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Radar observations of Venus at 3.6 centimeters
mARUS 3, 473-475 (1964)
Radar Observations of Venus at 3.6 Centimeters D. K A R P , W. E. M O R R O W , JR., AND W. B. S M I T H Li~woln Laboratory, ~ Massachusetts Institute o] Technology, Lexington, Massachusetts
Communicated by Carl Sagan Received October 23, 1964 Radar observations of Venus at 3.6 cm have shown that the planet appears extremely "rough" at this wavelength. Apparent reflectivity of the surface was found to be one-tenth that measured at longer wavelengths, raising the interesting possibility of absorption in the Cytherean atmosphere. Observations of the planet Venus during the inferior conjunction of 1964, using the Lincoln L a b o r a t o r y West Ford 3.6-cm bistatic radar, have yielded estimates of the scattering cross section and surface roughness at this wavelength. These observations were a sequel to a similar effort during the conjunction of 1962 which produced a negative result, although measurements of the r a d a r cross section of Venus at several lower frequencies and the parameters of the r a d a r indicated t h a t detection should have been possible. The 1964 experiment was instrumented for unusually low reflectivity a n d / o r wide signal bandwidth, since the negative result of 1962 was believed due to unknown r a d a r characteristics of the planet at the short wavelength. The experimental equipment included a Cassegrainian-fed 60-ft parabolic reflector antenna at the Pleasanton, California transmitting site, and a similar antenna at the Westford, Massachusetts receiving site. The r a d a r beamwidth was 0.14 degrees, and both antennas had 59.8 db ~ 0.5 db gain. Other system parameters were: transmitted power, 40 kilowatts; receiver noise temperature, 69°K at zenith and 74°K on Venus. The antenna pointing was derived from a celestial-to-sidereal converter, with manual bi-
ases added at both terminals, using radiometers operating on thermal radiation from the object planet for pointing optimization. Of course, an error in pointing the transmitting antenna due to light-time aberration was present, but was small compared with antenna beamwidth. Figure 1 shows ELEVATION TRANSIT = O . O 0 2 ° / s e c 2 SECOND INTEGRATION
i[
*i [.4 [[l[[JJ[IJll:!
TIME I DIVISION PER 5 SECONDS ! El111i111:liJIJl[
Fro. 1. Radiometric transit of Venus, 7 May 1964. At conjunction in June the diminished range increased AT by a factor (r,/r..) ~ ~ 3.0.
Operated with support from the U. S. Air Force.
the radiometer output during a transit of Venus, and illustrates the strong signal available for accurate pointing. The transmitting site radiometer operated at 7 7 5 0 Me/see (3.9 em). A time-variable frequency source was employed in the receiver, so as to compensate for time-variable doppler induced on received signals by the motions of Venus and of the transmitting and receiving sites.
473
474
D. KARP ET AL.
The expected doppler was predicted by an ephemeris method known to be precise for monostatic observations (Smith, 1963), and properly altered for bistatic observations. Transmissions were "keyed CW," alternately 2 see on and 2 sec off. The receiver was essentially a gated radiometer, in which receiver output energy was measured during the time when signal was expected, and separately when noise only was expected. Since the difference between these measurements (the energy of signals reflected from Venus) was small, the usual great care was exercised in maintaining receiver gain stability. In addition, the received signal was recorded on magnetic tape for subsequent studies. The goals of this experiment were restricted to a measurement of the radar scattering cross section at X-band, and of the frequency spread due to planetary surface roughness and rotation. There was no att e m p t to study the target in depth, in the sense of using short probing pulses [though some depth study is implied in doppler spread measurements (Cohen, 1959)], and no intent to observe Venus through an appreeiable part of its orbit. The reflected energy received (approximately 10-'-'" watts) at conjunction from Venus was found to be 0.9% plus or minus 0.3% of t h a t which would have been received from a smooth, perfectly conducting sphere of the same size. The intrinsic reflectivity of the surface material may, however, be less, depending upon the scattering law of the surface, hence upon the target "direetivity." The observed nominal reflectivity of approximately 1% m a y be compared with other measurements using longer wavelengths (Victor et al., 1961; Smith, 1963; ,lames et al., 1964) which yielded values greater than 10%. The difficulty in finding planetary surface materials of the necessarily low intrinsic refleetivity suggests t h a t the low value at X - b a n d m a y be due to absorption in the atmosphere (Barrett et al., 1964) of Venus; if so, a powerful new tool has been discovered with which to probe the Cytherean atmosphere. An estimate of the surface roughness at X - b a n d can be made from the measurement
of the doppler spectrum of the reflected signal. The measured spectrum (actual spect r m n convolved with a probing filter of 30 cps bandwidth) is shown in Fig. 2. Also MEASURED VALUES ( 3 8 rain i n t e g r o t i o n - J u n e 1964) - - - - - HYPOTHETICAL SPECTRUM FOR A UNIFORMLY ROUGH PLANET (convolved wilh probing filler)
~ o
0.8
~
0.6
I~:
0,4
+30"
400
/<'/5, zoo
o
200
400
soo
FREQUENCY RELATIVE TO EPHEMERIS PREDICTION (cps) F r o . 2. Radar spectrum of Venus at X-band. Tile hypothetical spectrum assumes a wing-towing doppler spread of 110 cps based upon a provisional value for Venus' rotation rate and axis (Shapiro, 1964). Parl, of the energy in the skirts of the measured curve is accounted for by instrnmentM behavior. The offset from zero is due to imprecise transmitter frequency.
