Andy Blackburn wrote:

At 18:30 28 December 2004, Eric Greenwell wrote:

How long ago did you ask him? GPS is much more accurate
in the last few
years, especially if using the WAAS ablities. But,
let's say you know
the distance to only +/- 100 feet (it's typically more
like +/- 30
feet), then flying only a mile (5000 feet) would be
a 2% error, or one
L/D point for a 50:1 glider. Good enough for us, I
think.


At 50:1 flying a one mile distance should yield an
altitude loss of 100', so I think a +/- 100' GPS error
in altitude could yield a 'measured' L/D of between
25:1 and infinity.

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).


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Eric Greenwell
Washington State
USA