1. Introduction

The Use of ExtraterrestrialRadio Sources in theMeasurement of AntennaParametersD.A. GUIDICE, Member, IEEEJ.P. CASTELLI, Member, IEEEAir Force Cambridge Research LaboratoriesfBedford, Mass. 01730

Abstract

Extraterrestrial radio sources, whose emission characteristics (flux

density, spectrum, angular size) and coordinates have been firmly

established by careful observations, have application in the

measurement of the effective area (aperture efficiency and gain)

of an antenna and its radiation pattern. The radio-emission char-

acteristics of the strong discrete (celestial) radio sources, of the

sun, and of the moon are presented. Problems encountered when

the sensitivity is insufficient for complete radiation pattern deter-

mination, when the width of the radio source is comparable to

the beamwidth of the antenna, when the illumination of the

antenna aperture varies with pointing direction, or when the

techniques are extended (after determining the gain of the

ground-based antenna) to the measurement of the effective rad-

iated power from a satellite are discussed

For large antennas, the transmitter-to-receiver distancerequired for conventional (antenna range) mnethlods ofboresight, gain, and radiation pattern ineasurements canbe excessive; the use of extraterrestrial radio sources torsuch measurements should be considered One method ofmeasuring the alignment of the beam axis is to boresightthe anitenniia oni a strong radio source witni- accuratelyknown celestial coordinates. The coordiinates of manystrong discrete radio sources are known to a fractionalminute of arc [8.

To measure antenna gain, one must know the absoluteradiation flux densitv incident upon the antenna. Theflux densities of the strong nontliermal radio sources areknowni to vary witlh frequency, but have measuredaccurately over the 100 to 10 000 MHz range [1] Dailymeasurements of solar flux density at various frequenciesare imade by several observatories around the world; aslong as the suIn is not highly disturbed, the flux densityvalues reported may be used for calibration for that day.The cyclical variations in flux density of the moon,caused by variation in its phase of illumination anddistance from the earth, have been well determined; forany day the lunar flux density at a given frequency canbe readily predicted.

For very large antennas, the strong discrete radiosources can be used for radiation pattern measurements toa sensitivity of about 15 to 20 dB below beam-axisresponse, adequate for beamwidth measurement. Forgreater sensitivity, the sun (a much stronger radio source)may be used, however, because of its troublesomeemission characteristics, the techniques involved may bemore complex.

11. Radio Sources Useful for Antenna Measurements

Discrete Radio Sources

Manuscript received March 10, 1970; revised September 11, 1970.

The four discrete radio sources most useful for effec-tive area (or gain) and antenna pattern measurements are:

Cassiopeia A, Cygnus A, Taurus A, and Virgo A. Table Igives their flux densities at frequencies between 100 and1O 000 MHz. From 100 MHz to 4 GHz, the absoluteaccuracy is about 3 percent for Cassiopeia A, 5 percentfor Taurus A and Cygnus A, and 5 to 10 percent forVirgo A; from 4 to 10 GHz, it is about 5 percent forCassiopeia A and Taurus A and about 10 percent forCygnus A The flux density of Cassiopeia A decreasesabout 1.1 percent per year; the values in Table I are for1964.0 (January 1964). The celestial coordinates (epoch1950.0) and the angular dimensions are given in Table II.Although the general shape of the spectra of these sources

below 100 MHz has been established [2], [3], [6], theabsolute accuracy of the flux density measurements isabout 10 to 30 percent. The limitation at low frequenciesis caused by confusion of the source emission with thecelestial background. The kinds of antennas (arrays withvery large baseline dimensions) used at low frequencies to

2EE6li TRANSACITIONS ON AF ROSIACI ANI) VL.CRNI( 5511 MS VOl AFS-7, NO. 2 MARC'H 1971226

TABLE I

Flux Densities of Cassiopeia A, Cygnus A, Taurus A, and Virgo A

-2 -1 -26Frequency Flux Density (W m*Hz xlO(MHz) Cassiopeia A* Cygnus A Taurus A Virgo A

