Conference reportGeneral Assembly of the International Union of Radio Science

Washington, USA, 10th-19th August 1981

The International Union of Radio Science is, as the nameimplies, an international body with the task of stimulating col-laboration and exchange of information in the field of radioscience. Its activities range quite widely, and include thepromotion of particular techniques and the examination ofspecific sponsorship of suitable meetings held by other bodies.URSI has an office in Brussels and is governed by a counciland its board of officers including the Secretary General,Prof. J. van Bladel of Belgium. There are nine commissionscovering the topics of electromagnetic metrology; fields andwaves; signals and systems; physical electronics; electro-magnetic noise and interference; waves in nonionised media,plasmas, ionospheric radio and propagation; and radio astron-omy. Activities within this country are co-ordinated, under theauspices of The Royal Society, by the British National Com-mittee for Radio Science which is chaired by Prof. A.L. Cullen,O.B.E., F.Eng., F.I.E.E., F.R.S. and has Sir Granville BeynonF.R.S., a former President of URSI, as one of its members.The IEE is also represented on the Committee.

One of the major highlights of the URSI calendar is theGeneral Assembly which is held every third year and reviewsprogress over the whole field of radio science. As can be im-agined, attendance at a General Assembly can be a stimulatingand an exhausting experience and the recent one in Washingtonwas no exception. Over a period of ten days some 90 sessionswere held, many of them containing up to ten or more papersbut including some excellent longer reviews. In addition to thevarious commissions, there were also five open symposia whichcovered remote sensing, millimetre and submiilimetre waves,mathematical models for radio propagation, interaction ofelectromagnetic waves with biological systems and waveinstabilities in space plasmas. Any attempt to summarise sucha wealth of material in a short article is doomed to failure, andthe best one can do is to outline just a few of the principaldevelopments which were reported.

Attempts to follow a particular path through the maze ofsessions were made more difficult by the strike of air-trafficcontrollers in the USA, which meant that some intendedspeakers did not arrive, while others were delayed by a day ortwo and were fitted into unscheduled slots later in the pro-gramme. Nevertheless the local organisation was good and thedisruption was kept to a minimum.

Electromagnetic waves

Activities in fields and waves are mainly directed towards anunderstanding of all aspects of the theory of guided electro-magnetic-wave propagation, radiation and scattering, and arestrongly influenced by developments in telecommunications.A particularly important topic is that of antennas to whichseveral sessions were devoted covering, inter alia, reflectorantennas, arrays, feeds, microstrip and VHF antennas, as wellas measurements. Applications to spacecraft are of especialinterest and topics suggested by CCIR,' listed in Table 1, werediscussed in detail. The generation of multiple beams from aspacecraft antenna poses fundamental problems, which arebeing addressed in Europe and the USA, while the number of

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beams and beam characteristics which can be obtained from areflector of given size is a key problem.

Phased arrays for the next generation of systems must meetstringent pattern control requirements. There is a need todevelop a fundamental theory which will show how the arrayproperties, including phase errors, influence the number ofnulls, null depth and bandwidth. The development of feed net-works to meet the more exacting phase constraints of the newphased arrays poses difficult engineering problems. Further-more, array elements are still too expensive and innovation isrequired to bring element costs down to the stated target of$10.

The development of feeds with low crosspolarisation eitherover wide bandwidths or in dual bandwidths continues to exer-cise research workers. Feeds with two depths of corrugationoffer the possibility of exceptional performance in two widelyseparated bands, while presenting a formidable challenge tothe theorist. The array feed has been investigated for multiple-beam and shaped-beam antennas but much more remains to bedone. The question whether an array feed, or a single shapedhybrid feed, offers best performance for a shaped-beam space-craft antenna for satellite TV broadcasting remains a centralproblem.

The performance of microstrip antennas still suffers fromthe lack of an adequate theory for surface-wave effects and theproblems of discontinuities. Calculation of the phase coef-ficient for a microstrip line is not yet accurate enough for thedesign of arrays, and present design procedures are empirical.A small error in phase coefficient has disastrous effects onarray performance. For example, a 1% error in a 36-wavelengtharray can cause 20 dB sidelobes. Present theory has 15%margins on accuracy; hence the seriousness of the problem.

The choice between near-field/far-field and far-fieldtechniques remains the main area of discussion in microwaveantenna measurements. With the expansion of the highermicrowave frequency bands the far-field condition requiresever longer test ranges. The use of measurements at two dis-tances within the Fresnel region is being explored as analternative to far-field measurements. Ultimately, an antennafacility located in geosynchronous orbit may be needed toovercome difficulties of ground reflection and to ensure far-zone conditions. Such a facility seems likely within a decade;although it is not clear whether this will be achieved by using acommuniciations satellite after expiry of its normal life or bythe use of a special satellite.

