[ntp:questions] How to measure computer clock error using PPS?

Miroslav Lichvar mlichvar at redhat.com
Wed Sep 3 09:04:48 UTC 2014

(hmm, it took over 3 months for this message to reach the list)

On Tue, May 27, 2014 at 03:03:47PM +0400, Vladislav Ross wrote:
> I have NTP server with Ublox LEA-6T GPS receiver. I want to determine my
> server's oscillator accuracy and stability. I've read about Allan deviation 
> and learnt how to make ADEV plot, but I don't fully understand how to
> use this method.
> My question is: what method should I use to determine server clock
> accuracy and stability using PPS as reference? Can I use ntpd to collect
> data about clock error? As far as I understand ntpd will adjust the
> clock, but I need freely running clock.

You could use the "disable ntp" directive in ntp.conf to disable the
clock discipline, but I'm not sure if ntpd will keep logging PPS
offsets as the clock will be slowly drifting away.

If you are on Linux, you might find the following tool useful:


It can be used like this:

ppsallan -p adev.plot /sys/class/pps/pps0/assert

This will collect raw PPS timestamps from the file in /sys and while
it's running, a rough graph of Allan deviation will be shown in the
console. On exit it will save the x, y coordinates of the plot to the
file specified by -p and the plotallan script can be used to create a
nice graph with gnuplot.

Miroslav Lichvar

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