You can make a reference oscillator almost as precise as an atomic clock with a temperature compensated crystal oscillator. They cost tens of dollars.

We've also been synchronizing grid generation since long before spaceflight. It seems that the core problems here have been solved a long time ago.

Then again, I don't know much about the grid at this scale. I very much doubt that a few PPM phase difference would actually matter, but I don't really know.

Either way, it still seems insane to rely on GPS for your sole source of truth, even with its remarkable reliability.