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

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

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

B M REDDY

National 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, that are relevant to radio communications from VLF to EHF bands. While the progress in sophisticated system electronics to aid radio 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 area of radio propagation with special emphasis on the Indian radio environment. The paper also gives examples of the variety of basic data that are presently available in India for radio system designers.

1 Introduction

Our radio environment is a precious natural re- source that has to be skilfully exploited to optimize a variety of civil and defence communication systems.

The progress of the radio wave between the transmit- ter and the receiver cannot be taken for granted; the terrestrial atmosphere imposes certain limitations though it also offers several advantages that can be uti- lized to promote radio communications. Some outstanding examples of such advantages for beyond- the-horizon communications are the existence of the ionosphere that can reflect HF band radiowaves, tropospheric refraction that extends line-of-sight ranges, troposcatter caused by omnipresent turbulence in the boundary layer and refraction of long waves around the earth's curvature. The increasing pressure on HF to EHF bands in terrestrial radio systems and satellite communications demands an a priori knowledge of ionospheric and tropospheric effects that contribute to system deterioration. It is essential to characterize the average morphology of the radio atmosphere and to account for the variability in these environmental factors to design robust radio systems that can deliver planned performance levels with some facility to adapt to changes in environmental factors. There are primarily two reasons for advancing the state of art in propagation research: firstly, the explosion in sophisticated electronics systems hardware with zero failure rate has made the medium uncertainty as the only li- miting factor in attaining super high performance. Sec- ondly, the high data transmission rates, facilitated by advanced systems and necessitated by economics of spectrum management require a much more accurate prediction of radio climatology than available currently.

2 The Ionized Medium

The ionosphere, extending from 60 km to about 1000 km altitude, plays a major role in aiding long distance HF communications as well as in deteriorating performance of satellite radio systems in the VHF, EHF and perhaps SHF bands. Perhaps the most attractive aspect of the ionospheric medium is its ability to support low bandwidth channels in the HF band for long distance communications with very inexpensive inputs. The reliability is usually limited to 90% though it can be stretched to 95% with proper planning in- cluding short-term predictions. The starting point in planning HF communications is to make long-term solar predictions, because ionospheric parameters are essentially controlled by the varying solar activity.

The Radio Communications group at the National Physical Laboratory, Delhi, has developed its own techniques for long-term solar predictions; comparis- ons during the last 3 solar cycles show that these predictions have been consistently better ^Ithan most of the predictions in the advanced countries. The rela- tionship between the sun-spot number and

10

F2 is usually not linear and it is advisable to fit second or third degree curves for different local times and latitudes and generate a set of constants. Such constants have been successfully generated by the group at NPL and are currently in use for routine predictions.

The most cardinal inputs required for planning HF communications are given in Table 1.Examples of only the salient features in HF communication predictions will be given in this paper with supporting dia- grams. Fig. 1 shows the monthly median values of

10

F2 (in MHz) for February 1987 predicted by NPL group and issued to users in September 1986. A similar prediction is also issued for the distribution of

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REDDY: RADIO PROPAGATION THROUGH IONIZED &NON-IONIZED MEDIA

Table 1-Inputs for HF Communication Predictions

D-Region Absorption E-Region Frequency Dependence etc.

10

E (Dawn, Dusk

and Night)

Es-Layer Blocking

Mode Interference (Auroral, Mid and Equatorial Lats.)

External Noise

(Atmospheric, Galactic, and Man-made)

Polarization Coupling Loss (Equatorial East-West)

Jungle-Mountains

F-Layer-/o F2, M( 3000)F2 Horizontal-Gradient's Response to Events;

Day-to-Day Variability

Frequency

+

^Power

'"

Time and Location Dependence

'"

Antenna Elevatio~ Angle

v - ^"2 ^/ ^/ ^,\ ^"---

V

-- ^-

^/

_/

^fJ-

^r----.

²

^r--..

0 I\.

3' .I _-5

r-, '-r-;

1-

/

^/ ^.: ^...^I'---.

r-,

^...3,

0

r- <, -

^/^V

_/ _V

_V^/"'^I--r",^I-- -7-t-.. <,

r- ^r-..

