[ntp:questions] Why does GPS time diverge from system time?
unruh at invalid.ca
Sat Apr 14 04:52:50 UTC 2012
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
> 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
>> 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
>> >> 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
>> > 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
>> > difference you are seeing is just the expected noise.
>> > Again for Windows and a USB connected GPS +5 and -5 ms is more or
>> > 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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