Signals and processing

The first session under signals and systems, covering micro-electronics, was marred unfortunately by the failure of half thespeakers to arrive, but there was a useful review of the impactof VLSI (very-large-scale integration) on telecommunications.

The hastening transformation to digital signalling and VLSIwill bring stored-program control and distributed intelligence;with new services to users of credit-card shopping, electronicmail and the ability to know who is calling before answeringthe telephone. The latter, in particular, might have an interest-ing effect on social behaviour. Technological developmentsby 1990 will include a 1 Mbit random-access memory at acost of only 0.01 $ per bit. The need for a change in appraochto the design of VLSI circuits was emphasised with theneed for system-architecture development to embrace delay

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Table 1: CCIR study programme 1C-1/4: characteristics of antennas atearth stations in the fixed satellite service

(i) Determination of the relationship between near- and far-fieldpatterns of earth stations in the fixed satellite service and other stationsusing the same frequency bands.(ii) Determination of coupling between antennas close to one another.

(iii) Determination of the effectiveness of pits or other devices forproviding shielding.(iv) Determination of the radiation characteristics of each beam of

multibeam antennas.(v) Determination of the limitations on the number of beams that can

be generated by a single antenna, and the determination of the mini-mum achievable angular separation between the beams.(vi) Determination of the polarisation discrimination performance of

antenna systems in all angular regions with particular regard to theantenna type.(vii) Determination of the desirable limits of steerability of earth-stationantennas for geostationary satellites.

times within the circuit, symbolic layout and design verifi-cation. There was also a stimulating talk on switched capacitornetworks.

The sessions on hardware for fast signal processing had twodistinct themes. The first covered the performances beingachieved with the various types of device, including semi-conductors, surface-acoustic-wave components and Josephsonjunctions. The second illustrated the considerable impact high-speed (providing it is cheap) electronics can have in radioscience; ranging from conventional radio astronomy, signalprocessing in very long baseline interferometry, spectralmeasurements and fast correlators for ionospheric studies.Among systems papers were those considering the effect, inspread-spectrum communications, of mutual interference andjamming, when frequency hopping is used, and the possibleapplication of satellite links to public mobile communications.

In the open symposium on millimetre and submillimetrewaves a series of papers on ultra-low-noise millimetre-wavereceivers was included. This concentrated mainly on super-conductor/insulator/superconductor (SIS) tunnelling junctionsin which some spectacular results have recently been obtained.SIS devices can be made into detectors and heterodyne re-ceivers capable of approaching quantum-limited sensitivity atmillimetre-wave frequencies. These nonlinear tunnelling devicesperform the same detector functions as standard Schottkydiodes and other resistive mixers but with an important physi-cal difference. At sufficiently high frequencies a single-particletunnel junction ceases to respond classically and becomescapable of detecting individual quanta. The results of thistransformation are dramatic and appear certain to revolutioniselow-noise receiver technology in the millimetre-wave region ofthe spectrum. One example is that of a tin/tin-oxide/tin junc-tion operated as a mixer at 36 GHz in which a single-sidebandconversion gain of as much as 4.3 dB has been observed at amixer noise temperature of only 10K.

Noise

Noise, as someone said of the poor, will always be with us, andwe seem to falling in line with another old adage 'If you can'tbeat 'em, join 'em' because, as the techniques for overcomingnatural noise sources are improved, the effect of manmadenoise becomes progressively larger. Several presentations onmanmade noise were concerned with mathematical represent-ations of noise and the analytical treatment of its influence oncommunications; but some experimental data were presentedon ignition noise, general noise levels in cities and noise frompower lines. The need for more data on noise from nonradiosources was stressed.

Studies of noise from lightning covered a wide range ofaspects, including the description and location of sources, pro-pagational factors and the characteristics of the received noise.

The discussions demonstrated an awareness of deficiencies inour knowledge of intracloud discharges and atmospherics atVHF and UHF frequencies. It was also recognised that theincreasing use of digital modulation and spread-spectrumtechniques calls for the evaluation of noise parameters whichhave not yet been widely studied. The results of studies ofsource distributions by satellite observations were presented,and considerable stress was placed on the need for improvedcollection and presentation of world-wide noise data for theplanning of communications systems.

