Indian Journal of Radio & Space PhysicsVol. 15, October & Pecember 1986, pp. 163-170

Fifty Years of Radio Scattering

WEGORDON

Rice University, Honston, Texas 77001, USA

Radio scattering from atmospheric irregularities has servedfor about half a century as a mechanism for both remote sensing ofcertain properties of the atmosphere and for reliable radiocommunication beyond the horizon to distances of 2000 kilometers.

In its earliest form, radio scattering was used to observelayers and their changes in the lower atmosphere. In mid-career,i.e., 25 years ago, it was introduced as a sensor of the proper~ties of the upper ionized atmosphere, and most recently it iswidely used to observe winds especially in the troposphere andstratosphere.

It is an honor to join in paying homage to an early giant ofatmospheric science.

As a communication mechanism, radio scattering by tropospher­ic irregularities, discovered quite by accident, was explained andexploited to provide links over distances of a few hundred kilo­meters. With time the terminals became more powerful and sensi­tive, the links became longer 'and the height of the scatteringirregularities increased climaxing with ionospheric forward scat­ter. Continuing the upward search for scatters led quite natural­ly to incoherent scatter. Powerful radars spawned MST's (meso­sphere, stratosphere, troposphere) wind measuring capabilities.

It is an honor and a privilegeto join in the Golden Jubilee Cele­bration honoring Professor S. K.Mitra, and I am grateful to ProfessorA. P. Mitra for inviting me to par­ticipate.

To put the celebration in pers­pective, we note that radio waveshave been propagating for billions of

years through the universe- sincethe beginning of time, give or take afew nanoseconds! Radio waves propa­

gating purposefully are less than 100

years old by a few years, if Hertz in1888 was first, i.e., another Hertzon another planet .in another solarsystem didn't precede him.

Radio wave propagation and th~communication revolution it spawned,however, are products of the twen­tieth century. Guglielmo Ma~coni,

inspired by Hertz, set the pace withhis bold experiment perhaps in partbased on Tesla's conjecture of a highreflecting layer in the atmosphere.In December 1901 Marconi transmitteda series of code S's from Poldhu inWales to St. Johns in Newfoundland,3000 km around the curve of the earth

using radio waves that ~ere supposedto travel in straight lines andinitiated wireless telegraphy acrossthe ocean. Heaviside and Kennellyl

"indisputably" explained the channelwithin a year, establishing the

present pattern in which theoreti­cians in no time at all explainobservations that experimentalistssweat over for years.

Heinrich Hertz generated, propa­gated, and received radio waves, andrecognizing the problem inherent inhis broad-band spark-source thought

163

INDIAN J RADIO & SPACE PHYS, VOL. 15, OCTOBER & DECEMBER 1986

that the communication of information

would not be feasible.

Microwaves whose beginnings wenormally associate with the remark­able mobilization of scientific and

engineering talent during World WarII were in fact produced, radiatedand received before the end of the

1800' s - 4 GHz in 1894 by Sir OliverLodge in a demonstration at a meetingof the American Association for the

Advancement of Science, and 60 GHz in

1897 by J. Chunder Bose2 in Calcutta.The ideas were not followed since

practical interests were in long'waves. In the course of this cele­bration I should like to learn more

about the other radio pioneers inIndia and in particular about S. K.Mitra •.

The radio scientists of India

joined the International Union ofRadio Science (URSI) in 1950. The

formal resolution was passed at theURSI General Assembly in Zurich. Thereports of the radio scientists ofIndia to the URSI provide a source of •information on the early work on theionosphere.

The Report of the NationalCommittee of India to the Zurich

General Assembly includes a note onthe activities for 1948-49 of theRadio Research Committee of theCouncil of Scientific and IndustrialResearch of India:

(1) General - The Radio ResearchCommittee of the Council of Scientif­ic and Industrial Research was cons­tituted in 1943 and has been func­

tioning ever since. The work of theRadio Research Committee covers all

aspects of research in wirelesscommunication and radar;

(2) Utilisation of CompletedSchemes - Several schemes of researchconducted under the ~adio Research

Committee have been completed anddefinite results achieved. Particu-

164

lar mention in this connection may be

made of the researches on salvagingof electrolytical condensers andreconditioning of lead acid bat­teries. A patent application on"Improvements in or relating to themanufacture of volume controls"

arising out of the scheme on manufac­ture of cheap radio sets by Dr. G. R.Toshniwal has already been filed;

