I would like to be able to read the time from an NTP server that is connected to a very accurate time source - an atomic clock, GPS clock, or similar. I can see an accurate clock here (http://www.atomic-clock.org.uk/atomic-clock.php) but would like an NTP version that I can use in some software that I'm writing.

Any help would be greatly appreciated.

EDIT: I've resolved the issue I was having (turns out it was my code, not the NTP server I was using)

4 Answers 4


Actually even if you find one NTP atomic based server(I dont think it exists) the NTP protocol isn't so accurate that your machine will have the same accuracy as the atomic clock. As wikipedia says:

NTP uses Marzullo's algorithm, and includes support for features such as leap seconds. NTPv4 can usually maintain time to within 10 milliseconds (1/100 s) over the public Internet, and can achieve accuracies of 200 microseconds (1/5000 s) or better in local area networks under ideal conditions.

An Atomic clock have a higher acurracy than NTP:

National standards agencies maintain an accuracy of 10−9 seconds per day (approximately 1 part in 1014), and a precision set by the radio transmitter pumping the maser.

  • 2
    Sorry, I was labouring under the misapprehension that most NTP servers are only roughly accurate. It turns out that my code was actually at fault, rather than the NTP server that I was using.
    – endian
    Commented Aug 23, 2011 at 14:37

I believe those listed via http://www.pool.ntp.org are all sync'd with stratum 1 systems; I'd suggest starting with some reading at http://support.ntp.org


Note that the NIST maintains an official pool of NTP servers synchronized with their atomic-based time system: NIST Internet Time Service Pool.


Several years later, it is actually possible to buy an off-the-shelf rack-mount NTPv4 server with an atomic clock inside. It is not exactly cheap, but it is not on the four digits either. It is also not as precise as a national-standards atomic reference clock class, but it is at least a thousand times more precise than what NTPv4 can actually handle.

I am not sure I can disclose specific vendors here, so the keyword to look for is "CSAC clock" (chip-scale atomic clock).

The typical high-end NTP server will have GPS input (plus others), and options of TCXO, OCXO and CSAC internal frequency references. The CSAC option is typically (as of 2016) either a Cesium or a rubidium atomic clock.

  • I think in this instance mentioning the vendor would be acceptable. Could you also clarify if these systems contain a radioactive source and if special handling would be required?
    – Burgi
    Commented May 31, 2016 at 15:46
  • For a lot less cash you could get a GPSDO with PPS and a CDMA card and connect them to your computer. It seems pretty obvious that OP does not need "atomic clock" level accuracy.
    – dfc
    Commented May 31, 2016 at 23:02
  • There are several vendors ofering berilium CSACs, and at least one that has a Cesium CSAC. Most of them have it as an option for their NTP server appliances (usually as an upgrade from OCXO). For a Cesium vapor CSAC, look for the Quantum S.45 from Symmetricom. The S.35 is also interesting, and based on Berilium. Examples for NTP servers that can take CSACs are the SyncServer S650 and S600 from Microsemi (there are many others as well). Atomic clocks are never radioactive, they are tuned using atomic ressonance, and that's where the atomic clock name comes from.
    – anonymous
    Commented Jun 7, 2016 at 12:19
  • Cannot edit the comment anymore, so... minor correction: The CSACs and MACs (miniature atomic clocks) are based on Rubidium lasers and Cesium vapor. They are not ITAR/export-restricted.
    – anonymous
    Commented Jun 7, 2016 at 12:40
  • As for too much precision for a NTP server, the main use for CSACs and MACs is as a hold-over reference (so that the clock does not stray during GPS/signal outages), i.e. as secondary references, unlike a national-standards atomic clock (which is stand-alone primary reference). The big deal about CSACs (Microsemi Quantum S.45 for example) is their size and power consumption, which are small enough for handheld devices. MACs (Microsemi Quantum S.35m for example) usually have more precision than CSACs (and use 10x the power for that), and are a good choice to replace OCXOs in a NTP server.
    – anonymous
    Commented Jun 7, 2016 at 12:40

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