GPS Navigation and Time Transfer for Pairs of Orbiting Space Vehicles

Duncan B. Cox Jr., 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 space vehicles in similar orbits. 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 otbiting near to each other, so that both the perturbations in their orbits and the errors in the GPS measurements to common satellites are significantly correlated. The receivers incorporate SA corrections, and can make measurements of pseudorange, accumulated- phase, and changes in accumulated phase, using P(Y)- code signals at the Ll and L2 frequencies. Common- mode time biases can be calibrated dynamically. State- of-the art accuracies for both absolute and relative navigation and time transfer are desired. This paper addresses the design of the navigation algorithms and the expected performance, and includes the results of computer simulations. An integrated navigation filter is employed, incorporating states of both vehicles. With 1100 km elliptic orbits at inclinations of 63’ and separations between vehicles of 100 km, excellent accuracies are projected: absolute radial position 4.5 m rms, relative radial position 7 cm rms, absolute time 6 ns, relative time 3 ns (with common- mode error), absolute radial velocity 4 mm/s, relative radial velocity 0.1 mm/s, absolute and relative frequency 7x10-“. Even better performance is achievable at shorter separation distances. For example, relative position accuracy of 2 cm has been demonstrated for a 10 km separation.
Published in:	Proceedings of the 1997 National Technical Meeting of The Institute of Navigation January 14 - 16, 1997 Loews Santa Monica Hotel Santa Monica, CA
Pages:	417 - 428
Cite this article:	Cox, Duncan B., Jr., Brading, John D. W., "GPS Navigation and Time Transfer for Pairs of Orbiting Space Vehicles," Proceedings of the 1997 National Technical Meeting of The Institute of Navigation, Santa Monica, CA, January 1997, pp. 417-428.
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