Improved User Position Monitor for WAAS

The majority of the monitors in the Wide Area Augmentation System (WAAS) [1] focus on errors affecting individual error components. For example, the User Differential Range Error (UDRE) monitor [2] protects against large satellite clock and ephemeris errors. Its purpose is preventing errors larger than roughly four times sigma(UDRE) from being broadcast to the users. The UDRE monitor does not necessarily need to react to errors on the order of two to three times sigma(UDRE), as such errors are expected to occur occasionally as part of a normal error distribution. Disallowing these smaller errors may limit the overall system availability. Although two sigma errors are not all that uncommon individually, it is very unlikely to have many independent two-sigma errors present at the same time. Such an occurrence could lead to unsafe position errors for the user. Consequently, WAAS has monitors to examine not just the individual component errors, but to also the aggregate effect of the component errors together. Chief among these monitors is the User Position Monitor (UPM) [3]. This paper describes an efficient new algorithm being developed for WAAS that significantly improves upon the prior UPM. This new algorithm is based on the sum of the squares of the normalized errors at each reference station. It has significantly improved the ability to detect error conditions that could be harmful to users. As a consequence, excess conservatism may be removed from other WAAS monitors, as this new UPM is better suited to identify potential threats. As these other monitors are also improved we expect this innovation to lead to higher availability while maintaining equal or better levels of integrity.