[superdug]

ok, but....

If you set two clocks to the same time and put one at the bottom of the ocean and one at the top of a mountain ... after time, they will drift apart ... so is this ultra precise time in space making up for gravity time distortion as well?

[colechristensen]

It’s much worse than that, last I heard we could measure the differences in time passage separated by only a few vertical feet.

Ultra precise time in space absolutely has to account for relativity changing clock rates based on how deep you are in the gravity well. GPS would be all but useless without it.

[bequanna]

The question is which time do we consider to be the “correct” time. Turns out, we’ve decided to use a clock in Colorado as the time of record and then occasionally sync that clock with GPS satellites.

https://timeandnavigation.si.edu/satellite-navigation/gps/sy...

[fanf2]

It’s several layers more complicated than that. https://dotat.at/@/2023-05-26-whence-time.html

The USNO Alternate Master Clock at Schriever SFB is not the clock of record. It is synchronized to the USNO Master Clock in Washington DC.

The USNO Master Clock generates the US DOD’s official time, but it is also not the clock of record. There is also NIST’s clock, which is the official time for civilian use in the USA. And the NPL’s clock in Teddington for the UK. And ESA’s clock in Noordwijk for Galileo. And the PTB’s clock in Braunschweig for Germany. etc. usw.

All these clocks and many more contribute their measurements and cross-comparisons to the BIPM in Paris on a regular schedule. The BIPM calculates a consensus timescale from these measurements, which takes the form of retrospective corrections published in BIPM Circular T.

Circular T is the time of record. But it is not the most accurate time available because of its relatively short averaging time.

The best time is TT, terrestrial time, a uniform timescale that ticks at the same rate as the SI second as measured on the rotating geoid, i.e. the notional surface of equal gravitational potential which is the general relativity equivalent of mean sea level.

Well, not TT itself, but TT(year). The BIPM periodically publishes retrospective corrections going back several decades, saying what the error in TT was back then based on their best understanding now.