[ntp:questions] high jitter on serial gps causes big time offsets

Brian Utterback brian.utterback at oracle.com
Tue Apr 9 14:40:00 UTC 2013

On 04/08/13 07:50, Nickolay Orekhov wrote:
> No I'm not a gps developer.
> Imagine that I've got very precise time according to GPS+PPS for a long
> period.
> After that GPS and PPS quality goes low and than quality of GPS serial high
> jitter clock will appear before low jitter PPS.
> Before selecting PPS, ntpd will do some adjustments according to GPS. And
> my highly disciplined time will be spoiled.
> When ntpd will switch to PPS my time will be disciplined again but there
> will be noticable period of time when GPS will spoil my local clock
> discipline and maybe frequency.
> That is the problem. I don't want ntpd to do small adjustments according to
> GPS serial that will spoil local clock. I just want to do steps according
> to GPS and do adjustments according to PPS.

Isn't the GPS providing the PPS as well? Are you saying that the PPS 
pulse is stopped being sent while the GPS sentences continue, without 
there being an indication in sentence that the time is invalid? There is 
no such thing of a poor quality PPS as far as NTP is concerned, the 
pulse is either there or it is not. Usually if you are using PPS ntpd is 
not even responsible for applying the offset corrections determined from 
the PPS.


Always code as if the guy who ends up maintaining your code will be a
violent psychopath who knows where you live. - Martin Golding
Brian Utterback - Solaris RPE, Oracle Corporation.
Ph:603-262-3916, Em:brian.utterback at oracle.com

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