PROCEEDINGS OF THE IRE

Radio Astronomy at the Meudon Observatory *E. J. BLUMt, J. F. DENISSEt, AND J. L. STEINBERGt

Summary-This paper describes the principal instruments usedfor radio astronomical observations at the Nancay field station. Theyare a variable spacing interferometer made of two 7.5-meter mirrorsmovable on rail tracks, several radio-telescopes and interferometersfor solar research at 3-cm wavelength, and a 32-antenna inter-ferometer operating on the wavelength of 177 cm. The second partof the paper gives some new results concerning the sources of solarradio emission and a phenomenon of scintillation at 3-cm wave-length apparently caused by the troposphere.

THE NANCAY OBSERVATORY

HE Nangay radio astronomy field station is lo-T cated 120 miles south of Paris in a region, up to

now, free from man-made interference. A building(Fig. 1) housing 15 persons, including rooms, dormitory,and workshops, was erected on a section of flat landabout one mile long in both the east-west and the north-south directions. It has been designed to meet the re-quirements for the first few years of operation of thestation. Other buildings are being built now. Electricalpower is distributed through the land by undergroundarmored cables. Observing sites in the station are con-nected by telephone and by special network for distri-bution of time signals, both solar and sidereal.The field station includes principally two levelled

platforms, 50 meters wide. The first, which is now inuse, is oriented east-west, with a total length of 1750meters; the second, nearing completion, is north-south.Along these platforms, every 50 meters, special plugsprovide ac power. Other areas of smaller extent are alsoprovided with facilities for observations.

The Variable Spacing Interferometer (Fig. 1)In the northern part of the platform that runs east-

west, a special rail-track of 6-meters gauge and 1500meters long has been built. On this track, antennas usedfor different purposes can be moved easily. During 1957,a second rail-track, oriented north-south will be com-pleted; it intersects the first one near its center.The variable spacing interferometer built on this T-

shaped track uses two parabolic antennas, 7.5 meters indiameter, of German origin (Giant Wurzburg). They areequatorially mounted; motion in right ascension isachieved by 3 electrical motors coupled by differentialgear boxes. Speeds of rotation are 15 minutes, 60 and360 per minute. The same system is used for declinationdrive with only two motors. Position angles are indi-cated by selsyns with an accuracy of about 1 minute ofarc. The mechanical stiffness of the mount is sufficientto maintain a pointing accuracy of the same order. This

* Original manuscript received by the IRE, November 6, 1957.t Observatoire de Meudon, Service de Radioastronomie, Paris,

France.

Fig. 1-One mirror of the variable spacing interferometer.

instrument is going to be used first on fixed frequenciesnear the 1420-mc hydrogen line for galactic and solarstudies.

The 3-CM Wavelength Equipments

Several equipments are operated on 3 cm essentiallyfor solar studies. Size, intensity, brightness distributionof bright solar regions can be investigated with an inter-ferometer operating on 9350 mc.' The two parabolicantennas are 2 meters in diameter, equatorially mounted,and separated by 60 meters. The base line is orientedeast-west and the antennas are automatically driven tofollow the sun. The resolving power of this instrumentis about 1.5 minute of arc near the meridian and as theangular separation between the interference fringesvaries with the diurnal motion of the sun, this apparatuscan be used as a variable spacing interferometer forbrightness distribution studies. Owing to the large dis-tance between each antenna and the main receiver,separate mixers and IF preamplifiers mounted close tothe antennas are used. The local oscillator wave is gen-erated at the electric center of the base line and trans-mitted through waveguide to each mixer. The 60-mcIF signals are transmitted by coaxial cables to the mainreceiver, where their frequency is converted to 10 mcand the bandwidth is reduced to 400 kc, to preventsmearing of the fringes. A three-section waveguide filteris used to eliminate one of the input sidebands.A 4-antenna interferometer with unequal spacing, al-

ready described in the literature,2 also is used on a 3-cm

1 I. Alon, M. R. Kundu, and J. L. Steinberg, Compt. Rend. Acad.Sci., vol. 244, p. 1726; March, 1957.

' J. Arsac, Rev. Opt., vol. 35, pp. 65, 136, 396; June, 1956.