shown in Fig. 2 is the expected spectrum for a "uniformly rough" sphere convolved with the probing filter. The half power spread of the actual spectrum is approximately 75 cps, and is close to the value expected for a uniformly rough sphere (Shapiro, 1964). Venus thus appears at X band (3.6 cm) to be like the lunar disk at optical wavelengths. Other measurements (Goldstein, 1964; Smith, 1963) have shown a tendency towards increasing roughness at shorter wavelengths, but the extreme roughhess at X - b a n d is surprising. ACKNOWLEDGMENTS
We gratefully acknowledge the contribution of Mr. H. H. J. Hoover and others under his supervision who administered the extensive experimental apparatus with concentration and endurance. We are also indebted to Dr. I. I. Shapiro for many interesting discussions concerning various aspects of the experiment. I~EFEREN CES BARRETT, A. H., A~'D STAELIN, D. H. (1964). Radio
observations of Venus and the interpretations. Space Sci. Rec. 3, 109.
3.6-CM RADAR OBSERVATIONS OF VENUS
COHEN, M. ~-~. (1959). "Radar Echoes from a Rough Rotating Planet." Cornell University Report EE428. GOLDSTEIN, R. M. (1964). Venus characteristics by Earth-based radar. Astron. J. 69, 12. JAMES, J. C., AND INGALLS, ]:~. P. (1964). Radar
475
observations of Venus at 38 Me/sec. Astron. J. 69, 19. SHAVIRO, I. I. (1964). Private communication. SMITH, W. B. (1963). Radar observations of Venus, 1961 and 1959. Astron. J. 68, 15. VICTOR, W. K., AND STEVENS, R. (1961). Science 134, 46.
Radar Observations of Venus at 3.6 Centimeters D. K A R P , W. E. M O R R O W , JR., AND W. B. S M I T H Li~woln Laboratory, ~ Massachusetts Institute o] Technology, Lexington, Massachusetts
Communicated by Carl Sagan Received October 23, 1964 Radar observations of Venus at 3.6 cm have shown that the planet appears extremely "rough" at this wavelength. Apparent reflectivity of the surface was found to be one-tenth that measured at longer wavelengths, raising the interesting possibility of absorption in the Cytherean atmosphere. Observations of the planet Venus during the inferior conjunction of 1964, using the Lincoln L a b o r a t o r y West Ford 3.6-cm bistatic radar, have yielded estimates of the scattering cross section and surface roughness at this wavelength. These observations were a sequel to a similar effort during the conjunction of 1962 which produced a negative result, although measurements of the r a d a r cross section of Venus at several lower frequencies and the parameters of the r a d a r indicated t h a t detection should have been possible. The 1964 experiment was instrumented for unusually low reflectivity a n d / o r wide signal bandwidth, since the negative result of 1962 was believed due to unknown r a d a r characteristics of the planet at the short wavelength. The experimental equipment included a Cassegrainian-fed 60-ft parabolic reflector antenna at the Pleasanton, California transmitting site, and a similar antenna at the Westford, Massachusetts receiving site. The r a d a r beamwidth was 0.14 degrees, and both antennas had 59.8 db ~ 0.5 db gain. Other system parameters were: transmitted power, 40 kilowatts; receiver noise temperature, 69°K at zenith and 74°K on Venus. The antenna pointing was derived from a celestial-to-sidereal converter, with manual bi-
ases added at both terminals, using radiometers operating on thermal radiation from the object planet for pointing optimization. Of course, an error in pointing the transmitting antenna due to light-time aberration was present, but was small compared with antenna beamwidth. Figure 1 shows ELEVATION TRANSIT = O . O 0 2 ° / s e c 2 SECOND INTEGRATION
i[
*i [.4 [[l[[JJ[IJll:!
TIME I DIVISION PER 5 SECONDS ! El111i111:liJIJl[
Fro. 1. Radiometric transit of Venus, 7 May 1964. At conjunction in June the diminished range increased AT by a factor (r,/r..) ~ ~ 3.0.
