Fixing the GPS Bad Attitude: Modeling GPS Satellite Yaw During Eclipse Seasons

Yoaz Bar-Sever, Joseph Anselmi, William Bertiger and Edgar Davis

Abstract:	It is well known that high-precision GPS navigation degrades during GPS eclipse seasons. We have determined that a major source of this degradation is the mismodeling of the yaw attitude of those GPS satellites that are in eclipsing orbits. The yaw attitude of GPS satellites is essentially random during an eclipse and for up to 30 minutes past exiting from shadow. Furthermore, commonly used models of the noon turn can be inaccurate for a period of up to 30 minutes. This leads to both measurement and dynamic errors. Errors in the radio metric measurements are introduced due to mismodeling of the carrier phase wind-up and the position of the GPS antenna phase center with respect to the spacecraft center of gravity. This measurement error can be larger than 10 centimeters. Errors in the satellite dynamics are introduced because the direction of the solar pressure force is mismodeled during the 30-minute recovery period after exiting the shadow and during the noon turn. We present an analysis of the effects of attitude mismodeling on precise positioning with GPS. A remedy was proposed for the GPS attitude control subsystem that will make yaw attitude modelable. In June 1994, the US Air Force has implemented the proposed modification to the attitude control subsystem. Details of the new model for the GPS satellite attitude during shadow events and noon turns are presented as well as the necessary modifications to navigation software packages. Early results using GPS under the new attitude control system are also presented.
Published in:	Proceedings of the 1995 National Technical Meeting of The Institute of Navigation January 18 - 20, 1995 Disneyland Hotel Anaheim, CA
Pages:	827 - 834
Cite this article:	Updated citation: Published in NAVIGATION: Journal of the Institute of Navigation
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