Local conditions can affect the GPS solution. GPS receivers use the same algorithm for solution, though they may differ in the number of satellites from which they can derive solutions. But most use 8-12 satellites, though a useful solution is possible with as few as 4. The big difference in accuracy comes mainly from the filtering methods used to accept or reject a given satellite's data stream. Not surprisingly, that filtering comes at a price. The more you filter, the greater the precision and accuracy, and the longer it takes to capture enough data for a solution.

Installation of the antenna plays a critical role in solution accuracy. It should be as high as practical above your car or boat or aircraft and away from any reflective surfaces like walls or bulkheads or trees. This minimizes errors due to multipath reflections -- the signal dances on all surfaces and arrives at the antenna in slight time delays. If the time delay is shorter than your GPS's filter is capable of detection, the GPS will provide an inaccurate solution.

But if you demand extreme filtering to get say, centimeter accuracy, you wont get the solution in time to prevent your boat from running aground or your aircraft from slamming into the runway at 400 mph. But you will know exactly where you stopped.

B