Comment by zahlman
2 days ago
> 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?
> If those other applications use their own local GPS clocks, what is the significance of NIST (and the 5μs inaccuracy) in their scenario?
Verification and traceability is one reason: it's all very well to claim you're with-in ±x seconds, but your logs may have to say how close you are to the 'legal reality' that is the official time of NIST.
NIST may also send out time via 'private fibre' for certain purposes:
* https://en.wikipedia.org/wiki/White_Rabbit_Project
'Fibre timing' is also important in case of GNSS signal disruption:
* https://www.gpsworld.com/china-finishing-high-precision-grou...
GPS gets its time from NIST (though during this incident they failed over to another NIST site, so it wasn't impacted).
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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
Yes, either Rb, Cs, or H standards depending on which GNSS system you're using.
For the most critical applications, you can license a system like Fugro AtomiChron that provides enhanced GNSS timing down to the level of a few nanoseconds. There are a couple of products that do similar things, all based on providing better ephemerides than your receiver can obtain from the satellites themselves.
You can get AtomiChron as an optional subscription with the SparkPNT GPSDO, for instance (https://www.sparkfun.com/sparkpnt-gnss-disciplined-oscillato...).
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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.