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At 00:00 29 December 2004, Eric Greenwell wrote:
My example was for GPS distance, and pressure altitude, to indicate that a distance measurement wasn't a problem. That's why a I later referred to flying off at least 1000 feet if GPS altitude was used. What I don't know is how much error change one can expect in GPS altitudes taken 5 or 10 minutes apart. The difference (GPS start height minus GPS finish height) might have a much smaller error than the altitude itself, which would allow shorter glides (500 foot loss if the differential error was only 5 feet, for example). Clarification noted - but distance measurement is a problem with GPS with respect to polar calculations. 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. 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. 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. The empirical evidence is that there is way too much randomness from the above noted effects to tease out a anything much beyond just how much randomness there in fact is on a typical flight. Maybe if you did fifty 10-mile runs on a dead calm day across five different airspeeds, you'd get less scatter - but I think that's more or less what Dick does, except he measures IAS directly, rather than having to figure it out from ground speed. 9B |
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