WAAS Network Time Performance with Site Data

Cheryl Griffith, Stephen Peck, Willy Bertiger

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 WMS's WNT realization to another. This paper expands on a previous paper presented at ION GPS-97 [Pe2] using simulated clock data as input. We have also included some sections from [Pe2] in this paper for completeness. Here, we will present the performance and validation results of the WNT algorithm using real GPS clock and receiver data from the National Satellite Testbed (NSTB). We will also give a sneak preview of the algorithm performance using the more precise WAAS receiver clock data, indicating that the WNT design should meet the aforementioned requirements, taking into account possible WMS switches. Two tests are performed. Using two different subsets (simulating WMS networks) taken from the NSTB 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. Next, we test how well WNT for a single WMS network agrees with GPS time.
Published in:	Proceedings of the 1999 National Technical Meeting of The Institute of Navigation January 25 - 27, 1999 Catamaran Resort Hotel San Diego, CA
Pages:	839 - 846
Cite this article:	Griffith, Cheryl, Peck, Stephen, Bertiger, Willy, "WAAS Network Time Performance with Site Data," Proceedings of the 1999 National Technical Meeting of The Institute of Navigation, San Diego, CA, January 1999, pp. 839-846.
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