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

David Woolley david at ex.djwhome.demon.co.uk.invalid
Fri Oct 24 06:40:53 UTC 2008


Richard B. Gilbert wrote:
> 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 

As I noted elsewhere, 10 seconds isn't possible for a GPS cold start, 
but most of the excess time is spent in obtaining the ephemeris.  He's 
probably counting GPS startup from initial power up, but NTP start up 
from when it first gets run.

> 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.

By polling sufficiently fast you can get good time accuracy, even if 
frequency takes a bit longer.

> 
> 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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