A New Algorithm for WAAS GEO Uplink Subsystem (GUS) Clock Steering

M. Grewal, Fullerton, P-H. Hsu, T.W. Plummer

Abstract:	Presently, the WAAS Wide Area Master Station (WMS) calculates WAAS Network Time (WNT) and estimates clock parameters (offset and drift) for each satellite. The GEO Uplink System (GUS) clock is an independent free running clock. However, the GUS clock must track WNT (GPS time) to enable accurate ranging from the GEO signal-in-space. Therefore, a clock steering algorithm is necessary. The GUS clock steering algorithms reside in the WAAS Message Processor (WMP). The WAAS Type 9 message (GEO navigation message) is used as input to the GUS WMP, provided by the WMS. In the new algorithm, the GUS clock is steered to the GPS time epoch. The GUS receiver clock error is the deviation of its 1 PPS from the GPS epoch. The clock error is computed in the GUS processor by calculating the user position error by combining (in the least square sense, weighted with expected error statistics) multiple satellite data (pseudorange residuals called MOPS residuals) into a position error estimate with respect to surveyed GUS position. The clock steering algorithm is initialized with the WAAS Type 9 message (GEO navigation message). This design keeps the GUS receiver clock 1 PPS synchronized with the GPS time epoch. Since the 10 MHz frequency standard is the frequency reference for the receiver, its frequency output needs to be controlled so that the 1 PPS is adjusted. A proportional, integral and differential (PID) controller has been designed to synchronize to the GPS time at GUS locations. Two sets of prototype clock steering results are shown. Clock adjustment commands as applied to the frequency standard to null the MOPS clock offset are given. This new algorithm also decouples the GUS clock from orbit errors and increases the observability of orbit errors in the orbit determination filter in the correction processor of the WMS. It also synchronizes GUS clocks at all GUS locations to GPS time. This paper shows the algorithm prototype results of GUS clock steering to GPS time. It also shows the improvements in the GEO (AOR-W) orbits when the GEO clock state is known, thereby making the GEO a valid ranging source. The GEO range errors with the known clock solution are found to be up to a factor of ~5 better than those for the fielded WAAS with real data using correction verification simulation. This will increase the availability and continuity of WAAS services since the signal is already processed by the user.
Published in:	Proceedings of the 16th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS/GNSS 2003) September 9 - 12, 2003 Oregon Convention Center Portland, OR
Pages:	2712 - 2719
Cite this article:	Grewal, M., Fullerton,, Hsu, P-H., Plummer, T.W., "A New Algorithm for WAAS GEO Uplink Subsystem (GUS) Clock Steering," Proceedings of the 16th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS/GNSS 2003), Portland, OR, September 2003, pp. 2712-2719.
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