391958

PROCEEDINGS OF THE IRE

wavelength for solar studies. The effects of the earthatmosphere on microwave propagation have been stud-ied, since 1954, with conventional equipment. The re-flectors are surplus searchlight mirrors of 1.5-meter di-ameter, whose vertical axis of rotation has been tiltedto be parallel to the earth's axis. A temperature con-trolled box fixed on the mount contains the mixer, localoscillator, and IF preamplifier.One such antenna is mounted on a fixed metal tower

10 feet high. A similar equipment is mounted on a speciallorry movable on the rail tracks.

The 32-Antenna Interferometer (Fig. 2)

This high resolution instrument was built for meas-uring the positions and intensities of bright solar regionsradiating in the meter range. It consists of 32 antennaserected on an east-west base; its total length is 1550meters. This array, working on the frequency 169 mc, isanalogous to a diffracting grating and its polar diagramin right ascension has several lobes of width 3.8minutes of arc between half-power points, separatedfrom one another by 20. This angular separation is largeenough to insure that only one individual lobe is pointedon the sun. Each elementary antenna is a parabolic mir-ror 5 meters in diameter and is illuminated by a dipoleand a reflector; it can be rotated around a horizontalaxis mounted on a concrete base.Each elementary antenna is connected, by electrically

equal lengths of transmission line, to the main receiverwhich is in a cabin near the geometrical center of thearray. Coaxial cables of a type developed by the FrenchPost Office are used throughout; losses are 25 db for 800meters length at 169 mc. Thus, the received signalsmust be preamplified near the elementary antennas.Adjacent pairs of antennas deliver their signals to oneof 16 preamplifiers used in the system. Each preamplifieris housed in a weather proofed container. The preampli-fier gain is 50 db, the bandwidth is 7 mc, and the noisefigure is 5 db. The circuit involves two cascode stages,2 amplifiers, and the output stage. The stability of thephase rotation across the over-all bandwidth of 2.5 mcof the complete instrument has been carefully tested forany abnormal phase rotation which would badly distortthe polar diagram of the array. Single tuned amplifierstages are used throughout. Stabilized supply voltagesand gain control voltages are fed to these preamplifersfrom the central cabin through neoprene insulatedcables. Each preamplifier embodies a noise diode andthe observer can check each individual amplifier noisefigure and gain from the central cabin. The coaxial ca-bles, whose total length is about 4 km, are deeply buried80 cm, so that variations in attenuation due to changesin temperature are avoided. With the branching systemadopted, the total electrical length from any elementaryantenna to the receiver can be held constant within 0.1wavelength (including the amplifiers).

Fig. 2-General view of the 32-antenna interferometer; one ofthe 3-cm radio telescopes is seen on the rail track.

Signals are recorded, both on electronic servorecorderswith linear response and a time constant of one secondand on fast recorders with a logarithmic scale and a timeconstant less thanl/50 second.The total collecting area of this interferometer is 640

square meters. The north-south branch of this inter-ferometer is going to be built in 1957-1958.The most powerful radio sources, including Hydra A

in consideration of its small apparent diameter, are cur-rently used to test the polar diagram and the sensitivityof this equipment.The Nangay field station is sponsored jointly by the

Paris Observatory and the &cole Normale Superieure;the maintenance of the station, building of the equip-ment, observations, and associated researches are carriedout by about 20 persons from the Meudon Observatory.

SOME RECENT RESULTS

Most of the recent observations have been carried outat the Nangay field station with the 32-antenna inter-ferometer and with the 3-cm equipment which has beendescribed above.The main part of the work for the last two years has

been concerned with studies of the solar activity on bothmeter and centimeter wavelengths; numerous observa-tions were devoted also to the study of atmosphericeffects on solar radiation observed at 3-cm wavelength;besides occasional observations of lesser importance, ageneral survey of the radio sources is now in progresswith the large interferometer.

