Re: [chrony-users] Multi-PPS+RTC Setup Questions

[Matt Corallo <ntp-lists@xxxxxxxxxxxxxxx>]

On 9/29/20 2:57 PM, Bill Unruh wrote:
> On Tue, 29 Sep 2020, Matt Corallo wrote:
>
> > On 9/29/20 2:25 PM, Bill Unruh wrote:
> > > The GPS PPS should be accurate to better than a microsecond. Now with two GPS
> > > lines coming in, there is a fight between the interrupt handlers which means
> > > that the second will be out by up to 10us while the first one handles the
> > > "first" interrupt.  That will of course depend on the hardware as well, and
> > > whether the the system has multiple CPU cores, and whether the interrupts can
> > > get distributed amongst the cores, etc. But certainly sub us accuracy and
> > > precision should be possible with GPS PPS. The NMEA is much much much worse
> > > than this-- there getting 10 ms accuracy can be difficult. So NMEA is one of the worst ways of getting accuracy 
> > > (usually much worse than
> > > internet ntp.) (PPS, network, NMEA, wristwatch is the order of accuracy.)
> >
> > Yep! Sadly the offset doesn't go away when one is set to tick at 5HZ and one at 4HZ, and also not when one is set to 
> > tick 1ms offset from "real time".
>
> Why would you have your GPS ticking at 5Hz? That does not help the accuracy of
> chrony, but may have harm esp in the NMEA timing.

Sorry, for clarity I'm only talking about the PPS line, not the fix calculation, and using it as a counterexample to the 
suggestion that the persistent 30us time difference my be due to interrupts colliding (though it *can* provide more 
datapoints for hiding interrupt noise).

> > > chrony already disciplines RTC, and the /var/log/chrony/rtc.log should show
> > > you what the RTC accuracy is. RTC is a pretty poor way of getting the time, since that clock has
> > > uncontrollable drifts-- not least from temperature variation inside the
> > > machine. So in short order it can be seconds away from UTC.
> >
> > Sorry it wasn't clear - in this case there is a secondary (DS3231) RTC that is being pulled in and disciplined, not 
> > the system RTC (which, indeed, is disciplined by chrony). The DS3231 is then used to prevent the GPS time from getting 
> > far off of what the DS3231 claims is possible.
>
> ?? This makes no sense. Any RTC is going to be far worse than GPS. There is
> simply no way that GPS should be off by enough that and RTC can help in any
> way, while the RTC can certainly be off by enough to make its time way off.
>
> If you are finding that that RTC helps then I would throw away the GPS devices
> and start again.

Err, I'm not suggesting I'm finding that, only that I find it a novelty to have a check against GPS losing 
signal/getting spoofed/etc, which an RTC can provide.

Matt

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