GPS-Based Position Determination for Geosynchronous Satellites

Roger Bryan

Abstract:	The rationale for the analysis presented herein is to validate (or not) GPS as a replacement for ground-based tracking for determining the positions of orbiting spacecraft. The focus of this paper is upon GPS-based position determination for spacecraft in orbits above the GPS constellation – specifically, spacecraft in geo-synchronous orbits. The GPS satellites broadcast their navigation signals to terrestrial users via a radio-frequency (RF) beam that is focused on the earth’s surface. Hence, the only GPS signals available to GEOsats are those that can “squeeze through” the slit formed by the Earth’s limb and the outer limits of the GPS earth-coverage beam – a slit only a few degrees wide. The geometry of this situation limits the number of GPS satellites simultaneously in view of the GEOsats to no more than two – not enough for an instantaneous position fix. Solutions to this problem would be to broaden the GPS’s “nadir-looking” antenna beam, or to put a “zenith-looking” antenna, with a hemisperical-coverage beam, atop the GPS satellites. Both these solutions, however, require modifications to the GPS spacecraft design, and increased transmitter power which approaches unattainable levels. An alternate approach exploits the limited GPS-to-GEOsat visibility that is available to provide position determination using sequential sightings, made over several days, of single GPS satellites. The signals received during these sequential sightings could then be processed via Kalman filtering techniques to produce the desired navigation solution. RF link calculations indicate that this approach is feasible, given nadir-looking GPS receiving antennas with modest gain (i.e., 15 dB) aboard the GEOsats. Clearly, none of these solutions is straightforward.
Published in:	Proceedings of the 1999 National Technical Meeting of The Institute of Navigation January 25 - 27, 1999 Catamaran Resort Hotel San Diego, CA
Pages:	587 - 592
Cite this article:	Bryan, Roger, "GPS-Based Position Determination for Geosynchronous Satellites," Proceedings of the 1999 National Technical Meeting of The Institute of Navigation, San Diego, CA, January 1999, pp. 587-592.
Full Paper:	ION Members/Non-Members: 1 Download Credit Sign In