[ntp:questions] ntpd + GPS + PPS + Linux (Ubuntu Lucid)

Rob nomail at example.com
Wed Jun 1 16:35:48 UTC 2011

Stephan Skrodzki <skrodzki at stevekist.de> wrote:
> Hi there,
> I am trying since some time to attach a SkyTraq Based GPS receiver to a
> serial port + pps which "basically" works fine by using gpsd 2.92 and
> ntpd 4.2.4p8.
> Still I get "time resets" every some hours which I do not really
> understand: i get less time resets (+1 sec ... -1 sec) when I configure
> only pps via shm. Configuring gps + pps via shm even gets worse...
> Ok, not to bother you with support requests on old version, I will go
> for the actual release and therefore I have a few questions:
> a) can I use PPS there without the PPSAPI? I do not really want to build
> a new kernel... having a short look it seems that there is no PPS
> support for the SHM interface (anymore?)?

gpsd plus ntpd does not require any PPS support in the kernel.
there is no specfic PPS support via SHM but the gpsd just provides a
pretty accurate clock to ntpd.  When your system is not overloaded
(CPU loading always around or over 1.00) there should be no problem
with sync

> b) could somebody hint me to a working ntp.conf for nmea gps + pps and
> no external ntp servers?

# SHM clock, requires running gpsd
server minpoll 4
fudge refid GPS

server minpoll 4 prefer
fudge refid PPS

> c) I did convince somebody at SkyTraq to provide me also a special NMEA
> sentence for discovering leap seconds, AFAIS I have to modifiy
> refclock_nmea.c in the same way as done for the GARMIN devices...

no experience with that.

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