[ntp:questions] very confusing results though using pps and nmea

Nicola Berndt nb at komeda-berlin.de
Mon Aug 11 15:18:06 UTC 2008


Hello!

Some might recall I asked some questions here and disappeared before 
getting my hands dirty. - Sorry for that, I had to go to hospital for a 
while and didn't touch the ntp-server project since. So now I'm back and 
everything is fine!

First of all I am trying to understand what is actually happening inside 
the computer, time-wise.

So I set up a nmea-driver using a u-blox usb-gps-device with the 
standard nmea driver and I set up a pps-device via the serial port using 
the atom driver and the linuxpps kernel-patch found in the enneenne wiki 
here: http://wiki.enneenne.com/index.php/LinuxPPS_support

Now using the following ntp.conf I can see that things are working, but 
not as expected. I would expect the pps clock to take over, no matter 
what happens, since I understand it is either there or not and it can't 
be wrong. Instead my system randomly marks either of these a false 
ticker every now and then..

Here the ntp.conf:
----------------------------------------------------
# PPS
server 127.127.22.0 minpoll 4 maxpoll 4
fudge 127.127.22.0 flag2 0

# NMEA GPS driver for u-blox receiver
server 127.127.20.0 prefer minpoll 4
fudge 127.127.20.0 flag3 0 flag2 0 time1 0.0

server 0.debian.pool.ntp.org iburst
server 1.debian.pool.ntp.org iburst
----------------------------------------------------


The output of ntpq -p changes offset and jitter values in large ranges 
for every source, also something that I can't believe to be true..


      remote           refid      st t when poll reach   delay   offset 
  jitter
==============================================================================
xPPS(0)          .PPS.            0 l    6   16   77    0.000  258.904 
30.207
*GPS_NMEA(0)     .GPS.            0 l    7   16  377    0.000  -263.23 
30.743
xnetzwerkteufel. 192.53.103.108   2 u    7   64  177   23.019  -159.57 
99.817
xceres.sternbaue 131.188.3.222    2 u   59   64   77   48.554  -133.36 
85.637

What am I doing so badly wrong? Can anyone help?

Best regards,
../nico berndt



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