Glider performance testing

I recently reread Richard Johnson's article, "Sailplane Performance
Flight Test Methods" in the May, 1989 issue of Soaring magazine. In
the article Johnson mentions the Deceleration Measurement method of
determining a performance polar. It had occured to me a couple of
years ago that such a method could be used in conjunction with GPS data
as a fairly simple way of measuring glider performance. I envision
importing the GPS time, altitude, and position data into a spread sheet
application to do the number crunching. Anyone out there ever tried
anything like that? Anyone have suggestions or caveats about doing it?

Myles Bradley