Spanner

(See https://docs.cloud.google.com/spanner/docs/true-time-externa...)

Does it need to be this close to NIST, or just relative to each other? Because the latter one is solved by PTP.

As far as I remember, near each other, but the comment I was replying to was specifically about needing accuracy.

It's been over a decade now since I managed the truetime team at google, things may have changed since :)

Spacecraft state vectors.

GPS

Not sure but synthetic massive aperture radio telescope would need syncing their local clocks.

I defer to the experts.

As far as I'm aware they just timestamp the sample streams based on a local gps backed reference. Then when they get the data/tapes in one computing center they can just run a more sophisticated correlation in software to smooth things out.

I work at a particle accelerator. We use White Rabbit (https://white-rabbit.web.cern.ch/) to synchronize some very sensitive devices, mostly the RF power systems and related data acquisition systems, down to nanosecond accuracy.

Telling people at the bar that I have an atomic clock at home.

cesium or rubidium?

rubidium + gps

As a very coarse number, 5µs is 1500 meters of radio travel.

If (and it isn't very conceivable) GPS satellites were to get 5µs out of whack, we would be back to Loran-C levels of accuracy for navigation.