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

Unruh unruh-spam at physics.ubc.ca
Wed Oct 1 13:40:44 UTC 2008

David Woolley <david at ex.djwhome.demon.co.uk.invalid> writes:

>Unruh wrote:

>> 128ms at 500PPM is 250sec or 4 min. It will take longer than that since the
>> rate will not peg out, but it should not be hours.

>It will follow the normal transient response, which has a first zero 
>crossing which I believe is at about 39 minutes for phase (RFC 1305) and 
>rather longer for frequency).  I'm not sure, but it may well overshoot 
>by more than 5ms, in which it could take rather longer to be acceptable 
>to the OP.

>It should get nowhere near being slew rate limited, so the 500ppm limit 
>is academic.

>> Maybe it is best to set the clock initiallly so that it is out by by more
>> than 128ms (Eg advance it by 10 sec) and then use -g.

>That would be my best trick, but if you have to use -g, you probably 
>don't know the time well enough to be sure that adding 10 seconds won't 
>actually put it in the 128ms band!

It could but the chances are small if he has an rtc which is not completely
broken. If 10 sec is no good, use 10 years. That will make the chance of
being within 128ms of 10 years vanishngly small.

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