PPS and selection algorithm issues

[Eric Towers]

Is prefer NMEA required or is a high quality source of interval labels
required?  (PPS is a great metronome, but the intervals have no labels
-- gotta get the correct labels from somewhere.)
--
    -- Eric Towers ! fuzzyeric at gmail.com

On Mon, Sep 23, 2024 at 9:41 AM Jim Pennino via users <users at ntpsec.org> wrote:
>
> On Sun, Sep 22, 2024 at 01:29:26PM -0700, Hal Murray wrote:
> >
> > > refclock nmea flag1 0 baud 9600 minpoll 4 time2 0.097
> > > refclock pps minpoll 4 flag2 0 time1 0.002
> >
> > Try adding a prefer to the nmea line.  THe PPS driver needs some place to
> > get the seconds.  The prefer keyword is a hack to do that.
>
> I tried this and it worked. All the numbers are back to being under 2
> microseconds where they were with legacy ntp.
>
> *NMEA(0)         .GPS.            0 l    7   16  377   0.0000  -0.8929   0.9211
> oPPS(0)          .PPS.            0 l    6   16  377   0.0000   0.0076   0.0017
>
> I also tried going back to having 3 sources without the prefer on the
> nmea line as it was with legacy ntp and got this:
>
> *NMEA(0)         .GPS.            0 l   10   16  377   0.0000   0.3839   0.8317
> xPPS(0)          .PPS.            0 l    9   16  377   0.0000   1.7825   0.9609
> +192.168.0.100   .PPS.            1 u   46   64  377   0.1330   0.9937   0.7232
> +192.168.0.101   .PPS.            1 u   51   64  377   0.1334   0.9648   0.6534
>
> So it appears that prefer is absolutely required in the nmea configuration
> line for the pps driver to work.
>
> This is a big change change from legacy ntp and really needs to be
> documented.
>
> If the Linux distros all deprecate legacy ntp and switch to ntpsec,
> every server on the planet with a nmea/pps reference clock is going to
> break.
>
> I would suggest the following:
>
> In the refclock command documentation, the fact that the prefer option
> is required for nmea/pps to work needs to be documented.
>
> It probably wouldn't hurt to repeat this in the nmea and pps driver
> documentation.
>
> The documentation home page needs an entry for transitioning from legacy
> ntp to ntpsec. The need for prefer needs to be here as well and some
> examples of going from psuedo ip addresses to the named driver method.
>
> I also noticed that nowhere in the documentation is the need for
> setserial and ldattch documented in setting up a stratum 1 server.
>
> Thanks for the help.
>
>
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