Title: Code-Carrier Divergence Monitoring for the GPS Local Area Augmentation System
Author(s): Dwarakanath V. Simili and Boris Pervan
Published in: Proceedings of IEEE/ION PLANS 2006
April 25 - 27, 2006
Loews Coronado Resort Hotel
San Diego, CA
Pages: 483 - 493
Cite this article: Simili, Dwarakanath V., Pervan, Boris, "Code-Carrier Divergence Monitoring for the GPS Local Area Augmentation System," Proceedings of IEEE/ION PLANS 2006, San Diego, CA, April 2006, pp. 483-493.
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Abstract: Code-carrier smoothing is a commonly used method in Differential GPS (DGPS) systems to mitigate the effects of receiver noise and multipath. The FAA’s Local Area Augmentation System (LAAS) uses this technique to help provide the navigation performance needed for aircraft precision approach and landing. However, unless the reference and user smoothing filter implementations are carefully matched, divergence between the code and carrier ranging measurements will cause differential ranging errors. The FAA’s LAAS Ground Facility (LGF) reference station will implement a prescribed first-order Linear Time Invariant (LTI) filter. Yet flexibility must be provided to avionics manufacturers in their airborne filter implementations. While the LGF LTI filter is one possible means for airborne use, its relatively slow transient response (acceptable for a ground based receiver) is not ideal at the aircraft because of frequent filter resets following losses of low elevation satellite signals (caused by aircraft attitude motion). However, in the presence of a code-carrier divergence (CCD) anomaly at the GPS satellite, large divergence rates are theoretically possible, and therefore protection must be provided by the LGF through direct monitoring for such events. In response, this paper addresses the impact of the CCD threat to LAAS differential ranging error and defines an LGF monitor to ensure navigation integrity. Differential ranging errors resulting from unmatched filter designs and different ground/air filter start times are analyzed in detail, and the requirements for the LGF CCD monitor are derived. A CCD integrity monitor algorithm is then developed to directly estimate and detect anomalous divergence rates. The monitor algorithm is implemented and successfully tested using archived field data from the LAAS Test Prototype (LTP) at the William J. Hughes FAA Technical Center. Finally, the paper provides recommendations for initial monitor implementation and future work.