[ntp:questions] Running GPS under NTP

Unruh unruh-spam at physics.ubc.ca
Thu Dec 11 06:45:41 UTC 2008


bruce.kling at exfo.com (Bruce Kling) writes:

>Hello,

> 

>I am currently running GPS as a separate process, independent of NTP and
>I am able to achieve microsecond accuracy   If I run GPS under NTP as a
>reference clock driver will this impact on the accuracy of time in any
>way?  Thank you.


Not sure what you mean. GPS is, in this context a hardware timing process.
What do you mean "running GPS as a separate process"? You have a program
which reads GPS, or which interrupts whent eh GPS PPS hits. How do you know
you have "microsecond accuracy" and what has microsecond accuracy? And what
does "impact the accuracy" mean?

The GPS PPS can be used to discipline the local lock on a computer using
ntp to a few microseconds standard deviation. I have a computer which does
that. You can see the results on
www.theory.physics.ubc.ca/chrony/chrony.html and look at string, which is a
computer running ntp driven by the GPS PPS signal. As you can see the
offsets tend to be around 3 usec. (mainly due to clock reading jitter, but
also due to the reaction of ntp to thermal fluctuations of the computer.)

(Mind you I have written a separate interrupt routine to read the clock on
the parallel port interrupt, feeding the info to ntp via the shm refclock,
so whether or not the various possible other refclocks do as well or better
or worse I have no idea.
)
 




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