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

Unruh unruh-spam at physics.ubc.ca
Fri Oct 24 06:21:27 UTC 2008

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

>David Woolley wrote:
>> Richard B. Gilbert wrote:
>>> To turn your equipment on after months of downtime and expect it to 
>>> lock on to the correct time with millisecond accuracy within seconds 
>>> is asking for a hell of a lot.
>> Not really.  He's starting a GPS receiver at the same time and that has 
>> to lock to 50ns.
>> Doing it on a general purpose computer is more difficult, but not 
>> particularly impossible.

>Even with GPS and a full four satellite fix, ten seconds to synchronize 
>is extremely ambitious!!  You can set the time to within whatever 
>precision the hardware and software support but that is only half the 
>problem.  You also need to set the correct clock frequency.  On a cold 
>start, the clock frequency is a moving target as the hardware warms up.

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. 

>I would expect to wait at least thirty minutes for the system to 
>stabilize with both the correct phase (time) and frequency.

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