As I understand it, GPS works by having transmitters with known locations send a timestamp, and calculating the time it took to receive it, so that you can triangulate your own position, (essentially an intersect of three or more circles with known centers).
I am interested in whether it is possible to create a "local" gps sender and using it with an unmodified gps receiver.
Sci-Fi Example to clarify
Let's assume that "suddenly" a country finds itself transported 100 years back in time, and all GPSs are rendered useless. Would they be able to (for example) establish GPS transmitter towers without having to reconfigure the GPSes, or would each GPS unit have to be programmed with information (eg. the coordinates) about the new transmitters?