SAR - AN INSTRUMENT FOR PLANETARY GEODESY AND NAVIGATION

A space borne synthetic aperture radar (SAR) placed in a near polar circular orbit of 300 km altitude around the planet Venus is considered. It is shown that radar surface images can be used to improve knowledge of physical parameters of Venus. Since the basic SAR measurement consists of slant range between the spacecraft and the planetary surface, the data are sensitive to both planetary topography and spacecraft position relative to planet-fixed coordinates. This paper presents an analysis to define achievable accuracy levels in determination of (1) the physical parameters of Venus, (2) global topography of the planet, and (3) refined orbital element estimates. The analysis presented is based on the demonstrated fact that the ascending orbit node in the plane-of-sky of a distant planetary orbiter is not observable in geocentric range-rate measurements obtained over relatively short time intervals. This lack of observability can be removed by using orbiter-based angular measurements, such as those obtainable with SAR data described in this paper, in conjunction with conventional Doppler determined orbits. The resulting refinement in spacecraft ephemeris (in particular the Eulerian angles i, omega , OMEGA ) in turn enhances the ability to sense the low order gravity field of Venus.