::::::::::--- _

E.><

200 400 600 BOO )000 1200

REFRACTIVITY [N]

Fig. 11- Radio refractivity profile from ground level to 1,000 km[The ionospheric profiles are calculated for a model ionosphere(Goodman 14). The tropospheric profile is from observed values at

Bombay during monsoon season at 5.30 a.m. local time.]

120

was subsequently updated by Sarkar et aLl5 in 1985.The revised atlas has used radiosonde data from 32stations in India and gives a number of parameterssuch as super-refraction, and sub-refraction occurr-ence, scatter paramerer C~etc. Figs 12 to 16give someexamples of the basic reference data available in theatlas.

3.1 Tropospheric Monitoring TechniquesThe wealth of data available over several decades

from the India Meteorology Department throughtheir their twice-a-day radiosonde flights continues tobe a valuable asset in view of the uninterrupted longseries of observations. However, they suffer primarilyfrom three faults. Firstly diurnal variation is not avail-able. Secondly, the slow response ofthe sensors intro-duces systematic errors. Thirdly, it is impossible tostudy steep gradients and small-scale turbulence be-cause of poor resolution. Kytoons on the other handcan give much better resolution though a single profilemay take more than 45 min and thus the refractivityprofile may be contaminated with large temporal var-iations. The kytoons cannot be operated under windyconditions. Acoustic echo sounders have emerged asuseful techniques for monitoring the inversions in theboundary layer. They are unique for ductingand othermorphological studies at radar sites and also for moni-toring pollution conditions.

Ground-based meteorological radars in the S,C andX bands have been used in recent years in advanced

-r-r '_,--_0..' r:.--i

·1 '''Q''~''Ii~CE.~flFIED copy

~.

1I,

l1

Fig. 12- Distribution of modulus of initial refractivity gradients (inN/km) for the month of May at 0000 GMT over the Indian sub-continent [Being a typical pre-monsoon month large values persist

all overthe country.]

REDDY: RADIO PROPAGATION TIfROUGH IONIZED & NON-IONIZED MEDIA

countries to yield raw video presentation with colourdisplay for detailed investigation on the cell-by-cellbasis in the precipitating clouds. Dual polarized rad-ars are perhaps most suited to investigate propagationimpairments especially in frequency reuse systems.However, radar observations in India are limited

QII'I~'I''Il1lr

CERTlFI£C' C~,)'(

.-----.t".-. --}---------:.-- __2..

tI

~

!.', ,"of<~'I" !

I

Fig. 13-Contours of effective radius factor kover the Indian sub-continent at 0000 GMT in May [The k-factor values are deter-mined from the initial refractivity gradient between the surface and

the250 mlevel.]

I ·----,·--------,---.---~I

g_.r_ .•"'T'1t'

eliOt ••. .:.•.. _:?1

-------;.. -----~ -------.;.-----~.-

Fig. 14-Sub-refraction occurrence probability (in %) for themonth of July at 0000 GMT [Sub-refraction statistics are neces-

sary to allow adequate Fresnel zone clearance in hilly terrain.]

merely to study super-refraction and ducting throughincreased range and ground clutter.

By far, the best technique for obtaining accurateRadio Refractivity profiles, with resolution highenough to yield turbulence parameters, is the in situmeasurement using a microwave refractometer

IrI

i+II

~. !!' -. --'j- -.-~-

t1.,.·1'),1l '.r~C[RTlfl[O COM

,] ......~ -_....~. ~.~.',_--,,-_..!.....---,I

•.

\1':f·

t·

.;-,,t

Fig. 15-Surface duct occurrence probability (in %) in the pre-monsoon month of May at 0000 GMT when the values are very

high due to the large humidity gradients

6r-----------------------------~EAST COAST

PRE MONSOON .OOOOGMTI

i·I

;: 3r •..)..,

°O~----~------~5-------L------~,oC~ (10~15m-2/3)

Fig. 16- Profile of the structure constant C~ over the east coast ofIndia during pre-monsoon conditions at 0000 GMT [C~ parame-

ter is essential in troposystems design.]

