In thinking about the speeding ticket post here while back besides tracking down a trooper on the interstate and asking him to check the speed (assuming he would) Would anyone know about the math involved in the accuracy of GPS units. Most of them state a velocity accuracy of (depending on how its stated) 0.05 meters/sec or 0.19 kph or 0.1 knots. In putting 0.19kph into a conversions calculator it looks like all these come up to the same level of accuracy which according to the calculator gives you an accuracy of 0.11806 miles per hour. My question is if this is constant at any speed or cummulative (assuming you are tracking more than 3 or 4 satellites. Would 60mph be +/_ 0.11806 or would it be 0.11806 per mph which would be 7.08 mph off. The specs also dont say if that is + or - or if it would always err on the slow or fast side? Any mathamatical kind of people with GPS backgrounds or expeience with GPS speed acccuracy? I guess I could buy a GPS and let a radar unit ride around with it using the radars patrol unit speed window to verify but did not know if someone had already looked into it. Thanks Kelly
Kelly, I would say that it would not be cumulative. I can't say for sure, of course, but that would be my guess. I sometimes have to use a GPS unit at work and when travelling in our trucks the GPS unit is usually right on with the speedometer. A GPS unit works by determining how long (speed) it takes the signal to reach the receiver. It HAS to have at least 2 satellites to work and the more the better. It is not uncommon for me to pick up 10 or 12 satellites on our units at any given time. I would say that the more satellites that you have, the more accurate the information is because of more inputs to triangulate...but I can't swear by it. I don't really have anything to judge it by. I'm sure that I've made this about as clear as mud, but I hope it helps some.
The accuracy of any GPS depends on how many satelites it is tracking. Regular GPS units are usually off 25 ft. or more. If the unit has WAAS and your picking up one of the WAAS satelites you can expect better than 25ft. accuracy. What that translates into speed accuracy figures I'm not sure. But I've stood still with a Waas equipped GPS and watched it say I was moving 1 mph.
I've used GPS to datalog vehicle speed. It is slow to respond but has good steady state accuracy. It is within 1 MPH, +/-. And no, you can't know which side it will err it is always +/-.
GPS Thanks for the input, I have only used one once and it was a borrowed unit. I will prob buy one since all three of my vehicles seem to say different speeds if we are running together.
