Most GPS systems will "coast" for a while when they loose satellite reception before notifying the user that they have indeed lost reception. Unfortunately you could be in that lake at the end of the road before the GPS tells you it doesn't really have a clue where the hell you are.
I have a factory installed unit in my Class B that doesn't have any overhang. The GPS antenna is installed under the dash as close to the front of the vehicle as possible and it should have a pretty clear view of the sky at all times. However, I've noticed that in some parts of the country (West Texas down close to the border) when traveling in certain directions and at certain times of the day I get a "lost satellite reception". Indeed, checking the location of the visible satellites at the time and which ones are above the mask angle to the horizon I can see that I don't have enough to get a good fix.
Back when I worked on systems that used GPS for the military one of the pieces of trivia I picked up was that the St. Louis area has the worst US GPS coverage. Many of the early GPS units only accessed the US satellites; newer GPS units and phones use both the US and the Russian satellites. I suspect the Russian satellites give better coverage over the US than the US satellites which were launched to provide the best coverage over Europe. This has to do with the main reason the US and Russia put their satellites up there in the first place. The WAAS feature uses ground stations to assist in locating where you are. So when looking for a new GPS system I think it's a good idea to make sure it uses WAAS and well as GLONAS (the Russian system) if you want the best accuracy you can get. And of course it helps to make sure the antenna is placed where it will have the best overall view of the sky.
