Clock drift. Not strictly a Linux question.

[Net Llama!]

On Fri, 16 Feb 2007, Roger Oberholtzer wrote:
>
> It's Friday. Time for a weekend-think-about-it question.
>
> I think I am doing my maths correct. I am checking a PC's clock against a
> high-end Trimble receiver (> $2000), using the pulse per second signal.
> Unless I am doing something wrong, I seem to see a 0.02 % linear (over the
> time I have looked) drift in the PC's clock (via gettimeofday()) compared to
> the pulse from the GPS. Doesn't that seem a bit large? Here is a bit of the
> sample data. There is one line for each pulse detected from the GPS.
>
> pulse   1: 07-047-12:51:23.741
> pulse   2: 07-047-12:51:24.741
> pulse   3: 07-047-12:51:25.741
> pulse   4: 07-047-12:51:26.741
> pulse   5: 07-047-12:51:27.741
> pulse   6: 07-047-12:51:28.740
> pulse   7: 07-047-12:51:29.740
> pulse   8: 07-047-12:51:30.740
> pulse   9: 07-047-12:51:31.740
> pulse  10: 07-047-12:51:32.740
> pulse  11: 07-047-12:51:33.740
> pulse  12: 07-047-12:51:34.739
> pulse  13: 07-047-12:51:35.739
> pulse  14: 07-047-12:51:36.739
> pulse  15: 07-047-12:51:37.739
> pulse  16: 07-047-12:51:38.739
> pulse  17: 07-047-12:51:39.739
> pulse  18: 07-047-12:51:40.738
> pulse  19: 07-047-12:51:41.738
> pulse  20: 07-047-12:51:42.738
> pulse  21: 07-047-12:51:43.738
> pulse  22: 07-047-12:51:44.738
> pulse  23: 07-047-12:51:45.738
>
> Every 5 seconds, the time is off by another millisecond. The PC clock seems to
> be running slower than the GPS.
>
> The PPS signal is detected via an ioctl() that monitors changes in serial
> lines. The GPS only gives this signal when it has access to multiple
> satelites. So, it is provides the signal, it is good. If there is inadequate
> coverage, it will not send the signal. I think the operator said there were
> 11 satelites at the time this was done.
>
> I know the real system clock update is at 100 Hz (a 2.6 kernel). Maybe that is
> involved here. I am not using ntp or any of that, so there should be no
> active code doing any funny clock time changes.
>
> Any thoughts?

The motherboard sucks?

-- 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Lonni J Friedman                        netllama at linux-sxs.org
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