The Effect of GPS Orbit Solution Strategies on Absolute Point Positioning

John B. Lundberg and Alan G. Evans

Abstract:	High accuracy real-time GPS positioning is dependent on the accuracy of the GPS satellite orbit and clock solutions as well as the quantity and quality of the observations collected by the user. First order position accuracies can be achieved in real-time using broadcast (predicted) orbit information with pseudorange measurements. The highest quality absolute positioning solutions are achieved in a post-processing mode using dual frequency carrier phase and pseudorange measurements with precision post-fit GPS orbit solutions. The broadcast satellite ephemerides are essentially 24-hour predictions of a near real-time orbit solution with accuracies on the order of a few meters RMS. The accuracies of the post-fit orbit solutions are on the order of 5 cm RMS URE. To support near real-time positioning, estimates of the GPS satellite orbits and clocks should be computed in near real-time and the results communicated to the user as quickly as possible. However, delays in acquiring observations from a set of reference stations, processing the data to compute the orbit and clock estimates, and transmission of results to the user will create a latency effect between the epoch of the latest estimate and the time at which the solutions are available to the user. This study examines the strategy of estimating the GPS satellite orbit and clocks states in near real-time using 900-second smoothed observations and extrapolating the near real-time orbit and clock solutions forward in time to offset these latency effects. The extrapolation periods investigated in this study ranged from 0 to 15 minutes beyond the last measurement epoch. The extrapolated solutions are compared to high precision post-fit satellite orbit and clock solutions. The extrapolated solutions are also used to estimate the locations of a set of fixed receivers. Extrapolation intervals of up to several minutes are found to have a small effect on the satellite and clock solutions and on the positioning solutions for a set of fixed receivers.
Published in:	Proceedings of the 14th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2001) September 11 - 14, 2001 Salt Palace Convention Center Salt Lake City, UT
Pages:	2579 - 2584
Cite this article:	Lundberg, John B., Evans, Alan G., "The Effect of GPS Orbit Solution Strategies on Absolute Point Positioning," Proceedings of the 14th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2001), Salt Lake City, UT, September 2001, pp. 2579-2584.
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