An Estimation Algorithm for Precision GPS Signal Monitoring

P.D. Groves, I.J. Richards, P. Briggs

Abstract:	A 12 channel P(Y) code monitor station has been developed to give an independent measure of the performance of the GPS space and control segments. Such information may be used to assist analysis of any incident or action that relied on GPS data. Examples range from a board of enquiry following an air crash to an assessment of the accuracy of a GPS guided munition. The monitor station comprises off the shelf hardware and specially developed software. The outputs include a smoothed history of the PPS pseudorange accuracy obtainable from each satellite tracked with a stand-alone receiver. To this end, an estimation algorithm has been developed to calculate the correlated pseudo-range errors. These comprise a combination of residual errors due to ephemeris prediction, the satellite clocks and ionosphere and troposphere propagation. A specialised Kalman filter algorithm is used to process the receiver pseudo-range and pseudo-range rate outputs. As the aim is to estimate the time correlated pseudo-range errors, the design philosophy is very different from that of a standard navigation filter which aims to estimate position and velocity. Prior knowledge of the antenna’s nominal position and motion, together with the use of a high performance frequency standard to drive the receiver clock, are exploited to refine estimation of the range errors. Factors accounted for in the design include ephemeris updates, antenna motion and the time correlation properties of both the range errors and the receiver tracking function. Sub-metre range error accuracies have been obtained, a performance comparable to codebased differential GPS. As the monitor station is designed to run continuously for many months, maintenance of Kalman filter stability is essential. To this end, a suite of pre-filtering and integrity monitoring techniques is used so that faults in one satellite channel, including multipath, can be prevented from corrupting the rest of the Kalman filter and the effects of any tuning problems may be mitigated. These techniques include spike filtering, transient detection, range checks and residual bias monitoring. For security reasons, the GPS receiver outputs only raw pseudo-range and pseudo-range rate data to the monitor station PCs. Therefore, ionosphere, troposphere, satellite clock and selective availability corrections must be applied by the bespoke software, running in a controlled environment. The opportunity has thus been taken to tailor the dual frequency ionosphere propagation correction in order to reduce the effects of tracking errors.
Published in:	Proceedings of the 2003 National Technical Meeting of The Institute of Navigation January 22 - 24, 2003 Disneyland Paradise Pier Hotel Anaheim, CA
Pages:	254 - 261
Cite this article:	Groves, P.D., Richards, I.J., Briggs, P., "An Estimation Algorithm for Precision GPS Signal Monitoring," Proceedings of the 2003 National Technical Meeting of The Institute of Navigation, Anaheim, CA, January 2003, pp. 254-261.
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