Introduction to Radio Astronomy

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PROCEEDINGS OF THE IREIntroduction to Radio Astronomy *F. T. HADDOCKt, MtEMBER, IRESummary-A general description of the nature of radio astron-omy, its differences from optical astronomy, a review of the earliestbeginnings of galactic and solar radio astronomy, and a listing ofother important observational discoveries is given. A nearly completebibliography of these early publications and of the principal reviewbooks and papers on Radio Astronomy is given.Some practical aspects and instrumental developments of possibleinterest to radio engineers are pointed out. The papers in this issue ofthe PROCEEDINGS on Radio Astronomy are discussed generally andindividually. A brief description of some new results on solar-burstspectra obtained at the Radio Astronomy Observatory at the Uni-versity of Michigan is presented.INTRODUCTIONUR CONCEPT of the Universe has been appreciablyenlarged by radio observations in recent years. It hasbeen found that radio waves are emitted from theMoon, Venus, Mars, Jupiter, and Saturn, from the Sun'sgaseous atmosphere, from the debris of exploded stars, fromneighboring galaxies of stars, from clusters of distant galaxies,from clouds of gas in the spiral arms of our Milky Way galaxy,from very distant Island Universes in collision, and from hun-dreds of invisible and unsuspected heavenly objects, popularlymiscalled radio stars. Radio studies have taken the lead inmapping the structure of our own galaxy, and have revealedthe fact that the Milky Way is a rather tightly wound spiralgalaxy. The solar system is traveling along the inner edge ofa spiral arm at a distance of 30,000 light years from the centerof rotation of this disk of stars and gas. Many astronomersbelieve that some of the fainter radio sources are at distancesbeyond the range of present optical telescopes and that withthe recent attainment of Doppler shift measurements on theradio hydrogen-line radiation from distant galaxies, cosmolo-gists may soon be given a fresh and penetrating view into thedepths of time and space.Although cosmic ray and meteor studies have made valu-able contributions, most of our knowledge of the Universehas been gained from electromagnetic light waves, falling onthe earth through a window in the atmosphere about fiveoctaves wide. Radio astronomy is now exploiting the existenceof a second window through the atmosphere 12 octaves wide.Radio waves shorter than a few millimeters are absorbed byatmospheric oxygen and water vapor; whereas the ionosphericlayers turn back radio waves longer than several decameters[11]. Thus radio astronomy observations are made at frequen-cies used in television, fm radio, microwave relay links, rocketand satellite telemetering and control, and radar.Light and radio waves are fundamentally the same exceptfor their length. Radio waves are thousands of times longerthan light waves; this large difference has two consequences ofgreat importance to astronomy. First, the interaction of radiowaves with interstellar gas, solar and planetary atmospheres,* Original manuscript received by the IRE, December 10, 1957.t University of Michigan, Ann Arbor, Mich.and cosmic dust clouds complements that of light waves.Large regions of the Universe are forever inaccessible to opti-cal study because of heavy obscuration by interstellar dustwhich, however, is transparent to radio waves. On the otherhand, tenuous ionized gas-regions such as exist around the Sunas its corona, and surrounding super-giant blue stars as brightgaseous nebulas, and even enveloping all the stars in spiralgalaxies, are transparent to light waves, but reflect, absorb, oremit radio waves. Thus radio methods offer new and powerfulmeans of studying these gaseous regions and, at the same time,penetrating the obscuration due to cosmic dust without theleast detectable effect. Second, the value of radio to astronomyis limited somewhat by the difficulty of obtaining high angularresolving power. Wave diffraction sets this instrumental limitas it does for optical telescopes, and since its blurring effect isproportional to wavelength, radio waves are very much harderto "focus sharply." For example, the most precise radio tele-scopes of today, operating on microwaves, are capable of pro-ducing angular resolution which is not quite as good as theunaided human eye with light waves. To compare with anoptical telescope in resolution a radio antenna would need to behundreds of miles in extent. However, position location is notthe same as resolving power; if a radio source is much brighterthan its surrounding background emission and its shape isassumed to be known, then its position can be located to amuch smaller angular uncertainty than is indicated by theresolving power of the antenna.With the largest optical telescopes it is practically impossi-dle to obtain the ultimate instrumental resolution given bywave diffraction theory because of the inhomogeneities in therefractive index of the earth's atmosphere. When the atmos-phere is very stable at the proper heights and stellar imagesare steady and sharp, the astronomer says that the "seeing" isgood. Pronounced twinkling, or scintillation, and "dancing" ofstellar images is poor "seeing." This "seeing" problem alsoexists in radio astronomy, with the additional complication,at the longer wavelengths, of ionospheric effects, including theFaraday rotation of the radio wave polarization, and occa-sional large absorption at decameter wavelengths. Undoubt-edly, "radio seeing" will also set a practical limit to the angularresolution obtainable at all wavelengths used in terrestrialradio astronomy.For the above reasons and for the fact that stars are rela-tively poor radio emitters, the radio telescope presents to us adustless and starless universe, populated largely by turbulentgas clouds.THE BEGINNINGSIt is very likely that the first attempt to measure the emis-sion of radio waves from an extraterrestrial source was that ofSir Oliver Lodge [2], well-known radio pioneer, who in a lec-ture before the Royal Institution of Great Britain on June 1,1894, only six years after Hertz discovered radio waves,stated, "I hope to try for long-wave radiation from the sun,filtering out the ordinary well-known waves by a blackboardIn lieu of the usual "Scanning the Issue" page, the organizer of this special issuepresents below, for the benefit of the general reader, a discussion of the contents of thisissue. He has preceded the discussion by an excellent introduction which will orient thereader and will interpret for him the importance of radio astronomy to both astronomersand radio engineers.