Operated with support from the U. S. Air Force.
the radiometer output during a transit of Venus, and illustrates the strong signal available for accurate pointing. The transmitting site radiometer operated at 7 7 5 0 Me/see (3.9 em). A time-variable frequency source was employed in the receiver, so as to compensate for time-variable doppler induced on received signals by the motions of Venus and of the transmitting and receiving sites.
473
474
D. KARP ET AL.
The expected doppler was predicted by an ephemeris method known to be precise for monostatic observations (Smith, 1963), and properly altered for bistatic observations. Transmissions were "keyed CW," alternately 2 see on and 2 sec off. The receiver was essentially a gated radiometer, in which receiver output energy was measured during the time when signal was expected, and separately when noise only was expected. Since the difference between these measurements (the energy of signals reflected from Venus) was small, the usual great care was exercised in maintaining receiver gain stability. In addition, the received signal was recorded on magnetic tape for subsequent studies. The goals of this experiment were restricted to a measurement of the radar scattering cross section at X-band, and of the frequency spread due to planetary surface roughness and rotation. There was no att e m p t to study the target in depth, in the sense of using short probing pulses [though some depth study is implied in doppler spread measurements (Cohen, 1959)], and no intent to observe Venus through an appreeiable part of its orbit. The reflected energy received (approximately 10-'-'" watts) at conjunction from Venus was found to be 0.9% plus or minus 0.3% of t h a t which would have been received from a smooth, perfectly conducting sphere of the same size. The intrinsic reflectivity of the surface material may, however, be less, depending upon the scattering law of the surface, hence upon the target "direetivity." The observed nominal reflectivity of approximately 1% m a y be compared with other measurements using longer wavelengths (Victor et al., 1961; Smith, 1963; ,lames et al., 1964) which yielded values greater than 10%. The difficulty in finding planetary surface materials of the necessarily low intrinsic refleetivity suggests t h a t the low value at X - b a n d m a y be due to absorption in the atmosphere (Barrett et al., 1964) of Venus; if so, a powerful new tool has been discovered with which to probe the Cytherean atmosphere. An estimate of the surface roughness at X - b a n d can be made from the measurement
of the doppler spectrum of the reflected signal. The measured spectrum (actual spect r m n convolved with a probing filter of 30 cps bandwidth) is shown in Fig. 2. Also MEASURED VALUES ( 3 8 rain i n t e g r o t i o n - J u n e 1964) - - - - - HYPOTHETICAL SPECTRUM FOR A UNIFORMLY ROUGH PLANET (convolved wilh probing filler)
~ o
0.8
~
0.6
I~:
0,4
+30"
400
/<'/5, zoo
o
200
400
soo
FREQUENCY RELATIVE TO EPHEMERIS PREDICTION (cps) F r o . 2. Radar spectrum of Venus at X-band. Tile hypothetical spectrum assumes a wing-towing doppler spread of 110 cps based upon a provisional value for Venus' rotation rate and axis (Shapiro, 1964). Parl, of the energy in the skirts of the measured curve is accounted for by instrnmentM behavior. The offset from zero is due to imprecise transmitter frequency.
shown in Fig. 2 is the expected spectrum for a "uniformly rough" sphere convolved with the probing filter. The half power spread of the actual spectrum is approximately 75 cps, and is close to the value expected for a uniformly rough sphere (Shapiro, 1964). Venus thus appears at X band (3.6 cm) to be like the lunar disk at optical wavelengths. Other measurements (Goldstein, 1964; Smith, 1963) have shown a tendency towards increasing roughness at shorter wavelengths, but the extreme roughhess at X - b a n d is surprising. ACKNOWLEDGMENTS
We gratefully acknowledge the contribution of Mr. H. H. J. Hoover and others under his supervision who administered the extensive experimental apparatus with concentration and endurance. We are also indebted to Dr. I. I. Shapiro for many interesting discussions concerning various aspects of the experiment. I~EFEREN CES BARRETT, A. H., A~'D STAELIN, D. H. (1964). Radio
observations of Venus and the interpretations. Space Sci. Rec. 3, 109.
3.6-CM RADAR OBSERVATIONS OF VENUS
COHEN, M. ~-~. (1959). "Radar Echoes from a Rough Rotating Planet." Cornell University Report EE428. GOLDSTEIN, R. M. (1964). Venus characteristics by Earth-based radar. Astron. J. 69, 12. JAMES, J. C., AND INGALLS, ]:~. P. (1964). Radar
475
observations of Venus at 38 Me/sec. Astron. J. 69, 19. SHAVIRO, I. I. (1964). Private communication. SMITH, W. B. (1963). Radar observations of Venus, 1961 and 1959. Astron. J. 68, 15. VICTOR, W. K., AND STEVENS, R. (1961). Science 134, 46.