Re: [gpsd-dev] [PATCH] HOWTO: calculate the average GPS offset

[Andy Walls]

Hi Gary,

Days seems like too long.

In my open-loop tests, it took about 4000 seconds (67 minutes) for the relationship between the ntp disciplined system clock and PPS to have a somewhat stable offset.  It took about another 1000 seconds for me to have confidence that it was.
(The plot of the peerstats data lets one see the stabilization.  A plot of 4 hours of data gives me confidence, YMMV. :) )  

In my open-loop tests, it also took about 4000 seconds (67 minutes) for the relationship between the ntp disciplined system clock and GPS serial stream to have a somewhat stable offset.  It then took the balance of 4 hours for me to have confidence that it wasn't going to get any more stable.  The serial stream jitters *alot*.

Looking at the peerstats for the GPSD/Shared Memory clock, and throwing out the initial 6000 seconds, I observed the following about the offset:

Mean: -0.455
Min:    -0.495
Max:   -0.413

So a jitter of about +/- 41 milliseconds in GPSD/Shared Memory time reports after NTP's disciplining of the system clock has a fairly stable offset from the PPS. :(

Regards,
Andy

On Mon, 2013-10-21 at 14:38 -0700, Gary E. Miller wrote:
> Yo Beat!
>
> Looks cool.  But I would only do this after an ntpd server has been running
> for days and is stable.  Mayb just say to repeat this procedure until
> stable?
>
> On Mon, 21 Oct 2013 23:29:21 +0200
> Beat Bolli <address@hidden> wrote:
>
> > ---
> >  www/gpsd-time-service-howto.txt | 30 ++++++++++++++++++++++++++++++
> >  1 file changed, 30 insertions(+)
> > 
> > diff --git a/www/gpsd-time-service-howto.txt
> > b/www/gpsd-time-service-howto.txt index 62aefda..c0592a3 100644
> > --- a/www/gpsd-time-service-howto.txt
> > +++ b/www/gpsd-time-service-howto.txt
> > @@ -459,6 +459,36 @@ If the offset is positive, reduce the time1
> > value and vice versa. A more detailed description of the output is
> > available at <<NTPQ-OUTPUT>>.
> >  
> > +In order to determine the correct GPS offset, do the following:
> > +
> > +1. Add these lines to ntp.conf:
> > +
> > +-----------------------------------------------------------------------------
> > +statsdir /var/log/ntpstats/
> > +statistics peerstats
> > +filegen peerstats file peerstats type day enable
> > +-----------------------------------------------------------------------------
> > +
> > +This enables logging of the peer server statistics.
> > +
> > +2. Start ntpd and let it run for at least four hours.
> > +
> > +3. Calculate the average GPS offset:
> > +
> > +-----------------------------------------------------------------------------
> > +awk '
> > +     /127\.127\.28\.0/ { sum += $5 * 1000; cnt++; }
> > +     END { print sum / cnt; }
> > +' </var/log/ntpstats/peerstats
> > +-----------------------------------------------------------------------------
> > +
> > +This prints the average offset.
> > +
> > +5. Adjust the "time1" value in ntp.conf by subtracting the average
> > +   offset from step 4.
> > +
> > +6. Restart ntpd.
> > +
> >  == Providing public NTP service ==
> >  
> >  <<NTP-FAQ>> has good advice on things to be sure you have done - and
>
>
>
>
> RGDS
> GARY
> ---------------------------------------------------------------------------
> Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97701
> 	address@hidden  Tel:+1(541)382-8588