"Tim.Ward" writes:

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.

I don't think this is quite right. Consider a 50:1 glider descending
100 meters:
It will go 100 x 50 = 5000 meters
I think vertical navigation errors are typically 1.5 times the
horizontal.
If horizontal accuracy is 10 meters, then vertical will be about 15
So when we've measured this exactly 50:1 glider sinking 100 meters, the
two worst cases we would read for a measurement a
(5000-Herror)/(100+Verror) = 4990/115 = 43.4
(5000+Herror)/(100-Verror) = 5010/85 = 58.9

To find and eliminate the random errors just have a second GPS receiver on the
ground under the area of the flight test. Comparing of the records would show
the GPS errors. (Any motion on a stationary receiver is GPS error.) We are
able to get better than millimeter accuracy (for earth plate motion studies)
using basically this technique, as you don't care where you are absolutely only
relative to the other point/receiver. We are interested in the polar at
higher speeds than plate motion speeds so some accuracy will be lost.

Steve