Currently, the dominant single-point GPS positioning error sources for military (and some civilian) users are satellite position and clock error. Any improvement in satellite position and clock accuracy results in a direct benefit to the user. 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. A simulation has been developed to determine the effects of using x-ray pulsar measurements on the GPS Operational Control Segment. This simulation uses a nonlinear batch least-squares approach to estimate the position, velocity, and clock errors of all satellites in the specified GPS constellation at a particular epoch time. Both pseudorange measurements and time-difference-of-arrival (TDOA) measurements from pulsars are generated and used. 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. However, 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 equivalent to or better than using tens of pulsars with very weak measurement error statistics. Finally, the results indicate that using TDOAs in the absence of pseudoranges for a limited amount of time may aid the OCS in keeping track of the GPS satellites until the ground station links to the constellation can be reestablished. Preliminary analysis shows that the benefits of implementing a TDOA scheme is evident for outage intervals of 20 hours or more.
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