Simple fix really.....don't go to a gas station :P
The gas station will have somesort of a roof, which will block the signal from getting through, you could hack the GPS and fit a stronger aerial onto it.
GPS works by time. All the sattelites have a precise time, which they broadcast. The GPS receiver than works out where the satelites are judging by the time. GPS microwave signals take time to reach the ground, I can't be bothered working out the exact time, but the further away a satelite is, the longer the signal takes to reach you.
For example, if you have four satelites, each one will give a slightly different time down to the nanosecond, whch the GPS receiver can then work out where you are! With a roof, those times can get a little skewed and you'll have "lost signal" while in all reality, you won't have lost the signal completely, the GPS receiver will get widely varied times, so the GPS receiver cannot pinpoint your location.
At any one time, there are always three or more GPS satelites above us ;-)
With the launch of the European GPS satelites, we will actually be able to accurately work out where we are....to a meter!! Currently, US GPS sattelites can work out only to 10 meter range.
Try to use a petrol pump near the edge of the forecourt, which should help you with your signals
