Comment by opello
2 days ago
The 5us inaccuracy is basically irrelevant to NTP users, from the second update to the Internet Time Service mailing list[1]:
To put a deviation of a few microseconds in context, the NIST time scale usually performs about five thousand times better than this at the nanosecond scale by composing a special statistical average of many clocks. Such precision is important for scientific applications, telecommunications, critical infrastructure, and integrity monitoring of positioning systems. But this precision is not achievable with time transfer over the public Internet; uncertainties on the order of 1 millisecond (one thousandth of one second) are more typical due to asymmetry and fluctuations in packet delay.
[1] https://groups.google.com/a/list.nist.gov/g/internet-time-se...
> Such precision is important for scientific applications, telecommunications, critical infrastructure, and integrity monitoring of positioning systems. But this precision is not achievable with time transfer over the public Internet
How do those other applications obtain the precise value they need without encountering the Internet issue?
> How do those other applications obtain the precise value they need without encountering the Internet issue?
They do not use the Internet: they use local (GPS) clocks with internal high-precision clocks for carry-over in case GNSS signal is unavailable:
* https://www.ntp.org/support/vendorlinks/
* https://www.meinbergglobal.com/english/products/ntp-time-ser...
* https://syncworks.com/shop/syncserver-s650-rubidium-090-1520...
* https://telnetnetworks.ca/solutions/precision-time/
If those other applications use their own local GPS clocks, what is the significance of NIST (and the 5μs inaccuracy) in their scenario?
6 replies →
TIL/remembered GNSS satellites have onboard atomic clocks. Makes a lot of sense, but still pretty cool. Something like this, I guess?
https://en.wikipedia.org/wiki/Rubidium_standard
3 replies →
A lot of organizations also colocate timing equipment near the actual clocks, and then have 'dark fiber' between their equipment and the main clock signals.
Then they disperse and use the time as needed.
According to jrronimo, they even had one place splice fiber direct between machines because couplers were causing problems! [1]
[1] https://news.ycombinator.com/item?id=46336755
If I put my machine near the main clock signal, I have one clock signal to read from. The comment above was asking about how to average across many different clocks, presumably all in different places in the globe? Unless there's one physical location with all of the ones you're averaging, you're close to one and far from all the others so how is it done without the internet?
If you must use the internet, PTP gets closer.
Alternate sources include the GPS signal, and the WWVB radio signal, which has a 60kHz carrier wave accurate to less than 1 part in 10^12.
Can you do PTP over the internet? I have only seen it in internal environments. GPS is probably the best solution for external users to get time signals with sub-µs uncertainties.