Other sessions considered noise-measuring techniques andthe effects of impulsive noise on communications.

Atmospheric and space propagation

The importance of propagation in, and through, the atmos-phere and in space, both from the point of view of a funda-mental understanding and for practical reasons, was reflectedin the number of papers and the enthusiasm of the discussion.Thus fifteen papers dealt with measurements on the middleatmosphere to evaluate scattering mechanisms, structure andchemistry, to take only one example.

Several papers .reflected the current interest in evolvingsatisfactory prediction methods for earth-space attenuation byrainfall. Such methods are only as valid as the data upon whichthey are based, leaving plenty of latitude at the present timebetween the fanciful and oversimplistic. Currently thereappear to be as many methods available as there are researchgroups working in the field. Other research programmes, whichshould lead to an improvement in modelling of the physicalstructure of rainfall for radio predictions, are on dual-polarisation radar and on a Doppler drop-size measurement inconjuction with a satellite beacon experiment.

There was an interesting theoretical paper justifying the useof the Pareto distribution for intense point rainfalls, which hasalready been adopted, on grounds of being a good engineeringapproximation and of convenience of use, by several groups.The first phase of a study to develop a climatic map for earth-space attenuation in the European region was presented inwhich contour maps of point-rainfall intensity were derivedfrom long-term extreme rainfalls.

Useful data on the operation of site diversity in particularlyadverse conditions (less than 10° elevation in a climate subjectto typhoons), diversity data on crosspolarisation performanceand scintillation statistics on earth-space links were reported.There was very little new material on rain or ice depolarisation.Other papers covered duct and other propagation effectsrelevant to line-of-sight microwave links, null broadening inmonpulse radars due to rain scattering and compensation ofdepolarisation in satelite communications; while a novel paperattempting to assess the cost of interference protection onmicrowave radio systems sharing the same frequency bandsdefined the allowable interference between services as thatwhich minimise the total cost of the systems concerned.

Two complete days were devoted to 'Mathematical modelsin radio propagation' which contained material of basic intersstto radio scientists.

The ionosphere

Of the main topics contained in 110 papers on the ionsphere,including irregularities, the influence of the ionosphere onradio propagation, the high-latitude ionosphere and scatteringof radio waves in the middle atmosphere, the topic on modi-fication of the ionosphere was the main highlight and waspossibly the most important and worrying. There is now awealth of exciting observational material available, and asuitable theoretical background is needed for its interpretation.The observation of artificially accelerated electrons with

IEEPROC, Vol. 129, Pt. A, No. 5, JULY 1982 353

energies as high as 20 eV was particularly interesting and dif-ficult to explain within the current theoretical framework. Thecoupling of high-power radio waves to the ionosphere throughthermal self-focusing was widely discussed, since it has import-ant practical implications for proposals to beam microwavepower to the ground from solar-power satellites (see laterSection).

The mechanisms involved in equatorial-spread-F generationalso received considerable attention, and the importance ofacoustic gravity waves as a source is now becoming apparent.Perturbation in the F-region will be generated by such waves iftheir propagation vectors have a strong component alignedwith a magnetic field. The process is greatly enhanced if thephase velocity of the wave matches the mean ionisation driftvelocity, giving rise to the so-called 'spatial resonance effect'.

The structure and dynamics of the high-latitude ionospherewere discussed, with emphasis on the fascinating results avail-able from the incoherent scatter systems at Millstone Hill andChatanika. The data provided by these systems has shed newlight on the nature of the bulk convection of plasma over thepolar cap. The importance of this flow and its associatedregions of stagnation, in the formation of the observed 'holes',troughs and ridges of ionisation, is now becoming clear. How-ever, the manner in which plasma enters the polar cap throughthe 'throat' region on the day side remains unresolved.

Waves in plasmas

The URSI definition of waves in plasmas is a broad one, in-cluding wave/plasma interactions, as well as electromagneticand electrostatic waves in laboratory, planetary and inter-planetary plasmas. A wealth of new results is becoming avail-able, particularly with the voyage of space probes to the outerplanets in recent years.

Earth, Jupiter and Saturn all emit nonthermal radio noise.The mechanisms governing the radio emissions from theseplanets and the source regions themselves are currently underintensive theoretical and experimental observations. TheEarth's radio spectrum has an intense component (the auroralkilometric radiation, AKR) which has been studied in somedetail (polarisation, power, fine structure) and its source ap-pears to be located in a narrow area ranging from 6000 to7000 km above the polar ionosphere for magnetically quietperiods. Jupiter's decametric radiation and the broadband kilo-metric component are associated with emission resulting fromparticle precipitation involving the satellite Io and its plasmasheath, and with auroral regions near the cloud tops of Jupiter,respectively. The recently discovered radio emissions fromSaturn include an intense component associated with itsnorthern auroral zone.