-The Editor- =1958 3PROCEEDINGS OF THE IREor other sufficiently opaque substance." He apparently did trysometime during 1897 to 1900 because he inserted into thethird edition of this lecture the following note: "I did not suc-ceed in this, for a sensitive coherer in an outside shed unpro-tected by the thick walls of a substantial building cannot bekept quiet for long. I found its (galvanometer) spot of lightliable to frequent weak and occasionally violent excursions,and I could not trace any of these to the influence of the sun.There were evidently too many terrestrial sources of disturb-ance in a city like Liverpool to make the experiment feasible.I don't know that it might not possibly be successful in someisolated country place; but clearly the arrangement must behighly sensitive in order to succeed."Inasmuch as the lecture then proceeded to discuss "a smallcomplete detector . .. which is quite portable and easily setup" consisting of a battery, galvanometer, and coherer de-signed for the detection of centimeter wavelength radiationarising from spark excited spheres, it is presumed that it isthis wavelength region that Lodge had in mind for "long-waveradiation from the sun." Over forty years later, in 1942,Southworth, at the Bell Telephone Laboratories in NewJersey, first detected and measured the waves that Lodgeapparently had in mind.The first discovery of radio waves of extraterrestrial originwas made by Jansky [3] in 1932, also at the Bell TelephoneLaboratories, while studying the horizontal direction of ar-rival of atmospheric static at 20 mc (X = 14.6). Fig. 1 is aphotograph of his antenna. He noticed a very steady weakhissing static of unknown origin whose direction of arrivalchanged gradually around the compass in about 24 hours.Further study revealed that the periodicity was stellar ratherthan solar as first suspected; in fact, the maximum intensityoccurred when the center of the Milky Way was in the an-tenna beam. Secondary intensity maximum occurred when thebeam passed through the plane of the Milky Way at positionsfurther removed from the center of our galaxy.Two suggestions put forth by Jansky relating to the originof this interstellar radio emission consistent with his observa-tions are of interest today. First, he stated that the mostfascinating explanation would be a disk-like distribution ofradio sources around the Earth like the stars. In a later paperhe mentioned that since he could not detect radio emissionfrom the Sun he supposed that some other class of heavenlybodies in the Milky Way must radiate a much greater ratiothan the Sun of energy in radio waves to that in the form oflight and heat. Second, he was immediately struck by thesimilarity between the sound produced by the received galac-tic radiation to that produced by the thermal agitation ofelectric charge in a resistor. Therefore he speculated that theradiation might be caused by some sort of thermal agitationFig. 1-Karl G. Jansky and his 14.6 meterrotatable directional antenna.Fig. 2-Grote Reber and his 31-foot reflector after it was acquired bythe National Bureau of Standards in 1947 and erected on a turn-table at their field station near Sterling, Va.of charged particles such as found not only in stars, but alsoin the very considerable amount of interstellar matter dis-tributed throughout the Milky Way with an effective tem-perature of 15,000C. Later observations have largely sub-stantiated these speculations.Later Reber [5] in Illinois, measured the angular distributionof this cosmic emission at the much higher frequency of 160mc with a sharper antenna beam. Fig. 2 is a photograph of hisantenna. He found several subsidiary maxima, one in the con-stellation of Cygnus. The intensity at this higher frequencywas much less than Jansky found at 20 mc. Reber suggestedthat the radiation was generated thermally by collisions be-tween free electrons and positive ions (called free-free transi-tions) in interstellar matter ionized by star light. The theoryfor this type radiation was first given by Kramers [6] to ex-plain continuous X-ray spectra, and is physically equivalentto the Lorentz absorption theory used in ionospheric propa-gation.Early in 1946, Hey, Parsons, and Phillips [7] in England,found marked short-period irregular fluctuations in intensityat 64 mc (X=5 m) in the direction of the same subsidiarymaximum in Cygnus. Their observations indicated that thefluctuating source subtended an angle less than 20. It appearedto them that such fluctuations could only originate from asmall number of discrete sources and not from widely dis-tributed interstellar matter. Although they were incorrect inattributing the fluctuations to the source itself (turbulent in-homogeneities in the Earth's ionosphere were responsible)they were correct in pointing out the importance of their dis-covery. It was the first clue of the existence of a discrete radiosource, which was established the following year by Boltonand Stanley [8] by observing that the size of the source wasless than 8 minutes of arc. Among the score or so which havebeen identified optically [9, 10], there are objects of highlyunusual types, new to astronomy, as well as well-known ce-lestial bodies. Jansky's supposition that there exist heavenlybodies whose radio energy output relative to that of their heatand light output is much greater than the Sun has been dra-matically borne out. The source in Cygnus on the above ratiobasis is a better radio emitter than the Sun by a factor of 1018(a billion-billion times). It is now believed that a large part ofradio emission from the Milky Way is due to discrete radiosources in a disk-like distribution like the stars [11], and that a4 JanuaryHaddock: Introduction to Radio Astronomycertain fraction, which is relatively more intense in the micro-wave region, arises thermally from free electron