WAAS Network Time Performance Using WRS Data

Cheryl Griffith, Stephen Peck, Peter Diamond

Abstract:	Raytheon Systems Company is currently designing the algorithms of the FAA’S Wide Area Augmentation System (WAAS). This paper will briefly overview the WAAS network time (WNT) algorithms and their integration into the overall system. The WAAS system is required to provide users with orbit, clock, and ionosphere corrections for single-frequency measurements of the GPS signal. The clock corrections will be made in the WAAS network time (WNT) reference scale. This reference scale is required to track the GPS time scale,-while at the same time providing the users with the translation to UTC. Other internal requirements are placed on the WNT bias and frequency jumps from the accuracy demanded in the WAAS fast corrections. The WAAS architecture requires that WNT be computed at multiple WAAS master stations (WMS) using potentially differing sets of measurements from potentially differing sets of receivers and clocks. This design must provide the user a seamless transition when switching from one WMSs WNT realization to another. This paper expands on two previous papers, one presented at ION GPS-97 using simulated clock data as input, and the other presented at the 1999 Technical Meeting using receiver data from the National Satellite Testbed (NSTB) and preliminary WAAS data. We included some introductory sections from previous papers here for completeness. Here, we will present the performance results of the WNT algorithm using GPS clock and receiver data from WAAS Reference Station (WRS) using the standalone WNT algorithm with the WAAS prototype software. We will also show some recent WNT algorithm results from the operational WAAS software, indicating that the WNT design should meet the aforementioned requirements, taking into account possible WAAS WMS switches. Two tests are performed. The first test shows how well WNT for a single WMS network agrees with estimated GPS. We will check this in the operational,as well as in the standalone/prototype, WAAS software. The second test will check the coordinated WNT algorithm. Using two different subsets (taken from a single WRS thread to simulate two WMS networks) of the WRS sites, we demonstrate that coordinated WNT as computed at the two different networks closely agree (on WNT), thereby ensuring a seamless transition from one WMS network to another. We will also show how WNT differs in the operational system between two actual WMSs.
Published in:	Proceedings of the 31th Annual Precise Time and Time Interval Systems and Applications Meeting December 7 - 9, 1999 Marriott's Laguna Cliffs Resort Dana Point, California
Pages:	161 - 172
Cite this article:	Griffith, Cheryl, Peck, Stephen, Diamond, Peter, "WAAS Network Time Performance Using WRS Data," Proceedings of the 31th Annual Precise Time and Time Interval Systems and Applications Meeting, Dana Point, California, December 1999, pp. 161-172.
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