[ntp:questions] Realistic Performance Expectation for GPS PPS fed ntpd jitter

Dan Drown dan-ntp at drown.org
Wed Oct 7 22:27:26 UTC 2020


Quoting Andreas Mattheiss <please.post at publicly.invalid>:
> I wonder what's a realistic ballpark for the jitter I can expect when
> feeding a GPS PPS into ntpd?

Assuming Linux, and using pps-ldisc or pps-gpio, I'd expect reported  
jitter to be around 1us

:
> The jitter values I get do, sorry, jitter. I guess it's a lot to do with
> the poor view to the sky, mainly. PPS from GPS is claimed to be accurate
> to say 10's of ns; my jiitter values are around 10 micro s approximately,
> on avarege, for a well settled system set up as described. They do zoom
> up, occasionally, to 60 micro s though. I would assume that it's
> unrealistic to expect ns accuracy from ntpd on a non-designated system.
>
> What do you think, is my 10 micro jitter within experience?

What are your GPS signal levels during these times?  Does your module  
lose lock?  What are your minpoll/maxpoll settings on your refclock?

You can try changing your timesource.  Many Linux systems will default  
to the TSC timesource, which is much quicker to read but is less  
stable in frequency compared to HPET.

The timesource settings are here:

/sys/devices/system/clocksource/clocksource0/available_clocksource  
/sys/devices/system/clocksource/clocksource0/current_clocksource


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