<h2>AAS 95-312</h2><h2>Precision Orbit Determination and Gravity Field Improvement Derived From TDRSS Orbit Determination</h2><h4>J. A. Marshall and F. G. Lemoine,  NASA Goddard Space Flight Center, Greenbelt, MD, S.B. Luthcke, J. C. Chan and R. G. Williamson,  C. Cox, Hughes STX</h4><h2> Abstract </h2>The nominal orbit accuracies achieved for the Tracking and Data Relay Satellites (TDRS) are between 40 and 60m and limit the level of orbit determination possible for its user satellites.  One can exploit the 3-4 cm radial RMS and 15 cm RMS total position accuracy available from ground based tracking of TOPEX/POSEIDON to improve the TDRS orbits to the 3-5m total position level.  Using these improved ephemerides, sub-meter positioning for typical user satellites is operationally possible.  Many of the TDRSS user satellites fly at altitudes and inclinations that are poorly represented in satellite-derived Earth gravity models.  Accordingly, TDRSS tracking data of these satellites are being processed and incorporated into the IGM-2 gravity field model.  The resultant orbit determination accuracies and the impact of these additional data on the gravity field will be discussed.
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