100 17300 12200 1750 1500150 12800 9300 1600 1130200 10300 7500 1500 890300 7700 6400 1320 660400 6140 4600 1230 560600 4550 3350 1100 400800 3700 2600 1040 320

1000 3100 2100 980 2651500 2330 1500 900 2002000 1850 1100 820 1553000 1380 690 730 1104000 1100 480 690 -

6000 820 290 625 -

8000 640 215 580 -

10000 500 160 550 -

*The flux density of Cassiopeia decreases about 1.1 percent per year; values given are for 1964.0.

TABLE II

Celestial Coordinates (Epoch 1950.0) and Angular Dimensions of the Four Strongest Discrete Radio Sources

Source Right Declination Angular CommentsAscension Diameter

Taurus A 05h31m31s +12'39.9 3' by 4!5 Radiation from the Crab Nebula; ellipticalshape with major axis at 140

Virgo A 12h28ml75 +21 59'0 core 0!6 Halo contributes 40 percent of flux densityhalo -6' at 1420 MHz, 55 at 400 MHz, 75 at 100 MHz,

almost all below 30 MHz

Cygnus A l9h57m44s +40037'4 each <0'7 Double source, components separated by 1.8on axis at 1100

Cassiopeia A 23h21in1 s +58 32.'8 4' 1 Circularly symmetric, probably ring shaped;flux density decreases 1.1 percent per year

_.__ _I__

resolve the source from the background are not the typesubject to accurate absolute determination of effectivearea.

The Sun as a Radio Source

The strongest extraterrestrial radio source is the sun.Excluding bursts, the flux density of the sun monoton-ically increases with frequency but has a varying spectralindex over the 100 to 35 000 MHz range. More significantthan the somewhat accountable variation with frequencyis the unpredictable variation of flux density with time.Again excluding radio bursts, the flux density of the sunin the 500 to 5000 MHz range during a disturbed periodmay be as much as five times greater than the flux

density of the quiet sun. The flux density of the quietsun, the measured monthly mean for July 1964, over thefrequency range from 100 to 10 000 MHz is shown inFig. 1. Flux densities from this curve are representativeof the flux densities for any time during thesunspot-minimum period and might be used for antennameasurements for any time during this period. Alsoshown is a curve of the monthly mean for a period ofmaximum sunspot activity (June 1958). Flux densitiesfrom this curve are only very generally representative ofthe actual flux density for any particular day during thesunspot-maximum period. Not only will the monthlymean for other months in the sunspot-maximum periodbe different, but the flux densities for any given daymight be greatly different from the monthly mean.

GUIDICE AND CASTELLI: EXTRATERRFSTRIAL RADIO SOURCES FOR ANTFNNA PARAMETER MEASURFMFNT 227

TN:F

in

2

zwa

x

-JU.

oo

100 1000 10000FREQUENCY- MHz

Fig. 1. Monthly mean of solar flux density as afunction of frequency during a sunspot-minimumperiod (July 1964) and during a sunspot-maximumperiod (June 1958).

Solar flux density measurements in the 600 to 10 000MHz range are made on a daily basis at several radioobservatories with absolute accuracies on the order of 5percent. For example, from the Algonquin Radio Obser-vatory (Ottawa) we have daily measurements at 2800MHz; from our Sagamore Hill Radio Observatory and theManila Observatory we have daily measurements at 606,1415, 2695, 4995, and 8800 MHz [5]. When determiningthe effective area or gain of an antenna using the sun asthe calibration source, one can obtain the solar fluxdensity at the operating frequency of the antenna byinterpolation.

