Re: Altitude reading on aircraft

Date:         30 Mar 98 04:31:14 
From:         ehahn@mitre.org (Edward Hahn)
Organization: The MITRE Corporation
References:   1
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In article <airliners.1998.496@ohare.Chicago.COM>, billaryclinton@hotmail.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 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 ?

That is correct.  GPS calculates the "geometric height", or altitude of the
receiver referenced to the WGS-84 ellipsoid.  Aircraft fly by barometric
altimeter, which of course varies according to weather patterns and
temperature.

Some suggest that in the future aircraft should use geometric height for
altimetry.  IMHO, that would be questionable for the following reasons:

1) An aircraft that is trimmed correctly in cruise will naturally fly the
barometric altitude - it will rise and fall with the local varience in air
density at a similar rate to what the barometric altitude changes.  An
aircraft tracking geometric height would be constantly adjusting power and/or
pitch to maintain a given altitude/speed.

2) GPS altitude under selective availability is accurate to only ~150 meters.
Even under differential correction, the residual error is expected to be
something like 5-7 meters.  (I'm assuming standard differential correction,
and not real-time kinematic correction.)  Barometric altimeters are already
designed to be accurate to about 20 feet when close to the ground, and most
transport aircraft have radar altimeters for approach and landing which are
extremely accurate.

3) Adopting GPS altitude would require universal equipage with GPS; while this
might be the wave of the future, it will not be realistically achieved for
some time. (At some future time, this objection may go away.)

The above are definitely my $0.02 only, and not an official position of my
company, BTW.

ed

>>>>  Ed Hahn    |    ehahn@mitre.org    |    (703) 883-5988  <<<<
The above statement is the opinion of the author.  No endorsement
or warranty by the MITRE Corporation is expressed or implied.
Really, I wouldn't kid you about a thing like this.