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 PPS.
-----Original Message----- From: questions-bounces+meder=whoi....@lists.ntp.org [mailto:questions-bounces+meder=whoi....@lists.ntp.org] On Behalf Of Rob Sent: Thursday, July 14, 2011 4:13 AM To: questions@lists.ntp.org Subject: Re: [ntp:questions] Single GPS/PPS time source gets marked as a falseticker Michael Eder <me...@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 receiver. _______________________________________________ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions _______________________________________________ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