It is assumed that between any two frequencies atwhich daily values are reported, the solar flux densitymay be represented by a power law, S cc vpU, where a is aconstant. Given the flux densities S(vj) and S(vk) re-ported at frequencies v1 and vk, the flux density S(v) atthe frequency v of the antenna system being calibrated isfound from the equation

5()= (v1)17vS(V) - [S( j S(vk),

with (1)

1(v) 109log (vlvk)log (Vjlvk)

Note that this expression does not require an explicitdetermination or an assumption of the spectral index a.Of particular interest to space communications engineersis the frequency region between 1415 and 2695 MHz,which contains the 1435 to 1535 MHz and the 2200 to

TABLE III

Values for r(v) in (1) for the 1435 to 1535 MHz Band with

vj= 2695 MHz and vk = 1415 MHz

Frequency (MHz) r(v)

1435 0.0221455 0.0431475 0.0641495 0.0851515 0.1061535 0.126

TABLE IV

Values for NOv) in (1) for the 2200 to 2300 MHz Band withv- 1415 MHz and vk = 2695 MHz

Frequency (MHz) r(v)

2200 0.3152220 0.3012240 0.2872260 0.2732280 0.2602300 0.246

2300 MHz telemetry bands. Values of 1(v) in (1) forthese telemetry bands are given in Tables III and IV.

Using only the flux density from the nearest frequency(vk), a simpler interpolation for S(v) can be made usingthe expression

S(v) = ~(?) S(Vk) (2)

with an assumed value of a. An investigation of the valueof a in the 1415 to 2695 MHz range has been made fromthe spectral curves of Tanaka and Kakinuma [12) andrecent daily flux density data from Sagamore Hill RadioObservatory. Average values of a ranged from about 0.62for low solar flux densities (quiet sun) to as small as 0.33for high flux densities (where a wide variation in a wasfound). Based on this survey, Table V has been puttogether, recommending values of a to be assumed forinterpolating from 1415 MHz (into the 1435 to 1535MHz telemetry band, for example) or from 2695 MHz(into the 2200 to 2300 MHz band). For values of S(vk)> 135 X 10-22 Wm-2 -Hz-' at 1415 MHz or > 180 X10-22 W.m-2 Hz-1 at 2695 MHz, a reliable assumption ofa is not possible and the use of (I ) rather than (2) isrecommended.

The apparent right ascension and declination for thecenter of the solar optical disk for any day can be foundin the American Ephemeris and Nautical Almanac(AENA) for that year. Also in the AENA, the value ofthe apparent angular diameter of the optical disk may befound; it varies from about 31'.5 to 32.5 over the one-year cycle, due to the variation in the earth-sun distance.

22 ii 1TRANSAC TIONS ON Al ROSPACE AND ELLC(TRONIC SYSTFMS MARCH 1971228

TABLE V

Recommended Values of a to be Used in (2) for Various Ranges of Flux Density Reportedat 1415 MHz or 2695 MHz

Range of Observatory Reported Flux-2 -1 -22Density (W m Hz x10 ) Values of a

to be Assumedvk = 1415 MHz vk = 2695 MHz

40 - 80 60 - 125 0.685 - 110 125 - lSS 0.5110 - 135 150 - 180 0.4

TABLE VI

Components of the Moon's Brightness Temperature (Average over the Whole Lunar Disk)Measured at Various Wavelengths

Wavelength Temperature (0K) Phase Lag t(cm) TmO Tml (degrees)

0.4* 217 64 240.8 211 40 301.6* 208 34 353.2* 211 14 409.6* 221 7 40

14.2 223 <4 _25 230 - _32* 230 - _40 224 - _60 217 - _70 217 - _

*Average values of measurements made at roughly the same wavelength by two or more observers(whose estimated errors are 10 percent or less).

The Moon as a Radio Source

For wavelengths of 30 cm or shorter, the moon is adependable radio source for precise antenna calibration.Unlike the strong discrete radio sources, the flux densityof the moon increases with increasing frequency. Unlikethe sun, there are no large unpredictable changes in theflux density of the moon. The small, gradual changes thatresult from cyclic variations in lunar phase (illuminationby the sun) and in the earth-moon distance are entirelypredictable.

