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

Richard B. Gilbert rgilbert88 at comcast.net
Wed Oct 1 12:45:53 UTC 2008

Unruh wrote:
> davidm at pipstechnology.co.uk (David McConnell) writes:
>> 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.
> Are you using the -g option? What is the poll interval for your gps clock?
> (Use 4 whcih I think is the default for gps clocks.)
> ntp is NOT designed for rapid convergence, but on a gps clock with poll
> interval 4 and -g it sure should be better than hours.
>> 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

On a COLD START, I would expect to have to wait for something like 
thirty minutes to get really TIGHT synchronization (5 milliseconds or 
less).  Once you have it, you should be able to hold it until you have a 
power failure or a hardware failure.

If you can't wait for synchronization, get a good UPS, a generator to 
back it up and NEVER shut down or reboot!

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