[time-nuts] looking for low-power system for gps ntp timekeeping NANOSG20

folkert folkert at vanheusden.com
Wed Jul 3 16:14:59 EDT 2013


> folkert at vanheusden.com said:
>      remote           refid      st t when poll reach   delay   offset  jitter
> ==============================================================================
> x127.127.28.0    .NMEA.           0 l    3   16  377    0.000  -994.05   7.857
> x127.127.28.1    .PPS.            0 l    7   16  377    0.000  -250.13 572.812
> +192.168.64.18   .PPS.            1 u   66  128  377    0.553   -0.055   0.033
> *192.168.64.2    .PPS.            1 u   99  128  377    0.237   -0.062   0.039
> +192.168.64.168  .PPS.            1 u   44  128  377    0.646   -0.013   0.321
> 
> Something is broken.  What NMEA device are you using?  Why are you using gpsd 

It is a garmin 18x lvc.

> rather than ntpd's NMEA driver?

Oh for convenience. I need to patch ntpd to use linux pps (afaik) and on
other systems I successfully run gpsd with ntpd (read: low jitter).

> It looks like the NMEA side it is off by a second.  Some devices do that.  
> You can fix that with some fudging.

Yes, I'm not so worried about the nmea part, I might even decide to
remove it from the configuration. What I mean is: if I set the clock to
the current time, than it is only a matter of keeping it at that with
the pps.

> The PPS stuff is off by 250 ms.  Are you using the wrong edge?  (Got a scope 
> handy?)

Well, the offset is not constant. If you look at
http://keetweej.vanheusden.com/~folkert/nanosg20.png you see it is
between 0 and -1ms. I now notice that it is not random-ish but consists
of a few specific values:

folkert at belle:~$ cat peerstats | grep 127.127.28.1 | awk '{ print $5; }' | cut -c 1-6 | sort | uniq -c | sort -rn
    503 -0.874
    497 -0.875
    486 -0.999
    467 -1.000
    365 -0.249
    332 -0.250
    201 -0.857
    188 -0.125
    184 -0.142
    140 -0.124
    131 -0.000
    113 0.0000
    108 -0.166
     24 -0.833
      5 -0.200
      2 -0.856
      2 -0.799
      2 -0.199
      1 -0.167
      1 -0.143
      1 0.0002
      1 0.0001

This is with 3754 lines.

If I clean it a little:

1000x -0.8745
953x  -0.9995
697x  -0.2495
328x  -0.1245

and so on.


Folkert van Heusden

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