[ntp:questions] Query about NTP accuracy

Dave Hart davehart at gmail.com
Sat May 23 05:40:14 UTC 2009

>>One of the keys to getting tight synchronization is a stable and
>>accurate source of time.  A GPS timing receiver can supply time of the
>>required quality.  Note that GPS receivers designed for navigation
>>service are generally pooly suited to timing service, and vice versa.
> You must have a GPS which delivers PPS, and a good interrupt service
> routing to timestamp the PPS transition.
> You must run an operating system like Linux or BSD which actually has
> time support (interpolation) in the kernel.

Best possible practice is to use a GPS timing receiver rather than a
navigation/general purpose unit, and use FreeBSD.  I am not sure I'd
lump Linux in with it.  And if 100 microsoeconds or so is close
enough, Windows 2003 or earlier with ntpd 4.2.5p162 or later works
well despite the lack of interpolation in the kernel.  Later Windows
versions challenge ntpd's Windows interpolation code with a
higher-precision system clock (too high precision for ntpd
interpolation to work well, but still only a half-millisecond or so).

Dave Hart

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