Sunday, July 1, 2007
Can GPS replace ILS?
All GPS navigators (with or without WAAS) use signals from an artificial constellation of 24 NAVSTAR satellites. The satellites each carry an ultra-accurate, cesium-based atomic clock, and broadcast a timing signal at precise intervals. The GPS navigator picks up signals from three or more satellites (more is better), compares the timing signals and then determines its location on the surface of the earth with accuracy in the tens of meters. With a signal from a fourth satellite, it can calculate altitude.
For IFR use, pre-WAAS navigators include a barometric input used as a check on the calculated altitude and additional logic (Receiver Autonomous Integrity Monitoring, or RAIM) that compares the calculated position by using all satellites in view. RAIM constantly checks to see if the calculated position and altitude from all satellites agrees within a preset limit. If not, a flag is dropped indicating to the pilot that the GPS position can't be trusted—which is why pilots filing IFR with pre-WAAS GPS navigators are required to file a non-GPS alternate.
Some pre-WAAS GPS navigators can use the barometric input to provide advisory vertical guidance, but there are always errors in barometric altitude (and there could be additional errors in the computed GPS altitude), so instrument approaches with this equipment are limited to nonprecision minimums: typically no better than one mile from the end of a runway and 500 feet above the ground. And you must be prepared to break off the approach at any time if RAIM decides the computed position is unreliable.
WAAS uses three additional satellites in geostationary orbit some 22,000 miles above the earth's surface—at that altitude they take 24 hours to orbit and don't appear to move in the sky. WAAS transponders on the satellites broadcast signals that tell a WAAS-equipped navigator which of the regular GPS satellites can be believed and which should be ignored; they also add differential corrections to the GPS signal based on information from ground stations. The result is supposed to be so reliable that the FAA doesn't require any other kind of navigation equipment to file IFR, and it increases accuracy to a few feet in both horizontal and vertical directions, without any requirement for a separate barometric input. Navigators built this way meet Technical Standard Order (TSO)-C146a; at this writing, only two are in use: the GNS-480 (formerly Apollo CX-80) and GNS-430W/530W, both from Garmin.
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