Wave particle interactions in the magnetosphere continue tobe a subject of intensive investigation. The characteristics ofemissions stimulated by ELF and VLF wave injections havebeen observed in an extensive range of experiments. UK workincludes the theoretical analysis of emissions triggered by theSiple experiments in Antarctica, which enables precipitatedelectron flux densities to be calculated. Papers concerned withthe remote 'determination of plasma-wave spectra coveredsatellite and ground-based measurements as well as relatedtheoretical ray-path calculations, ducted and non-ducted pro-pagation etc.

The temporal and spatial plasma density variations play arole, inter alia, in the conversion mechanism through which'electrostatic' become 'electromagnetic', i.e. transverse, waves.Under certain circumstances, waves can be converted to be-come auroral hiss and kilometric radiation. Investigation of thesource and propagation of terrestrial myriametric radiation(TMR) shows how the waves couple (where the wave frequency

and the local plasma frequency are equal), so that TMR spectracan be used to determine the plasma density profile at theequator.

Significant progress has been made in understanding regionF ionospheric plasma irregularities. Gravity waves and field-aligned plasma densities (bubbles or plumes) give rise to'spread F ' due to the Rayleigh-Taylor instability mechanismassociated with the resultant high-density gradients. Advanceswere reported in the study of plasma irregularities in the equa-torial ^-region electrojet (using radars and rockets) and, theor-etically, using linear (Farley-Buneman) theory as well as non-linear approaches. Equatorial F-region scintillation observationshave been explained quantitatively in terms of a refractivescattering theory.

Our colleagues in electric-power engineering need not feelleft out of this discussion, since they figured strongly in atleast two of the sessions in Washington. In one of the sessionstheir efforts were found to be interesting, if faintly unwelcome,while in the other attempts at being helpful by harnessing solarenergy turned out to be ineffective. Thus power-line (kHz)radiation (PLR) stimulates strong wave-particle and wave-waveinteractions with consequential electron precipitation into theionosphere under certain circumstances. Amplified PLR has aconsiderable effect on trapped particles in the magnetosphere.It appears that such ULF power-line radiation may be at leastpartly responsible for the electron slot between the inner andouter magnetospheric radiation belts. Many observations ofthe ELF and ULF wave field, associated with PLR in differentparts of the world, were described.

An interesting bonus from the study of plasma waves usingcomputer simulation came from an important contribution onthe symbolic manipulation of equations which is likely to havewide application throughout physics and engineering, forexample in the analysis of multiple-input multiple-outputcontrol systems.

Radio astronomy

By a fortunate coincidence 1981 was an historic year for radioastronomers since it saw the 50th anniversary of the first de-tection of extraterrestrial radio waves by Karl Jansky, whoread a paper on his discovery at the Washington URSI meetingof 1932. Thus it was appropriate for a session to be devoted tothe history of radio astronomy, particularly since the retiringpresident of URSI, Prof. Christiansen, is a radio astronomer.Jansky discovered this form of radio noise accidentally, whileinvestigating radio interference, at 20.5 m wavelength, andrecognised that this cosmic 'static' is associated with the centreof our galaxy. Jansky's work, though largely unrecognised, in-spired the radio engineer and amateur enthusiast Grote Reberto build a 10 m dish in his 'backyard'. With this instrumentReber eventually confirmed Jansky's discovery and mappedthe radio emission from the Milky Way at a frequency of160 MHz. Reber wrote up his work and sent it to Astrophysical

Journal, but it was only after scrupulous refereeing and a visitby the then editor, Otto Struve, to Reber's home laboratorythat the journal agreed to publish. Reber now works inTasmania on a low-frequency array and has made interestingcontributions to various scientific disciplines, includingbotany.

Subsequent contributions were given by some of the greatpost-war pioneers of radio astronomy, and Christiansen re-viewed early solar work in Australia which followed Hey'swartime discovery of solar radio emission. The Australianwork led to the construction of high-resolution arrays of tele-scopes and the eventual synthesis of the solar emission. Apaper by Sir Bernard Lovell (read by Prof. John Davies)described developments at Jodrell Bank culminating in the

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development of the 250 ft-diameter radio telescope in 1956,and Kerr described the initial radar and radio astronomy workin Australia which led on to the story of the detection of the21 cm line of neutral hydrogen. The early Russian work whichbegan after the war, and the beginnings of radio astronomy inHolland were also outlined. It is interesting to note that in theUSSR and Holland radio astronomy was developed mainly byestablished astronomers, whereas in the rest of the world theearly work was done by radio engineers.

