Re: [gpsd-users] Raspberry Pi, GPIO PPS and NTP

[Gary E. Miller]

Yo Chris!

On Thu, 9 Jun 2016 08:34:34 +0100
Chris Hastie <address@hidden> wrote:

> On 09/06/16 07:50, Gary E. Miller wrote:
> >> I would like to be able to use ntp's gpsd-ng json driver (type
> >> 46).  
> > Uh, don't do it.  The only way to make it work properly is
> > undocumented.
> >
> > Wait until it gets properly fixed.  The SHM drivers work better, and
> > work as documented.  
> 
> Intrigued now :)

Then read the comments at the top of ntpd/refclock_gpsdjson.c

And ponder this config fragment, it is the only gpsd json config that
works stably:

    # gpsd JSON
    server 127.127.46.128 minpoll 4 maxpoll 4
    fudge 127.127.46.128  time1 0.001500
    server 127.127.46.0 minpoll 4 maxpoll 4
    fudge 127.127.46.0 time1 0.142

> I thought I'd try various configurations and see which works best.

Welcome to the Monkey House.

> I've had the SHM driver going for about 9 hours now

ntpd often goes crazy when you restart it.  Sometimes scary crazy.  Do
not look at it for at least 24 hours after a restart.  Maybe 48.

I have seen jitter decrease by a facto of 1,000 over 48 hours.

To keep from going crazy staring at ntpq, you need to install this:

    https://github.com/ddrown/chrony-graph

It will graph things for you, and compute a lot of good stuff for you,
like 95% and 99% ranges.

When I look at my graphs I can see each and every ntpd restart, not good.

WHat I also see is a strong 24 hour cycle in the data.  This is due to
the GPS birds returning to the exact same geometry every sidereal day!
So make no judgements on less than several days of combined data.

RGDS
GARY
---------------------------------------------------------------------------
Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
        address@hidden  Tel:+1 541 382 8588

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