collisions withpositive ions in interstellar gas clouds [12, 13].The first documented recognition of the reception of radiowaves from the Sun was made in 1942 by Hey [14] in Englandand independently by Southworth [15] in New Jersey. Bothwrote reports which had restricted circulation because of thewar and were not published until later. Reber [16] in Wheaton,Ill., independently discovered and identified solar radiation at160 mc in the following year and was the first to publish theevidence. He devoted only a few lines to the Sun in this paperon galactic radiation.Southworth used radar receivers and paraboloids in a de-liberate attempt to detect radio emission from the Sun. Hewas successful on June 29, at 10,000 mc and at 3000 mc aweek or two later. He correctly attributed this emission tothermally generated radiation. He has recently given a gooddetailed account of this work [17] and of his early associationwith Jansky at the Bell Telephone Laboratories in NewJersey. These measurements initiated the study of the radioundisturbed or "quiet" Sun. Now we will describe the dis-covery of radio bursts, the second main class of solar radioemission.Undoubtedly there have been many unpublished observa-tions of high hissing noise levels arising before or after radiofadeouts, but the first publication was in 1936 when Arakawa[18] noted that, "High noise accompanied these sudden fadings(4-20-mc band), especially on local circuits. The effect mustbe related to solar activities, since they occur only on daylightroutes." And two years later Heightman [19] wrote, "At suchtimes (when fadeouts occur) the writer has often observed thereception of a peculiar radiation, mostly on frequencies over20 mc, which on a receiver takes the form of a smooth thoughloud hissing sound. This is presumably caused by the arrivalof charged particles from the Sun on the aerial."During this period scientists [20] were studying the associa-tion of short-wave radio fadeouts with solar flares (chromo-spheric eruptions), terrestrial magnetic pulses, enhanced at-mospheric static, and sudden increases in noise level men-tioned above. It appears that the question of the origin of thisenhanced noise attracted the least attention. In 1939 onepaper appeared in Japan by Nakagami and Miya [21] whichdescribed an experiment made near 15 mc to measure theangle of arrival of noise "like that from a grinder" attaining alevel of 40 db above the background just before and shortlyafter a fadeout. They found that this noise occurred only dur-ing the day and that its direction of arrival was from highelevations, but they did not note the significance of the corre-sponding elevation of the Sun. They concluded that this noiseprobably originated in or near the E layer.On February 26-28, 1942, a number of British Army radars,operating at frequencies between 55 and 80 mc, sufferedsevere interference. This was investigated by Hey and wasdescribed in restricted reports in 1942 and 1945. He had foundthat the direction of arrival of the interference was from theSun and that during this period there was a large sunspotgroup on the disk of the Sun. Hey concluded that it was diffi-cult to imagine any other explanation than that the radiationoriginated at the Sun in association with the sunspot activity.In January, 1946, Hey [22] published a letter in Nature,briefly describing the 1942 event and, in a postscript, F. J. M.Stratton described the solar flares and geomagnetic effectsassociated with the solar event. Appleton and Hey [22, 23]pointed out that the previously reported noise from the Sunwas many times greater than that to be expected from the Sunas a black-body radiator. The relationship between the intensesolar radio noise and sunspot activity was substantiated inSydney by Pawsey, Payne-Scott, and McCready [24] in 1946.Ryle and Vonberg [25] in England first published inter-ferometric measurements made to obtain the size of a radioemitting region. They used two antennas spaced 140 wave-lengths apart to give a multilobed pattern analogous toMichelson's method for measuring stellar diameters. Anemitting source on the Sun during July, 1946, was found to beless than 10 minutes arc, less than 3 the diameter of the Sun.In 1947, McCready, Pawsey, and Payne-Scott [26] publishedresults obtained earlier in February, 1946, giving similar re-sults on the source size but using a cliff interferometer whichconsists of only one antenna 250 feet above the ocean andutilizing the reflected ray to produce a multilobed patternat low angles of elevation, analogous to Lloyd's mirror in-terferometer.The polarization of the intense radio emission from sunspotregions was first detected and found to be circularly polarizedduring July, 1946. Observations were conducted by three in-dependent groups [25, 27, 28] in England and Australia.Since then both the basic thermal component from the"quiet" Sun and the nonthermal intense bursts from the activeSun have been subjected to detailed study at various fre-quencies from a few mc to a few 100,000 mc.HIGHLIGHTS OF OTHER OBSERVATIONAL DISCOVERIESIn 1946 and 1947, Hey and Stewart [28] in England firstused radar equipment to study meteors. They gave the firstdefinite proof that most transient echoes from the E-layer re-gion were from meteors. With Parsons [29] they made the firstradar measurement of meteor velocities.Dicke and Beringer [30] in 1945 made the first measurementof the thermal radio emission of the Moon. They used thechopper-type microwave radiometer, at 24,000 mc, whichDicke [31] had developed at the M.I.T. Radiation Laboratoryin order to measure thermal microwaves from atmosphericoxygen and water vapor. Piddington and Minnett [32] inSydney made the first extended series of observations of thelunar emission at various phases in 1949.Radar echoes from the Moon were first obtained in 1946 byscientists at the U. S. Army Signal Corps Laboratory [33]at 111.5 mc, and a little later by Bay [34] in Hungary at 120mc using a novel electrochemical integrator.An outstanding discovery was made in 1951 by Ewen andPurcell [35] at Harvard University by the detection of the1420-mc line radiation from the ground state of atomic hydro-gen, which occurs as gas largely in the spiral arms of the MilkyWay. The detection