[ntp:questions] Single GPS/PPS time source gets marked as a falseticker

Michael Eder meder at whoi.edu
Tue Jul 12 12:01:15 UTC 2011

Hi Folks,

For our buoy deployments we have GPS and a highly accurate oscillator
conditioned by the GPS PPS.  Unfortunately when we let this run for longer
periods we see both the GPS and PPS being marked as false tickers.  There is
a lot of information on the web about this, but I have not found a
definitive solution.  What is the best way to configure NTP when you only
have a GPS NMEA string and pulse per second?   In our situation it is
feasible that the NMEA may not be always be available while the PPS is, we
are seeing a problem when both the NMEA and pulse seem to be working

I have also seen some discussion about issues regarding what needs to happen
when NTP first comes up in this scenario.  Is it a problem to have the PPS
always available even before the NMEA string can fully set the time?

I did try setting minclock and minsane, to two and one respectively, but
this did not seem to change the situation.

Any suggestions on how to configure for this situation and/or things I
should be aware of would be much appreciated.  

Thanks in advance,

Kind Regards,

Michael Eder

Added this to the cfg file
#Look for two clocks instead of three for falsetickers - tos minclock 2
minsane 1 but still get the following.

ce000:/etc# ntpq -p
     remote           refid      st t when poll reach   delay   offset
xSHM(0)          .NMEA.           0 l   12   16  377    0.000  -407.31
xSHM(1)          .PPS.            0 l    8   16  377    0.000  -256.43

Seems like one clock (pps) does better than pps and gps.

ce000:/etc# ntpq -p
     remote           refid      st t when poll reach   delay   offset
*SHM(1)          .PPS.            0 l   12   16  377    0.000   -6.243

Michael Eder
Woods Hole Oceanographic Institution

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