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

Charles Elliott elliott.ch at verizon.net
Sat Apr 14 12:40:39 UTC 2012


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

	Clockstats.20120413 System Time vs. SysTime - GPSTime.jpg and
Clockstats.20120413 System Time vs. SysTime - GPSTime.png are graphs of
System Time (X) versus (Sys Time - GPS Time) (Y).  You might note that the
difference between System Time and GPS Time does indeed vary between 346 ms
and 1382 ms.  The cycle appears to be twice a day.

	The original data and resulting graph are in "clockstats.xlsm,"
which is in Excel 2007 format.  If you would prefer another format, pls let
me know.

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: Saturday, April 14, 2012 12:53 AM
> To: questions at lists.ntp.org
> Subject: Re: [ntp:questions] Why does GPS time diverge from system
> time?
> 
> On 2012-04-14, 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=92A71A
> > 1C4B64
> > 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.
> 
> On the graphs I see 30ms to 130ms, not 300 to 1300.
> The former I could well expect.
> 
> >
> > 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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> >> questions at lists.ntp.org
> >> http://lists.ntp.org/listinfo/questions
> 
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