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Geometry and availability of satellites is the key to GPS. If you are in an "urban canyon" (tall buildings all around) that block the satellite signals, accuracy will be poor. It takes a minimum of 3 satellites to do 2-D positioning, but 4 or more is MUCH better. In a fairly open environment, you should be seeing 6-10 satellites at all times, and the accuracy will typically be +/- 20-30 ft.
Do not confuse the accuracy of the navigation & mapping software with the accuracy of GPS. If the number of available satellites drops, the software may put more emphasis on the inputs from other sources, such as the speedo, the steering angles, etc. Mapping accuracy may degrade fairly quickly.
For more GPS info, try:http://www.trimble.com/gps/index.htm
A former GPS Engineer
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