[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.