was shortly confirmed by Muller andOort [36] in Leiden and by Christiansen and Hindman [37]in Sydney.The possibility that this radio spectral line might be de-tected from the galaxy was first pointed out by van de Hulstin a Leiden Observatory talk in April, 1944, and was publishedthe following year [38]. His talk was stimulated by a recentpaper in the Astrophysical Journal by Reber on "CosmicStatic." The importance of this line is realized when it ispointed out that atomic hydrogen in the ground state con-stitutes the major fraction of interstellar matter and is un-detectable by optical methods. It has also made possible theradio measurement of velocities by means of the Doppler shift.This has been extremely valuable. In 1955, Lilley and McClain[39] at the U. S. Naval Research Laboratory reported the"red-shift" of a distant radio source by observing in absorp-tion the 1420-mc hydrogen line Doppler-shifted to 1340 mc.This indicated a line-of-sight recession of 16,830 km in goodagreement with the optical determination.An unexpected discovery was made in 1955, by Burke andFranklin [40] at the Department of Terrestrial Magnetism inWashington, D. C. when they detected bursts of radio emis-sion from the planet Jupiter at 22 mc. When Shain [41] inSydney was notified of this, he was able to identifyJovianbursts on his galactic radiation records at 18 mcback to 1950,and found that a source of emission was localized on Jupiter.In 1956 and 1957, Mayer, McCullough and Sloanaker [42]1958 5PROCEEDINGS OF THE IREat the U. S. Naval Research Laboratory detected for the firsttime microwave emission, at 10,000 mc, from Venus, Jupiter,and Mars and the partial polarization of radiation from theCrab Nebula [43]. In 1956, Kraus [44] at Ohio State Universityreported the detection of Jupiter-type bursts from Venus at27 mc. To date, attempts by others to confirm this have notbeen successful. In 1957, Drake and Ewen [45] at Harvard at8000 mc confirmed the detection of the thermal emission fromJupiter and detected for the first time radio emission from theplanet Saturn. In the spring of 1957 there appeared a brightnaked-eye comet denoted Arend-Roland, 1956h. Many radioastronomers attempted to detect radio emission from it; mostwere unsuccessful. However, scientists in Belgium at 600mc, and in Bonn, Germany, at 1420 mc claimed possiblesuccess [46].No attempt has been made above to cover all the importantobservational results nor any of the theoretical work. Littlehas been mentioned of solar-terrestrial relationships, and, aswith this entire issue, next to nothing has been given onmeteors, principally because the meteor work warrants sepa-rate treatment and has been studied by people not otherwiseengaged in radio astronomy, with the important exceptions ofthe Manchester group under Lovell, and, in the past, by thegroup under Hey. For an adequate survey of radio astronomy,including the neglected topics, the reader is referred to the fol-lowing selected bibliography of books and monographs [47-54],technical review papers [55-73] and popular articles, which areusually well-illustrated [74-86]. A nearly complete bibliog-raphy of radio astronomy has been compiled by Martha StahrCarpenter [87] of Cornell University. The research staff of theRadiophysics Laboratory [88] in Sydney has compiled a bib-liography with short abstracts for the period 1954-1956.THE IMPORTANCE OF RADIO ASTRONOMYTO ENGINEERINGThe history of technology shows clearly that our engineer-ing and technical progress depends more and more on the re-sults of research that has been motivated by curiosity aboutthe nature of our physical world. A popularly cited example isthe technology of nuclear power which is based on the resultsof over fifty years of research in pure physics. It is not possibleto predict if a new field of engineering will arise from resultsof radio astronomy investigations, but we already have severalapplications.In 1943, Southworth [89] applied for a patent, which waslater granted, claiming a method of artificial vision for scan-ning a terrain in overcast weather for object location, for sceneidentification, etc., using reflected radio waves emitted by theSun,The direct use of radio waves from the Sun for all-weathernavigation [90] is an attractive application for ships. TheCollins Radio Company [91] has developed, under a U. S.Navy Contract, a radio sextant for tracking the Sun all day.It utilizes the radiation from the Sun near a wavelength of onecentimeter and a fire-control radar-type scanning antennawith a modified Dicke-type receiver. An experimental version[92] was tested by the U. S. Navy a number of years ago. Itworks through heavy overcast with an accuracy somewhatbetter than can be obtained in clear weather with the standardoptical sextant. With improved sensitivity, it should be prac-tical to track radio "stars" 24 hours a day under overcast skies,as predicted by the writer [93] shortly after the detection ofcentimeter wave radio sources.With the advent of large gain antennas and sensitive low-noise receivers, the use of radio sources for antenna and re-ceiver calibrations will become both more practical and neces-sary. Specially designed antennas of modest size at micro-wavelengths can provide matched transmission line termina-tions with equivalent noise temperatures near absolute zero.These may be useful in making noise level measurements oflow-noise receivers, masers, etc. With larger antennas ofknown gain, the brighter radio sources can potentially provideabsolute noise level increments for international standardiza-tion. Conversely, absolute antenna gain can be measured byusing a calibrated receiver, or by using a standard gain an-tenna and a calibrated attenuator with an uncalibrated re-ceiver. At present, the absolute flux density at the earth fromthe brighter radio sources is known to accuracy of about 20to 50 per cent, but some effort and much importance is beinggiven to the determination of the absolute energy spectrumof radio sources. The Sun and radio sources have been widelyused by radio astrononmers to measure relative antenna pat-terns. At meter wavelengths the energy from the Sun is fre-quently variable and the galactic background emission levelis well above absolute zero. Therefore the