GPS is a Satellite service, Just like DISH or DIRECTV.. you might want to read a few posts about the "Evil Signal Eating Tree" as it applies to GPS as well
My GPS works if it has a CLEAR view of the southern sky.. But My RV is wood and fiberglass and glass so not a lot of obstruction there.. My car, however, is steel, if I set the GPS up in the windshield area it works, going south but I've had cases where it looses sync going north because the roof is in the way.
I've had it loose sync on a "Tree lined street"
And I've had it loose sync due to lots of steel/concrete buildings or overhead wires.