Validation of the WAAS MOPS Integrity Equation

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 it 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 integri ty requirements are always met. In the future, it may also be possible to utilize this information to increase the availability of the system. INTRODUCTION Integrity of a system is often extremely difficult to prove. One must demonstrate safe performance in the past and an expectation of continued safe operation even in the face of potentially unknown threats. Past safe performance can be demonstrated easily, but it may be difficult o r impossible to gather enough data to meet stringent requirements at 10-7 or 10-9 levels. In addition, there is the question of whether all possible fault modes were adequately tested. In fact it would not be possible to prove the integrity of any system to a true skeptic. In order to gain confidence in any system, one must be able to predict the performance of the system under both nominal and faulted operation. For example, if the errors have a gaussian distribution with certain means and variances under “fault free” and various faulted modes of operation, performance can be predicted if enough data is collected to determine those values. This system must also be robust against general fault modes to ensure safety in the face of unexpected errors. The Wide Area Augmentation System (WAAS) [1] protects the users of the service by providing timely alarms and bounds on the error in the position solution. These bounds, called protection levels, provide a n indication of the quality of service. In order for the system to be usable, the protection levels must be below predefined thresholds known as alert limits. The most challenging aspect of the system is to generate bounds which are large enough to always protect the user but small enough to permit the operation. At the center of this challenge is the integrity equation. INTEGRITY EQUATION The WAAS MOPS integrity equation is based on the concept that the actual pseudorange errors can be conservatively bounded at and beyond the 10-7 probability Mauna Loa Honolulu Sitka Fairbanks Kotzebue