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.
replyP.S. There is WR workshop this week with some talks being publicly available on CERN's indico website.
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.
replyIt'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.
replyTwo-way time transfer measures the round-trip propagation time. As a result, it's not directly relevant to the accuracy.
replySo then you need to know distance / roundtrip-length within centimeter precision as well (below 29.98 cm for sub-nanosecond precision… to be precise).
replySince cm precision is often not possible, is roundtrip-length an estimated average from prior roundtrips?
The roundtrip time is measured and compensated. Even NTP does this. Knowing the distance is not necessary for time synchronization.
replygPTP 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/
replyHmm 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.
replyIt doesn't matter if the length changes provided:
reply* you measure the round trip time often enough
* the shift affects light in both directions equally
delay is easy
replyjitter kills
... why would cm precision be often not possible?
replyThe 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)
replySub-nanosecond synchronization is getting into the relativity is measurable realm.
That means you get a free clock cycle every 2-3 hours on top of a mountain compared to sea level!
replyDatacenters in spaaaace!
replyBut they travel at 8 km/s so actually that cancels benefits? EDIT: checked, not enough to cancel them completely.
replyI wonder I'd that's the math for the ludicrous space data center ideas "floating" around...
replyJust 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.
replyYes, it uses phased locked loops and measures phase difference between the master clock and the local clock.
replySimilar 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
replyyes, it needs custom built hardware to work.
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