An Analysis of Satellite Integrity Monitoring Improvement for WAAS

T. Wu and Stephen Peck

Abstract:	Raytheon is currently developing under contract with the Federal Aviation Administration (FAA) a GPS-based navigation system that is intended to become the primary navigational aid for commercial aviation during all phases of flight—from enroute through precision approach (initially LNAV, VNAV). This innovative system, named the Wide-Area Augmentation System (WAAS), will make use of a network of reference stations distributed throughout the U.S. National Airspace System. These reference stations will collect GPS measurements and send them to master stations. The master stations will process the data to provide correctional and integrity information for each GPS satellite and ionosphere gridpoint. The key function of the WAAS signal is to provide integrity information that meets the stringent safety-of-life requirements. The integrity messages set limits on the magnitude of the WAAS correction errors, allowing the user to determine horizontal and vertical protection levels (HPL and VPL). These protection levels are calculated from two types of integrity bounds. The satellite and clock errors are bound by the user differential range error (UDRE) along with the associated message type 28 (MT28). The ionospheric errors are bound by the grid ionospheric vertical error (GIVE). The actual probability of exceeding these protections levels (probability of hazardously misleading information denoted PHMI) must be less than 1 x 10-7/hr (enroute users) and 1 x 10-7/150sec (precision approach users) for any user within the service volume at any time WAAS is operational. This paper looks at the integrity bounds for the satellite position and clock errors. WAAS integrity is ensured by the UDRE monitor algorithms. The safety philosophy demands that the probability of missed detection (failure of the UDRE monitor to detect any satellite error greater than the integrity bound) must be less than PHMI. In addition, since monitoring receivers are not safety certified in the current system, this probability must be met even if one monitoring receiver is malfunctioning in the worst possible way. The current UDRE monitor implementation produces a UDRE value based on the missed detection probability assuming the satellite error is common to all receivers. The integrity bound is further inflated by MT28, which broadcasts the normalized covariance matrix based on all monitoring receiver measurements. A purely covariance based UDRE (and MT28) would yield smaller integrity bounds (that meet integrity requirements), but this was not implemented initially due to the complications of determining the covariance overbound given one monitoring receiver is malfunctioning. This paper proposes a method to compute the UDRE and MT28 based on a covariance solution. Several variations of a covariance based method are proposed, trading computational complexity vs. performance (integrity bound). Slightly more than one day of actual WAAS reference receiver data is used to evaluate the performance of these methods, using the current WAAS implementation for baseline performance. The MT28 and UDRE message bit quantization is ignored in this analysis, since it tends to distort performance comparisons. Computationally feasible methods are shown to achieve a 25% to 35% average reduction in the integrity bound. Simulated data demonstrates this reduction should increase as the WAAS receiver network grows.
Published in:	Proceedings of the 15th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2002) September 24 - 27, 2002 Oregon Convention Center Portland, OR
Pages:	756 - 765
Cite this article:	Wu, T., Peck, Stephen, "An Analysis of Satellite Integrity Monitoring Improvement for WAAS," Proceedings of the 15th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2002), Portland, OR, September 2002, pp. 756-765.
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