GPS-disciplined oscillators are based around full GPS receivers that decode the GPS signals and compute time from them in the usual way, then use that to steer

GPS-disciplined oscillators are based around full GPS receivers that decode the GPS signals and compute time from them in the usual way, then use that to steer the local oscillator. I think the better ones have a fixed-position mode which makes use of the fact the ground station doesn't move to get slightly more accurate timing, but otherwise they're basically the same as any other GPS receiver.