[catoc]

So then you need to know distance / roundtrip-length within centimeter precision as well (below 29.98 cm for sub-nanosecond precision… to be precise).

Since cm precision is often not possible, is roundtrip-length an estimated average from prior roundtrips?

[fsh]

The roundtrip time is measured and compensated. Even NTP does this. Knowing the distance is not necessary for time synchronization.

[RossBencina]

gPTP estimates the link delay and the peer clock ratio, see for example this random link I just found for you: https://blog.meinbergglobal.com/2024/03/27/what-is-gptp/

[Gravityloss]

Hmm one would expect heat expansion to change the length of fiber over tens of kilometers. Does it also affect light speed in the fiber? I think consumer fiber is not buried very deep on average, but maybe for these use cases you use something hefty anyway.

[tempay]

It doesn't matter if the length changes provided:

* you measure the round trip time often enough

* the shift affects light in both directions equally

[netjiro]

delay is easy

jitter kills

[numpad0]

... why would cm precision be often not possible?

[dudinax]

Because you're using fibres laid years ago. Often pulse velocity isn't known to better than one part in 1000.