1966AJ 71. . 902E the ASTRONOMICAL JOURNAL VOLUME 71, NUMBER 9 NOVEMBER 1966 Radar Observations of Venus at 3.8-Cm Wavelength J
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902E . 71. THE ASTRONOMICAL JOURNAL VOLUME 71, NUMBER 9 NOVEMBER 1966 Radar Observations of Venus at 3.8-cm Wavelength J. V. Evans, R. P. Ingalls, L. P. Rainville, and R. R. Silva 1966AJ Lincoln Laboratory * Massachusetts Institute of Technology, Lexington, Massachusetts (Received 12 July 1966) During the early part of 1966 the Lincoln Laboratory Haystack Radar (X = 3.8 cm) was employed to observe the planet Venus. The radar cross section observed at this wavelength was only about one-tenth as much as has been reported at wavelengths of 12 cm or longer. Separate observations of the planet Mercury yielded cross sections similar to those previously obtained at longer wavelengths and lent confidence that the radar equipment was functioning properly. Thus our measurements support the low value for the cross section reported by Karp et al. (1964). The echo spectrum is found to be comparatively narrow, suggesting the solid surface as the reflecting agent. No echo power can be observed outside the frequency limits expected on the basis of the known rotation rate and the center Doppler shift measured using a separate X = 23 cm radar. If any energy is reflected by the atmosphere it appears to be only a small portion of the total. Con- firmation that the surface is the scattering agent has been provided by near simultaneous ranging experi- ments at two frequencies, which showed that the echo delay times agreed to 5 /¿sec, i.e., well within the experimental accuracy. The depolarized component of the echoes was observed and found to be of the order of 14 dB weaker than the expected component. An analysis of the scattering law for the planet has been made, and when compared with the law observed at longer wavelengths indicates that about 4-6 dB two-way vertical absorption occurs in the atmosphere of Venus. Variations in the observed cross section with time (by as much as a factor of 2) support the view that atmospheric attenuation is responsible at least in part for the low cross section observed. A comparison of the positions of anomalously reflecting regions observed in these observations with the positions reported by Carpenter (1966) during the 1964 inferior conjunction permits the Venus rotation period to be established as 244± a few days. I. INTRODUCTION reflections from the limbs of the planet. Since the IN a previous paper (Evans et at. 1965) we described absorption along each ray would increase toward the limbs (due to the greater path length in the Venusian radar observations of Venus at a wavelength of 23 cm using the Lincoln Laboratory Millstone Hill radar atmosphere) one would expect the signal spectrum to be during a seven-month period centered on the inferior narrowed. In fact, if atmospheric absorption is re- sponsible for lowering the cross section by a factor of 10 conjunction which took place in 1964. These results, it should be difficult to recognize any energy at fre- together with those of other observers in the wavelength quencies corresponding to reflection from near the range 12.5 cm-6 m, indicate a radar cross section for limbs. Thus the frequency spectrum observed cast Venus of about 15% of the projected area of the disk doubt upon an explanation of the low cross section as (Evans et al. 1965). This is higher than encountered being due to atmospheric attenuation of a signal for the moon, Mercury, or Mars and implies a rather reflected from the surface. compacted type of surface material (Pettengill and Shapiro 1965; Muhleman et at. 1965). Alternative explanations for this anomalous behavior were considered. One of these was that the 3.6 cm Separate observations of Venus made during the reflections might arise not from the surface itself but same conjunction using the Lincoln Laboratory West from scatterers (e.g., rain or ice crystals) in the Venusian Ford radar system at 3.6-cm wavelength indicated a atmosphere (I. I. Shapiro, private communication; radar cross section of only 1% (Karp et al. 1964). This R. E. Newell, private communication). The large low value, if attributable to the effect of atmospheric Doppler width of the signals would then be attributable absorption, would place definite constraints upon the to the random motion of these scatterers and the number of possible models for the atmosphere of Venus atmospheric absorption would be required to be so high that could be constructed to provide the right amount (e.g., 10-20 dB one way) that effectively little or no of attenuation and satisfy existing radio and infrared energy would be returned from the planetary surface. temperature measurements (Barrett and Staelin 1964; This explanation required relatively large scatterers. If Walker and Sagan 