RADIO OCCULTATION STUDIES OF THE VENUS ATMOSPHERE WITH THE MAGELLAN SPACECRAFT .1. EXPERIMENTAL DESCRIPTION AND PERFORMANCE

While primarily designed for radar studies of the Venus surface, the h igh effective isotropic radiated power (EIRP) from the Magellan spacec raft makes it an ideal transmitter for use in radio occultation measur ements of the refractivity and absorptivity of the Venus atmosphere. S uch experiments have been conducted involving transmissions at 2.3 GHz and 8.4 GHz (13 cm and 3.6 cm, respectively), during spacecraft ingre ss. Since the stability of the spacecraft transmitter is critical for accurately determining the Doppler shift and amplitude attenuation cre ated as the ray penetrates the atmosphere, the spacecraft transmitter was locked to a 2.1 GHz uplink from a 70-meter DSN station which also received the signals. Because of the high directivity of the spacecraf t antenna, and the significant ray bending in the deep Venus atmospher e, a spacecraft tracking maneuver was designed to keep the spacecraft antenna pointed in the direction of the refracted ray path back to Ear th. This tracking maneuver, plus the high EIRP of the Magellan transmi tter has yielded 3.6 cm refractivity and absorptivity profiles down to the 35 km altitude and 13 cm profiles down to the altitude of critica l refraction (approximately 33 km). The statistical uncertainties in t he derived profiles are significantly lower than those previously obta ined, resulting in extremely accurate profiles of H2SO4 (g) abundance, as discussed in an accompanying paper by J. M. Jenkins, P. G. Steffes , D. P. Hinson, J. D. Twicken, and G. L. Tyler (1994, Icarus 110, 79-9 4). (C) 1994 Academic Press, Inc.