(3) Schemes in Operation - TheSchemes at present in operation underthe Radio Research Committee are:

(i) Manufacture of radio valves inIndia by Prof. S. K. Mitra, Calcutta;(ii) Ionospheric investigation byProf. S. K. Mitra, Calcutta; (iii)Theoretical investigations of theupper atmosphere by Prof. M. N. Saha,Calcutta; (iv) A new technique ofinvestigating the ionosphere by Mr.B. M. Banerjee, Calcutta; (v) Polari­sation of down coming waves by Dr. S.R. Khastigir, Banaras Hindu Universi­

ty, Banaras.

In 1952 at Sydney the Report ofthe National Committee to Commission

IlIon the Ionosphere included:

(1) Institutions - In India,researches on the ionosphere werefirst started in 1930 in the Univer­

sity of Calcutta under Professor S.K. Mitra. The ionosphere stationparticipated in the Polar Year obser­vations of 1932-1933 when regularrecords of E and F layer virtualheights were kept. The Council ofScientific and Industrial Research

sanctions grants for researches to beconducted at the Universities and

other research organizations. Duringthe last two years, ionosphericinvestigations were conducted underits auspices at (a) University ofCalcutta under Profesor S. K. Mitra

and under Professor M. N. Saha; (b)Banaras University under Dr. S. S.Banerjee and under Dr. S. R.Khastgir; (c) Physical ResearchLaboratory under Dr. K. R. Ramana­than.

·.. --

GORDON: FIFTY YEARS OF RADIO seA TIERING

25

Professor S. K. Mitra along withK. S. Krishnan and A. P. Mitraattended the Assemblies in 1952

(Sydney) and 1954 (The Hague).

(3) Research Reports - mentionrates of electron production in bothregions E and F2; the seasonal varia­

tion in temperature; coefficient ofrecombination; the earth's magneticfield at region F2 heights; 27-dayrecurrence tendency; solar tidaldrifts; formation and structure ofthe D-layer; structure and theproperties of sporadic E-region; Esat the geomagnetic equator; the Ztrace; the sodium twilight glow;

dissociation of N2 molecules; andSolar Eclipse.

Fig. 2. The results of Breit and Tuve.

S. K. Mitra and SyamS in 1935announced that they had detectedregular reflection of radiowaves atvertical incidence from an equivalentheight of about 55 km. About a yearlater on March 8, 1936, Mitra andBhar6 made a further announcement

that they had recorded echoes ofradio waves from still lower heights(Figure 3). Soon after Colwell andFriend7 from West Virginia, U.S.A.,reported the recording of echoes fromregions between 55 and 5 km and alittle later Watson Watt, Bainbridge­Bell, Wilkins and Bowen8 announcedthat they also had occasionallyrecorded echoes from similar heights.

Observatoriesin India (see

(2) IonosphericWorking RegularlyTable at the end) .

The first direct measurements of

the ionosphere were made in 1925 byAppleton and Barnett3 in London byvarying the phase of a CW signal andnoting the interference, Fig. 1,between the direct and reflected

waves at a nearby receiver; in thesame year, Breit and Tuve4 trans­mitting and receiving pulses recog­nized ground and sky waves, Fig. 2,laying the foundation for ionosondenetworks and predictions of themaximum usable frequencies forvarious communication paths andtimes.

• 6

••••• 3 ~ n-: (.).c. rePoo ($5 kID.); (6), C. resIon (10 1aL).

These observations from three

independent sources situated in threewidely different parts of the worldshowed that the atmosphere below 90km is also stratified particularlyduring daytime into ionized layerslike the upper ionosphere. Thelayers were found to be groupedroughly in three different heights;one at about 55 km another round 30

km and the third in the upper tropo­sphere between 5 and 15 km. It isnow generally agreed that the 55 kmlayer, which might be said to havebeen rediscovered by Mitra and Syam,should retain the original designa­tion - the D layer - as suggested byAppleton and that the 30 km. and the

TimeI I I I r I

!M)() IHl5 9010 9-16 9020

Fig. I. The results of Appleton and Barnett.

o

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101-~g••

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165

INDIAN J RADIO & SPACE PHYS, VOL. 15, OCTOBER & DECEMBER 1986

).,

i

Fig. 4. Tropospheric scatter results.

