In the interests of mis - information being quashed:
I was right about GPS vs Pressure altitude(PA). Tim advised me he had
GPS and Pressure Altitude transposed in his spreadsheet. So his
analysis of flight records now agrees with the theoretical analysis
in my article and my manual analysis of the IGC files that a few very
helpful people sent me. Thanks again guys. As we fly gliders mostly
in summer when it is warmer than ISA, most of the time GPS altitude
is greater than PA. Not necessarily so in winter on wave flights.
If you find this isn't so it may be because you have an early GPS
receiver in the FR with some heavy filtering (early Colibris in
particular or weird processing of GPS and PA) or your pressure sensor
in the FR is off significantly.
It also looks to me like the pressure altitude is the anomaly in
Geoff Vincent's Flarm. I'd need a few more files to be sure.
If you want to compare GPS and PA do remember to take the offsets
into account before takeoff and after landing. In this case the FRs
that don't automatically start on movement or climb are probably
better. Pressure varies from day to day and the pressure sensor in
the FR is fixed to 1013 Hpa reference pressure.
If you have an early Colibri don't attempt to use the GPS altitude
for final glide calculations. Some other FRs seem to show gaps in the
GPS altitude record or in the case of one recent FR design the GPS
receiver seems to be one optimised for 2D navigation and ground
vehicle dynamics with maybe some dead reckoning of GPS altitude under
some circumstances . You can't really use that GPS altitude for final
glides either.
The CAI 302 traces seemed to be quite good with only the odd
gap(Garmin module), the EW Microrecorder seems to have an excellent
GPS receiver and I would expect FLARM GPS data to be very good also
as the GPS module is one where the user can set it for flight
dynamics and full time 3D navigation. Always assuming you have a good
antenna location with no shielding by the airframe or your body.
This is one of the reasons we use our own GPS module in the B500 and
now B800 where we use GPS altitude for final glides. We know how the
GPS is set up.
The glider cares about geometric altitude for how far it can glide,
not PA. PA is a requirement for airspace compliance. In my experience
given that the surface pressure will change during the day and the
errors inherent in using cockpit statics as well as other problems in
electronic pressure sensing, GPS altitude is superior to PA for final
glides. Do remember to give yourself a margin as your glider may not
really glide as well as assumed in the polar in your electronic final
glide computer. In some B800 configurations you'll get a real time
display of how well it glides against the assumed polar on every glide.
See the article on www.borgeltinstruments.com
Mike
Borgelt Instruments - manufacturers of quality soaring instruments
since 1978 ABN: 75532924542
phone 0746 355784
fax 0746 358796
cellphones 0428 355784
0429 355784
email: [email protected]
website: www.borgeltinstruments.com
P.O.Box 4607 Toowoomba East, 4350
Queensland Australia
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