above remarksapply principally to centimeter wavelengths; at millimeterwavelengths the signal-to-noise ratio on radio sources is smalland atmospheric absorption is troublesome, although the Sunand the Moon are relatively easy to detect.The directional pointing of antenna beams of sensitiveradars can be calibrated on extraterrestrial radio sources, afterdue correction for total refraction through the earth's at-mosphere. Perhaps more important is the determination ofthis total refraction of waves from a radio source or moonecho. This can be determined with the use of a calibrated an-tenna system or interferometer. Knowledge of total refractionand its vagaries at different frequencies is important in radioguidance, detections, and tracking systems involving high fly-ing objects, satellites, etc.The present results of radio astronomy provide us with afairly good estimate of the minimum radio noise levels thatcan be expected on earth and in outer space over the radioastronomy frequency spectrum. Depending on the frequencyband, naturally generated noise originates principally fromthe Earth, its atmosphere, the Sun, the Milky Way, or variousdiscrete radio sources. Less accurate estimates can be made ofoccasional large increases in noise level caused by radio burstsfrom the Sun, Jupiter, and, perhaps, Venus and Saturn. Withthe large antennas and low-noise receivers now being built orplanned for various applications, it is becoming increasinglyvaluable to know the probability of radio noise outbursts andthe level of the steady background emission from the variousastronomical objects.The study of solar radio radiation is becoming increasinglyimportant in the prediction of radio propagation conditionsinvolving the ionosphere. As noted in the introduction there isan association between increased radio noise from the Sun andsudden ionospheric disturbances, or radio communicationfadeouts. At present it is not possible to predict the occurrenceof a solar flare or a fadeout, but recent evidence [94] suggeststhat the characteristics and timing of radio solar bursts withrespect to solar flares make it possible to predict whether ornot a geomagnetic storm will follow within a few days. Thesestorms are associated with [95] earth current changes, auroras,and anomalous ionospheric phenomena, called ionosphericstorms, which can seriously disrupt radio communications,especially in high latitudes. Since these radio "blackouts" canlast several days their prediction is greatly desired by com-munication people.The day-to-day variations in solar activity affect the iondensity of all layers in the ionosphere, and hence the choicefor optimum frequency for communications that involve theionosphere. The sunspot number, or area, is the most com-monly used index of solar activity, although the daily level ofradio flux around 3000 mc is very closely correlated with sun-spot area and appears to be a better solar index for some ap-plications. For instance, it can be measured precisely and con-tinuously in any weather. In 1957, Denisse and Kundu [96]found a close and linear relation between 3000-mc solar flux6 JanuaryHaddock: Introduction to Radio Astronomyand the ionizing flux of the E layer. The correlation is betterthan when using sunspots in place of the radio solar flux.THE CONTRIBUTION OF RADIO ASTRONOMYTO INSTRUMENTATIONOn the instrumental side, radio astronomers have developedto a high state certain specialized antenna and receiving sys-tems. An outstanding innovation by Hanbury Brown andTwiss [97] is the conception, development, and use of a newtype of radio and optical interferometer for measuring thediameter of radio sources or, in an optical analog, opticalstars. Their instrument detects the signals from two spacedantennas, or telescopes, independently, and then correlatesthe low-frequency detector outputs. The system has provedto be largely free of the effects of atmospheric and ionosphericseeing. This is expected since the relative phases are lost andonly the correlation in their intensity fluctuations is measured.This is radically different from the familiar Michelson in-terferometer. It can be used with a very great separation be-tween antennas. They have measured the optical diameter ofthe star Sirius [98] to be 0.0068 0.0005 second of arc, in goodagreement with an astronomical estimate. This post-detectioninterferometer has two weaknesses however. It requires a goodsignal-to-noise ratio and, consequently, it can only be used onthe brighter objects, and since phase information is lost itcannot be used to determine an unsymmetrical component inthe brightness distribution across the object, as can be ac-complished with a predetection radio interferometer.Many different and effective techniques have been developedto measure the size, position, brightness, radio spectrum, dis-tribution, and polarization of radio sources in the sky and onthe Sun. They have been well described in the literature andseveral outstanding developments are represented in thisissue of the PROCEEDINGS [99]. Radio astronomy has stimu-lated the conception, the design, and the construction of manylarge, high-gain reflector-type antennas [100]. One of the firsthigh-gain antennas is the 50-foot precision paraboloid at theU. S. Naval Research Laboratory. It was used in three in-vestigations described in this issue [101]. The largest steerableantenna known in the world is the 250-foot paraboloid [102]in Manchester, England.Although low-noise receivers are being developed for manyapplications, it is in radio astronomy that high stability of thereceiver gain, pass band, and noise figure can be exploitedbecause of the frequent possibility of long-time integration ofthe receiver output to obtain greater sensitivity. An exampleof good radiometer stability has recently been announced byDrake and Ewen [45]. Their 8000-mc receiver has an hourlydrift rate in noise level equivalent to 0.10K antenna tempera-ture change.Trexler [103], at the U. S. Naval Research Laboratory, hasreported the transmission and reception of voice modulated200-mc signals reflected from the Moon, without appreciablechange in fidelity. The writer can attest that he could not de-tect any change in quality of the voice transmissions aftertheir flight to the