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^V ^-: ^V/^v ^.,--9 ^') ^~

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^'"\

-,

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_1'\

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⁸ ^1-

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^-

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/

I \

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1"'-

^./

/

\ ^V

FEBRUARY 1987 80~

:z:!;; ^1>0

o:z:

40

OZ 04 08

ZONE E

10 1Z 16 18 ZO ZZ Z4

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 information about the F2 region altitude. These parameters are enough to make a prediction of the median MUF for the month of February 1987 between any two points in this longitude zone. The elevation angles for antenna optimization can also be calculated from the F region height parameter. Fig. 2 shows the distribution offa E for the month of January and for low solar activity period such as the year 1987. The height parameter fot the E region can be taken to be constant for

105km. The fa E morphology is necessary for planning short and medium distance links as well as to take precautions regarding obscuration of frequencies meant for F region communications.

2.1 Day-to-Day Variability in F Region Parameters

The state of art allows credible predictions as far a the median MUF parameter is concerned. Such median predictions by definition hold good only for 50%of the time. The F region is subject to a large day-to-day

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

JANUARY (LOW SOLAR ACTIVITY)

35

30

0>

:3

^L5

wo ::J '::::20

~

<[

...J

15

0.4

Fig. 2 -Contour maps of monthly median values offo E in MHz over the Indian sub-continent in'local mean tittle vs latitude frame 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~E r-,

<:' JULY AUG 'SEPT OCT

08 10 12 14 1& 18

LOCAL MEAN TIME (HRS)

20 22

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

variability which is apparently unrelated to any specific solar or magnetic event. It will be necessary to scale down the working frequencies below the predicted MUF by various degrees to attain increased reliability levels. The extent of this variability is dependent on geographical location, local time, season and solar activity; to optimize the frequency usage, it is imperative to know the morphology of this day-to-day variability.

Rush et al?have studied the day-to-day variability of fa F2 and hmF2 at mid-latitudes and have concluded that fa F2 variations are more important than h^mF2 variations in affecting HF communications. Some studies=' were reported from India in recent years using the long series of Indian ionospheric data. Fig. 3 shows the coefficient of variation info 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 this large day-to-day variability is two-fold: since all the us-

600 (I)

!:I00

LOW SOLAR ACTIVITY-WINTER

JANUARY 196!:1 LATITUDE

Ge09" Ge0""'!l.

(I ) KODAIKANAL IQ2°N O.lIoN (2) AHMEDABAD 23.o°N 14.0oN (3) DELHI 28.6°N 19.2°N

24

~!... 400

QI

Z300 Z

~ 200 Z

<[

J: 100

U

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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 inN;following local sunrise

can be appreciated forthe Indian zone.]

ers have to choose frequencies much lesser than predicted MUF values, the available frequency spectrum is reduced; at frequencies far less than the MUF values, the power requirements steeply go up resulting in expensive 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 and Temporal Electron Density Gradients

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

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REDDY: RADIO PROPAGATION THROUGH IONIZED & NON-IONIZED MEDIA

shows the percentage change in electron density compared to the previous hour for winter during the low solar activity period of 1965. As can be expected, the changes are spectacularly large at Kodaikanal and be- come modest as the latitude increases. The problems posed due to such rapid dawn transition are multi-fa- ceted. Typically HF link operators employ one daytime frequency and one nighttime frequency. The use of nighttime frequency during sunrise will require much larger power than is normally permitted while the frequency aliocated for the daytime will not be supported during such transition by the ionosphere.

Also, point-to-point links normally use inexpensive tuned 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 for the transition period. This third frequency has to be judiciously selected from a study of long series of observations during transition periods. The problems posed by large spatial gradients are particularly serious in the equatorial zone though similar problems do exist in the mid-latitude trough region. For example, if we consider the equatorial anomaly peak in the northern hemisphere to be at 15° north geomagnetic latitude and if the north-south HF circuit is operating such that the 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 on the ionosphere for the northern circuit will continu- ously come across increasing levels of electron density on two counts, namely, the one due to the vertical gradient as the radiowave penetrates higher into the ionosphere and the other due to horizontal gradient as the wave progresses towards the direction of increasing electron density. On the other hand, for the same link in the north-to-south direction the horizontal gradient is reversed while the vertical gradient still continues to be positive. Fig. 5 shows how the maximum usable frequency changes as the horizontal gradient transcends from negative to positive values for three different angles of incidence at the ionosphere. It was observed from actual observations from top-side sounder satellite that horizontal gradients of three electrons per cc per metre do exist in the equatorial ionosphere which may yield MUF values ranging between 20 MHz and 39 MHz for a predicted MUF of 28 MHz. Thus HF communication in one direction only is possible if a frequency predicted assuming zero gradient is used.

