[ntp:questions] Prefer Peer Plays Ping-Pong

John Ackermann N8UR jra at febo.com
Sun Sep 10 19:34:26 UTC 2006


Sorry, couldn't resist the subject line...

I am using a HP Z3801A GPS-disciplined oscillator with both the hpgps
and atom drivers in order to use the PPS signal as well as the serial
data stream.

I'm finding that the system keeps switching sync sources, like so:

10 Sep 15:11:41 ntpd[42461]: synchronized to PPS(0), stratum 0
10 Sep 15:13:51 ntpd[42461]: synchronized to GPS_HP(0), stratum 0
10 Sep 15:14:54 ntpd[42461]: synchronized to PPS(0), stratum 0
10 Sep 15:15:58 ntpd[42461]: synchronized to GPS_HP(0), stratum 0
10 Sep 15:17:03 ntpd[42461]: synchronized to PPS(0), stratum 0
10 Sep 15:19:10 ntpd[42461]: synchronized to GPS_HP(0), stratum 0

I suspect this may be due to the (relatively) much greater jitter on the
serial data stream.  Here's the current billboard from ntpq -p:

$ ntpq -p
remote      refid          st t when poll reach   delay   offset  jitter
==============================================================================
+GPS_HP(0) .GPS.            0 l   11   64  377    0.000   -0.620   3.171
oPPS(0)    .PPSZ.           0 l    9   64  255    0.000    0.001   0.004


Here is my current config file (with logging and other irrelevant stuff
pruned out):

$ cat /etc/ntp.conf
# /etc/ntp.conf for tock, using Z3801A and PPS

enable pps

# Z3801A
# note:  "ttl 1" sets comm params for Z3801A
server 127.127.26.0 ttl 1 prefer
fudge 127.127.26.0 time1 -0.960
fudge 127.127.26.0 refid GPS

# PPS
server 127.127.22.0
fudge 127.127.22.0 refid PPSZ
fudge 127.127.22.0 flag3 1
fudge 127.127.22.0 true

Any suggestions on how I can force ntpd to lock to the PPS source and
avoid this ping-pong effect?  Any other suggestions on improving the
configuration in general?  (By the way -- this is on FreeBSD 5.4 and ntp
4.2.0a).

Thanks,

John



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