Space Navigation, the GPS Backside Antenna, and Relativity

Jurn-Sun Leung

Abstract:	Recently, a hemispherical-pattern backside antenna is proposed for the next upgrade opportunity to enhance GPS coverage for space navigation. With the present constellation, GPS visibility drops below 4 on the average at 2.2 earth radii from the center of the earth, and falls below 1 at about GPS altitude. This paper investigates the advantage of the backside antenna and also discusses the benefit and problems of deploying the hemispherical-pattern antenna in the forward (nadir pointing) direction. When less than 4 satellites are visible, GPS navigation accuracy is fundamentally limited by the uncertainty in user clock bias. For this reason, the relativistic effect on space clocks complicates GPS space navigation further. The typical size of this effect on space and launch vehicles, in a time interval of 1 hour, can accumulate to a clock shift of the of a few p-seconds. Precise navigation requires proper compensation for this trajectory-dependent effect. When less than 4 GPS measurements are available, such correction requires either an accurate clock model with input on a good knowledge of the vehicle dynamics, or measurement from auxiliary sensors such as an IMU.
Published in:	Proceedings of the 1998 National Technical Meeting of The Institute of Navigation January 21 - 23, 1998 Westin Long Beach Hotel Long Beach, CA
Pages:	137 - 142
Cite this article:	Leung, Jurn-Sun, "Space Navigation, the GPS Backside Antenna, and Relativity," Proceedings of the 1998 National Technical Meeting of The Institute of Navigation, Long Beach, CA, January 1998, pp. 137-142.
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