I'd be moderately surprised if it was necessary: atomic clocks are only really king of time stability over longer time periods (days/months). If this just needs

I'd be moderately surprised if it was necessary: atomic clocks are only really king of time stability over longer time periods (days/months). If this just needs to stay in sync over the course of a few hours (after syncing outside with GPS, for example), then it's much more plausible to do it with a high-precision crystal or silicon MEMS oscillator (SiTime has already become a big player in the phone market because of their low power consumption for the precision they offer, and they have some really impressive higher-end offerings that wouldn't currently fit but perhaps could be made to).