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Old 11th Jan 2012, 21:45
  #11 (permalink)  
Join Date: Jul 2008
Location: EDDF
Age: 39
Posts: 119
GPS altitude is indeed essentially geometric height above the ground / sea / ellipsoid, and therefor it is not suitable for altitude keeping, for that you need a pressure altimeter. So from an ATC perspective GPS altitude is not be acceptable.

But GPS height is acceptable for measuring your vertical position relative to the ground. For that purpose it is used in aircraft systems, for example in EGPWS and in GPS / GBAS based landing systems.
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