[ntp:questions] NTP, GPSD & PPS

Miroslav Lichvar mlichvar at redhat.com
Wed Dec 10 10:24:56 UTC 2014

On Tue, Dec 09, 2014 at 03:29:45PM +0100, Sander Smeenk wrote:
> I run a stratum 1 server which has a Garmin LVC 18x connected to its ttyS0.
> The GPS provides a PPS signal via serial and i use gpsd to provide the
> NMEA sentences and pulse data in shared memory to NTP.
> This partly works. NTP syncs against the PPS signal but the NMEA signal
> is always marked as falseticker even though i managed to bring down the
> offset to -1.5ųsec average by fudging the time a bit. The NMEA signal
> offset fluctuates a lot. From ~ -65ųsec to ~ +75ųsec.

> 1) Can i get a 'true PPS sync' with this setup?
> Eliminating gpsd so 'ntpq -p' shows 'oSHM(1)' instead of '*SHM(1)' ?

If it's a recent version of gpsd which uses kernel timestamps (check
for KPPS messages in "gpsd -D 4" output), you may already have a "true
PPS sync". The PPS and NMEA timestamps are paired in gpsd, so it's not
necessary to add the NMEA source to ntpd. To avoid problems with the
falseticker, you can remove the source from ntpd configuration or use
the noselect option to never use it and only monitor it.

Miroslav Lichvar

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