Re: Altitude reading on aircraft

Date:         30 Mar 98 04:31:19 
From:         "Doug Haluza" <>
Organization: MindSpring Enterprises
References:   1
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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 ?

I don't know the details of the "flight tracker" system, but I assume it is
an output from the aircraft's navigation system. There are several reasons
for an altitude variation between your handheld GPS and the aircraft's
indicated altitude.

1) High VDOP (Vertical Dilution of Precision, the vertical component of
PDOP) caused by poor satellite geometry due to the ability to see less than
half the sky out the side window.

2) SA errors which are, on average, 1.5 times greater in vertical distance
than in horizontal distance. With poor VDOP, the error is even greater.

3) Geoid/Ellipsoid height differences (aircraft altitude is referenced to
the MSL Geoid and GPS uses an arbitrary Ellipsoid). Many receivers have a
Geoid height correction, but the lower cost receivers have a less accurate
Geoid model.

4) At cruising altitude (> 18,000 ft MSL) aircraft altimeters are set to
standard pressure, and are not corrected for surface pressure variation.
This can cause a difference between pressure altitude and true altitude in
excess of 1000 feet. This is not a problem for air navigation because all
aircraft use the same setting, and have the same "error".

The magnitude of the error you saw was well within this envelope.