Particularly noteworthy new facilities described are theVLA, the Jodrell Bank multi-telescope radio-linked interfero-meter (MTRLI), the Molonglo Synthesis Telescope and theClark Lake Observatory. New developments which should beoperational before the next General Assembly are the im-pressive millimetre wave telescopes at Nobeyama in Japan(45 m single dish and 5-element interferometer) and the 30 mtelescope being built for the Franco-German institute IRAM(Institute for Radio Astronomy and Millimetre Wavelengths)on Pico Valeta in Spain.

Among the many interesting astronomical results reportedwere the maps of radio sources from MTRLI, where up to2000:1 dynamic range has been achieved in some cases. Map-ping of the hydroxyl emission associated with late stars hasrevealed an expanding shell of gas and new spectral-line COdetections were reported. New receiver developments weredescribed and all the VLA L-band parametric amplifiers willbe replaced by cooled FET systems in the near future.

Considerable attention was also paid to the problem ofensuring highly accurate surfaces on large precision telescopes(an RMS accuracy of 50jum has been achieved over 10.4 m)and to the hardware requirements for fast processing of thevast quantities of data, as mentioned earlier.

Special topics

It would be possible to write at considerably greater length onthe excellent sessions containing many papers on remotesensing of the atmosphere and of the earth's surface, and onoptical communications; but let me conclude with more com-ments on what has been described as the biggest potentialengineering project up to the year 2000. This is the scheme forcollecting solar radiation in space and beaming it down to thesurface of the earth, which was covered by Prof. W.E. Gordon,in one of three general lectures.

A system of 60 geosynchronous satellites is envisaged, eachcomprising a collector 5x 10km in area linked to a sun-tracking dish. With a collector of 1 kg/m2 the total mass persatellite would exceed 50000 metric tonnes. The thermalradiation would be converted into microwave radiation at2.5 GHz and beamed down to earth from a transmittingantenna 1 km in diameter. The receiving antenna, of area10 x 14 km, would contain rectifiers capable of handling a

power of 5 GW and feeding it into the electricity grid system.The total power from the 60 satellites would be 300 GW. Theoriginal scenario envisaged the construction of two satellitesper year starting in the year 2000 with an estimated cost, attoday's prices, of $100 x 109 for the first, and $11 x 109 foreach succeeding one. Who said that electric-power engineeringis not exciting?!

Quite apart from the gigantic engineering problems involvedthere are other major considerations. Firstly, what will be theworld's power requirements in the 21st century and how farwill existing resources be unable to meet them? There havebeen many different predictions, including one that envisages adisastrous fall in the standard of living and hence of power re-quirements. What effect would such an enormous amount ofpower transmission have on the atmosphere? The passage ofSkylab produced a large hole in the electron population in theionosphere which took four hours to recover, so that the effectof 60 large satellites could be serious and even a slight degreeof coupling of the microwave power into ionospheric wavesmight have an unpredictable and unwelcome result. Theradiation level would only fall to a safe level some 44 km fromthe receiving antenna; the consequences for HF and otherforms of communication and for the climate are unknown,and the possibilities of a transmitting antenna going out ofcontrol and sweeping a scorched path over the surface of theearth are intriguing. All in all there was a certain relief felt inthe lecture hall at the closing remarks which were to the effectthat, while the idea is not dead, the programme is (at leasttemporarily) dead.

Finally

With such a mass of offerings it is perhaps inevitable that thestandards of presentation should have been variable. However,the writer was well satisfied overall and found this GeneralAssembly as stimulating as the two previous ones he attendedin providing an excellent, if exhausting, opportunity of hearingabout progress over the whole field of radio science. A parti-cularly pleasing finale was the election of Prof. A.L. Cullen asVice President of URSI.

Acknowledgments

Grateful acknowledgment is made to the UK nationaldelegates to the various commissions of URSI for providingindividual reports from which this article has been prepared.The author hopes he has done justice to their contributionsand to the initiative of Sir Granville Beynon, Prof. A.L. Cullenand Mr. A.E. Bailey who conceived, and approved, the idea ofan overall report of the engineering and scientific proceedingsat the General Assembly of URSI.

PROF. W.A. GAMBLING

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