2.3 Magnetic Storm Effects on HF Communication

Magnetic storms are known to be a consequence of interaction between the terrestrial upper atmosphere and enhanced solar wind. While the ionospheric storm behaviour" -⁸ is complex and defies a unique description, the following simplified picture is useful

in modelling HF communications. During the main phase, the electron densities are depressed at mid and high latitudes while they increase to a lesser extent at low latitudes. The F region height however, increases at all latitudes. The increased height and enhanced fo F2 at low latitudes partly compensate for each other

and the MUF values undergo but marginal changes compared to the predicted values. This is one reason that continues to retain HF communications at low latitudes as attractive. While the predictability of the storm-inducing solar event itself remains elusive, it is now possible to predict the storms after seeing the so-

45r---,---,

40 :::EX 35 ru

zlJ.J

::l 30 ClJ.J 0::u, lJ.J 25

--'CD

<t

Cfl 20

t

::l

:::E::l 20.4 MHz :::E

X 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

DELHI GEOMAG. LAl 19.2••••

SU •••• ER Ap >30 ,60

IA. "0 ::l~

~ 20

wCI -40

~

::l-60

0:

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

o " 8 12 16 20 2<t

LOCAL TIME (HRS 1ST)

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

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

2.4 Trans-Ionospheric Propagation Effects

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

radio systems are due to time delay, refraction and scintillation fading (inter-pulse interference). While the predictive capability for quantifying such deterioration does not exist, it is possible to develop morphological models so that appropriate corrections can be applied to mitigate the problems. Fig. 8 shows the percentage of time that reception is unacceptable for different data transmission rates at 1.4 GHz. Such models help in cutting down data transmission rates to ac- ceptable levels of reception for different solar epochs and latitudes. Fig. 9 shows elevation angle error inmil- liradians at Kodaikanal for different elevation angles.

Fig. 10 shows a summary of scintillation data at 4 GHz recorded from INSAT-1B signals during

1984-85. The scintillation indices were rather low, partly because of the low solar activity and partly because Delhi is at the fag end of the scintillation belt. It is known that very intense scintillations do occur even at 4 and 6 GHz at low latitudes during higher solar epochs.

ARWC-NEWDELHI

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

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CONSTRAINTS ON DIGITAL COMMN.

W...J

~ 4.0

>-_0..

~ 3.5 u<!

Z ::>3.0

I.4GHz

±6" G.M LATITUDE MEDIUM SOLAR ACTIVITY -

HIGH SOLAR ACTIVITY---

,

I I I I

w I

i2.5 /

~ I/

o /

~2~ /

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 of percentage of time for medium and high solar activity epochs at a frequency of 1.4 GHz at a low latitude station [These are derived

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

0.7

ELEVATION ERROR KODAIKANAL

1000km WINTER 1980-81 0.6

0.5

0.4 cQ u

E' E 0.3

~

a::o

~ 0.2 w

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 angles at Kodaikanal at a frequency of 500 MHz using ray tracing tech-

niques on model profiles calibrated by ionosonde data

3The Non-Ionized Medium

The terrestrial atmosphere below 60 km can be considered to be non-ionized for practical purposes of radio communications. However, the neutral atmosphere above about 10 km is of little interest to radio communication systems. Thus the neutral atmosphere below 10 km, known as the troposphere, is the only non-ionized medium that we will consider here.

SUMMER 0 WINTER.

(0) AUTUMNAL EQUINOX --1984 - -- 1985 zo

~ 10

...J ...J

~ 5 u

U)

I.L 0 ~+--'-+-'--+-'--+-"=!-'-+----l o

z 1984-1985

of=

:320

...J

f=

~ 15

U) I.L

010 wu zw

g: ⁵

~u

go~~~~~

18 20 22 00 0204 06 LOCAL TIME (HRS) (b) VERNAL EQUINOX

--- 1984 --- 1985 w

~ 10 w

0:

tr

~ 5 uu o

OULr-~++~+-'--~

18 20 22 00 02 04 06 LOCAL TIME(HRS)

Fig. lO-Seasonal distribution of nighttime occurrence of scintillations (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 is well mixed so that the percentage concentration of each constituent does not change with altitude except for water vapour. The tropospheric propagation mechanisms arise due to the fact that the concentration of the gases, especially water vapour change with altitude and also due to the fact that the temperature lapse rate in the troposphere sometimes can even re- verse. Refraction, scatter and attenuation are the three principal features of the troposphere which exercise profound influence on the propagation mechanisms.

To summarize, the following characteristics of the troposphere are most relevant to radio communications:

a Pressure decreases exponentially with altitude.

