alt Hacker NewsGPS uses the atomic clocks on the satellites though.
(Random search result from space force https://www.ssc.spaceforce.mil/Newsroom/Article/4039094/50-y... claims that cell phone tower-to-tower handoff uses GPS-mediated timing (only microsecond level though.)
The satellite clocks are designed to run autonomously for a few days without noticeable degradation, and up to a few weeks with defined levels of inaccuracy, but they are normally adjusted once a day by the ground stations based on the timescale maintained by the USNO. That, in turn, uses an ensemble of H-masers.