I think there is a lot of misinformation about GPS. Heres the spec for the garmin 35 which I use in my plane connected to my video feed.

One thing I do know is that the gps reads "absolute" speed. It doesn't care if you going vertical or horizontal. It reads the absolute distance from you previous position, and divides it by the time it took you to get there (1 sec usually, but thats programmable).

This GPS is typical for the consumer market. About 120 bucks.

Notice the "Velocity accuracy".
0.1 m/s RMS. I'm not sure how to read this. Usually accuracy is a percentage of error, IE +/- 1%. If I'm reading this right its saying there is a maximum of 1/10 meter per second error and any speed.
That would suggest the faster you go the more accurate the reading. Am I wrong?? Opinions?


BTW, Selective Availability is the deliberate induction of error by the government. But I have read that they stopped doing that.


http://www.garmin.com/products/gps35/spec.html
also
http://www.garmin.com/manuals/64.pdf

Accuracy:

Differential GPS (DGPS): 5 meters RMS
Non-differential GPS: 15 meters RMS (100 meters with Selective Availability at maximum)
Velocity accuracy: 0.1 m/s RMS steady state (subject to Selective Availability)

Dynamics: 999 knots; 6g's