93-165
Very-Long-Baseline-Interferometry Measurements Of Planetary Orbiters At Mars And Venus
Peter M. Kroger, William M. Folkner, Byron A. Iijima and Claude E. Hildebrand*
Abstract
Very-long-baseline interferometric (VLBI) measurements ot the positions of planetary orbiters (or landers) relative to nearby extragalactic radio sources provide information on the positions of the planets in the inertial reference frame described by the measured positions of the radio sources. This information is important for interplanetary spacecraft navigation by radio frequency techniques since it establishes the relationship between the reference frame of the target planets with the reference frame in which the spacecraft position measurements are made. Any angular offset between the two reference frames will result in a systematic error in the planet relative angular position of the spacecraft.
The first attempts to use radio interferometric techniques to measure the positions of planetary orbiters were made in 1980 with the Viking Mars orbiter and again in 1983 using the Pioneer Venus orbiter. The angular accuracy of these early measurements was on the order of 200 nrad. This work describes more recent VLBI measurements made in 1989 of the Soviet Martian orbiter Phobos 2 and several measurements made since September of 1990 of the Magellan spacecraft orbiting Venus. Both the Phobos and Magellan measurements recorded data with the Mark lll VLBI systems located at antennas of NASAs Deep Space Network (DSN). the much wider bandwidth of this recording system and the availability of ionospheric calibrations should allow angular accuracy approaching 5 nrad to be achieved with these measurements.
* The authors are affiliated with the Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, California 91109-8099.