I have seen GPS altitude at 19,200 when the panel altimeter set to local
pressure said 17,990. I figure that gives me about 1200 more feet of
"headroom" to play in below Class A airspace in the USA. In the high
mountain country of the western US, GPS altitude gives much better final
glide calculations than pressure altitude.

Hopefully, the feds won't take away this extra useable attitude by switching
to GPS altitude for ATC.

Bill Daniels

"Mike Borgelt" wrote in message
...
On Tue, 08 Jul 2003 02:45:18 GMT, "Peter Kovari"
wrote:

Last season I find some great discrepancy between my GPS altitude and
indicated altitude by my standard mechanical altimeter. The differences
were
minimal at ground level, 2-300ft at 10,000' and 7-800ft at 17,000msl.
I had the altimeter checked by a certified repair station, who certified
it
within acceptable tolerances, and it is still off. The question I have
therefore, how accurate is the GPS altitude?
Peter K.



Peter, The pressure altimeter measures the difference between some
reference(the setting in the subscale window) and the ambient pressure
where you are. If you take that layer of atmosphere and heat it the
two pressure levels move apart, hence for the same altimeter reading
you are actually higher above the reference level. The GPS altitude
and pressure altimeter will read the same within instrument and GPS
system errors in an ISA standard atmosphere. In soaring we mostly fly
in thermals in warmer than standard atmospheres hence the GPS will
show a higher than pressure altimeter number.

On a really hot day at 10,000 feet you could get an error of 800 feet
GPS vs pressure altimeter. i.e.pressure alt 10,000 GPS 10,800 feet.

As to why flight computers don't use GPS altitude - the B2000 does.
I was about to build the pressure altitude module for it when SA got
turned off and the GPS altitude accuracy got to be at least as good
and mostly much better than pressure altitude for glider performance
purposes.( there are pads for a socket for that module on the main
circuit board)
Using a Garmin 35 GPS source set for 3D nav only with no averaging
and no dead reckoning I get the very isolated single reading GPS
altitude glitch on examination of the flight record. Never noticed in
flight. A simple software patch could take these out as they are
always totally weird and nothing like the readings either side in
time.

Having calibrated quite a few IGC approved FR's of various makes the
pressure sensor accuracy in them all can be unimpressive and I
wouldn't use it for final glides.

For badges and records ballooning and I think the rest of aviation
converts pressure readings from barographs etc to geometric altitude.
It is long past time we did this in gliding. The Ballooning people
have a nice worksheet to do this. FR's would get considerably cheaper
if the pressure altitude requirement was dropped.

Mike Borgelt

Borgelt Instruments