Validation of the WAAS MOPS Integrity Equation

Todd Walter, Andrew Hansen, and Per Enge

Abstract:	There has been widespread growth in the number of differential augmentation systems for GPS under development or in operation. Such systems are being developed by both civil authorities and commercial interests. These systems serve a variety of users and applications including precision approach for aviation, where the system provides vertical and horizontal guidance. Precision approach has very strict requirements for accuracy, integrity, continuity, and availability, and these become more stringent as the decision height decreases. To date, it appears that the Wide Area Augmentation System (WAAS) will be able to meet the accuracy requirements all the way down to a 200 ft decision height. The primary concern for such a system is that is always maintain integrity. The WAAS Minimum Operational Performance Standards (MOPS) specifies how users combine error confidences from the different sources to form a position bound. The service provider guarantees that the error at any user location is smaller than the respective bound with a sufficiently high confidence. This paper describes the validation of the integrity equation. Actual data from the National Satellite Test Bed (NSTB), a prototype for WAAS, is compared side-by-side to simulated data. The difference between actual and expected performance is investigated in detail. It is shown that compared to the real data, the assumptions used in the integrity equation are conservative. Integrity is maintained both in the simulated data and in the live data. The comparison of the two data sets provides insights as to the actual probability distribution of the errors in the live data and about correlations between different error components. This knowledge helps to ensure that the full integrity requirements are always met. In the future, it may also be possible to utilize this information to increase the availability of the system.
Published in:	Proceedings of the 55th Annual Meeting of The Institute of Navigation (1999) June 27 - 30, 1999 Royal Sonesta Hotel Cambridge, MA
Pages:	217 - 226
Cite this article:	Walter, Todd, Hansen, Andrew, Enge, Per, "Validation of the WAAS MOPS Integrity Equation," Proceedings of the 55th Annual Meeting of The Institute of Navigation (1999), Cambridge, MA, June 1999, pp. 217-226.
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