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Old 12th Jan 2012, 08:58
  #14 (permalink)  
Join Date: Jul 2008
Location: EDDF
Age: 39
Posts: 119

In this document it is briefly explained how GPS height can be used in EGPWS.

Note that an aviation GPS receiver does not only calculate horizontal position and geometric height. It also calculates a number of quality indicators based on the satellite geometry. These quality indicators give the accuracy & integrity of the calculated position & height.

In the past the accuracy of the GPS signal was purposely degraded by the US govt for military strategic reasons, the so called Selective Availability (SA). This affected both horizontal and vertical position accuracy of GPS receivers. Since May 2000 SA is turned off and the accuracy is greatly improved.

Older generation GPS receivers (probably >80% of the population) have a hard coded assumption that SA is still ON, so the quality indicators they put out are worse that the actual quality of their position. New generation GPS receivers are SA-aware and give better quality indications, although they are still conservative.

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