[skulk]

Haven't looked into this in depth but sub-nanosecond sync for systems up to 10km apart is interesting since 10km is about 33 light microseconds. There is some trickery going on.

[nuccy]

In our lab tests phase lock jitter between WR client and master is about 10ps (picoseconds) over 50km fiber (with temperature change of the fiber, so WR actively compensates elongations), so relative clock of one system can be transmitted with about that accuracy to another.

P.S. There is WR workshop this week with some talks being publicly available on CERN's indico website.

[xxs]

Even though you're commenting on While Rabbit post, it took some time to understand "WR" is white rabbit, esp. since describing the pico seconds in brackets.

[elromulous]

It's totally possible to achieve synchronization better than light transmission time. For the purposes of synchronization, the speed of light delay, and any other delay are indistinguishable, and need not be distinguished.

[colechristensen]

The gravity well time dilation is about 3.5 nanoseconds per meter per year near the surface of the earth. (time changes rate with altitude in a gravity well)

Sub-nanosecond synchronization is getting into the relativity is measurable realm.

[mike_hock]

That means you get a free clock cycle every 2-3 hours on top of a mountain compared to sea level!

[brookst]

Datacenters in spaaaace!

[Gravityloss]

But they travel at 8 km/s so actually that cancels benefits? EDIT: checked, not enough to cancel them completely.

[pwndByDeath]

I wonder I'd that's the math for the ludicrous space data center ideas "floating" around...

[OutOfHere]

Just because you don't understand how something can work doesn't make it ludicrous. People like you are hell bent on destroying what's left of the Earth by turning it into a computer. If we left progress to those without an imagination, we wouldn't even have had a working calculator.

[ooterness]

Two-way time transfer measures the round-trip propagation time. As a result, it's not directly relevant to the accuracy.

[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.

[UltraSane]

Yes, it uses phased locked loops and measures phase difference between the master clock and the local clock.

[geerlingguy]

Similar to the much smaller scale Wi-Wi setup that was on HN a few days ago (which came after WR): https://news.ycombinator.com/item?id=48209055

[kcexn]

If both systems have a good clock. Then the synchronization messages only need to contain the time delta to correct the time (phase?) drift to achieve full synchronization.

[SiempreViernes]

yes, it needs custom built hardware to work.