[ntp:questions] How do I know my GPS-based NTP server is actually working properly?

David J Taylor david-taylor at blueyonder.not-this-bit.nor-this-bit.co.uk
Sat Nov 3 10:43:22 UTC 2007

John Ioannidis wrote:
> Thanks for the reply, but this is not answering my question. I wasn't
> asking how to *configure* the beast (I've already done this
> successfully), I was asking how to verify that it actually works as
> documented!
> What measurements do I have to take that will show the difference
> between a setup that's actually using the PPS signal from the GPS
> receiver and one that's not (because, for example, the DCD line on the
> motherboard is cracked, to give a stupid example).
> Cheers,
> /ji


There is information on how I monitor here:


The GPS-locked accuracy is some three orders or magnitude better than 
Internet only.  Compare the top graph (+/- 20 microseconds) and the lower 
graphs (+/- 100 milliseconds).  In my particular case, with a limited view 
of the sky from quite far north (56 degrees), The drift which starts when 
there are too few satellites to allow the GPS-18 to declare "lock" is very 
obvious.  You could simulate that by disconnecting the GPS, of course.  My 
simple notes are here:


Search for the word "pixie" to see the effect of PPS_SYNC, with extra 
lines in the "ntpdc -c kern" output.


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