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

c Hydrometeors and water vapour absorb radiowave energy above several GHz.

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

e Tropospheric turbulence aids long distance scatter communications but, also causes interference from unwanted transmissions.

f Super-refraction and ducting cause anomalous propagation conditions for radar and microwave radio systems.

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

Our present knowledge of the radio climatology of the troposphere is rather meagre for Indian conditions. It has been observed ¹³ that very large errors would be committed if we use tropo path loss techniques developed in more advanced countries. For ex-

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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 than winter field strengths. This we know is almost never true over the entire Indian sub-continent. The inter- face of land and sea causes very serious problems in the Indian coastal regions. The warm ocean surfaces around India abound with perennial ocean ducts of varying thicknesses increasing radar ranges, but also causing severe radar blindholes. The total refractivity of 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, Mis the ionospheric component of refractivity, 1(s) is the air temperature (in K), p{s) is the atmospheric pressure (in mbar), e(s) is the partial vapour pressure (in mbar),f is the radio frequency (in MHz), N;is the electron density (m - 3), and sis the distance parameter.

Fig. 11 shows (after Goodman 14)a typical refractivity profile from ground level onwards past the ionosphere. The tropospheric part is for Bombay, from Sar- kar etal.^15• A considerable amount of work has been published on refractivity variability both in India and abroad. A cc.nprehensive radio refractivity atlas was published as early as in 1977 by Majumdar et al.¹⁶and

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.]

was subsequently updated by Sarkar et aLl5 in 1985.

The revised atlas has used radiosonde data from 32 stations in India and gives a number of parameters such as super-refraction, and sub-refraction occurrence, scatter paramerer C~etc. Figs 12 to 16 give some examples of the basic reference data available in the atlas.

3.1 Tropospheric Monitoring Techniques

The wealth of data available over several decades from the India Meteorology Department through their their twice-a-day radiosonde flights continues to be a valuable asset in view of the uninterrupted long series of observations. However, they suffer primarily from three faults. Firstly diurnal variation is not available. Secondly, the slow response ofthe sensors intro- duces systematic errors. Thirdly, it is impossible to study steep gradients and small-scale turbulence because of poor resolution. Kytoons on the other hand can give much better resolution though a single profile may take more than 45 min and thus the refractivity profile may be contaminated with large temporal variations. The kytoons cannot be operated under windy conditions. Acoustic echo sounders have emerged as useful techniques for monitoring the inversions in the boundary layer. They are unique for ductingand other morphological studies at radar sites and also for monitoring pollution conditions.

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

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

·1 '''Q''~''Ii~

CE.~flFIED copy

~.

1

,I

l 1

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

all overthe country.]

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REDDY: RADIO PROPAGATION TIfROUGH IONIZED &NON-IONIZED MEDIA

countries to yield raw video presentation with colour display for detailed investigation on the cell-by-cell basis in the precipitating clouds. Dual polarized radars are perhaps most suited to investigate propagation impairments especially in frequency reuse systems.

However, radar observations in India are limited

QII'I~'I''Il1lr CERTlFI£C' C~,)'(

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

t

_I

~!.'

"^,^of<^,

~'I

" !

I

Fig. 13-Contours of effective radius factor kover the Indian subcontinent 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 the month 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 through increased range and ground clutter.

By far, the best technique for obtaining accurate Radio Refractivity profiles, with resolution high enough to yield turbulence parameters, is the in situ measurement using a microwave refractometer

r

I_I

+

i^I

I

~. !!' -. --'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 .OOOOGMT I

i· I

;: 3 r•..)

. _.

_,

°O~----~---~5---L---~,o

C~ (10~15m-2/3)

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

ter is essential in troposystems design.]

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mounted on a small aircraft that can spiral vertically up in a short time. Such a solid-state, digital refractometer was designed at the National Physical Labor- atory, Delhi, and was fabricated in collaboration with Defence Electronics Application Laboratory, Dehra- dun. The refractometer was mounted on the CESSNA aircraft of lIT, Kanpur, and flown during 1983 and 1985. Fig. 17 shows a sample refractivity profile thus obtained. Assuming frozen-in turbulence, the temporal variations in refractive index were analyzed to yield the structure constant C~ and the scale sizes of the atmospheric turbulence appropriate to the scattering volume. The profiles of variance of refractive index fluctuations derived from the refractometer data are compared with those derived from radiosonde profiles in Fig. 18. Comprehensive measurements for statistically significant periods are required for de- signing tropo systems and in a number of earth-space systems.

