[Pool] High Offset / Jitter

David Lord david at lordynet.org
Tue Mar 1 00:32:32 UTC 2011


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On 28 Feb 2011 at 20:25, Rob Janssen wrote:

> Alby wrote:
> > Rob:
> >
> >   I updated my ntp.conf and restarted ntpd and got the following:
> >
> >
> >
> >
> > godzilla:/etc# ntpq -p
> >       remote           refid      st t when poll reach   delay  
> >       offset
> > jitter
> > ====================================================================
> > ==========
> > +time.nist.gov   .ACTS.           1 u   50   64   17   56.635  
> > +-5.536
> > 6.242
> > +ntp-nasa.arc.na .GPS.            1 u   45   64   17   91.943   
> > +8.023
> > 1.997
> > +tick.usask.ca   .GPS.            1 u   47   64   17   94.198   
> > +4.766
> > 1.470
> > *ntp.alaska.edu  .GPS.            1 u   46   64   17  129.114   
> > 2.305 3.711
> > +timekeeper.isi. .GPS.            1 u   47   64   17   92.737  
> > +-0.813
> > 9.839
> > +ntp2.usno.navy. .USNO.           1 u   43   64   17   70.944  
> > +-4.711
> > 8.011
> > xSHM(0)          .GPS.            0 l    6   16  377    0.000 
> > -872.25 24.157
> >   SHM(1)          .PPS.            0 l    -   16    0    0.000   
> >   0.000
> > 0.001
> >    
> Ok, the offiset of the serial data is too much for gpsd to lock on the
> PPS. This is a problem with the latest firmware of your GPS. You will
> have to hope that the manufacturer fixes it, the previous version was
> OK.


I used "tos mindist 0.4" to overcome the problem with my
18x-LVC and version of ntp I was using on NetBSD.

Without that line ntpd tries to lock to GPS serial data
and then is out of range for PPS so coninuously flips
between the two. I've not tried GPS with a more recent
ntp which is supposed to better handle this situation.

Of course the 18x might be too far out. I had a brief 
look on the Garmin site but could only find the current
firmware. My unit was purchased mid 2009 and has not 
been updated.

It's possible I was using serial or parallel pps at the
time but there was no significant difference when using 
serial DCD.

$ ntpq -p
     remote       refid st t poll reach delay offset jitter
===========================================================
+GPS_NMEA(0) .GPSb.      0 l   64  377  0.000 -7.687 13.108
oPPS(0)      .PPSb.      0 l   16  377  0.000  0.000  0.002
- -k6x400      192.168.??  2 u   64  377  0.619  0.785  0.631
+me6000gi    .MSFa.      1 u   64  377  0.722  0.905  0.519
- -p4x2400c     92.168.??  2 u   64  377  0.774  0.690  0.381

*************************
# /etc/ntp.conf:

tos             mindist 0.4

server 127.127.20.0 mode 1 prefer minpoll 6 maxpoll 8
fudge  127.127.20.0 time1 0.651 refid GPSb

server 127.127.22.0 minpoll 4 maxpoll 6
fudge  127.127.22.0 refid PPSb flag3 1

server k6x400     minpoll 6 maxpoll 8 
server me6000g    minpoll 6 maxpoll 8
server p4x2400c   minpoll 6 maxpoll 8
*************************

David

- -- 
David Lord <david at lordynet.org>
<ftp://ftp.lordynet.org/pub/pgpkeys/lg_david.pkr>
<http://www.lordynet.org/pub/pgpkeys/lg_david.pkr>



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