Radar observations of Venus at 3.8 CM wavelength

Abstract

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 a'. (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-wa 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.