Precision Orbit Determination in the Geosat Orbit

Kenn Gold, George Born, Kelly Irish, Angie Reichert, Rob Markin, Patrick Binning, Penina Axelrad, Scott Mitchell, William Frazier, Willy Bertiger and George Hajj

Abstract:	The Geosat exact repeat orbit will be used by the Navy’s Geosat Follow-On (GFO) satellite, and is being considered for the Earth Observation System (EOS) radar altimetry mission. Both of these missions will perform very accurate oceanographic altimetry from space, and will carry Global Positioning System (GPS) receivers for tracking. Orbits accurate to a few centimeters are required to maximize the science return from the altimetry data. This level of orbit accuracy has been achieved for the TOPEXPOSElDON mission with GPS, but because the Geosat orbit is at a much lower altitude, the effects of atmospheric drag and gravity are much more severe. Atmospheric drag is expected to be two orders of magnitude greater, and gravity model errors are expected to be a factor of three higher (Tapley, 1994). An orbit accuracy study for a satellite in the Geosat orbit has been completed with JPL’s GIPSY-OASIS II software. The radial orbit accuracy attainable was investigated through the use of simulation analysis. Certain models with time varying effects can not be treated in a covariance analysis and must be simulated. In the simulation mode, orbits and measurement data are generated with a given set of models, and are subsequently processed with a different set of models to form an orbit solution. The differences between the solution orbit and the orbit used to generate the data set are due entirely to model differences. In this study, the effects of gravity model error, atmospheric drag, GPS orbit error, multipath reflection, the Earth’s gravitational constant (GM), ocean and solid Earth tides, ground tracking site location error, tectonic plate motion, timing and polar motion, radiation pressure and spacecraft attitude error are treated both individually and as a group. The fidelity of such an error analysis is directly dependent upon the realism of the error models. Most of the error sources chosen for this study were agreed upon by leading experts in the field of orbit determination at the Jet Propulsion Laboratory, the University of Texas at Austin, models, thus reducing the effect of dynamic model error. The analysis techniques and error models were validated to some extent by applying them to the TOPEX orbit. Preliminary and conservative results show that radial orbit accuracy in the Geosat orbit will be approximately 5.5 cm for GFO (five GPS satellite receiver) and 4.3 cm for EOS ALT-R (ten GPS satellite receiver).
Published in:	Proceedings of the 1995 National Technical Meeting of The Institute of Navigation January 18 - 20, 1995 Disneyland Hotel Anaheim, CA
Pages:	579 - 591
Cite this article:	Gold, Kenn, Born, George, Irish, Kelly, Reichert, Angie, Markin, Rob, Binning, Patrick, Axelrad, Penina, Mitchell, Scott, Frazier, William, Bertiger, Willy, Hajj, George, "Precision Orbit Determination in the Geosat Orbit," Proceedings of the 1995 National Technical Meeting of The Institute of Navigation, Anaheim, CA, January 1995, pp. 579-591.
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