[ntp:questions] Slow convergence of NTP with GPS/PPS

David J Taylor david-taylor at blueyonder.neither-this-part.nor-this-bit.co.uk
Sat Oct 25 06:07:47 UTC 2008

David Woolley wrote:
> David J Taylor wrote:
>>   "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.
> Assuming default parameters and worst case initial error, and no PPS,
> it will start with an error of 128ms and take about 40 minutes before
> it crosses zero.  It will then overshoot, so, might take several
> cycles to settle within 5ms.  Using minpoll 4 may get it to cross
> zero within the 15 minutes, but the overshoot may still violate the
> criteria.

OK, David, so having a GPS/PPS source and distributing that to the clients 
should be far better?  How much better?

Isn't the 128ms a worst-case estimate, because NTP would set the time by 
stepping initially if the appropriate parameter is specified?  Wouldn't 
the initial offset then be near zero?


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