Kalman Filtering Technique for Spacecraft Differential GPS Navigation

Luigi Galvagni, Lucio Caporicci and Claudio Soddu

Abstract:	Differential GPS operation greatly reduces the effects of SA (Selective Availability), ionosphere, troposphere and other slowly-varying errors for users in a local area around a reference station. If a user and a local GPS reference receiver are sufficiently near, space vehicle and propagation errors are common to both and can be removed or significantly reduced by differential techniques. A reference receiver at a known location measures the ranges to GPS satellites. These measured ranges include the actual ranges to the satellites plus several errors, but the actual ranges can be derived from the station and satellite coordinates. However, different error sources have different spatial correlations. The ephemeris, clock bias and SA errors slowly change with increasing separation distance between remote and reference receivers and can be considered almost constant also for great separation distance. On the contrary, propagation errors are deeply depending on altitude and satellite viewing angle, that quickly change as the separation of the two receivers increases. In particular, this situation occurs when a spacecraft differential navigation is considered. An algorithm based on the Kalman filtering technique has been developed to optimally process the measurements from the ground reference station and estimate the several errors. Then, it is possible to achieve corrections of the deeply correlated error sources (range bias and range-rate bias) and the values of several parameters relevant to the mathematic models of tropospheric and ionospheric effects. These parameters allow an on board computation of the propagation corrections starting from estimated spacecraft position and velocity. Some simulations have been performed that demonstrate a sensible accuracy improvement with respect to the GPS stand alone navigation.
Published in:	Proceedings of the 4th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1991) September 11 - 13, 1991 Albuquerque, NM
Pages:	271 - 284
Cite this article:	Galvagni, Luigi, Caporicci, Lucio, Soddu, Claudio, "Kalman Filtering Technique for Spacecraft Differential GPS Navigation," Proceedings of the 4th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1991), Albuquerque, NM, September 1991, pp. 271-284.
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