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

unruh unruh at invalid.ca
Sat Apr 14 03:31:15 UTC 2012

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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