The average lunar brightness temperature can bedivided into two components. The constant component,Tmo, which is the time average over one lunation (solarillumination cycle), has no strong wavelength dependence.The variable component, Tm 1, reaches up to 25 percentof the constant component at millimeter wavelengths, butdecreases with increasing wavelength.

The lunar brightness temperature is approximated(experimentally) by the following expression:

Tm(X) = Tmo + Tm i (X)cos ((27rt/P)- (X)) (3)

where Tm (X) is the average radio brightness temperature

of the moon, t is the time of observation in units of P, Pis the lunar synodic period (29.53 days), 27rt/P is thephase angle of the visible lunation measured from fullmoon (t = 0 at full moon), and t(X) is the phase lag. Inthe infrared, the phase lag behind the visible lunar phaseis very small; from the infrared to 3 cm, it increases withincreasing wavelength; at longer wavelengths, it does notappear to become appreciably greater.

Table VI gives some of the measured values of Tm o,Tm 1, and t at various wavelengths [4], [13], [14].Averaging the measurements of Tm 0 over the entirewavelength range from 0.4 to 70 cm, one obtains Tm 0 =2190K ± 1 1°K, which agrees with all the measured valueslisted for each wavelength in Table VI to within 5 per-cent.

The flux density of the moon is obtained from theexpression

2kTmQ2mS= (4)

where k is Boltzmann's constant, Tm is the averagebrightness temperature of the moon, and Qm is the solidangle subtended by the moon (equal to (7r/4)0d2, whereOd is the apparent angular diameter in radians). Because

GUIDICE AND CASTELLI: EXTRATERRESTRIAL RADIO SOURCES FOR ANTENNA PARAMETER MEASUREMENT 229

;0-20 r rTr

2i

o _ .,_,y

N

-- 6Ad =33i 30

- d 31M30

C: _ 9d = 29' 30"

10

10/23L -12 4 6 8 0 20 40

FREQUENCY - GHz

Fig. 2. Flux density of the moon as a function of fre-quency for various angular diameters. It is assumed that_ 0

Tm = 219 K.

the apparent angular diameter of the moon varies betweenroughly 29r5 and 33'5, its flux density can vary by 25percent.

Assuming Tm = 219 K, the flux density of the moonas a function of frequency (for several values of apparentangular diameter) is given in Fig. 2. For X < 10 cm, thevariable component Tm 1 becomes important, and onemust determine the actual value of Tm from (3). Theangular diameter of the moon and the apparent celestialcoordinates of the center of the moon for any day (on anhourly basis) may be obtained from the AENA for thatyear. When the moon is near the galactic disk or near anyof the strong discrete radio sources, its use as a radiosource for antenna calibration should be avoided.

Ill. Radiation Pattern Measurements

The Use of Discrete Radio Sources

Radiation pattern measurements can be made using thestrong discrete radio sources, whose celestial coordinatesare known (see Table II). Using Cassiopeia A or CygnusA, one can measure the off-axis response to about 20 dBbelow beam-axis response for large antennas (diameter

T ~ ~11 1-

MINUTE--OF TIME

/

MINUTE-OF TIME

CASSIOPEIA Aa *X58 33'

DRIFT TIME BETWEEN HALF-POWER LEVELS - 4.5 MINUTES

CYGNUS A8 - + 40 37'

DRIFT TIME BETWEEN HALF-POWER LEVELS 5 3.2 MINUTES

TAURUS AB - + 21 59'

DRIFT TIME BETWEEN HALF-POWER LEVELS - 2. 5 MINUTES

I MINUTE- -OF TIME

Fig. 3. Drifts through the radio sources Cassiopeia A, Cygnus A,and Taurus A, using an equatorially mounted 84-foot parabolicantenna operating at 1250 MHz. The difference in drift timebetween half-power levels is due to the different declinations ofthe radio sources. From these drifts, the beamwidth was found tobe 35 minutes of arc.

> lOOX). While these sources are useful in determining thehalf-power beamwidth of an antenna, they usually cannotbe used to make the precise measurements of sideloberesponse that might be required, for example, in deter-mining the main beam efficiency of an antenna.