Ducting, in fact, is an effic­ient means of carrying waves aroundthe horizon, as long as the ductexists. Without a duct, the Navy wasobserving much weaker but still use­ful signals. Those observationsstimulated Booker and me12 to look

for an explanation. The explanationwe proposed was based on radio scat­tering in the troposhere by irregu­larities tied to the turbulence

normally embedded in the atmosphere.Like Crawford's observation, here wasanother example of radio scatteringin the troposhere, and it nicelyexplained the distant observationsover the water. This became known in

the literature as troposheric scat­ter. The Caribbean Islands today,for example, are linked by tropo­spheric scatter as are the NATOcountries in Western Europe.

RANGE IN IlAUTICAL IIILES

Katzin11 reported some experi­

ments in the Atlantic conducted bythe U. S. Navy. In 1945, they dis­covered that radio waves traveled far

beyond the radio horizon (Figure 4),and good communications could beestablished between ships that werewell beyond the line of sight. Theextended communication range couldnot be accounted for by radio duct­

ing, a common overwater phenomenon.

20 40 110 80 100 120 140 110 110

lEGEND

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16"14-·... -.. .•;,0\

111-24----~ 411-'4---- ~ A 41114--0 -' 40•. lD~•. 50•• 0 I1-\'k \1IIII1IIT•• 0.;10

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At an early stage, 110 wrote apaper saying that those echoes wereproduced by differences in refractiveindex between bubbles of air and

their surroundings. Those differ­ences would produce a small reflec­tion or more precisely a small scat­tering. The differences arise in twoways: (1) convection from a heatedground and (2) shear at a boundary oftwo air masses.

In the desert of Arizona in 1945Arthur Crawford of the Bell TelephoneLaboratories had a very sensitivemicrowave radar which he pointedvertically in the atmosphere. Theradar was sufficiently sensitive thatit could detect weak echoes through­out the troposphere. Sometimes theseechoes were randomly distributed inheight and time, and at other timesthey would be concentrated in thevicinity of particular heights. Thatis, they occurred more or less inlayers.

Reflections of radio waves from

these low heights raise points ofgreat theoretical interest. Firstly,what is the mechanism of the reflec­tions? The usual Eccles Larmor

theory of bending of radio waves byan ionized atmosphere is, on account

of the high collisional frequencybetween the electrons and ions and

the gas particles, certainly inopera­tive. Secondly, in any mode ofreflection in which a large densityof ions or electrons is necessary the

question arises - what is the originof the ionization.at such low heightsof comparatively high atmosphericdensity? These three paragraphsquoted from Mitra, Bhar and Ghosh9provide the earliest report of radioscattering from a turbulent atmo­sphere, and I shall use this as mypoint of departure.

lower layers should be called C

regions - C2 and C1 for instance.

166

GOROON:FIFTY YEARS OF RADIO SCATIERING

Troposcatter takes a powerfultransmitter and large antenna on eachend because the scattered signals areweak. But, it does provide reliablecommuncation' channels. Particularlywhere line-of-sight microwave linksare not practical (longer overwaterpaths) or where fewer terminals arewanted. Having had some success withradio scattering in the troposphere,Booker and I both were looking forother· ways to extend the idea. Iconcentrated on the stratosphere andproduced a paper13 on stratosphericforward-scatter for communicationspurposes and showed that distance ofthe order of 600 miles were achiev­able (the distances are largerbecause the scatters are in facthigher). Henry Booker, on the otherhand, thought of forward scatter froma rather higher height in the vicini­ty of 90 or 100 kilometers, and thiswas a very successful endeavor. 14 Hepredicted forward scatter from iono­spheric turbulence. Scattering inthis layer has characteristics thatare quite different .because therefractive index is dominated by acomponent that depends on the elec­tron density difference rather thanthe temperature and moisture. Al­though I wasn't directly connectedwith Booker's success, it representeda progression in the altitude of thescattering. I was intrigued by theprospect of scattering in the atmo­sphere at heights that would be inthe upper part of the ionosphere. Itwas in the spring of '58 when it allcame to a climax. One evening Icalculated what turned out to be theradius of the electron or the scat­tering cross-section of an electron.It's an extremely little bit scat­tered by a single electron but if theionosphere has large numbers of elec­trons present that product begins tomake the scattering cross-sec,tion ofsome volume of the ionosphere not sominute, and so it was worth lookingat the possibility of doing a commun­ications scheme involving the Fregion.

