Yesterday, a DJI dealer tested before and after adding foil inside the Phantom to see if it really helps improve GPS satellite lock.
TL:DW summary: they got 7 satellites to lock within 1 minute in stock configuration. After adding foil (and they show you how to do it), they got 8 satellites — after 1 minute they got one more. (And it really bounced between 7 and 8, so it didn’t really, really help.)
But at least it doesn’t hurt!