Radiation pattern measurements using radio sources arecustomarily made by letting the sources drift through theantenna beam and recording the response. Fig. 3 showsobservations made at our Sagamore Hill Radio Obser-vatory using the 84-foot parabolic antenna. The actualhalf-power beamwidth is the same for all three cases; thedifference in the time for the source to pass through thebeam is due to the difference in the declinations of thethree sources. The antenna half-power beamwidth can becalculated from the elapsed time between the half-intensity points of the drift curves; one minute of time isequal to 1 5 rminutes of arc times the cosine of thedeclination of the source.

The Use of the Sun

To measure the off-axis response of an antenna tolevels more than 20 dB down, the sun may be used. Thesun, however, has certain disadvantageous properties.During sunspot activity, the solar flux density and appar-ent center of emission vary unpredictably. The angular

210F111 RANSAC HIIONS ON AFIROSPA I. ANI) I' I.CYRONIC SYST}iMS MAR(CIH 1971

--

2 30

TABLE VII

Beam Pattern and Main-Beam Solid Angle Expressions for Two Types of Circularly SymmetricAperture Distribution

Aperture Distribution Beam Power Pattern Main-Beam Solid Angle

Gaussian Gaussian e-2 QM= 1.133 02Uniform Bessel function [2J1(0)/pJ2 QM= 1.008 02

Fig. 4. Two-dimensional antenna pattern of a 25-meter parabolicradio telescope obtained by the use of the sun [11]. Operatingfrequency was 400 MHz. The contours are given as percentagesof the beam-axis sensitivity.

.05

0I

rt \59 / ~~~~~~~~~~~~u4p(I

80 40 00 40 E

diameter of the sun, which at low frequen(iably greater than the diameter of its opticalsignificant in comparison to the beamwiantennas. Corrections for this latter problemmade for antenna half-power beamwidth chowever, for precise measurement of sidelocalculation and application of the correctioreasy.

In making radiation pattern measurementna using the sun, the flux density and ceemission must remain constant for the rmeasurements. Another small antenna and r,be used to track the sun and monitor itsduring this period. If there are no drastic c]density, the center of radio emission (reoptical center) should also stay the same.

An example of the measurement of(two-dimensional) radiation pattern of ansensitivity of 33 dB below beam-axis resposun is shown in Fig. 4 [11]. The 25-meter ptelescope operating at 400 MHz had halfwidths of 2.2 degrees in the H plane and I

sOT an anten-nter of radio)eriod of theadiometer canflux density

hanges in fluxlative to the

the completeantenna to a)nse using theiarabolic radio-power beam-1.7 degrees in

the E plane. From the two-dimensional radiation pattern,the main-beam solid angle 2M, the antenna solid angleQA, and the main-beam efficiency 7NM = 2M/2A were

directly determined.

Indirect Approach for Obtaining Main-Beam Efficiency

right To determine the main-beam efficiency of an antennadirection, looking when a two-dimensional map of the radiation pattern

Into the sky down to the level of -30 dB or better cannot be made,an alternate approach is to use certain formulas to cal-

Polariza tion: X4electric vector culate the main-beam solid angle and the antenna solid

angle from other measured antenna parameters. Themain-beam solid angle is calculated from

QM =kdOEOH =kdO2

where kd is a factor depending on the shape of the powerpattern (hence, on the aperture distribution), OE and OHare the half-power beamwidths in the E and the H planes,and 0 is the geometric mean of OE and OH. For a beam

t° with circular symmetry, the factor kd has a value between1.01 and 1.13, as shown in Table VII [10]. The antennasolid angle is calculated from the expression QA = X2/Ae,where Ae is the effective area of the antenna. The main-beam efficiency is then calculated from .M/2A.

