[time-nuts] verifying synchronization with PPS

[Graham / KE9H]

Several comments:

What level of accuracy do you mean by "synchronized"?

Plotting a lightly loaded Linux box, which is extracting time from the
network via timesyncd, against a GPS 1PPS signal, I observe typical time
excursions within +/- 10 ms, and extreme excursions over a 24 hour period
of up to 20 ms.

So, I would not count on network based time on a Linux box to be better
than +/- 20 ms, probably worse with heavy loading of the OS.

A simple navigation GPS receiver PPS output, without any additional
corrections can provide accuracy of less than 0.1 microsecond.

The owner of this list has designed an excellent dual input timing device
called the TICC, which can compare two PPS signals with resolution of 8 or
9 decimal places, for projects like this.

https://tapr.org/kits_ticc.html

--- Graham

==

On Mon, Jul 8, 2019 at 9:03 AM Steve Summit <scs at eskimo.com> wrote:

> This is a different sort of question than what seems to be
> discussed here usually, and I apologize if it's wholly off-topic,
> but I'm guessing there are some time nuts here who might be able
> to give me some pointers.
>
> If I have two boxes with clocks that are supposed to be perfectly
> synchronized, and I need to verify this, and they're physically
> right next to each other, and they both have PPS outputs, one
> way to verify the synchronization would be to compare the two PPS
> outputs with a 'scope.  (I'm guessing there's probably a dedicated
> instrument out there specialized for the task of comparing two
> such PPS signals for frequency, phase and jitter, and if I were
> a proper time nut myself I guess I'd know that.)
>
> My specific question concerns the case that one of the boxes is
> a computer, for example, a Linux box with time kept using ntpd
> or chrony (perhaps also listening to a PPS signal coming in on a
> serial port).  But I've never seen a computer with a PPS *output*.
>
> Is this a reasonable thing to be thinking about, or am I going
> about it wrong?  How would *you* positively verify synchronization
> of such a system?  Me, I'm pursuing this idea because too much
> of the time (at least in my own, perhaps parochial experience)
> synchronization seems to be "verified" either by saying "Yes,
> we configured it properly", or by observing a status output
> from ntpd or chrony claiming the requisite synchronization
> has been achieved, but in neither case by making a definitive,
> independent, external, empirical determination.
>
> (Perhaps PPS is overkill for this situation; perhaps it's the
> case that given the inherent inaccuracies of software-based
> timekeeping, a software-based query mechanism -- perhaps using
> ordinary NTP or PTP protocols -- would be sufficient for
> externally assessing synchronization.)
>
> I guess it's not a *completely* unreasonable thing to be thinking
> about, because I've found a couple of web pages (for example
> https://www.raspberrypi.org/forums/viewtopic.php?t=115554 and
> https://github.com/jsln/pps-gen-gpio) describing how to implement
> a PPS output under Linux.  And I do realize that trying to
> minimize the jitter and latency in this situation (given that
> the principal drivers for the hypothetical output are inherently
> software-based) presents considerable difficulties.  But taking
> all of that into consideration, I'm wondering what others think
> of the approach.  Thanks for any comments.
>
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