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

Michael Eder meder at whoi.edu
Tue Jul 12 19:06:13 UTC 2011

We are using a Garmin GPS 16x.  I should clarify these are platforms in the
middle of the ocean so the sole time source is the GPS and PPS.
We have measured the time delta between the GPS and the PPS and the GPS
while delayed to process the NMEA string has a relatively low jitter and is
quite reliable.

-----Original Message-----
From: questions-bounces+meder=whoi.edu at lists.ntp.org
[mailto:questions-bounces+meder=whoi.edu at lists.ntp.org] On Behalf Of Rob
Sent: Tuesday, July 12, 2011 1:25 PM
To: questions at lists.ntp.org
Subject: Re: [ntp:questions] Single GPS/PPS time source gets marked as a

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.

questions mailing list
questions at lists.ntp.org

More information about the questions mailing list