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

David Woolley david at ex.djwhome.demon.co.uk.invalid
Wed Oct 1 07:22:43 UTC 2008


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!




More information about the questions mailing list