From: perry @ piermont.com ("Perry E. Metzger") Aren't things even worse? Since the satelite signals are not authenticated with anything like public key methods, couldn't I just synthesize a signal appropriate to any spot on the planet, knowing the positions of the satelites relative to that spot?
In the case of the C/A (civilian) code, absolutely. In the case of the P code (military) only if you have the key, but in that case, yes. As Jim Bell pointed out, there are boxes you can buy for suitable amounts of money that are "GPS simulators" -- they construct out of whole cloth the signals you would receive if you were at location X with satellites {Y1, Y2, ...} overhead. However, if all you set out to do is fool a location authenticator, deriving shifted location data from the actual satellites is far easier and bound to be much cheaper. There is one limitation this has that the simulator approach doesn't -- delaying real signals requires having access to the same satellites (or a large enough subset, i.e., 3-4 of them) that the checking station has overhead. paul