Thanks John Orton, Derek Ruddock, Owen Jones and Matthew Scutter for the files.
I think I've got enough there now to show that the basic physics in
my article on our website www.borgeltinstruments.com is correct.
For those who asked what I was trying to do - it was to verify the
predictions from the physics of the atmosphere.
The flights done to the higher altitudes on hot days show exactly
what is predicted. The warmer the atmosphere, the greater the
percentage difference between GPS and Pressure Altitude with GPS >PA.
In absolute terms the higher you go the greater the difference in
feet and the warmer than ISA , the greater the difference in feet
after correcting to the same on the ground. Wave flights done in
likely colder conditions close to ISA show reasonable agreement
between GPS and PA.
I didn't have the pressure calibrations for the loggers but they are
unlikely to be all that far out in the 3000-3500 meter range (most
are better than 2% in my calibration experience).
Interesting observation:
The older GPS receivers are definitely not as good for altitude with
the exception of Garmin 25 module which is excellent as used in some
302s.That receiver only has the odd anomalous GPS altitude point
which is trivial to remove in software in near real time.
The ublox TIM-LP as used in later Colibris and Flarms (as far as I
know all Flarms use this) is also good device.
One recent type of FR uses a new GPS module which did show excellent
performance most of the time but a worrying anomaly that persisted
for some time near the end of one flight. This would not be possible
to process and remove. It may also be a pressure sensor issue (brakes
open, sideslip etc???).
That leaves us to explain what Tim Shirley found where he says that
he's found PA>GPS alt on average.
Anybody got a flight done to 10,00 or so on a hot day with an EW
Microrecorder? That unit uses a Sirfstar GPS module.
You can see what pressure sensor and GPS are used by opening the igc
file with a text editor. It is in the header on all recent approved FRs.
Mike
Borgelt Instruments - manufacturers of quality soaring instruments since 1978
phone Int'l + 61 746 355784
fax Int'l + 61 746 358796
cellphone Int'l + 61 428 355784
email: [email protected]
website: www.borgeltinstruments.com
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