121


mounted on a small aircraft that can spiral verticallyup in a short time. Such a solid-state, digital refrac-tometer was designed at the National Physical Labor-atory, Delhi, and was fabricated in collaboration withDefence Electronics Application Laboratory, Dehra-dun. The refractometer was mounted on the CESSNAaircraft of lIT, Kanpur, and flown during 1983 and1985. Fig. 17 shows a sample refractivity profile thusobtained. Assuming frozen-in turbulence, the tem-poral variations in refractive index were analyzed toyield the structure constant C~ and the scale sizes ofthe atmospheric turbulence appropriate to the scat-tering volume. The profiles of variance of refractiveindex fluctuations derived from the refractometer da-ta are compared with those derived from radiosondeprofiles in Fig. 18. Comprehensive measurements forstatistically significant periods are required for de-signing tropo systems and in a number of earth-spacesystems.

3.2 Large Diurnal VariabilitySome of the outstanding problems presented by the

Indian troposphere to designers of microwave systemof high reliability are caused by large variations in thehumidity at very high temperature levels. The diurnalvariability of radio refractive index profile which de-

S6'0~--------~~--------------------'~~REFRACTOMeTER FLIGHT OVER

KANPUR liT AIRSTRIP

9Ih~~NEI9S3 7.26

7.27

7.28

7.29

3200.•...~l-IC)

