WAAS Offline Monitoring

S. Gordon, C. Sherrell, B.J. Potter

Abstract:	The performance of the Federal Aviation Administration's (FAA's) Wide Area Augmentation System (WAAS) is often summarized by three characteristics of the services it provides: availability, continuity and integrity. Integrity is a measure of the probability that a WAAS user computes Hazardously Misleading Information (HMI). The top level WAAS integrity requirement is stated as follows: “The probability of HMI shall not exceed 1?10-7 per approach (or hour) at any point in the service volume. Undetectable conditions must be extremely rare and follow no predictable pattern. Detectable failures must meet this allocation under the worst detection scenarios.” WAAS validates the integrity requirement by using a mathematical proof in conjunction with data driven analyses. This proof uses fault tree analysis to show that WAAS meets the top level integrity requirement. The foundation of the proof is predicated on a set of carefully constructed assertions. These assertions require periodic monitoring to ensure that the physical environment has not changed in a manner that would invalidate the claim. Certain satellite failure modes have a priori event probabilities that must be detected and corrected in a reasonable amount of time in order to limit the user’s exposure to the failure. In each case, the assertion has been carefully scrutinized against the following criteria: a. The hazard will occur with a sufficiently small probability. b. The hazard does not have a major impact on the user’s integrity. c. An action exists which can correct the problem. The application of such criteria was made after careful consideration from the WAAS Integrity and Performance Panel (WIPP). Offline monitoring is applied in cases where the WIPP, using engineering judgment, concluded that even though the system still would likely meet the integrity requirements even in the face of the hazard, it would be prudent to limit the exposure time to a possible degradation of integrity performance due to some unanticipated change in environmental conditions. WAAS has instituted offline monitoring to satisfy this periodic monitoring requirement and the capability to perform this monitoring now resides with the WAAS Operations Group at the FAA's Aeronautical center in Oklahoma City. The monitoring is focused on specific environmental and satellite performance threats. The current environmental factors monitored are reference station measurement error (primarily multipath), ionospheric storms, and movement of reference station antennas. For satellite specific threats, the monitoring includes code carrier coherence, ephemeris, signal quality, and clock runoff. The results from this monitoring are reported quarterly to the WIPP. This paper will present the WAAS offline monitoring capability including the specific analyses conducted, representative results, criteria used for pass/fail and the process that WAAS follows should it be determined that a particular assertion may be violated. This process is of particular interest since it determines if the anomalous condition needs to be addressed and the allowable exposure time. The paper will also provide a few examples of WAAS anomalies that this processing is intended to capture.
Published in:	Proceedings of the 23rd International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2010) September 21 - 24, 2010 Oregon Convention Center, Portland, Oregon Portland, OR
Pages:	2021 - 2030
Cite this article:	Gordon, S., Sherrell, C., Potter, B.J., "WAAS Offline Monitoring," Proceedings of the 23rd International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2010), Portland, OR, September 2010, pp. 2021-2030.
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