GPS Satellite Timing Performance Using the Autonomous Navigation (Autonav)

Douglas Martoccia, H. Bernstein, Y. Chan, R. Frueholz and A. Wu

Abstract:	In the GPS Block II/IIA system, time and frequency errors of the satellite clocks are estimated and controlled by the GPS operational (OCS) control segment. When there are two or more GPS Block IIR satellites in orbit, the clock error states (time and frequency errors) can be estimated by the clock Kalman filter on-board the satellite using Autonav. In the Autonav mode, the satellite performs pseudorange measurements with other satellites that are in-view using an UHF cross-link. The received pseudorange measurements are compensated. These compensations include selective availability, UHF antenna offset, plasmasphere effects (dual frequency measurements), and transmitter and receiver delays. The measurements and other mission data are cross-linked to other GPS IIR satellites. The clock Kalman filter uses the pseudorange differences to adjust and propagate clock states. Since the measurements contain inter satellite clock differences only, they allow the system to keep the satellite clocks synchronized but the Autonav constellation mean clock is unobservable. Clock performance has been obtained using a Monte Carlo technique. The following topics are addressed: (1) performance comparison between the GPS Autonav clock Kalman filter and the National Institute of Standards and Technology (NIST) AT1 time control algorithm, (2) Block IIR Autonav time performance during the satellite build-up to a 24 satellite constellation, (3) Time performance of a mixed constellation of Block IIR and Block IIF satellites when they do not interface with each other.
Published in:	Proceedings of the 11th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1998) September 15 - 18, 1998 Nashville, TN
Pages:	1705 - 1712
Cite this article:	Martoccia, Douglas, Bernstein, H., Chan, Y., Frueholz, R., Wu, A., "GPS Satellite Timing Performance Using the Autonomous Navigation (Autonav)," Proceedings of the 11th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1998), Nashville, TN, September 1998, pp. 1705-1712.
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