[ntp:questions] Learnings: FC8 Linux ntpd syncs to NMEA message but not PPS

Rob nomail at example.com
Fri Jan 30 19:45:19 UTC 2009


Phil.Newlon at wendysarbys.com <Phil.Newlon at wendysarbys.com> wrote:
> - If you have NMEA output from the GPS in your ntp.conf file
> (127.127.20.0), expect it to have "reach 0" reported when you finally get
> PPS (127.127.28.0) working.  You can have NMEA or PPS from your serial
> input, you just can't have both at the same time from the same GPS on the
> same port.

That is in fact the reason why NTP support was built into gpsd.

There can be only one process reading the NMEA output, and as gpsd is
already doing that (or puts the GPS in binary mode and reads that output),
ntpd cannot read it.

So, gpsd reads the data and puts it into an SHM segment for ntpd to use.
For convenience, it can also read the PPS signal and put that into another
SHM segment.

When you use this method (gpsd and ntpd), the clock can be synced without
further reference or pre-setting.  Of course, you can always have additional
servers configured in ntpd.




More information about the questions mailing list