3.2 Large Diurnal Variability

Some of the outstanding problems presented by the Indian troposphere to designers of microwave system of high reliability are caused by large variations in the humidity at very high temperature levels. The diurnal variability 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- I C) LU I

1720

7.30Vl

a:

I

7.31;;;

~

I-

.32.J

<f

u 7.33

:l

7.3.5 7.36

~~~~~~~~~~~~~~~~~7.37

180 200 220 2040260 280 300 320 340 360 380 400 REFRACTIVITY ( N-UNITS)

Fig. 17- A sample radio refractivity profile obtained using the microwave refractometer(The large amount ofturbulence which is obviousin thisprofile will not be seen in a conventional radiosonde

profile.)

NORTHERN PLAINS

- AEFRACTOMETER -- --RAOIOSOND E-c;

S- SUMMER

w-WINTER <t200GM)

w S

2.0

E ~

->< II 1.5 \

I- \1

I II

o \\

W .\

I \\

1.0 \\

\ \

0.5 \ \

\ \

W S

0

10-11 10^-9

Fig. IS-Comparison of height profiles of the variance of refractive index fluctuations and the structure constants C~measured with 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:::>

zI

~

>- 400

~>

I- U

<f Il::u,

LU Il::

350

BOTTOM SCALE

.,

-10 -5 o 5 10 15

TEMPERATURE, ·C

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

temperature shown at the top.]

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-"E

~2

---

III

~

26 ^SKY ^PATH ^Z0

22.2GHz >=

OQZ+0245R <t

24 ::J

rn ZW

".20 ^18GHz ^I-

Z A=O.OI+OI98R I-

'2 ^<t

I-

<I 16 ^U

0) <;:

ZW U

I- 12 w

l- II GHz ^Q.

<I

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

cides the k factor as well as the height of the common volume in troposcatter systems is particularly large in the tropical climate. Fig. 19 shows the variation of calculated atmospheric refractive index with temperature at 60% relative humidity. At lower temperatures, for example, when the temperature changes from

- lOoe to

+

lOoe the variation in refractive index is very little in contrast to much larger variations in the refractive index values at higher temperatures say from +20⁰

e

to +40°C. For higher relative humidity values which are common in India, the changes at high temperatures are more spectacular. Hence it is essential to take note of the importance of humidity and warm temperatures whose variations can cause dras- tic changes in the k factors, for Indian climatic conditions.

3.3 Attenuation due to Hydrometeors

Out of all types of hydrometeors such as rain, hail, ice, fog, cloud or snow, rain is the most relevant one for tropical latitudes in causing absorption and scatter of

8r---~

7

WALLOPS ISLAND VA SUMMER 1973

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

of rain cells measured at that value.]

(CONVERTED TO 25 k m EQUIVALENT 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 radiometric 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 depolarization, antenna gain degradation and bandwidth coher- ence reduction. There have been some scanty rain attenuation measurements using radiometers as well as line-of-sight (LOS) microwave systems in India but no results are available on depolarization as well as on the vertical extent of rainfall statistics necessary for pre- dicting earth-space path attenuation. Fig. 20 shows mean rain rate profiles for measured surface values as estimated from radar reflectivity measurements by Goldhirsh and Katz" at 2.8 GHz at Wallops Island.

The very rapid fall off starts above 3to 4 km and such statistics at tropical latitudes are necessary to deter- mine the total path attenuation for satellite radio systems. Fig. 21 shows attenuation at 11, 18 and 22.2 GHz over New Delhi from radiometric measurements^18.Rain attuenation from LOS link observations at7 GHz(pathlength42 km)areatsoshown.Anaver- age rain cell length of2.S km was assumed fornormal-

7

Ql

6":

N

I 5(!)r- 4 ~<t

Z 3~t:i

2 ~ ILl~

110-

<t

140 -ATTENUATION

x xx RAIN RATE

~ 120 s:

E

¹⁰⁰

E

ILl 80

t:i

cr 60 Z<i 40

cr

20

o 0

104 10-3 10-2 10-1 100 10

PERCENTAGE OF TIME ORDINATE EXCEEDED

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

102,---.

10-I

150mm/hr

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

12.emm/hr

/.

I

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

/. PALMER

/. _._.- JOSS (WIDE-

, SPREAD RAINI

-2

10 ~-L~-LI~O---~IOLO---I~~

FREQUENCY(GHz)

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

various distributions are only marginal.]

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

