Re: Altitude reading on aircraft

Date:         30 Mar 98 04:31:20 
From:         k_ish <kenish@ix.netcom.com>
Organization: Netcom
References:   1
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I use a GPS in my Cessna, and it is very accurate in X and Y, but not Z
(altitude).  A friend is a GPS engineer with Litton, and I asked him
why.  Although I am an engineer, too, I quickly did not understand his
explanation!!  The first reason he gave was that the earth is not a
perfect sphere.  The GPS models this deviation based on latitude, but
how accurate and extensive this internal model is depends on the GPS.
His explanation quickly became complex, involving math, propigation
velocities of signals, etc.

A typical error on my GPS is 500 feet.  He said this was pretty good.

Ken Ishiguro