A Proposed Integrity Equation for WAAS MOPS

In recent years there has been widespread growth in the number of proposed differential augmentation systems for GPS. Such systems are being developed by both civil authorities and commercial interests. The most stringent application of these systems is 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 foot decision height. However, the requirements on integrity and availability will require special attention. In order to guarantee integrity, confidence bounds must be sufficiently large so as to always cover the error of the broadcast correction. However, to maintain availability, the confidence bounds cannot be unduly conservative. As such, careful choice of the integrity equation is paramount. This paper derives an integrity equation for the position solution starting from the expected distribution of errors on the corrected pseudorange measurements. We show that the details of the integrity equation depend on these expected error distributions. As a further complication, the integrity requirement applies individually to each aircraft conducting precision approach and not to an ensemble average over the airspace as a whole. Thus, there is some question as to how to calculate and apply confidence bounds. An additional constraint is the bandwidth limitation on the corrections. There are a limited number of bits with which to send corrections and confidences to the user. An integrity equation which significantly increases the amount of information broadcast to the user is not practical. The proper integrity equation must take all of these constraints into consideration. We propose an equation which satisfies the integrity requirement while maximizing availability. Additionally this equation is fully compatible with the WAAS Minimum Operational Performance Standards (MOPS) and requires no additional bits beyond the current specification. The bits used for integrity also serve to weight the measurements for the position solution. The proposed integrity equation has been verified through Monte Carlo simulation. In addition, this equation is applied to real data from the National Satellite Test Bed (NSTB) network.