GPS is funded by your tax dollars and is a completely passive system. What that means is, there is no way for the satallites to know your position or even what equipment you are using. GPS works by triangulating your position across multipul microwave streams being emitted by orbital satallites. The GPS Reciever, or the software that drives it, then uses that information to plot your position on a map.
Here is an overview of GPS. Another fine product brought to the world by the United States of America.
Here is specific information about how GPS works.