The Effect of Selective Availability on Orbit Space Vehicles Equipped with SPS GPS Receivers

William M. Lear, Moises N. Montez, Lonie M. Rater and Lubomyr V. Zyla

Abstract:	This paper discusses the use of GPS in real time navigation on NASA orbital space vehicles using SRS GPS receivers. The Space Station Freedom is an example of such a proposed system. In particular we shall consider the effect of the Selective Availability error source on orbital vehicles equipped with SRS receivers. A unique performance criteria which arises for orbital space vehicles is the navigation accuracy of the vehicle’s semimajor axis estimate. The semimajor axis is a Keplerian orbital element arising out of celestial mechanics which combines position and velocity accuracies into a single parameter and is a measure of how accurately the vehicle orbit can be predicted forward in time. Accurate real time prediction is important to support vehicle guidance, collision avoidance and mission planning. The authors survey the various selective availability models which have been described in the literature as well as analyze some actual data to see firsthand an example of selective availability. The SA error source is broken up into GPS satellite ephemeris errors (epsilon error) and the clock dither error. Simulations of an onorbit Gl?S receiver and several navigation filter designs are presented to assess the impact of each SA error component. Proposed onorbit GE’S navigation filter designs are tested against a catalog of Selective Availability error models. The authors relate the statistical smoothness of the clock dither error to onorbit navigation accuracy. In addition we examine the significance of the fidelity of the onorbit dynamics model in the navigation filter in achieving high accuracy semimajor axis estimates. Lastly we discuss the use of special filter compensation techniques (state variable augmentation) to mitigate the effect of selective availability.
Published in:	Proceedings of the 5th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1992) September 16 - 18, 1992 Albuquerque, NM
Pages:	825 - 840
Cite this article:	Lear, William M., Montez, Moises N., Rater, Lonie M., Zyla, Lubomyr V., "The Effect of Selective Availability on Orbit Space Vehicles Equipped with SPS GPS Receivers," Proceedings of the 5th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1992), Albuquerque, NM, September 1992, pp. 825-840.
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