This monitoring is referred to by another acronym, RAIM, which stands for Receiver Autonomous Integrity Monitoring. In a strange twist, that phrase actually provides a good description of what RAIM does. Without going deeply into the details (which could be an article in itself), RAIM automatically monitors system integrity, i.e., accuracy. If there are at least the requisite five GPS satellites visible for basic fault detection (FD), or six for an enhanced version called fault detection and exclusion (FDE), continuous testing determines (from the satellite availability, geometry and other criteria) whether the accuracy meets the minimum required. If not, the pilot will be notified within six seconds, and depending upon factors like when the alert was received, the aircraft’s position on an instrument approach, etc., the requirements might allow for continuing an approach or a mandatory suspension of the use of GPS, thus requiring a change to another navigation technology (e.g., VOR/ILS).
Although WAAS actively monitors for potential accuracy issues, the fact is that degraded accuracy is usually a result of not having enough operational satellites in optimal view. Because the satellites’ orbits are known, there are applications that predict RAIM availability through some GPS units, other applications and websites; RAIM information can also be obtained through an FSS briefing or through DUATS GPS RAIM Prediction. Pilots should verify expected accuracy before beginning a flight in any questionable weather.