[ntp:questions] Why does GPS time diverge from system time?

Charles Elliott elliott.ch at verizon.net
Sat Apr 14 04:21:14 UTC 2012

What you say is largely true.  However, there is no explanation for why the
system time and the GPS time are diverging in clockstats and peerstats.  See
the graphs of peerstats at
The center line is the GPS device.  The line above it, the one that wanders
is a Sure GPS device w/o pps.  The 3 lines that go from quadrant III to
quadrant II are three computers connected to 8 stratum 2 servers via ntpd.
These shold be correct.  Hence the GPS device does appear to wander.  But is
that because the GPS device is not influencing the system clock? 

There is no relationship between the difference between system time and GPS
time shown in the clockstats file and the computed offset in the loopstats
file.  The GPS device may not be influencing the system clock.  The GPS time
and the system time should converge.  Instead the difference between them
goes almost monotonically from 300 ms to 1300 ms, oscillates between 1300
and 300 ms for a minute or so, then returns to 300 ms, only to count up
again.  It repeats this pattern every day.

Charles Elliott

> -----Original Message-----
> From: questions-bounces+elliott.ch=verizon.net at lists.ntp.org
> [mailto:questions-bounces+elliott.ch=verizon.net at lists.ntp.org] On
> Behalf Of unruh
> Sent: Friday, April 13, 2012 11:31 PM
> To: questions at lists.ntp.org
> Subject: Re: [ntp:questions] Why does GPS time diverge from system
> time?
> On 2012-04-14, Chris Albertson <albertson.chris at gmail.com> wrote:
> > On Fri, Apr 13, 2012 at 12:01 PM, Charles Elliott
> <elliott.ch at verizon.net>wrote:
> >
> >> ...   The
> >> average offset since the restart from the offsets computed by NTPD
> >> and output in the loopstats file is -0.054193 (ms), while the stdDev
> >> is 1.507155.  The maximum and minimum offsets are about +5 and -5
> ms,
> >> respectively, while the overwhelming majority of offsets are between
> >> +- 3 ms.
> >>
> >
> > If you are running Windows with a USB connected GPS, I'd say your
> > performance is good.  Better than some others have reported using
> that
> > setup.
> >
> > You said the GPS and system times were monotonically diverging.  I
> > don't see that from the numbers you post.  To prove that you'd need
> to measure
> > system and GPS times every couple hours for several days.   I think
> the
> > difference you are seeing is just the expected noise.
> >
> > Again for Windows and a USB connected GPS +5 and -5 ms is more or
> less
> > what you should expect.
> I would say that for any operating system and using only the nmea
> output of the gps, +- 5ms is doing well. The gps devices do not work
> very hard at making sure that the nmea outputs come at a specific time
> with respect to the "seconds"  turnover. They more of less relegate the
> nmea output to time they are not terribly busy timing sattelites as I
> understand it. If however he is using the PPS from the gps, then 5ms is
> pretty terrible. It should be a factor of 1000 times better than that.
> >
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