[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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