[ntp:questions] Sub-millisecond NTP synchronization for local network
Richard B. Gilbert
rgilbert88 at comcast.net
Thu Dec 4 22:48:38 UTC 2008
David Woolley wrote:
> Richard B. Gilbert wrote:
>
>>
>> My Solaris 8 SPARC system seems to be able to synch with the GPS
>> receiver in about 30 minutes. I may reboot that system once a year
>
> How do you define "synch"? NTP has an infinite impulse response, so
> will never completely synchronize. What Unruh means is the time until
> the residual error is comparable with the measurement error specified
> for the receiver, which is one microsecond for cheap GPS receivers and
> 50 nano-seconds, according the the GPS performance specification. The
> error also has to stay that low, e.g. the first zero crossing doesn't
> count because the error will overshoot on the other side.
>
<snip>
I don't really give much thought to "defining" synchronization. If
pressed, I'd say "remaining within one millisecond of the correct time".
The GPS receiver generates a PPS signal accurate to within 50
nanoseconds. I don't have the equipment necessary to measure the loss
of accuracy getting the time into the computer. I'm not sure I'd be
able to use it if I did have it.
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