1966). However, the frequency small ice crystals or rain drops are invoked the scatterers spectrum of the signals obtained at 3.6 cm in 1964 would have cross sections in the Rayleigh region, i.e., indicated a considerable amount of Doppler broadening. proportional to (f/X)4, where r is the radius of the parti- The half-power width of the signals was 75 cps, though cle. In this case it becomes rather difficult to introduce one would have expected a half-power width of only enough particles to obtain a total cross section as large 20 cps or less due to the combined effects of surface as 1% of the projected area of the planet. Of course, a roughness and rotation rate as determined at longer situation in which there is both scattering from the wavelengths. In addition, there was some evidence in surface and from rain is possibly one way out of this these early measurements for the existence of echo difficulty. energy at frequencies outside the limits expected for The first opportunity to repeat these short-wave- * Operated with support from the U. S. Air Force. length measurements with better precision arose around 902 © American Astronomical Society • Provided by the NASA Astrophysics Data System 902E . 71. OBSERVATIONS OF VENUS AT 3.8 cm 903 Table I. Parameters of two Lincoln Laboratory radars. during transmission to place the greatest amount of the transmitted power on the planetary surface. West Ford 1966AJ Haystack bistatic radar Radio waves having a circular polarization of one Location Tyngsboro, Westford, Massachusetts sense were transmitted and both senses of circular polar- Massachusetts Pleasanton, California ization were available for reception. For all types of Frequency 7750 Mc/sec 8350 Mc/sec measurements the transmitter was energized for a Wavelength ~3.8 cm ~3.6 cm length of time equal to that expected for a pulse to Antenna diameter 120 ft Transmitting 60 ft Receiving 60 ft travel to the planet and back, and was then turned off Antenna gain 66.1 dB 59.8 dB for an equal interval of time during which the received 59.8 dB signals were sampled and recorded. In the simplest Beamwidth 0.07° 0.14° experiments the transmission consisted of an un- 0.14° Maximum average 105 kW 40 kW modulated carrier wave. In others, pulses of 4, L and transmitter power 0.5 msec were employed at repetition frequencies of 15, Maximum peak 105 kW 40 kW 50, and 60 cps (approx), respectively. Also, a pulse transmitter power compression scheme was employed for accurate range System temperature ~120°K 74°K on Venus determinations. In this scheme phase-coded pulses Total waveguide 0.5 dB were transmitted and a matched filter delay-line was and other losses employed at the receiver to effect the compression (Evans et al. 1965). the time of the 1966 inferior conjunction of Venus Throughout each receiving period the receiver tuning (which occurred on 25 January). The new measure- was adjusted continuously to compensate for the ments were made at a wavelength of 3.8 cm using the Doppler shift of the signals arising from the motion of recently completed Lincoln Laboratory Haystack radar the planet’s center with respect to the observer. This system. At the same time a fresh set of measurements was accomplished by changing the frequency in 0.1-cps was made using the Millstone Hill 23-cm radar, so that steps at a linear rate which best matched (in a least- a direct comparison between the two would be available. mean-square sense) the expected change in Doppler This paper reports the results of the 3.8-cm-wavelength frequency during that particular receiving period. The observations. The 23-cm observations are reported in a error introduced by a linear fit is less than ±1 cps at companion paper. In the sections which follow we times when the planet is at an evaluation that is briefly describe the radar system employed (Sec. II) sufficiently high (10°) to permit useful observations and the operating and data reduction procedures near inferior conjunction. Since the over-all Doppler (Sec. HI). The principal results are then presented in broadening due to the apparent rotation of the planet turn, viz., the identity of the scattering agent (IV), the with respect to the observer at this wavelength is never cross section (V), the scattering law (VI), and the less than 110 cps (limb-to-limb) this 1-cps error is of no depolarization of the signals (VII). A discussion of the serious consequence. At times removed from inferior results follows. conjunction the error in the Doppler compensation introduced by this linear approximation is somewhat II. EQUIPMENT larger, but the resolution with which it is useful to explore the signal spectrum is correspondingly poorer Table I lists the parameters of the 3.8-cm Haystack owing to the lower total echo power and its greater radar. For completeness we have included the parame- spread in frequency.