Solar ObservationsAs is well known, the solar activity in the meter

wavelength range is mainly characterized by the occur-rence of highly fluctuating sources (called R centers) ofvery intense radio emission labelled in the literature asnoise storms or enhanced radiation. The highly fluctuat-ing character of these sources, which are always accom-panied by type I bursts, make their study rather diffi-cult and, in spite of very ingenious investigations carriedout in Australia, little is known about this outstandingphenomenon.

40 January

Blum, Denisse, and Steinberg: Radio Astronomy at the Meudon Observatory

Fig. 3-Drift curves of the sun; above: moderately active;below: quiet.

The large interferometer allows the sun to be scannedeach day during 2 hours around local noon with a re-solving power of 3.8 minutes of arc. The location of solarradio centers of activity is given with an accuracy of afraction of a minute and the diameters of sources assmall as one minute can be measured.

Daily observations have been carried out since May,1956. From the drift curves (see Fig. 3) obtained eachday and reduced, the location and diameter of the Rcenters are deduced and plotted on a diagram that showsthe one dimensional evolution of the sources of activityacross the sun's disk (see Fig. 4).3.4The radio centers generally last for several days, but

their displacements from day to day are highly irregularand it is often rather difficult to relate without am-biguity a given R center with an optical spot. Fromtheir mean speed of rotation and their position whenthey appear on the limb, one can measure the heights ofthe R centers that are found highly variable with a meanvalue around 400,000 km above the sun's surface. Inci-dentally, this height is much larger than the critical fre-quency height calculated from standard data on thesolar corona. Diameter of radio centers can vary fromone day to another, ranging from less than one minute to6 minutes, rarely more.Very often, for periods of several weeks, no center of

radio emission is observed, in spite of the occurrence oflarge and active optical spots; then one radio centerflares up, soon followed by several others widely sepa-

'E. J. Blum, A. Boischot, and M. Ginat, Compt. Rend. Acad.Sci., vol. 243, p. 19; July, 1956.

' Y. Avignon, E. J. Blum, A. Boischot, R. Charvin, M. Ginat,and P. Simon, Compt. Rend. Acad. Sci., vol. 244, p. 1460; March,1957.

Fig. 4-Diagram showing daily position in right ascension of theR centers; E-W is the horizontal diameter of the optical disk.Peaks amplitudes are indicated in 10` 1Wmiic(cps)Y'.

Fig. 5-Drift curves of the sun on November 20, 1956, showing:a) a type IV burst (storm phase), b) an R center.

rated in heliographic longitude, showing some kind ofcollective property that does not seem to have anyoptical counterpart (see Fig. 4).

In the course of this investigation a new type of solarradio emission was clearly identified.6 The 200-mc ob-servations of Dodson, Hedeman, and Owren have indi-cated that outbursts of radioemission associated withsolar flares can be separated into two distinct phases.In the first or main phase the noise level rises suddenlyand lasts for about 10 minutes; in the second, or stormphase which starts soon afterwards, the level rises muchmore gradually and stays very high for hours. The radia-tion of the storm phase is circularly polarized and wasbelieved to be identical to the noise storms.

Observations with the large interferometer disclosednew information on the storm phase which indicatesthat it is an entirely new type of solar radio emission.Fig. 5 shows drift curves obtained on the sun with anordinary R center and an outburst (storm phase) to-gether on the disk. The difference between the two typesof radiation is striking: the outburst (called type IV byBoischot6) is completely free of type I bursts character-istic of noise storms. Furthermore, the diameter of thequiet type IV source is larger than the diameter of theR centers; it can reach about 10 minutes of arc betweenhalf-power points. Sometimes, especially at the begin-

6 A. Boischot, Compt. Rend. Acad. Sci., vol. 244, p. 1326; March,1957.

1958 41

PROCEEDINGS OF THE IRE

ning rapid motions of the order of 500 km are observed.All of these properties make the source of the stormphase an object distinct from the R centers or, at least,a very extreme case. It is interesting to speculate aboutthe possibility that this emission is due to synchrotronradiation emitted by fast electrons produced along withthe cosmic rays that are known to be associated withcertain flares; this suggestion is supported by the veryhigh intensity of the emission that precludes a thermalorigin, and the smoothness and large size of the sourcethat is likely to be produced by a microscopic process.