izing the LOS values. Fig. 22 shows the cumulative distribution of both rain rate and attenuation at 7 GHz for different seasons during 1977-78 (Ref. 19). The rain rate measurements were made with a fast response rain gauge having an integration time of lOs at Delhi. Fig. 23 shows" specific atenuation due to rain calculated for 1 to 1000 GHz using drop size distributions of Laws and Parsons, Marshal and Palmer and Joss with a rain temperature of 20°C. These results are obtained by a series expansion solution of the Mie scattering coefficients. It can be seen that above about 50 GHz the attenuation levels off and even drops slightly at higher frequencies. The most important thing to notice is that there is very little difference between the three distributions, with the obvious conclusion that dropsize distribution of models available are adequate for rain attenuation purposes though they may be inadequate to estimate depolarization effects.

Thus, the most important information required in any geographical region is good rainfall statistics obtained with faster response rain gauges.Such an attempt has been initiated by the National Physical Laboratory, New Delhi, by establishing a network of 7 fast response rain gauges all over India.

3.4 Attenuation due to Water Vapour

Regarding gaseous absorption in troposphere, the first three absorption bands are centred at frequencies of 22.2 GI Iz for water vapour and at 60 GHz and at

118.8 GHz for molecular oxygen.While the distribution of oxygen is well known and invariant, water vapour shows considerable variation. An atlas" was brought out by the NPL giving water vapour profiles in the 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 ' over surface during monsoon at 0000 GMT [Water vapour attenuation problem will be more serious for terrestrial links since the whole

path will be in the lower troposphere, 1

24 shows just one example. The importance of water vapour attenuation can be realized from the fact that the intense rain cells are usually a few kilometres in length whereas terrestrial LOS systems will have to contend with hundreds of kilo metres of humid atmosphere.

3.5 Radar Propagation and Targeting Problems

The surface-based evaporation ducts over warm tropical oceans result in the so-called anomalous microwave propagation. Ironically, the anomalous propagation conditions exist most of the time and it is necessary to provide a capability to ship-borne systems to assess these effects and to display in quantified terms the expected performance of naval surveillance, electronic warfare and comminication systems. The Naval Oceans Systems Centre of USA has developed an integrated refractive effects prediction system'?

(IREPS). This particular model was developed using asymptotic forms of the plane wave reflection coeffi-

40'

SELECTEDRE~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 from observations in specific areas

21

<0'

SELECTED REPRESENTATIVE AREAS MONSOON

o

^RADAR ^RANGE(km I.).." 3cm 30'

~''" ""'" ~"m

2O' '050

ill!

'"

"

~ ⁱ _[EJ

>- ,0r-'

., _'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 ranges within the evaporation ducts for a 3 em radar

(12)

REDDY: RADIO PROPAGATION THROUGH IONIZED &NON-IONIZED MEDIA

90 50

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

14 WINTER

PROBABI LITY

85 50 24 PRE-MONSOON

80 28 22

70

94 ₇₀ ₅₀

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

43 30

25 15 8POS1'-MONSOON

1000

E .><

UJ

\!) Z

<l Q:

0 300

z~

0 Q:

\!)

100--~--

J

50~ -J ~ ~~ L- ~

-=~

+10 -50 -100 -150

INITIAL REFRACTIVITY GRADIENT (LlN)

-200 -250

Fig. 27 - Tropospheric height error contours for a target heightof 5000 ft [The height errors indicated on the countours are in 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 NPL group using ray theory for the Indian environment have met with some success. Some of the basic data generated to implement a more elaborate wave mode approach are shown in Figs 25 and 26. The duct heights are maximum over the equator and fall off gradually towards higher latitudes. Larger duct heights mean trapping of lower frequencies as well as more frequent radar detection problems.

Surveillance and ack-ack gun radars are usually corrected for a kfactor of 4/3. Tropospheric refraction not only increases the range but,also substantially alters the elevation angle especially in tree-top flying.

A three-dimensional programme was developed at NPL to trace the ray in the troposphere and a compu- ter programme was evolved to estimate the range and elevation angle errors if an appropriate refractivity profile isfed to the programme. With very little effort, the surface refractivity measured on real-time basis can be used as a first-order approximation. Fig. 27 shows the height error contours for a site in India. The probability distributions of initial refractivity gradient for different seasons are indicated on the top of the fi- gure.

4 Conclusion

The 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 practical radio systems. This is not purported to be a review of the Indian work; the i'lustrations given are only to demonstrate the serious .ess of the problems. The examples given here do not include the results from the Defence Laboratories in India or from the Depart- ment of Telecommunications and the All India Radio.

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