Date: 30 Mar 98 04:31:16 From: firstname.lastname@example.org (CL6013R) Organization: AOL http://www.aol.com References: 1
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>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 ? YES , the airplanes altimiter is set 29.92 inches of mercury during cruise (any time above 18,000'). True altitude may vary by a couple of thousand feet due to air pressure changes , but if we are all at 29.92 then we all read the same altitude and maintain proper verticle seperation.