Other observations are progressing with the two an-tenna 3-cm interferometer; diameters of localized re-gions of steady emission which account for the slowlyvarying component were found of the order of 5 minutesof arc, and valuable results were also obtained on theoutbursts' (Fig. 6).

Solar Scintillations in the 3-CM Wavelength Region

Several experiments are concerned with the study ofsolar scintillations that were discovered to exist on awavelength of 3 cm.6 This phenomenon has a twofoldinterest. As such, it limits the seeing for radio astronomi-cal observations, especially at low elevations, but it isalso a powerful tool for studying atmospheric inhomo-geneities probably responsible for the anomalous radiopropagation within or beyond the horizon.The first studies were devised7'8 to assess the atmos-

pheric origin of the scintillations. Comparing records ofthe sun's radiation obtained with two different radiom-eters, it was found that identical patterns of scintilla-tions are obtained as long as the two antennas are notseparated by more than about 100 meters; the correla-tion becomes weak when the receivers are more than200 meters apart (see Fig. 7). The correlation co-efficient between the two records is roughly given by:p(D) = e-(Dl70), where D in meters is the distance be-tween the two lines of sight of the radiometers.The amplitudes and pseudoperiods of the scintilla-

tions increase with the zenithal distance; their occur-rence is strongly related to the existence on the sun oflocalized regions of steady emission whose diameter isabout equal to 5 minutes of arc. The whole disk of thequiet sun appears to be too large in diameter to pro-duce scintillations. On the contrary, some radio out-bursts of the very narrow diameter of the order of 1minute of arc, give rise to large increases in scintillations.

Experiments to find the origin of the scintillations andheight of the turbulent layers were performed along twolines; in one series of experiments comparisons weremade of scintillations observed on 3 cm and 33 cm. Thesame kind of scintillations were observed on both wave-

6 . Kazes and J. L. Steinberg, Compt. Rend. A cad. Sci., vol. 240,p. 493; January, 1955.

I. Kazes, Compt. Rend. Acad. Sci., vol. 245, p. 636; August, 1957.8 . Kazes and J. L. Steinberg, Compt. Rend. Acad. Sci., vol. 245,

p. 782; August, 1957.

Fig. 6-Interferometric records of 3-cm double bursts showingdifferent fringe visibility; the burst on the left has the smallestdiameter.

!v..AJ.1J

b

1n

Fig. 7-Simultaneous record of solar scintillations with two anten-nas at distance D: a) D=60 m; good correlation (p=0.88), b)D=460 m; no correlation (p =0.05).

lengths, but they were rather weaker on the longer waveshowing that the phenomenon was very probably not ofionospheric origin. In another series of observations onecomponent of the wind existing in the turbulent layerwas estimated by measuring time lags existing betweentwo records obtained simultaneously with different an-tennas. Some results are given in Fig. 8 where themeasured winds are compared with the correspondingcomponent of the windsmeasured by the Trappes Meteo-rological Station situated 200 km northwest of Nangay.It is quite noticeable that the radio winds and the windsmeasured of the height of the tropopause are always ofthe same sense and also of comparable magnitude. Theseresults suggest that the observed scintillations take

January42

Strum: Considerations in High-Sensitivity Microwave Radiometry

e .. . * ..e

* * ~4 to

Fig. 8-Comparison of speed V7(m/s) of radio winds measured ondirection: * :N-S, o:S-N, +:N-0, -:S-E, with the same com-ponent Vm of meteorological winds measured at different heights,from left to right: 500-1500 m, 5000-6000 m, tropopause(9000-13,000 m).

place in the low atmosphere in a region close to thetropopause.

