[binbag]

"the BIPM collects time measurements from national timing laboratories around the world"

I'm really interested in how this is done with multiple clocks over a distance. Can anyone explain? It feels like it would be very difficult since asking "what time is it there?" at the timescale of atomic clocks is kind of a bit meaningless? And that's before considering the absolute local nature of time and the impossibility of a general universal time per relativity.

[fanf2]

The term of art you want for searchengineering is “time transfer”.

There are a variety of mechanisms:

* fibre links when the labs are close enough

* two-way satellite time transfer, when they are further apart

* in the past, literally carrying an atomic clock from A to B (they had to ask the pilot for precise details of the flight so that they could integrate relativistic effects of the speed and height)

* there’s an example in the talk, of how Essen and Markowitz compared their measurements by using a shared reference, the WWV time signal.

[crote]

I believe an important aspect is that the actual time offset between the clocks doesn't matter all that much - it is the drift between them you care about.

True UTC is essentially an arbitrary value. Syncing up with multiple clocks is done to account for a single clock being a bit slow or fast. It doesn't matter if the clock you are syncing with is 1.34ms behind, as long as it is always 1.34ms behind. If it's suddenly 1.35ms behind, there's 0.01ms of drift between them and you have to correct for that. And if that 1.34ms-going-to-1.35ms is actually 1.47ms-going-to-1.48ms, the outcome will be exactly the same.

This means you could sync up using a simple long-range radio signal. As long as the time between transmission and reception for each clock stays constant, it is pretty trivial to determine clock drift. Something like the DCF77 and WWVB transmitters seems like a reasonable choice - provided you are able to deal with occasional bounces off the ionosphere.

Of course these days you'd probably just have all the individual clocks somehow reference GPS. It's globally available, after all.

[fanf2]

It isn’t just the difference in rate. The main content of Circular T https://www.bipm.org/en/time-ftp/circular-t is the time offset of the various national realisations of UTC. Another important aspect is characterizing the stability of each clock, which determines the weighting of its contribution to UTC.

The algorithm behind Circular T is called ALGOS.