CieS iS apprec-disk, may be

idth of large IV. Determination of Effective Area and Gainccan be easily

letermination; General Techniquebe levels, the For a "point source" (i.e., a source whose angularis may not be diameter is much smaller than the beamwidth of the

antenna), the effective area is expressed by

Ae= 2kTAAe= s (5)

where k is Boltzmann's constant, TA is the measuredantenna temperature due to the source above that due tothe sky background, and S is the flux density of thesource.

To calibrate the antenna temperature due to the radiosource, a comparison noise source of known excess noisetemperature is substituted or coupled into the system bymeans of a low-loss coaxial switch (low frequencies) or adirectional coupler (higher frequencies). For frequenciesof several hundred megahertz or less, a temperature-limited thermionic diode is used as the noise source. Its

GUIDICE AND) CASTELLI: EXTRATIERRISTRIAL RAI)IO SOURCIS FOR ANTENNA PARAMETER MIEASUREMENT 231

excess noise temperature is directly proportional to itsplate current, which can be varied. For microwave fre-quencies, a gas discharge tube is used. This device putsout a constant excess noise temperature, on the order of11 0000K (the exact value for individual units is cali-brated using the 100IK temperature difference of amatched load immersed in boiling and then freezingwater).

When one obtains the effective area of an antenna,aperture efficiency 71a and gain G follow from the simpleexpressions

Tla =AelApand

G=R i- Ae

0

w

0C-

(6)

(7)

where Ap is the physical (geometric) area of the col-lecting aperture, and kR is the radiation (ohmic) effic-iency, which for most large antennas can be assumedequal to unity.

O Fn (89,O) =-e(P)'

< Fn (9,#). [ 2J, (p sinG) 2p Fnn8 J

(3Fn(9, ) [sin (Psin 612

LO0I - I I

0 0.5 1.0 1.5 2.0RATIO OF SOURCE DIAMETER Ed TO ANTENNA HPBW OH

Fig. 5. Correction factor K for a circular source ofuniform brightness as a function of Od/OH for threerepresentative antenna power patterns.

Corrections for Measurements Using an Extended Source

The apparent flux density SA of a source whoseangular diameter is not small compared to the half-powerbeamwidth is equal to 2kTA/Ae. The true flux density ofsuch a source is

2kTAS = 2A K = KSA (8)

where K is an antenna correction factor, which must beused when the angular diameter of the source is morethan about one-fifth the antenna half-power beamwidth.This correction factor is given by

K2sff Fn (0 O)dQ (9)source

where QS is the solid angle subtended by the radio sourceand Fn (0,4)) is the normalized power pattern of theantenna. For half-power beamwidths much greater thanthe angular diameter of the source, K approaches unityand the apparent flux density is essentially the true fluxdensity.

Consider a radio source having Gaussian distributionwith elliptical symmetry, where the half-intensity widthsalong the principal axes are OSI and OS2. For a Gaussianmain beam of an antenna with circular symmetry andhalf-power beamwidth OH. the correction factor K isgiven by

K= (1S 1 12( ft 2

OH2 ~~OH2/

Consider next a radio source that is a circular disk ofuniform brightness temperature with angular diameter Od*

This may be taken as the case for the moon or the sun(when not highly disturbed). Curves for the correctionfactor K for three representative types of antenna radi-ation patterns are given in Fig. 5, based on expressionsderived by Ko [9]. For 0dl0H < 1, the curves for thethree types of radiation patterns are almost identical, withK : (1 + 0.18(Od/OH)2 )2 .

Measurement of Ae for an Antenna with VariableAperture Illumination

If an antenna does not maintain the same apertureillumination for different beam pointing angles, its effec-tive area will vary with pointing angle. An example ofsuch an antenna is the 1000-foot-diameter radio telescopeof the Arecibo Ionospheric Observatory, in which beampointing is achieved by moving the feed relative to thefixed spherical-cap reflector.

