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

Chris Albertson albertson.chris at gmail.com
Sat Apr 14 05:07:51 UTC 2012


The NMEA spec says that the NMEA data shall be output within the second to
with it applies.   This means a GPS with no PPS is allowed to have a full
second of error.   Most are much better but a few are even worse.   It
looks like the offset is a saw tooth function.  The GPS looks like it is
periodically resetting the time between the second and when the data is
sent.  th error builds up and is then reset.

NMEA is not a precision time signal, not even close.

On Fri, Apr 13, 2012 at 9:21 PM, Charles Elliott <elliott.ch at verizon.net>wrote:

> 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
>
> https://skydrive.live.com/redir.aspx?cid=92a71a1c4b64fd41&resid=92A71A1C4B64
> FD41!253&parid=root.
> 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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-- 

Chris Albertson
Redondo Beach, California


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