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

Unruh unruh-spam at physics.ubc.ca
Fri Oct 24 17:34:17 UTC 2008


"Richard B. Gilbert" <rgilbert88 at comcast.net> writes:

>David J Taylor wrote:
>> Unruh wrote:
>> []
>>> With a minpoll of 4, just setting the phase correctly with zero drift
>>> compensation would at worst be out by 1ms by the next reading. And
>>> you can get a pretty good estimate of the drift, even if it is
>>> changing. The temp coefficient is not 10PPM/degree C. More like 1 or
>>> less. That means the first few measurements gives a pretty good
>>> estimate of the drift ( ie to a few PPM) and then the finer
>>> corrections can come while things settle down.
>> 
>> It isn't NTP which is the limit, but the GPS receiver acquiring lock from 
>> scratch after an indeterminate period of being switched off.  The GPS 
>> could take minutes to lock and declare that it has valid time.
>> 
>> David 
>> 
>> 

>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.

Actually it is relatively easy to determine the frequency as well in a
short time. and then refine it. It is NOT easy if you use a Markovian strategy
like ntp uses. 




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