4 ms·

> 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

> 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.ntp.org/support/vendorlinks/ * https://www.meinbergglobal.com/english/products/ntp-time-server.htm https://www.meinbergglobal.com/english/products/ntp-time-ser... * https://syncworks.com/shop/syncserver-s650-rubidium-090-15200-652/ https://syncworks.com/shop/syncserver-s650-rubidium-090-1520... * https://telnetnetworks.ca/solutions/precision-time/ https://telnetnetworks.ca/solutions/precision-time/

lysace 10mo ago
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 https://en.wikipedia.org/wiki/Rubidium_standard
- CamperBob2 10mo ago
  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-oscillator.html https://www.sparkfun.com/sparkpnt-gnss-disciplined-oscillato...).
  - mschuster91 10mo ago
    > You can get AtomiChron as an optional subscription with the SparkPNT GPSDO, for instance (https://www.sparkfun.com/sparkpnt-gnss-disciplined-oscillato 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.
    - CamperBob2 10mo ago
      Yep, for hardware sold in low volumes, what looks like a healthy or even abusive profit margin turns out to be barely adequate to justify getting out of bed in the morning. There are so many artificial bureaucratic barriers to entry these days, and it's not getting better.
herpderperator 10mo ago
If those other applications use their own local GPS clocks, what is the significance of NIST (and the 5μs inaccuracy) in their scenario?
- Denvercoder9 10mo ago
  GPS gets its time from NIST (though during this incident they failed over to another NIST site, so it wasn't impacted).
  - iJohnDoe 10mo ago
    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).
    - sq_ 10mo ago
      I think GP might’ve been referring to the part of Jeff’s post that references GPS, which I think may be a slight misunderstanding of the NIST email (saying “people using NIST + GPS for time transfer failed over to other sites” rather than “GPS failed over to another site”). The GPS satellite clocks are steered to the US Naval Observatory’s UTC as opposed to NIST’s, and GPS fails over to the USNO’s Alternate Master Clock [0] in Colorado. [0] https://www.cnmoc.usff.navy.mil/Our-Commands/United-States-Naval-Observatory/Precise-Time-Department/The-USNO-Master-Clock/The-USNO-Master-Clock/Time-Dissemination-at-the-USNO/USNO-Alternate-Master-Clock-AMC/ https://www.cnmoc.usff.navy.mil/Our-Commands/United-States-N...
      - sq_ 10mo ago
        I find this stuff really interesting, so if anyone's curious, here's a few more tidbits: GPS system time is currently 18s ahead of UTC since it doesn't take UTC's leap seconds into account [0] This (old) paper from USNO [1] goes into more detail about how GPS time is related to USNO's realization of UTC, as well as talking a bit about how TAI is determined (in hindsight! - by collecting data from clocks around the world and then processing it). [0] https://www.cnmoc.usff.navy.mil/Our-Commands/United-States-Naval-Observatory/Precise-Time-Department/Global-Positioning-System/USNO-GPS-Time-Transfer/Leap-Seconds/ https://www.cnmoc.usff.navy.mil/Our-Commands/United-States-N... [1] https://ntrs.nasa.gov/api/citations/19960042620/downloads/19960042620.pdf https://ntrs.nasa.gov/api/citations/19960042620/downloads/19...

lysace 10mo ago

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 https://en.wikipedia.org/wiki/Rubidium_standard

CamperBob2 10mo ago
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-oscillator.html https://www.sparkfun.com/sparkpnt-gnss-disciplined-oscillato...).
- mschuster91 10mo ago
  > You can get AtomiChron as an optional subscription with the SparkPNT GPSDO, for instance (https://www.sparkfun.com/sparkpnt-gnss-disciplined-oscillato 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.
  - CamperBob2 10mo ago
    Yep, for hardware sold in low volumes, what looks like a healthy or even abusive profit margin turns out to be barely adequate to justify getting out of bed in the morning. There are so many artificial bureaucratic barriers to entry these days, and it's not getting better.

herpderperator 10mo ago

If those other applications use their own local GPS clocks, what is the significance of NIST (and the 5μs inaccuracy) in their scenario?

Denvercoder9 10mo ago
GPS gets its time from NIST (though during this incident they failed over to another NIST site, so it wasn't impacted).
- iJohnDoe 10mo ago
  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).
  - sq_ 10mo ago
    I think GP might’ve been referring to the part of Jeff’s post that references GPS, which I think may be a slight misunderstanding of the NIST email (saying “people using NIST + GPS for time transfer failed over to other sites” rather than “GPS failed over to another site”). The GPS satellite clocks are steered to the US Naval Observatory’s UTC as opposed to NIST’s, and GPS fails over to the USNO’s Alternate Master Clock [0] in Colorado. [0] https://www.cnmoc.usff.navy.mil/Our-Commands/United-States-Naval-Observatory/Precise-Time-Department/The-USNO-Master-Clock/The-USNO-Master-Clock/Time-Dissemination-at-the-USNO/USNO-Alternate-Master-Clock-AMC/ https://www.cnmoc.usff.navy.mil/Our-Commands/United-States-N...
    - sq_ 10mo ago
      I find this stuff really interesting, so if anyone's curious, here's a few more tidbits: GPS system time is currently 18s ahead of UTC since it doesn't take UTC's leap seconds into account [0] This (old) paper from USNO [1] goes into more detail about how GPS time is related to USNO's realization of UTC, as well as talking a bit about how TAI is determined (in hindsight! - by collecting data from clocks around the world and then processing it). [0] https://www.cnmoc.usff.navy.mil/Our-Commands/United-States-Naval-Observatory/Precise-Time-Department/Global-Positioning-System/USNO-GPS-Time-Transfer/Leap-Seconds/ https://www.cnmoc.usff.navy.mil/Our-Commands/United-States-N... [1] https://ntrs.nasa.gov/api/citations/19960042620/downloads/19960042620.pdf https://ntrs.nasa.gov/api/citations/19960042620/downloads/19...