[ntp:questions] Slow convergence of NTP with GPS/PPS
Richard B. Gilbert
rgilbert88 at comcast.net
Fri Oct 24 14:24:41 UTC 2008
David J Taylor wrote:
> Richard B. Gilbert wrote:
>> And ntpd could take many minutes to bludgeon the local clock into
>> submission! It's easy to determine and set the correct time. It's
>> less easy to determine and set the correct frequency and thus KEEP the
>> correct time.
> Wasn't the OP looking for about 0.5s accuracy? Ah, no, that was someone
> else. Here we're aiming for:
> "Our application requires good time accuracy (better than 5ms) but it
> also needs to get there quickly (as quickly as possible, but ideally
> taking no more than about 15 minutes)."
> I would have thought that a GPS and NTP would have been able to achieve
> that, at least with a current drift file.
Try it some time! Ntpd makes a mad dash for zero offset, overshoots,
and then "rings" for a while. Eventually it gets that tight synch that
we like to see but it does take about thirty minutes.
I think I mentioned this here at the time I observed it. I believe that
I also remarked that I could have done it a lot faster by hand if I had
the "control knobs".
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