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

Richard B. Gilbert rgilbert88 at comcast.net
Tue Sep 30 19:03:39 UTC 2008

David McConnell wrote:
> 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)?

If you are using a recent version of ntpd, start it with the "-g" 
switch.  That will cause it to set the clock to the correct time once 
only!  If you have a good drift file, you should be synchronized in 
thirty seconds or so and be within ten milliseconds, or less, of the 
correct time.

Try not to reboot unless absolutely necessary.  I realize that some 
versions of Windows need fairly frequent reboots but there are are 
versions that should run for many days or weeks between reboots.  My 
desktop system is W/XP SP2 and has been up for at least a couple of 
months and may stay up for another couple of months.


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