Moon and back, as recorded on magnetictape. However, the background noise level during recordingwas high and fidelity was not easy to judge. In any case, itappears that the Moon offers unusual broadcast possibilities.This prospect is not particularly appreciated by radio astron-omers since the Sun and man-made interference already pre-clude daytime observing for certain programs, and the exclu-sion of moon-lighted nights would leave them not much betteroff, for observing time, than the optical astronomers.After this attempt to point up some practical aspects ofradio astronomy and its potential importance to technology,it is well to mention that astronomers, and many other "non-applied" scientists, feel that the principal value of engineeringis its ability to contribute to man's quest for knowledge of hissurroundings.COMMENTS ON PAPERS IN THIS ISSUEAmong the papers selected for this issue are a number de-scribing antennas and receivers of advanced or novel design,a number giving recent observational results of astronomicalor geophysical interest, and a few of more direct practicalvalue; three or four papers on theoretical aspects of radiogeneration, and two papers describing the newly establishedU. S. National Radio Astronomical Observatory in GreenBank, W. Va. Since about 80 per cent of the contents of thisissue are from U. S. scientists with the balance from groups inAustralia, Belgium, Canada, England, France, and Japan, theissue does not give a representative view of the over-all field.There is nothing from the two large and active groups inEngland at Cambridge and Manchester Universities, the in-fluential Leiden Observatory group in Holland, nor from therapidly growing centers in Germany and the U.S.S.R. On theother hand there are several papers of outstanding instru-mental interest from the large group in Australia (papers byMills, et al., Shain, Christiansen and Mathewson, and Wildand Sheridan) and two papers from Japan on their speciality(by Suzuki and Tsuchiya, and Akabane) of radio polarimetersfor solar burst analysis. There is a review paper from the largeand growing center in Paris (by Blum, Denisse, and Steinberg)which may serve to introduce many U. S. readers to a pioneer-ing group in radio astronomy which is not so well known hereas are the groups in English-speaking countries.The emphasis in this issue on the U. S. effort is naturalpartly because there has been an awakening interest here inradio astronomy. Many U. S. universities are now in theprocess of planning or setting up programs in this field. Thestimulation of such activity was an important motive behindthe creation of the National Radio Astronomy Observatorywhich will make available large radio telescopes and isolatedsites to universities which would not otherwise go in for radioastronomy. It is believed that this country has begun to con-tribute its share to the development of basic discoveries, manyof which were made here, but followed up principally in Eng-land and Australia.The papers by C. M. Jansky, Jr., on his brother's discoveryof extraterrestrial radio waves, and by Reber, describing hislonely follow-up of Jansky's work, bring out the fact thatradio astronomy research began here and remained here for adecade. But after World War II, England and Australialargely took it over and developed it to the point where opticalastronomers could no longer ignore its potential value. Reber'spaper is a gem. In down-to-earth language he presents a vividpicture of his persistent, and often frustrating, efforts to de-tect radio waves from the Milky Way at frequencies wheretechniques were still in an experimental state of development.Emberson and Ashton describe the activities and plans ofAssociated Universities, Inc. in setting up and operating theU. S. National Radio Astronomy Observatory, with NationalScience Foundation support. Ashton is the design engineerfor their 140-foot steerable precision reflector which is nowplanned to be in operation in 2 or 3 years. Although it is notas large as the 250-foot paraboloid in Manchester, England,it should be usable at wavelengths which are 5 or 10 timesshorter. In the following paper, Findlay describes the un-wanted noise levels in Green Bank, W. Va., the location of theNational Radio Astronomy Observatory.Drake and Ewen describe the development of an outstand-ing broad-band microwave radiometer and observations madewith it. They have detected radio emission from the planetSaturn and from planetary nebulas for the first time, using a28-foot reflector. Strum presents the design considerationsthat arose in the development of this radiometer which candetect input noise level changes of a few hundredths of a de-gree Kelvin. McCoy considers the possibility of developinglow-noise crystal mixers for millimeter wavelengths. He con-cludes that it will be possible to obtain low receiver noise1958 7PROCEEDINGS OF THE IREfigures which do not increase noticeably with frequency from10 kmc to 50 or 100 kmc. This is an exciting prospect whichin practice may compete for some time with masers in fu-ture millimeter-wave astronomy.Mills, et al., present the design and performance details oftheir large synthetic pencil-beam antenna at 3.5 meters, com-monly called a "Mills Cross." This antenna-receiver systemhas an effective beam width of less than one degree and isbeing used to complete the largest catalog of radio sourcesextant. Many interesting results have been found with thisinstrument. Shain, of the same laboratory, describes a similarinstallation at 15 meters wavelength with which he has ob-tained a map of galactic background emission which showsclearly the regions where the background is reduced by ab-sorption due to interstellar ionized clouds of hydrogen in thespiral arms of the Milky Way.Kraus writes about a very large fixed reflector antennaof novel design now under construction by his radioastronomy group at Ohio State University. He plans to con-duct surveys of the sky for radio sources in the depths of spaceat several frequencies simultaneously. The antenna beam willbe reflected by a flat, tiltable sheet in order to cover a widerange of elevation angles.The next two papers, by Bracewell, apply informationtheory analysis to the efficient reduction of data and