Ptebbe,

The 3 meter resolution is the max (I'll use this loosely) error. If you set the GPS receiver in a fixed location and collect the position data over a 24hr period, you will notice that the position wonders +/- 3 meters (ocassionally you have a larger than normal spike that could have been caused by mulipath, satellite geometry, etc..). The speed will also wander slightly.

Off the shelf receivers are much better now that SA (selective availability) is turned off. I have tested Magellan receivers and found them to be very accurate even when travelling at less than 20mph.

Azhar