Comment by throw0101d
2 days ago
> 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...
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).
That is not correct at all. How did you arrive at that conclusion?
GPS has its own independent timescale called GPS Time. GPS Time is generated and maintained by Atomic clocks onboard the GPS satellites (cesium and rubidium).
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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...).
> You can get AtomiChron as an optional subscription with the SparkPNT GPSDO, for instance (https://www.sparkfun.com/sparkpnt-gnss-disciplined-oscillato...).
That's one hell of a healthy profit margin there O.o
The SiTime MEMS oscillator is about 100€ for one single chip, the mosaic-T GPS receiver is about 400€. Add 50€ for the rest (particularly the power input section looks complicated) and 50€ for handling, probably 600€ in hardware cost... sold for 2.500€.
The real money I think went into certification and R&D for a low-volume product - even though most of the hard work is done by the two ICs, getting everything orchestrated (including the PCB itself) to perform to that level of accuracy is one hell of a workload.
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