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