One of the diagrams in the paper refers to both locations using GPS to derive local time, and the diagram shows a single satellite. In fact GPS uses at least 3 and possibly 5 satellites, and each satellite has its own atomic clock. The satellites all transmit on the same frequency, and when the signals arrive back on earth, timing differences as well as relativistic effects are combined to give local time.
I'd like to see how accurate they BELIEVE their local clocks are. Sub-nanosecond?