I need offline-verifiable, time stamped, GPS signals or its equivalent.
Since cellular protocols include time stamps, encryption keys and other data, perhaps it's possible to re-use this infrastructure to provide some degree of "signed location data". I'm guessing that each tower or base station has a unique key, or emits a signed ID of some type.
Disregarding whats currently legal in the US/UK, what is theoretically possible?
What is the most practical /legal approach?