[LinuxPPS] High values of jitter/estimated error.

[Stefano Lucetti]

Hello all, first post on the list, googled a bit but not found solution to my problem.
Sorry for the length, but I try to add all the relevant info.
I used to run an ancient version of ntpd on a PPSKit-patched 2.4.(3?) system, having the PPS pulse fed by a Motorola Oncore GPS UT Evaluation kit. That system is not available any more, and looking for documentation on how to set up a new NTP server I stumbled upon the LinuxPPS page.

So, I have my brand new NTP server up and syncing. Kernel is 2.6.20 with LinuxPPS 3.0 (compiled with the hints from http://ml.enneenne.com/pipermail/linuxpps/2007-January/000544.html - so "Low latency" desktop and 1000Hz clock), ntpd is compiled from source (ntp-4.2.4p0, using configuration as indicated in the wiki for Oncore drivers), using the following configuration files:

###/etc/ntp.conf ###############

driftfile /var/log/ntp/ntp.drift
logfile /var/log/ntp/xntpd

statsdir /var/log/ntp/
statistics loopstats peerstats clockstats
filegen loopstats file loopstats type day enable
filegen peerstats file peerstats type day enable
filegen clockstats file clockstats type day enable

enable pps 
#enable kernel

pps /dev/oncore.pps.0 hardpps

server 127.127.30.0 prefer
fudge 127.127.30.0 refid GPS

#########################

### /etc/ntp.oncore ############

MODE 4
ASSERT
STATUS /var/log/ntp/ONCORE

##########################

pps is enabled on serial port using ppsctl (as in the wiki), and the serial is flagged as low_latency using setserial.
After initial 20-25 minutes for settling, the ntp server starts syncing, but after 2 to 3 hours the jitter keeps on rather high values (about 250 usec), when I remember that I had down to 10-20 usec with PPSkit. Hardware is unchanged (Compaq nx9005 laptop, mobile Athlon XP2600+, 512MB Ram, PPS through integrated serial port /dev/ttyS0 using a signal level adapter based on the MAX232).
Output of ntptime and ntpq -p:

nx9005:~# ntpq -p
     remote           refid      st t when poll reach   delay   offset  jitter
========================================================
*GPS_ONCORE(0)   .GPS.            0 l    9   16  377    0.000   -0.290   0.146

nx9005:~# ntptime
ntp_gettime() returns code 0 (OK)
  time c9e05d13.da0bd000  Mon, Apr 30 2007 14:18:27.851, (.851743),
  maximum error 5574 us, estimated error 190 us
ntp_adjtime() returns code 0 (OK)
  modes 0x0 (),
  offset -107.000 us, frequency 38.965 ppm, interval 1 s,
  maximum error 5574 us, estimated error 190 us,
  status 0x1 (PLL),
  time constant 4, precision 1.000 us, tolerance 512 ppm,

Can someone advise me on how to reduce the jitter? I know my conf files are not optimal, but apart from adding the LAT/LONG I don't know how to tune it further. Any help will be greatly appreciated.

Ciao
Stefano