WAAS Single Frequency GEO Operation Field Test and L1 Signal Code-Carrier Coherence Results

Laura Cheung and Po-Hsin Hsu

Abstract:	The Wide-area Augmentation System (WAAS) utilizes Geostationary Earth Orbit (GEO) satellites to provide users with correction and integrity information for GPS satellites, as well as provide an additional ranging source that is always in view within the GEO footprint. Maintaining coherence between the L1 downlink signal’s code and carrier is an important prerequisite for using a satellite signal as a ranging source. For GPS satellites, this is achieved by generating a coherent signal on-board the satellite payload. However, because GEO signals are generated on the ground, the signal must be “pre-corrected” for uplink errors in order to generate a GEO satellite output that is coherent. Estimation and correction of ionospheric (iono) delay, typically the largest single pseudorange error source, requires a secondary GEO broadcast signal, even though the user only requires the L1 signal for WAAS messages and ranging. Incorporating a new satellite into WAAS requires very long lead times – typically years – to design, implement, and integrate new hardware and software. Once in service, satellite components can degrade or fail over time, and other than redundancy, there are few options for mitigation. Under failure conditions which only affect the secondary signal, however, it may be possible for the satellite to continue to operate as a message source for users, even when its ranging functionality has been impaired. Due to very long replacement timeframes, the ability to operate in such a degraded mode would be desirable as a stop-gap measure until a replacement satellite is available.. This paper demonstrates the feasibility of operating a GEO as a WAAS message source with a single (L1 only) downlink signal frequency. Analysis of field test data from a live GEO shows that CCC performance is degraded as compared to dual frequency operation but could still provide WAAS messages to users.
Published in:	Proceedings of the 24th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2011) September 20 - 23, 2011 Oregon Convention Center, Portland, Oregon Portland, OR
Pages:	2446 - 2452
Cite this article:	Cheung, Laura, Hsu, Po-Hsin, "WAAS Single Frequency GEO Operation Field Test and L1 Signal Code-Carrier Coherence Results," Proceedings of the 24th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2011), Portland, OR, September 2011, pp. 2446-2452.
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