The effective area of the Arecibo radio telescope as afunction of zenith angle was determined for the 20 to 40MHz frequency range by Guidice [7] using the sourceTaurus A, whose declination is close to the latitude(18021'N) of the antenna's location. The antenna temper-ature was measured as Taurus A was tracked frommaximum zenith angle (20 degrees, the telescope limit) tominimum (3.6 degrees, the nearest approach of the sourceat transit) and out to maximum again. To separate theantenna temperature contribution due to the source [TAin (5)] from that contributed by the sky background,Taurus A was allowed to drift into and out of the beamat the beginning and end of the calibration run. The smallchanges in the background-contributed antenna temper-ature caused by the changes in illumination spillover ofthe feed with pointing angle were calculated and cor-rections applied. The effective area at various zenithangles was then calculated using (5).

22IERUANSAC rIONS ON AFROSIA(CE ANI) FLiCT RONIC SYSTE:MS MARC'H 1971232

TABLE VIII

Polarization Coefficients for Various Combinations of Polarization

Polarization of Polarization of Transmitter (or Astronomical Sources)Receiving Antenna Right Circular Left Circular Linear Random

Right Circular 1 0 1/2 1/2Left Circular 0 1 1/2 1/2Linear 1/2 1/2 Oto 1 1/2

Fig. 6. Block diagram of equipment used to measure theeffective radiated power on the 401-MHz telemetry channelof military satellites.

84 FTA NTENNAI

LIIR COUPLER \ROTATABLELINEARFEED

NOISEG E NERATOR>> ~~(VARIABLE )MILLIWATTMETER

PRECISION SIGNALATTENUATOR GENERATOR(VARIA#LE) 401 MHz

TELCO COLLINS CHART401 MHz IR-390 RECORDER

CONVERTER RECEIVER

An Application of the Effective Area Determination

After one determines the effective area of an antennathrough the use of a radio source, a further practicalapplication is the measurement of the effective radiatedpower from a satellite. The power received by a ground-based antenna from a satellite is given by

AePr=Cp(I-L)Pt 4 R2Gt(rr,Rr)

where Cp is the polarization coefficient, L is the atmos-pheric (or ionospheric) loss term, Pt is the transmittedpower, Ae is the effective area of the receiving antenna, Ris the range to the satellite, and Gt(Or,4r) is the gain ofthe transmitting antenna in the direction of the receivingantenna. Table VIII gives values of Cp for various combi-nations of polarization for the transmitter and the receiv-ing antenna. For frequencies greater than 50 MHz and lessthan 5 GHz, we can assume L 0 for elevation anglesabove 100.

Measurements of the effective radiated power of the401 -MHz telemetry channels of a number of militarycommunications satellites were made at our Sagamore HillRadio Observatory using the 84-foot-diameter parabolhc

antenna. A block diagram of the equipment is shown inFig. 6. The satellites were distributed in positions abovethe equator so that they drifted from west to east about20 degrees of longitude per day.

To determine effective area, the antenna was pointedon and then off Cassiopeia A. The noise generator outputwas adjusted to match the peak of Cassiopeia A on thechart recorder, and the temperature was read from ameter on the noise generator. The effective area wasfound to be 191 meters2 (qa = 0.37). With the antennapointed toward the satellite, the recorder output waspeaked by adjusting the declination and hour angle of theantenna, the receiver frequency, and the polarizationangle of the rotatable linear feed (the satellite antennaswere linearly polarized). The antenna was then moved offthe satellite, the signal generator (1-mW output) con-nected, and Pr measured by adjusting the precisionattenuator to match the satellite signal reading on therecorder. Assuming roughly isotropic radiation from thesatellite antenna systems, the effective radiated powerfrom most of the satellites was found to be on the orderof 250 mW (measured to an accuracy of ±0.5 dB).

Acknowledgment

The authors gratefully acknowledge the contributionsof other members of the Radio Astronomy Branch ofAFCRL who have helped make some of the types ofmeasurements described in this paper. A longer, moredetailed presentation on this subject is given in anin-house report, AFCRL-68-0231, available from theauthors upon request.

