anybody ever check GPS speed against a calibrated radar gun (or some other very accurate speed measuring device)?
RADAR is exact. The counting unit can take a speed out to the .000s digit. It never rounds up though. What the operator sees is the true ones digit- So 59.999 MPH will have a readout of 59 MPH.
Things must have changed since my days in motors. All our manuals from the various manufacturers used to say +/- 2mph was the margin for error. That was in the basic operator course too.
Here in Australia the mobile "kiosks" as you call them [ours are electronic LCD/LED sign units mounted on trailers, not sure what type of radar unit they use but they are often wrong- based on the consistency of several of my GPS units and compared to my speedo].Towns around my area regularly have radar operated "YOUR SPEED IS" mobile kiosks parked at major entrances to the town to slow people down. They have always matched the speed on my Zumo 550 exactly.
that 9' error in position has ZERO bearing on the speedand the best accuracy was 9'. Now I'm not a mathematician but even I know that +/- nine feet really won't have a bearing on overall speed calculations... unless I'm putting around at .5 MPH.
the .002% of the time yer GPS speed is off, its usually waay off, usually as your going thru urban canyons, and your reciever uses signals momentarily with a different set of geometryRandy, do you know what causes the crazy numbers that a GPS may output for the highest speed reached. I have 303mph on one unit right now and it has never been on an airplane.