LUI

1720

7.30Vla:I

7.31;;;~I-

.32.J<fu

7.33:l

7.3.5

7.36

~~~~~~~~~~~~~~~~~7.37180 200220 2040260 280 300 320 340 360380 400REFRACTIVITY ( N-UNITS)

Fig. 17- A sample radio refractivity profile obtained using themicrowave refractometer(The large amount ofturbulence which isobvious in this profile will not be seen in a conventional radiosonde

profile.)

122

NORTHERN PLAINS - AEFRACTOMETER

-- - -RAOIOSOND E- c;S- SUMMER

w- WINTER <t200GM)

w S

2.0

E ~->< II

1.5 \I- \1I IIo \\W .\I \ \

1.0 \ \\ \\ \\ \\ \\ \

0.5 \ \\ \\ \

W S

0-11

10-9

10

Fig. IS-Comparison of height profiles of the variance of refrac-tive index fluctuations and the structure constants C~ measuredwith microwave refractometer, with those estimated from radio-

sonde data

TEMPERATURE :C

500+~15 +~2rO~__+,2~5 +T30~__+~3~5 +_4~0 +_4~5

RELATIVE HUMIOITYE60%

450

If)I-

Z:::>I

z~>- 400~>I-U<fIl::u,LUIl::

350

BOTTOM SCALE.,-10 -5 o 5 10 15

TEMPERATURE, ·C

Fig. 19- Effect of temperature on radio refractivity for a relativehumidity of 60% [The lower curve is for colder temperaturesshown on the scale at the bottom while the upper curve is for higher

temperature shown at the top.]


E-"

~2 ---III

~26 SKY PATH Z

022.2 GHz >=OQZ+0245R <t24 ::J

Zrn W". 20 18GHz I-Z A=O.OI+OI98R

I-

'2 <tI-<I 16 U0) <;:ZW UI- 12 wl- II GHz Q.<I

A=O,QI+o.o945R (f)

cides the k factor as well as the height of the commonvolume in troposcatter systems is particularly large inthe tropical climate. Fig. 19 shows the variation of cal-culated atmospheric refractive index with tempera-ture at 60% relative humidity. At lower temperatures,for example, when the temperature changes from- lOoe to + lOoe the variation in refractive index isvery little in contrast to much larger variations in therefractive index values at higher temperatures sayfrom + 200e to + 40°C. For higher relative humidityvalues which are common in India, the changes at hightemperatures are more spectacular. Hence it is essen-tial to take note of the importance of humidity andwarm temperatures whose variations can cause dras-tic changes in the k factors, for Indian climatic condi-tions.

3.3 Attenuation due to HydrometeorsOut of all types of hydrometeors such as rain, hail,

ice, fog, cloud or snow, rain is the most relevant one fortropical latitudes in causing absorption and scatter of

8r-------------------------------~7

WALLOPS ISLAND VASUMMER 1973

Fig. 20-Altitudinal distribution of mean rain rate for given sur-face values as estimated from radar reflectivity measurements atWallops Island, USA [The numbers in parenthesis are the number

of rain cells measured at that value.]

(CONVERTED TO 25 k mEQUIVALENT PATH)

7GHz

~~::~~30~~40~?50~?60~~70~~AO~~~~0~,O~IO====---

RAIN RAfE,rnm/hr

Fig. 21-Rain attenuation at 11,18 and 22.2 GHz at Delhi from ra-diometric measurements along with rain attenuation in a 7 GHz

LOS link assuming a rain celllength of 2.5 km

radio wave energy as well as in producing depolariza-tion, antenna gain degradation and bandwidth coher-ence reduction. There have been some scanty rain at-tenuation measurements using radiometers as well asline-of-sight (LOS) microwave systems in India but noresults are available on depolarization as well as on thevertical extent of rainfall statistics necessary for pre-dicting earth-space path attenuation. Fig. 20 showsmean rain rate profiles for measured surface values asestimated from radar reflectivity measurements byGoldhirsh and Katz" at 2.8 GHz at Wallops Island.The very rapid fall off starts above 3 to 4 km and suchstatistics at tropical latitudes are necessary to deter-mine the total path attenuation for satellite radio sys-tems. Fig. 21 shows attenuation at 11, 18 and22.2 GHz over New Delhi from radiometric measure-ments 18. Rain attuenation from LOS link observationsat7 GHz(pathlength42 km)areatsoshown.Anaver-age rain cell length of2.S km was assumed fornormal-

7Ql

6":N

I5(!)r-~

4 <tZ

3~t:i2 ~

ILl~110-<t

140 -ATTENUATIONx xx RAIN RATE

~ 120s:E 100E

ILl 80t:icr 60Z<i 40cr

20

o 0104 10-3 10-2 10-1 100 10

PERCENTAGE OF TIME ORDINATE EXCEEDED

Fig. 22-Cumulative distribution ofrain rate and rain attenuationfor different seasons during 1977-78

102,---------------.

-I10

150mm/hr

50mm/hr/;::.;:::.= ...~..::-.:=:.:::..::....-

12.emm/hr

/.I

h -- LAWS aPMSONS/. - - -MARSHALL a

/. PALMER/. _._.- JOSS (WIDE-