References

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[31 A.H. Bridle, "Flux density of Cassiopeia A and Cygnus A at10.05 MHz," Observatory, vol. 87, pp. 60-63, 1967.

[41 J.P. Castelli, "Lunar thermal emission measurements andrelated antenna considerations," Air Force CambridgeResearch Labs., Hanscom Field, Bedford, Mass., Rept.AFCRL-64-313, 1964.

(5] J.P. Castelli and G.A. Michael, "The Sagamore Hill Solar

GUIDICE AND CASTELLI: EXTRATERRESTRIAL RAI)IO SOURCES FOR ANTENNA PARAMETER MEASURFMENT 233

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[71 D.A. Guidice, "March 1966 observations of the GalacticSpur in the 20 to 40 MHz range," Air Force CambridgeResearch Labs., Hanscom Field, Bedford, Mass., Rept.AFCRL-67-0546, 1967.

[8] W.E. Howard and S.P. Maran, "General catalogue of dis-crete radio sources," Astrophys. J., vol. 10, pp. 1-350,1965.

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[11] C.L. Seeger, G. Westerhout, R.G. Conway, and T. Hoekema,"A survey of the continum radiation at a frequency of 400Mc/s," Bull. Astron. Inst. Netherlands, vol. 18, pp. 11-41,1965.

[12] H. Tanaka and T. Kakinuma, "The absolute calibration onthe flux density of solar radio emission," in URSI Progressin Radio Science, 1963-1966. URSI: 1967, pp. 1314-1324.

[13] V.S. Troitskii, "Investigation of the surfaces of the moonand planets by thermal radiation," Radio Sci., vol. 69D, pp.1585-1611, 1965.

[14] V.S. Troitskii, V.D. Krotikov, and N.M. Tseitlin, "Measure-ment of radio emission from the moon in the 30-60 cmband," Astron. Zh., vol. 44, pp. 413415, 1967.

Donald A. Guidice (M'70) was born in New York City, N.Y., on October 12, 1934. Hereceived the B.E.E. degree from Manhattan College, New York City, in 1956, and theM.S. and Ph.D. degrees from Ohio State University, Columbus, in 1959 and 1969,respectively.

From 1956 to 1959 he worked as an Electronic Engineer (and an Air Force officer)at the Air Force Avionics Laboratory (and its predecessors), Wright-Patterson AFB,Ohio, where he directed applied research efforts on special signal-processing techniquesfor Doppler navigational radar and on ring laser techniques for angular rotation sensing.Since January 1964 he has worked as a Research Physicist at the Air Force CambridgeResearch Laboratories, Hanscom Field, Bedford, Mass. His research has included theinvestigation of the atmospheric 5-mm-band radiation for spacecraft vertical-sensingapplications, observations of discrete radio sources and the Galactic Spur at frequenciesbelow 40 MHz, and the investigation of the solar coronal electron density usingpulsars.

Dr. Guidice is a member of Eta Kappa Nu, Sigma Xi, and the American Astronomi-cal Society.

John P. Castelli (M'58) was born in Lexington, Mass., on November 7, 1916. Hereceived the A.B. and M.A. degrees from Boston College, Boston, Mass., in 1938 and1939, respectively.

During World War II he served in the Army Air Force as a Radar Officer. Since1946 he has been with the Air Force Cambridge Research Laboratories, HanscomField, Bedford, Mass. Until 1955 he worked mainly on rocket projects and variousphases of radar systems. From 1955 until the present, he has been doing microwaveradio astronomy research. He had the responsibility for AFCRL Radio Solar Eclipseexpeditions in 1961, 1963, and 1966. He is now Leader of the Solar Section of theRadio Astronomy Branch and is responsible for establishing and maintaining a solarradio facility at Sagamore Hill, Hamilton, Mass., to make continuous observations ofthe sun throughout much of the radio spectrum, and for analyzing and reporting theresults of the measurement program.

Mr. Castelli is a member of the American Astronomnical Society and Commission Vof the International Scientific Radio Union (URSI).

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