Re: Altitude reading on aircraft

Date:         30 Mar 98 04:31:18 
From: (Wayne T. McCandless)
References:   1
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In article <airliners.1998.496@ohare.Chicago.COM>, wrote:
>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 altitude sometimes deviated by several hundred meters

Due to the geometry of the GPS constellation with reference to a
terrestrial observer (i.e., you in a B747), the altitude resolution is the
weakest GPS "axis".  This arises because the signals from the satellites
that are in view have much less "diversity" in the altitude signal than
they will for latitude, longitude, and time.

W. T. McCandless
San Jose, California