Besides the topics outlined here, the Meudon Observ-atory Group is also actively engaged in research con-

cerned with solar-terrestrial relationships, absolutemeasurements, galactic structure, theoretical studies, etc.Work on solar terrestrial relationship is mainly due

to Simon. He has first confirmed and extended9 thefindings by Denisse of a definite relation between theoccurrence of solar enhanced radiation on meter wave-

lengths and the increase of geomagnetic activity. Hewas able to demonstrate furthermore a very close con-

trol of the solar radio noise on the geomagnetic pertur-bations following the onset of the brightest chromo-spheric flares.10

Along different lines, Kundu and Denissell found a

9 P. Simon, Ann. Astrophys., vol. 19, p. 122; May-June, 1956.10 P. Simon, Ann. Gtophys., vol. 12, p. 167; July-September, 1956."1 J. F. Denisse and M. R. Kundu, Compt. Rend. Acad. Sci.,

vol. 244, p. 45; January, 1957.

close and linear relation between the 10-cm solar radia-tion and the ionizing flux of the E layer as measured bythe index (foE)4/cos Z. This relation shows that the E-layer ionization is more closely related to the 10-cm solarradiation than to the sunspots themselves or to thecoronal green line intensity. This result stresses the in-terest of the 10-cm solar radiation as an index of solaractivity.

Observations by Le Roux led to a complete mappingof the sky brightness on the wavelength 33 cm."2'3 Onthe same wavelength absolute measurements were alsocarried out on the brightest radio sources and on thesky.'3

Theoretical works are concerned with the physics ofplasma,'4-'7 the theory of ionospheric interaction (Lux-enbourg effect),'8 and the studies by Arsac of antennaperformances and design and image formation inradio astronomy.'9 20

12 J, F. Denisse, E. Le Roux, and J. L. Steinberg, Compt. Rend.Acad. Sci., vol. 240, p. 278; June, 1955.

13 J. F. Denisse, J. Lequeux, and E. Le Roux, Compt. Rend. Acad.Sci., vol. 244, p. 3030; April, 1957.

14 M. Bayet, J. L. Delcroix, and J. F. Denisse, J. Phys. Radium,vol. 16, p. 274; November, 1955.

15 M. Bayet, J. L. Delcroix, and J. F. Denisse, J. Phys. Radium,vol. 17, p. 923; December, 1956.

18 M. Bayet, J. L. Delcroix, and J. F. Denisse, J. Phys. Radium,vol. 17, p. 1005; December, 1956.

17 M. Bayet, J. L. Delcroix, and J. F. Denisse, Compt. Rend. Acad.Sci., vol. 244, p. 171; January, 1957.

18 M. Bayet, J. L. Delcroix, and J. F. Denisse, Ann. Telecom.,1957, to be published.

19 J. Arsac, Optica Acta, vol. 3, p. 55; June, 1956.20 J. Arsac, Aust. J. Phys., vol. 10, p 16; June, 1957.

Considerations in High-SensitivityMicrowave Radiometry*PETER D. STRUMt, SENIOR MEMBER, IRE

Summary-This paper discusses considerations in high-sensi-tivity microwave radiometry, especially as applied to systems havingtemperature thresholds significantly less than 10K. Many considera-tions that have been ignored in previous analyses are shown to beprominent. The influences of the background radiation from space,atmospheric oxygen, atmospheric water vapor, and earth-bound radi-ators are shown to set a threshold level. Fluctuations in gain andtemperature of the antenna, the waveguide system, the comparisonsource, the noise balancer, the receiver, and other amplifying com-ponents are shown to set another threshold. linpedance-modulationeffects set still another threshold. The intrinsic internal receivernoise establishes an irreducible threshold. Continuous nonswitchedtypes of radiometers usually are not suited for high-sensitivity appli-

* Original manuscript received by the IRE, November 11, 1957.t Ewen Knight Corp., Needham, Mass.

cations because the present state of the art in gain stability is notadequate. The square-wave switched system is most likely to yieldsatisfactory results. In these systems the optimum performance isobtained when the magnitude of the signal within the system isminimized at approximately the level at which measurements will bemade. This result establishes the requirement for noise balancingwhich may be either continuously adjusted or adjusted once for eachmeasurement.

INTRODUCTIONT HE observation of radio sources at microwave

frequencies requires a high degree of sensitivity.Nonthermal radio sources have a noise spectrum

that compensates approximately for changes in antennagain for a given aperture size. For this reasQn there is

1958 43