[DonRotolo]

Differential GPS uses a local GPS transmitter whose exact location is known (and not moving). The local transmitter then compares its actual location with that from the satellites and transmits a correction fcator. This is effective over several hundred meters to perhaps a few miles (with degrading accuracy over distance).

The main issue I see is that even the best differential GPS is only good to about a foot in a dynamic environment (something that is moving). Yes, surveyors can get fractions of an inch, but only with observations over relatively long periods of time. That's not really good enough I think. Plus, the issue of receiving GPS signals inside a building add significant challenges.

Inertial navigation with external checks seems to be the ideal FRC navigational method. Inertial navigation tries to measure where you are, and the checks (such as ultrasonic ranging) help keep things real.