[ntp:questions] Re: Prefer Peer Plays Ping-Pong

user at domain.invalid user at domain.invalid
Fri Sep 15 20:21:18 UTC 2006


John,

This problem has come up many times. The clock selection algorithm 
intersects the correctness interval to cleave the truechimers from the 
falsetickers. Problem is, the two correctness intervals sometimes do not 
overlap due to jitter. See the tos mindist command to set the minimum 
distance to something larger than 5 ms, say 10 ms.

Dave

John Ackermann N8UR wrote:
> 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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