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

David McConnell davidm at pipstechnology.co.uk
Tue Sep 30 13:04:18 UTC 2008


Hi

We are using Linux ntpd with GPS/PPS reference clock to discipline the time
on our systems.

Our application requires good time accuracy (better than 5ms) but it also
needs to get there quickly (as quickly as possible, but ideally taking no
more than about 15 minutes).
(The Linux/ntpd is running on a remote embedded device that is frequently
restarted - possibly once a day or so - so we cant wait hours for
convergence).

Currently ntpd can take hours to achieve the desired acuracy.

So, the question is simple - is there any way to significantly speedup the
convergence of ntpd (using GPS/PPS reference clock)?

We would be prepared to compromise somewhat on accuracy and jitter.
(Currently accuracy and jitter values are excellent with jitter as low as 1
microsecond and accuracy better than 10 uS but it can take a day or two to
get there).

It does not seem unreasonable to expect that the ntpd could achieve the
required accuracy within 15 minutes or so - but nothing we have tried seems
to work.
Have tried modifying some of the tinker values, but we dont really
understand what they all do - and have not really had any success.

So to summarise:

1) Is it possible to speedup ntpd convergence (using GPS/PPS reference
clock)?
2) If so, how - and what are the tradeoffs?

Any help appreciated
David





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