I have an older garmin gps18x I use for ntp and it gets fantastic reception from a window shaded by trees. I wanted to test a monitor script so I stuffed it in a metal can, capped with a thick plate, and used excessive tinfoil to seal...If i recall it only dropped 2 satellites near the horizon, leaving 4+ with good signal. I suspect the signal was leaking in through the cable.

During past year running i don't believe its lost 3d fix once (not in the can)