, SPREAD RAINI

-210 ~-L~-LI~O------~IOLO------I~~

FREQUENCY(GHz)

Fig. 23-Specific attenuation from 1 to 1000 GHz for differentrain rates and dropsize distribution [The differences between the

various distributions are only marginal.]

123


izing the LOS values. Fig. 22 shows the cumulative dis-tribution of both rain rate and attenuation at 7 GHzfor different seasons during 1977-78 (Ref. 19). Therain rate measurements were made with a fast re-sponse rain gauge having an integration time of lOs atDelhi. Fig. 23 shows" specific atenuation due to raincalculated for 1 to 1000 GHz using drop size distribu-tions of Laws and Parsons, Marshal and Palmer andJoss with a rain temperature of 20°C. These results areobtained by a series expansion solution of the Miescattering coefficients. It can be seen that above about50 GHz the attenuation levels off and even dropsslightly at higher frequencies. The most importantthing to notice is that there is very little difference be-tween the three distributions, with the obvious conclu-sion that dropsize distribution of models available areadequate for rain attenuation purposes though theymay be inadequate to estimate depolarization effects.Thus, the most important information required in anygeographical region is good rainfall statistics obtainedwith faster response rain gauges. Such an attempt hasbeen initiated by the National Physical Laboratory,New Delhi, by establishing a network of 7 fast re-sponse rain gauges all over India.

3.4 Attenuation due to Water VapourRegarding gaseous absorption in troposphere, the

first three absorption bands are centred at frequenciesof 22.2 GI Iz for water vapour and at 60 GHz and at118.8 GHz for molecular oxygen. While the distribu-tion of oxygen is well known and invariant, water va-pour shows considerable variation. An atlas" wasbrought out by the NPL giving water vapour profiles inthe troposphere at a number of places in India and Fig.

I t-' 'I i7,-- ----r-r'-f'1'-j-i 1 i~:~rl'T~,- - ~ii, : .::~o<~:'-'. . :+ --t- --+--- --!- " - - ,'- ---- - - --+- - ~" I ' ; . • , ••••••...•

, . -' '.' .,"

',-- -+ T-

"i-- j -L,

!l- -t--I : ,

rr!',. - 1 ---!r-+---+-+--~-+---------!-~

2'-:,

Fig, 24- Distribution of water vapour concentration in g/m ' oversurface during monsoon at 0000 GMT [Water vapour attenuationproblem will be more serious for terrestrial links since the whole

path will be in the lower troposphere, 1

124

24 shows just one example. The importance of watervapour attenuation can be realized from the fact thatthe intense rain cells are usually a few kilometres inlength whereas terrestrial LOS systems will have tocontend with hundreds of kilometres of humid atmos-phere.

3.5 Radar Propagation and Targeting ProblemsThe surface-based evaporation ducts over warm

tropical oceans result in the so-called anomalous mic-rowave propagation. Ironically, the anomalous prop-agation conditions exist most of the time and it is ne-cessary to provide a capability to ship-borne systemsto assess these effects and to display in quantifiedterms the expected performance of naval surveillance,electronic warfare and comminication systems. TheNaval Oceans Systems Centre of USA has developedan integrated refractive effects prediction system'?(IREPS). This particular model was developed usingasymptotic forms of the plane wave reflection coeffi-

40'SELECTED RE~ESENTATlVf AREAS

~lCNSOON [J DUCT HEIGHT Imj

'd'[]~I ~

0'

20' 30' 4~" 60~

LONGI TUDE

100"

Fig. 2S-Evaporation duct heights over the ocean derived fromobservations in specific areas

21

<0'SELECTED REPRESENTATIVE AREAS

MONSOON o RADAR RANGE (km I.).." 3cm

30' ~''" ""'" ~"m

2O' '050 ill!'""~ i [EJ>- ,0 r-'., 'i§--'

0' 'd cJ'H [!]P]PO ~~ ~

'0'

~ aJ,20' 30' 6O' TO' '0' 100·

LONGI rUOE

Fig. 26 - Diagram showing the large variability in the radar rangeswithin the evaporation ducts for a 3 em radar


90 50

lIH (METRES) CONTOURS FOR 5,000 ft TARGET HEI5HT

14 WINTER

PROBABI LITY

85 50 24 PRE-MONSOON

80 28 2270

94 70 50

200r-__9T8tr9_0r-rTrr~5nOTT-r ~2r5 ~.- ~ ~~~M~0~N~S~0~0~N~

43 3025 15 8 POS1'-MONSOON

1000

E.><

UJ\!)Z<lQ:

0 300z~0Q:\!)

100--~-- J50~ -J ~ ~~ L- ~ -=~

+10 -50 -100 -150INITIAL REFRACTIVITY GRADIENT (LlN)

-200 -250

