[mgaunard]

Another protocol that's not mentioned is PPS and its variants, such as WhiteRabbit.

A regular pulse is emitted from a specialized high-precision device, possibly over a specialized high-precision network.

Enables picosecond accuracy (or at least sub-nano).

[nuccy]

As a user of WhiteRabbit, I can confirm a sub-10ps sync (two clocks phase lock) over 50km fiber connection for variable temperature of fiber (biggest problem of clock sync over fibers is temperature induced length change of the fiber itself, which needs to be measured and compensated).

[RossBencina]

Out of interest, how do you measure a sub-10ps phase lock between devices 50km apart?

[willis936]

The standards-compliant endpoints do all of the work. They count clock cycles for ping pong messages and share with each other the length of time so time-of-flight is tracked and compensated for.

[nuccy]

As of now, for testing, the two WR endpoints are sitting on the same desk with 50km fiber in a thermal chamber (simulating temperature changes in the soil), but in future they will be separated indeed.

[namibj]

Run 2 or 3 separate concurrent sync's and statistically compare the resulting clocks, for example.