Plane & Pilot
Thursday, June 11, 2009

WAAS


GPS Approaches for Every Airport?


waasThe benefits of transitioning from pilotage to dead reckoning, four course ranges, ADF, VORs, Loran and then to GPS have been nothing short of spectacular.
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waas1) Accuracy
WAAS is substantially more accurate than the existing global positioning system. A direct quote from the FAA’s website, “Using WAAS, GPS signal accuracy is improved from 20 meters to approximately 1.5–2 meters in both the horizontal and vertical dimensions.” GPS has always been more accurate in the horizontal plane, but WAAS now ensures a high level of accuracy in the vertical plane as well. In addition, the WAAS implementation automatically monitors the aircraft’s GPS accuracy and will instantly alert a pilot when the accuracy is degraded.

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.





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