Originally posted by bikenski:
The 2-dimensional locator is quite accurate, but you can't get a lock on enough satellites from the tiny window to get an accurate elevation. 35,000 Feet often varies by 500 feet or more.
FYI, there are three reasons that GPS altititude varies from the aircraft's altitude.
The primary reason is that above 18,000 ft., airlines change the barometric pressure on the altimeters to 29.92. Altitudes above 18,000 ft. are called Flight Levels, indicating that the altitude is not true, but is the altitude that cooresponds to this barometic pressure. Among other advantages, using 29.92 universally above 18,000 ft. allows airlines to maintain appropriate vertical separation without having to adjust the barometric pressure every 100 miles or so.
Secondarily, GPS altitude variation is roughly twice the level of horizontal variation. If you get an EPE of 100 ft., the altitude error can be as much as 200 ft.
And finally, GPS altitude is the altitude over a theoretical simplified spheroid of the Earth and can vary from the real altitude over the Earth.
Also, I wholeheartedly recommend
http://joe.mehaffey.com for comprehensive info on GPS!
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