Fig. 27 - Tropospheric height error contours for a target height of 5000 ft [The height errors indicated on the countours arein metres. The probability distributions of initial refractivity gradients for different seasons are indicated on the top of the fi-

gure.]

cients for the purpose of waveguide analysis for a trili-near ducting environment. Similar efforts by the NPLgroup using ray theory for the Indian environmenthave met with some success. Some of the basic datagenerated to implement a more elaborate wave modeapproach are shown in Figs 25 and 26. The ductheights are maximum over the equator and fall offgradually towards higher latitudes. Larger ductheights mean trapping of lower frequencies as well asmore frequent radar detection problems.

Surveillance and ack-ack gun radars are usuallycorrected for a k factor of 4/3. Tropospheric refrac-tion not only increases the range but, also substantiallyalters the elevation angle especially in tree-top flying.A three-dimensional programme was developed atNPL to trace the ray in the troposphere and a compu-ter programme was evolved to estimate the range andelevation angle errors if an appropriate refractivityprofile is fed to the programme. With very little effort,the surface refractivity measured on real-time basiscan be used as a first-order approximation. Fig. 27shows the height error contours for a site in India. Theprobability distributions of initial refractivity gradientfor different seasons are indicated on the top of the fi-gure.

4 ConclusionThe objective of the paper is to give a bird's eye view

of the propagation problems encountered over the In-dian troposphere and ionosphere especially to practi-cal radio systems. This is not purported to be a reviewof the Indian work; the i'lustrations given are only todemonstrate the serious .ess of the problems. The ex-amples given here do not include the results from theDefence Laboratories in India or from the Depart-ment of Telecommunications and the All India Radio.

References1 Reddy B M. ,\g.i!amal S, Lakshmi D R, Sastry S & Mitra A P,

Solar Terrecnai Prediction Proceedings, Vol I, edited by RF Donne" US Department of Commerce, Boulder,USA:' I-n-. 118.

2 Rush CM.MillerD& GibbsJJ,RadioSci( USA ),9( 1974)749.3 Aggarwal S, Lakshmi D R & Reddy B M, Solar Terrestrial Pre-

dictions Proceedings. Vol I,edited by R F Donnelly (US De-partment of Commerce, Boulder, USA), 1979, 134.

4 Aggarwal S, Indian J Radio &Space Phys, 14 (1985) 73.5 Lakshmi DR, Aggarwal S,Pasricha P K & Reddy B M,lndian J

Radio & Space Phys, 8 (1979) 101.6 Matsushita'S, J Geophys Res( USA), 64 (1950) 305.7 MaedaKI & SatoT, Proc lnst RadioEng(Australia),47 (1959)

232.H Reddy B M, Brace L H & Findlay J A, J Geophys Res( USA), 72

(1967)2709.

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9 Lakshmi D R, Reddy B M & Sastri S, Indian J Radio &SpacePhys, 12 (1983) 118.

10 Whitney H E & Basu S, RadioSci(USA), 12 (1977) 123.11 Szuszezewicz E P, Rodriguez P, Singh M & Mango S, Radio Sci

(USA), 18 (1983) 765.12 Klobuchar J A, Rastogi R G, Reddy B M & Dasgupta A, Pro-

ceeding of the Indo- US Workshop, Jan 30-Feb 3 1984 (Na-tional Physical Laboratory.New Delhi-12), 1984, 479.

13 Sarkar S K, Dutta H N & Reddy B M, lEE Conference Publica-tion, No 219,1983, p 229.

14 Goodman John M, NRL Memorandum Rep 4339, (Naval Re-search Laboratory, WashingtonDC, USA) 1980,4339.

15 Sarkar SK,Pasricha P K, Dutta H N, Reddy B M & KulshresthaS M, Atlas of Tropospheric Radio Propagation Parametersover the Indian Subcontinent(National Physical Laborato-ry, New Delhi) 1985.

126

16 Majumdar S C, Sarkar S K, Mitra A P,Kulshrestha S M & Chat-terjee K P, Atlas of Tropospheric Radio Refractivity over theIndian Subcontinent (National Physical Laboratory, NewDelhi) 1977.

17 Goldhirsh J & Katz I, IEEE Trans Antennas &Propag( USA),27 (1979) 413.

18 Raina M K, Rain attenuation over Delhi with microwave radi-ometers at 10 &11 GHz, Ph D thesis, Delhi University, Del-hi 1978.

19 Sarkar S K, Ravindran V R, Ramakrishna M, Banerjee P K &Dutta H N, Indian J Radio &Space Phys, 9 (1980) 47.

20 Ippolito LouisJ, Proc IEEE (USA), 69 (1981) 697.21 SarkarSK,DuttaHN,PasrichaPK&ReddyBM,AtlasofTro-

pospheric Water Vapour over the Indian Sub-continent{Na-tional Physical Laboratory, New Delhi) 1982.

22 Baumgartner G B (Jr), Hitney H V & Pappert R A, Proc lEE(GB),130(1983)630.

constraints in radio propagation through ionized and non...

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