[ntp:questions] How to debug GPS PPS?

David Lord snews at lordynet.org
Fri Feb 12 21:17:38 UTC 2010


xyz-2041 wrote:
> OK.  Couldn't get the Oncore unit to work in any way
> shape or form.  I bought a DB9 LED tester like this:
> 
>   <http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=260546635257>
> 
> Had PPS going to pin 1, DATA OUT to pin 2, and DATA IN to pin 3.
> Still
> no good.
> 
> Bought a Garmin GPS 25-LVC, put it in an enclosure, commented
> out the Oncore driver and added to ntp.conf:
> 
>   # Garmin / PPS driver:
>   server 127.127.20.0   prefer
>   fudge  127.127.20.0   refid GPS
> 
> Restarted:
> 
>   /etc/rc.d/ntpd restart
> 
> Checking the status:
> 
>   # ntpq -c pe
> 
>        remote           refid      st t when poll reach   delay
> offset  jitter
>   ==========================================================
>   xGPS_NMEA(0)     .GPS.            0 l   15   64  377    0.000
> -716.52 105.934
> 
> 
> Looks like it is being read, but is off by quite a bit.  Any ideas on
> how
> to get it to behave?

For my gps18x-LVC I have following config:

tos orphan 10
tos mindist 0.4 # I could reduce this now
server 127.127.20.0 mode 1 prefer
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 serv1
server serv2
server serv3

ntpq -p me6000
    remote     refid poll reach  offset jitter
+GPS_NMEA(0) .GPSb.   64   377 -12.625 21.361
oPPS(0)      .PPSb.   16   377   0.002  0.002
serv1                 64  377    0.039  0.660
serv2                 64  377   -0.532  0.513
serv3                 64  377   -2.486  0.639

Note the GPS_NMEA is all over place. I only just changed to
above config (looks to be giving lower PPS offset/jitter) and
restarted ntpd but yesterday with almost same config
peer_summary has:

        ident  cnt  mean    rms    max
127.127.22.0 5238 0.003  0.031  0.066
127.127.20.0 1339 8.017 21.307 92.970

David





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