Back in 2005 four M.I.T. students teamed up to apply some of the school's expensive testing gear to a few different tin foil hat designs. The intention was to determine if any effect can be measured with foil on one's head and what level of protection it might provide.

I give them an A for taking advantage of the facilities to further learn how a piece of equipment might operate and prove a real world point.

They discovered that a tin foil helmet actually amplifies frequencies between 1.2 and 1.4 Ghz. According to the FCC these channels are reserved for GPS signals and other satellite communications.

I've never actually seen anyone wear a tin foil hat and I'm pretty sure it's just an urban myth, but it seems like the conclusion of the experiment was that not only does a tin foil hat reduce your chances of meeting a nice girl in college but it can increase small electromagnetic fields within millimeters of your brain. Don't do it...not even as a joke for Halloween.

On a more serious note this might suggest that one could make a tin foil helmet for a hand held GPS unit to increase a weak or no signal situation.