An Extended Propagation Ephemeris with Graceful Degradation

M.A Walker, J.L Garrison

Abstract:	A new GNSS ephemeris, valid for several days, is presented. This new ephemeris demonstrates two improvements over prior extended propagation ephemerides. First, the new ephemeris exhibits „graceful degradation? in that orbit accuracy will initially be high and then gradually decrease with time, as opposed to a uniform accuracy over the entire fit interval. Second, the extended propagation ephemeris can be fused with the broadcast ephemeris, when available, to provide accuracy comparable to the broadcast ephemeris during a 3 hour period, and with a gradual decrease in accuracy outside of that time window. These improvements could enhance the operation of GNSS receivers in weak signal environments and applications that require a navigation solution from short data blocks (preventing the reception of the full 30 second broadcast ephemeris). In a typical scenario, a receiver could make the best use of the broadcast ephemeris, or the most current update of the extended propagation ephemeris, when either becomes available. Navigation would still be possible at a reduced accuracy during times in which neither is available through use of an older extended propagation ephemeris, stored in memory. The extended propagation ephemeris is demonstrated on an ensemble of 700 orbit arcs, obtained from the International GNSS Service (IGS) precise orbits covering a 6 month period between January and July of 2003, separated in steps of 6 hours. For a 24 hour ephemeris, 88% of these cases met the goal of an RMS error below 15 m during the first 6 hours and 94% of cases met the goal of an root mean squared (RMS) error below 80 m for the full propagation length. For a 72 hour ephemeris, 92% of these cases met the goal of an RMS error below 25 m during the first 6 hours and 83% of cases met the goal of an RMS error below 170 m for the full propagation length.
Published in:	Proceedings of the 22nd International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2009) September 22 - 25, 2009 Savannah International Convention Center Savannah, GA
Pages:	2029 - 2035
Cite this article:	Walker, M.A, Garrison, J.L, "An Extended Propagation Ephemeris with Graceful Degradation," Proceedings of the 22nd International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2009), Savannah, GA, September 2009, pp. 2029-2035.
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