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

Michael Eder meder at whoi.edu
Thu Jul 14 19:15:45 UTC 2011

Not using gpsd, just writing the NMEA time and receive time into SHM (0)
like gpsd does.  The pps does the same to SHM (1).  Effectively the pps code
just increments the second from the NMEA string and writes it to SHM.  We
need certain values from the NMEA string so have not looked into anything
but the ASCII strings.  PPS comes in on a high priority interrupt so it gets
serviced very quickly.  Actually the PPS conditions an oscillator that we
use to keep time in the event we lose the GPS (out in the ocean on a buoy).
The tests I am running are in the lab so we are always getting the NMEA and


-----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: Thursday, July 14, 2011 4:13 AM
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:
> We have looked at our GPS on a scope, the PPS it is dead on and the 
> NMEA (just one sentence) is also reliable with about a 680 ms latency 
> and 10 ms jitter.

Again, are you using gpsd?

If so, you may want to try removing the (huge) 680ms offset inside gpsd
instead of in ntpd.
There is a place in the code where a fixed offset is added to time obtained
using NMEA (because the gpsd author does not want configurable items) and it
cannot be correct for every possible receiver.

Again, it is better to switch from NMEA to the native binary protocol of the

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