Andy Blackburn wrote:

Without knowing the technique Dick Johnson uses, or
the specs on a specific pressure transducer, it's hard
to know if measuring pressure altitude through a digital
transducer is more or less accurate than the traditional
method. I'd guess it's a close call, but that has nothing
to do with GPS.

I don't know the specifics of Dick's instrument, but a digitally logged
unit has got to be better, maybe a lot better, than trying to read a
mechanical altimeter with a little vibrator on the panel!


The main source of error, is being able to turn GPS
ground speed (or distance) into IAS reliably by subtracting
wind speed and adjust for altitude.

I think the pilots trying this are not doing that, but instead rely on
their airspeed to give them IAS. This does require calibration of the
ASI for real accuracy.

An even greater
source of error is trying to use fixes from a typical
soaring day with airmass movements and pilot control
inputs, airspeed changes and flightpath deviations.

I agree completely, and expect the best data to come on the same kind of
day Dick uses; that is, not a soaring day!

It's this air mass movement that drives the Akafliegs to using "sacred
cows" to measure the motion of the test glider relative to the "cow".



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Eric Greenwell
Washington State
USA