planningof the observing programs. The first paper considers the in-formation content of interferometer records, and the secondevaluates the loss of data due to noise and to the finite antennabeamwidth.The next fourteen papers all pertain to the Sun. The papersby Medd and Covington and by Hey and Hughes deal with thecalibration of absolute solar radio flux at 10 cm. The valuablefeature here is that both have obtained the same absolute fluxvalue from the Sun on June 30, 1954. On this date the Sunwas eclipsed by the Moon and it was in a very undisturbedstate. The N.R.L. eclipse expedition to Sweden also obtainedexcellent solar measurements on this day at the same wave-length and have deduced the relative brightness distributionacross the solar disk from the measured eclipse curve [104].The paper by Coates is the first detailed report in radioastronomy of observations at a wavelength less than 8 mm.The technique, the atmospheric measurements, and the solarresults are all of interest. He gives evidence of excess radiationfrom sunspot regions. Christiansen and Mathewsen describethe evolution of an instrument for obtaining high angularresolution for solar studies. They are principally interested inmeasuring the size, shape, and intensity of sunspot radiationnear 20-cm wavelength [105].The next three papers describe the receiver, the antennasystem, and some observational results of dynamic spectra ofsolar bursts over the frequency range of 100 to 600 mc. Thistype of solar investigation was initiated and developed byWild and his colleagues in Australia at frequencies from 40-240 mc, and was a major advance over that of recording solarradiation at a number of single frequencies. Goodman andLebenbaum describe the three mechanically tuned super-heterodyne receivers and the output display. Jasik developedthe antenna feed system for use with a 28-foot paraboloidreflector. The feed consists of four confocal broad-band an-tennas for simultaneous operation at the reflector focus. Thisdevelopment is an advance in the utilization of a paraboloidalreflector. Maxwell and Goldstein describe their site, equip-ment, and operation. They show photographs of their records,illustrating several types of solar bursts.The Sun has been observed at the University of Michiganwith similar equipment since August 28, 1957. As each receiversweeps almost over an octave, giving complete coverage ofthe 100- to 580-mc frequency band every 0.3 second, the out-put is displayed as an intensity modulated line on a precisioncathode-ray tube. This is photographed on a slowly moving(1 cm/min) 35-mm film, producing a frequency-time recordwith solar intensity appearing as variations in photographicdensity. The combination of the film characteristic and thelogarithmic response of the receiver permits the recording of awide range of signal intensities in considerable detail. OnSeptember 3, at about 12:15:10 U.T., three simultaneousbursts were recorded with "U-Type" spectra (as found byMaxwell), having a duration of a few seconds, with their low-frequency "reversing" points at 130 mc, 250 mc, and 375 mc.Between 130 mc and 125 mc there existed a continuous in-terfering signal which could have masked a few megacycles ofthe low-frequency limit of the "fundamental" frequency burst;the lowest frequencies could also have been attenuated in theSun, as suggested by Wild, Murry, and Rowe when they pre-sented evidence of the second harmonic. Maxwell has reportedevidence of two simultaneous "U-Type" bursts in a 2 to 3frequency ratio but without evidence of a "fundamental" fre-quency. Classical Type III burst groups (fast drift bursts)associated with flare onsets have been recorded, however, withan unexpected broad-band, burst-free, continuum emissionbeginning 3 to 4 minutes after the Type III event and lastingfrom 1- to 22 hours, with a pronounced low frequency cut-off(above 300 mc), but showing a tendency to drift to somewhatlower frequencies. It is believed that this continuum emissionis distinct from that associated with Type I noise storms ob-served at lower frequencies. It is similar in character, and infollowing a Type III event, and in duration to the emissionfound by Boischot and called Type IV. Denisse has suggestedthat Type IV emission is due to synchrotron radiation, on thebasis of its source being large and greatly elevated in the solarcorona (4 or 5 R]?). This spectral type has also been noted byMaxwell, et al., in their paper in this issue. Fig. 3 is a photo-graphic record of the outstanding event of September 12,1957. It shows a typical Wild Type III fast burst group be-ginning at 15:15 U.T. at all frequencies. Then one minutelater there begins a Type II slow drift from high to low fre-quencies. The fine detail showing its composition as many fastbursts similar to the Type III bursts has not been reportedbefore. After this burst there appears a general brightening ofthe entire high-frequency band, entirely free of detailed struc-ture, lasting 150 minutes. The September 13, 1957, recordshows also a Type III burst group again preceding a highfrequency "smooth" emission, this time lasting 90 minutes.Dodson reviews some results of attempting to correlate op-tical flares on the Sun with radio bursts, ionospheric dis-turbances, and geomagnetic storms. A valuable finding is adescription of a radio event associated with optical flareswhich indicates the probable occurrence of a geomagneticstorm within a few days, described earlier in this paper.Wild and Sheridan describe the recent development of asweep-frequency interferometer for studying meter-wave solarbursts and noise storms. This instrument measures the sizeand position of the emitting region on the Sun and its dynamicspectrum simultaneously, or it can measure the burst polariza-tion characteristics and spectrum. The output records arecomplex but contain much information. It appears that ap-preciable effort is required to reduce the data completely.The next four papers deal with the measurement of thepolarization characteristics of solar radio emission. Cohenpresents a general review of the problem and then describesthe Cornell University meter wave equipment. Suzuki andTsuchiya, and Akabane describe meter wave and microwavepolarimeters, which have been developed