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

Rob nomail at example.com
Tue Jul 12 17:25:09 UTC 2011

Michael Eder <meder at whoi.edu> wrote:
> 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
> properly.

Are you using gpsd?
If so, please use the binary protocol for the GPS receiver in use
instead of NMEA.  NMEA often has problems with the timestamps.
(in fact, NMEA says the timestamps are time-of-fix which does not
have to be the current time but can be some time in the near past)

I see you already have been tweaking the NMEA time to compensate for
problems.  This does not work well with gpsd.  When you have a systematic
time offset for NMEA you should try to solve it either by stopping
the use of NMEA or by fixing the problem in the receiver with a
firmware update.

E.g. there is a recent Garmin receiver that has unusable NMEA time.

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