GPS Autonomous Relative Navigation and Time Transfer for Orbiting Space Vehicles

Duncan B. Cox Jr. and John D. W. Brading

Abstract:	GPS can be applied advantageously to the problem of autonomous relative navigation and time transfer for two vehicles with communications links between them. This application is significantly different from the navigation of one vehicle with respect to a reference station which has essentially perfect knowledge of position and time. The focus in this paper is on applications where the two vehicles are orbiting space vehicles. A GPS receiver is placed in each vehicle, and a communications system is used to pass information from one vehicle to the other so that relative navigation and clock-correction estimates can be computed in each vehicle. The vehicles are orbiting near to each other, so that the perturbations in their orbits are significantly correlated and the errors in the GPS measurements to common satellites are significantly correlated. The receivers incorporate SA corrections, and can make pseudorange and accumulated- phase measurements at L 1 and L2. Aiding/correction signals are limited to attitude data from on-board sensors and signals sent from one vehicle to another. State-of-the art accuracies are desired. This paper addresses the design of the navigation algorithms and the expected performance trends, and illustrates the trends with the results of computer simulations. Several important issues are addressed, particularly, (a) the benefits of utilizing a joint relative- navigation filter, rather than two independent navigation filters, (b) means of incorporating matched (same satellite, same epoch, different vehicle) and unmatched measurements, (c) the benefits of incorporating accumulated-phase measurements, rather than “delta- pseudorange” (delta-phase) measurements, (d) the effects of measurement nonlinearities, (e) modeling the accumulated-phase ambiguity states, (f) the benefits and limitations of using accumulated phase measurements in enhancing the relative accuracy of time transfer, and (g) the dependence of relative navigation accuracy on absolute navigation accuracy and on the distance between the two user vehicles.
Published in:	Proceedings of the 8th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1995) September 12 - 15, 1995 Palm Springs, CA
Pages:	217 - 228
Cite this article:	Cox, Duncan B., Jr., Brading, John D. W., "GPS Autonomous Relative Navigation and Time Transfer for Orbiting Space Vehicles," Proceedings of the 8th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1995), Palm Springs, CA, September 1995, pp. 217-228.
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