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

[Chris Hastie]

On 09/06/16 19:25, Gary E. Miller wrote:
>> 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've tried something very similar, but to no avail

  # GPSD-NG driver
  server 127.127.46.0 minpoll 4 noselect
  fudge 127.127.46.0 time2 0.625 stratum 5 refid GPSD
  server 127.127.46.128 minpoll 4 maxpoll 4
  fudge 127.127.46.128 time1 0.00013 flag1 1 stratum 0 refid PPS

I see the serial clock OK, but the PPS clock continues to report a reach
of 0. This is what led me to wonder if getting the PPS from GPIO instead
of a serial link was the problem. For things to work at all gpsd has to
be started with the arguments /dev/gps0 /dev/pps0, but I can't see a way
to associate pps0 with gps0. As I mentioned, I can see that gpsd checks
the path attribute of /dev/pps0, but I can see no way to alter that. I
did spot a comment from you at
https://lists.nongnu.org/archive/html/gpsd-users/2015-07/msg00012.html

> If you look at the gpsd INSTALL doc you'll find how to associate the
> /dev/ppsX directly with the serial port to save the extra effort.

but I failed to see anything in INSTALL that looked like it might fit.

>> 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.

Well things were very much improved by adding a minpoll 4 here:

  server 127.127.28.2 prefer minpoll 4

so I think I might stick with SHM for now.

> To keep from going crazy staring at ntpq, you need to install this:
> 
>     https://github.com/ddrown/chrony-graph

I'll take a look at that when I have time. It looks to be written for
chrony—does it work with ntpd too? I have a bunch of graph scripts I've
been using for years for ntpd which I hastily adapted to keep an eye on
this project

http://ntp.oak-wood.co.uk/blackthorn.offsets.php

> 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.

Interesting. I had assumed the diurnal cycle was about the temperature
going up and down in my office.

Thanks for your help

Chris