The scattering is based on thefact that the electrons are there,and the electrons scatter an incidentradio wave. Since each electronwould scatter independently, thescattering from many electrons isincoherent so that the total scat­tering might amount to something.That something, was it large enoughthat it could be detected, that itcould be used as a communicationmeans? Whenyou do the calculation,and ask howmuch power do you have toput into a transmitter, and how bigan antenna do you have to build, thenyou are quickly discouraged. Youdiscover that in terms of a communi­cation link, it clearly isn't econom­ical.

To illustrate it, jump ahead fora moment. You know how big theArecibo antenna is (1000 ft. diame­ter) and how it has a powerful trans­mitter associated with it. If youwanted to communicate between twopoints on the ground, they now couldb~ a few thousand kilometers apart,and you would have a rather depend­able communication scheme - exceptthat it would take an Arecibo antennaon each end, and that's clearly notvery practical.

Nowlet's go back to the point.Communication is not feasible usingstate-of-the-art hardware, but couldyou study the medium? That is, couldyou measure the number of electronsthat were present at each height inthe ionosphere and, therefore, getsome detail that was previously notavailable through the ionosphere?It's not hard to write an expressionfor the backscatter from the elec­trons if you know the scatteringcross-section. If you know somethingabout the available hardware: thereceivers, and the transmitters, thenthe only unknown is the antenna. Inround numbers at least, you need anantenna which is 1000 feetlS indiameter, and a thousand feet indiameter is a rather large antenna,

167

INDIAN J RADIO & SPACE PHYS, VOL. 15, OCTOBER & DECEMBER 1986

by anyone's standards.

Having gotten this far with theidea, I began to have discussionswith Henry Booker, who was happy todiscuss the details of the calcula­tions, and he pointed out that whenhe was a graduate student back atCambridge he had made the same cal­culation. That is, he had calculatedthe scattering cross section of as.ingle electron, and in fact it hadbeen done a long time ago, of course,by people like Thomson.

The thing that was different in1958 was that it was possible to usethis cross-section with availablehardware and with a thousand footdish, if it could be built. Theexcitement in this experience atCornell must echo that at Calcuttawhen S. K. Mitra and his collegueswere developing the equipment fortheir pioneering observations.

One of the students at Cornell,not long before that, had been astudent of Henry Booker's named KenBowles. We talked to Ken about inco­herent scatter, and he was a veryfine engineer. He immediatelygrasped what was going on and knewthat the Bureau of Standards happenedto have a rather large field orantennas in Illinois and a powerfultransmitter.

In October I gave a paper onincoherent scatter. I started thepaper by saying to the audiencesomething like the following, "Mypurpose is to tell you about inco­herent back scatter from the iono­sphere, and the possibility of build­ing a tool to make use of it instudying the ionosphere. And then Iwant to tell you about a telephonecall that I just had." And so I wentthrough the paper that had been pre­pared and then added the conversationthat I'd just had on the phone withKen Bowles, who had received thefirst incocherent back scatter echoes

168

from the ionosphere. A dramatic

touch that was added to that meeting.

After that day in October whenhe made his first observations, he,of course, repeated the observationsmany times, and two important thingscame out of the observations16­incoherent scatter was real, elec­trons did back scatter. In my view,the electrons were going to behavecompletely independently, and, there­fore, the broadening of the trans­mitted frequency by the scatteringcould be associated simply with thethermal motions of the electrons -andwould have a band width which mightbe on the order of a megahertz or so.

What Ken observed, however, wasthat the bandwidths were more like akilohertz rather than a megahertz.This, he correctly deduced, wasassociated with the fact that inaddi tion to the electrons beingpresent, there were ions present, ofcourse, and that the Coulomb attrac­tion between the electrons and theions was sufficient that the spectrumthat came back was controlled by themotion of the ions, rather than bythe motion of the electrons. Thatwas important for many reasons.Obviously, if the ions have an influ­ence on what's happening, then onecan learn something about the ions,or the ion motion and ion tempera­ture, as well as the electron temper­ature, as long as that isn't com­pletely suppressed by the ion con­trol.

