[ntp:questions] GPS_NEMA, but high offset and jitter?

David J Taylor david-taylor at blueyonder.co.uk.invalid
Mon Mar 1 09:43:41 UTC 2010

"Kelsey Cummings" <kgc at microshaft.org> wrote in message 
news:4b8b4844$0$1956$742ec2ed at news.sonic.net...
> David J Taylor wrote:
> ...
>> I would knock something up if there were enough interest, but it would
>> be a Windows program so of limited interest to the OP.
> That's the idea anyway.  At the time, I was convinced the PPS signal was
> showing clearly and it was but with the led transistor in backwards the
> PPS line was getting pulled down to 1.4v or thereabouts.  Not enough for
> the serial port to register it.  It would have been useful to have
> something that just watched for the DCD pulse rather than trying to
> figure out what nptd was or was not seeing.
> With that fixed, everything synced right up.  Seems reasonably stable
> with jitter at 4ms and offset fluctuating from -4ms to 4ms.  More than
> good enough for our needs.
> -K

In case it's some help, there's a simple serial port LED monitor program 
now available here:


It works on my Windows XP system when run as Administrator - no other 
testing done, but reports are welcome.  NTP must be stopped (and any other 
COM port program) while the tester is used.  The DCD LED flashes white 
when a working GPS is connected.

As Bill comments, I hope your milliseconds are actually microseconds! 
Even with Windows, I see well under 250 microseconds offset (depends on 
temperature, typically less then 50 microseconds) and less than 10 
microseconds jitter (average 3 microseconds) on the XP stratum-1 server.


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