in Japan. The shortpaper by Bracewell and Stableford is a theoretical one whichwill be of value in interpreting records of solar radio emission.The three papers by Roman and Yaplee, by Wells, and byKo report measurements and discussions of spectra of radiosources and cosmic background radiation. Marshall presents atheory of galactic radio emission involving synchrotron emis-sion from high energy interstellar electrons being deviated by8 JanuaryHaddock: Introduction to Radio AstronomyMFiare 11*- aximum100-150-200-250-300 -350-400-450-500-550-I15:14- vIV15:20 U T15:16 15:18Universal time- (September 12, 1957)Fig. 3-A log-intensity modulated display, recorded at the University of Michigan, of dynamic spectra of an unusual solar radio burst complexat the time of a solar flare. Frequency increases vertically downwards and time increases to the right. The horizontal bright lines, bothsolid and broken, are man-made signals. The broad horizontal banded structure is due to receiver gain and noise level variations, and tothe combining of three different receiver outputs. The fine uniform vertical lines are the individual scan lines. The vertical columns ofdashes are the frequency and time markers. The Type III, fast drift, bursts begin on all frequencies near 15:15 U.T.; the Type II, slowdrift, burst begins shortly afterwards on the higher frequencies and drifts downward in frequency. The detailed structure of this slowdrift event appears to be composed of individual fast drift bursts of restricted frequency range. The broad-band, featureless, brighteningat the higher frequencies following the Type II burst lasted 150 minutes. This is believed to be the new Type IV burst.galactic magnetic field.The next five papers deal with radio spectral line radiation,in contrast with the continuum radiation which we have beenconsidering; the first four are concerned with the 21-cm linefrom atomic hydrogen. The paper by Barrett is a tutorialpaper on radio spectroscopy for the radio astronomer; he de-scribes some radio spectral lines that warrant future investiga-tion. Lilley and McClain review the techniques of measure-ment of the 21-cm line in absorption when viewed against abackground of bright continuum emission from a radio sourceor sources, and discuss the potentialities of this technique.Menon has reported a very interesting astronomical study,using the 21-cm radiation from gaseous regions surroundingclusters and associations of stars within our galaxy. Heeschenand Dieter describe some results and many fascinating pos-sibilities of measuring the 21-cm line radiation from externalgalaxies and clusters of galaxies. The theoretical paper byField discusses the various mechanisms in outer space whichcan affect the excitation temperature of the hydrogen line.He has been interested in the possibility of detecting verydilute hydrogen gas in the regions between external galaxies,an important cosmological problem. Since there is nothing inthis issue on the spiral arm structure of our galaxy as revealedby the 21-cm line studies, the reader is referred to a recentpublication [106] on this topic which shows a map of the spiralarms.Mayer, McCullough, and Sloanaker give an account of theirrecent detection and measurement of microwave emissionfrom the planets Venus, Mars, and Jupiter. Drake and Ewen,as mentioned earlier, confirmed these detections and also de-tected Saturn. It is believed that this emission is thermalplanetary radiation. This work has opened up a new approachto the study of planetary surfaces. At the other end of theradio astronomy spectrum, Kraus reports the measurementsof nonthermal bursts of radiation on 11 meters from Jupiterand the first detection of bursts from Venus. This type ofemission is not the same as that observed at microwaves andis probably generated in the atmosphere or ionosphere of theplanet, although the phvsical cause of excitation may reside inor on the solid part of the planet. The report of radio burstsfrom Venus has not yet been confirmed by other observers.Coutrez, Hunaerts, and Koeckelenbergh present evidenceof the detection of 600-mc radiation from the recent naked-eyecomet, Arend-Roland, visible in the spring of 1957. A fewobservers have claimed the possible detection of this cometaryradiation on different radio frequencies. However, a greaternumber have tried without success to detect this comet.The next three papers deal with the Moon. Gibson has madea careful series of measurements of the thermal radiation fromthe moon at 8.5-mm wavelength. Observations like these shedlight on the thermal and radio characteristics of the outer fewfeet or inches of the Moon's crust. Trexler reports the radardetection of the Moon using a 220-foot paraboloid scoopedout of the ground and paved. His results show clearly that thelunar echo is from a small localized region at the center of thelunar disk, less than 200 miles in diameter. As mentionedearlier, Trexler demonstrated the potentialities of using theMoon in a radio communications link to obtain hemisphericalcoverage of the earth at high frequencies. Yaplee, et al., alsofind that lunar echoes are returned from a localized centralregion, but at a wavelength of 10 cm, in contrast to the 150cm used by Trexler. Yaplee, et al., have made an effort towardmeasuring the precise distance to the Moon. They use aCesium clock to get sufficient frequency control, and expectto obtain a distance measurement with an uncertainty of500-1000 feet.UN.UUa-asU1958 9PROCEEDINGS OF THE IREBooker has reviewed the work on radio source scintillationsmade at meter wavelengths and gives a theoretical discussionof the observations. Lawrence and Penfield, in separatepapers, describe an observational investigation of phase andamplitude scintillations of radio sources due to the ionosphere,and the two-element interferometer and receivers developedfor these measurements. Aarons, Barron, and Castelli describevhf and microwave scintillations of radio sources. The latterstudy has received little attention outside of France andshould be of interest to both radio engineers and astronomers.Little and Leinbach report the first detailed account of highlatitude ionospheric absorption using radio sources. 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