The Use of X-Ray Pulsars for Aiding GPS Satellite Orbit Determination

Abstract : This research proposes the use of an existing "signal of opportunity" - namely x-ray pulsars - to improve the accuracy and robustness of the GPS satellite and clock estimation algorithm. Improvement in satellite and clock accuracy results in a direct benefit to the user. A simulation has been developed to determine the effects of using x-ray pulsar measurements on the GPS Operational Control Segment. The epoch-specific position, velocity, and clock errors of all GPS satellites in the constellation were estimated using both pseudoranges and time-difference-of-arrival (TDOA) measurements from pulsars. The primary measure of accuracy is a constellation Signal-In-Space Range Error (SISRE). Results indicate that marginal SISRE improvements (approximately 1%) can be achieved if the x-ray detector is accurate to an order of approximately 40 m for the strongest pulsar. Increasing the accuracy of the x-ray detector by a factor of 100 can yield accuracy improvements up to 26% over the pseudorange-only based GPS system. Additionally, results show that using only 1 strong pulsar to create TDOA observations, may be comparable to using tens of weakly timed pulsars. Pulsar geometry analysis showed that the geometry does have a significant impact on the overall system performance. Results indicate that using TDOAs in the absence of pseudoranges may aid the OCS in keeping track of the GPS satellites until the ground station links can be reestablished.