Altitude reading on aircraft

Date:         24 Mar 98 11:38:49 
From:         billaryclinton@hotmail.com
Organization: Deja News - The Leader in Internet Discussion
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On a flight from Frankfurt to Johannesburg I used my Garmin GPS 12 XL and I
compared the readings with those on the movie screen when "flight tracker" is
on.
The speed and heading matched, the position as well (I checked the GPS
readings on a map which map position matched on the flight tracker graph),
but the altitude sometimes deviated by several hundred meters (up to 2000
ft). Is this because aircraft use "logical" altitude, determined by local air
pressure and GPS'es by satellite geometry ?

Klaas

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