The theory of incoherentscattering had to be put on soundgrounds. Salpeter 17 proceeded todo that and at more or less the sametime people like Farley andDaugherty18 produced a theory of backscattering in some detail, and therewere others who did the theory from avariety of approaches. All of theapproaches, fortunately, resulted incommon results. The results werethat the back scatter spectra would

+-

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GORDON: FIFTY YEARS OF RADIO SCATIERING

W.

contain not only things like theelectron density but also ion temper­ature, electron temperature and per­haps something about the ion compo­sition.

Incoherent scatter observationsare revealing the secrets of theionosphere at Arecibo in Puerto Rico,Jicamarca in Peru, Millstone Hill inthe U.S., Sondrestrom in Greenland,Tromso in Norway, St. Santin inFrance, Shigeraki in Japan and soonin India as a capability of a pro­posed MSTradar (mesosphere, strato­sphere" .!.roposphereh

Atmospheric irregularities areproviding wind velocity profiles forresearch (MST) and in networks (ST)for meteorological purposes.

HF, which many thought wouldbecome obsolete with the increasedusage of short waves and satellites,is still of great interest in commun­ication, in beyond-the-horizon radar,and in ionospheric studies (plasmaheating) •

Healthy activity is bearingfruit in all parts of the spectrum.The marrige of computers and communi­cation into information systems putspressure on spectrum use challengingthe engineers for better, faster andcheaper information exchanges.Remote sensing in its wide array 'ofapplications opens avenues for infor­mation gathering about resources onand in the earth, on conditions ofatmospheres and oceans and diagnosingand treating humanailments.

The propagation of radio wavesin media of all types provides fasci­nating problems for radio engineersand scientists, theoreticians andexperimenters. I know of no way tolook 100 years ahead but it is clearthat the next decade or two will beexciting. The technical meetingsshould be alive with stories of newdiscoveries by theorists with the

vision of a Maxwell and experimenters'with the courage of a Marconi and theforesight of a Mitra.

References

1. Heaviside, 0., Encycl. Brit., 33,215 (1902) and A. E. Kennelly,Elec. World, 39, 473, 1902.

2. Southworth, G., Froc. IRE, 50.3,1200. 1962. -----

3. Appleton, E. V., and M. A. F.Barnett, Proc. Roy. Soc., 109,621, 1925 and Nature, 115, 333,1925. ----

4. Breit, G. and M. Tuve, Phys. Rev.(2), 28, 554, 1926.

5. Mitra, S. K., and P. Syam,Nature, 135, 953, 1935.

6. Mitra, S. K., and J. N. Bhar,Science and Culture, 1, 782,1936. --

7. Collowell, R. C. , and A.Friend, Nature, 137, 782, 1936.

8. Watson, 'R. A. et al., Nature,137, 866, 1936.

9. Mitra, S. K., J. H. Bhar and S.P. Ghosh, Indian J. of Phys., 12,455, 1938.

10. Gordon, W. E., Proc. IRE,.E., 41,1949.

11. Katzen, M., Proc. IRE, 35, 891,1947.

12. Booker, H. G., and W. E. Gordon,Proc. IRE, 38, 401,1950.

13. Gordon, W. E., Proc. IRE, 45,1223, 1957.

14. Bailey, D. et a1., Proc. IRE,..!l,1185, 1955.

15. Gordon, W. E., Proc. IRE, 46,1824, 1958. -

16. Bowles, K. L., J. Res. NBS, 65D1,1, 1961. ----

17. Salpeter, E. E., Phys. Rev., 12.,1528, 1960~

18. Dougherty, J. P., and D. T.Farley, Proc. Roy. Soc, 259A, 79,1960.

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INDIAN J RADIO & SPACE PHYS, VOL. 15, OCTOBER & DECEMBER 1986

Station

Delhi

28°35' N7r5' E

Calcutta22°33' N

88°22' E

BombayWN73° E

MadFas

139 N80° 15' E

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170

, I'

Nature

of Apparatus

Manual (Panoramicrecorder will be

utilized)

Semi-automatic

(Automatic recorderwill soon be under

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Manual

Manual

Manual

IonosphericParameters Recorded

(U.R.S.I. Nomenclature)

foF2, hpF2, M(3000)F2and MUF's

foE, fEs, foF2, h'Es

hpF2, h'F2, M(3000)F2and MUF's

foF2, hpF2, M(3000)F2

(only for 0600-1800 hrs)

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(only for 0600-1800 hrs)

foF2, hpF2, M(3000)F2

(only for 0600-

1800 hrs)

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