[ntp:questions] NTP, GPSD & PPS

Nomen Nescio nobody at dizum.com
Tue Dec 9 20:17:34 UTC 2014

William Unruh <unruh at invalid.ca> wrote:

> On 2014-12-09, Sander Smeenk <ssmeenk at freshdot.net> wrote:
> > Hi,
> >
> > 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 -1.5??sec average by fudging the time a bit. The NMEA signal
> > offset fluctuates a lot. From ~ -65??sec to ~ +75??sec.
> Who cares? If the system is locking on to the PPS that is far far
> greater accuracy than you will ever get from nmea. Over 10000 times
> better. Not sure what your preceived problem is. The nmea is only good
> for getting the seconds time. Not more accurate time. You do not want it
> involved in time setting if the PPS is working. 


The PPS pulses and NMEA sentences need to be "close enough" before
ntpd will accept them. Usually that means the the NMEA sentence needs to
arrive during the bounds of the second it is naming.

It is not uncommon to have to adjust the offset to fix this problem.

> > The GPS provides 9600bps serial comms. Would it help to speed this up to
> > 19200bps? I've already disabled all NMEA sentence output for sentences that
> > "aren't useful for timekeeping" but at this moment i have to use external
> > clocks to sync against.
> I presume that the unreadable (to me) symbol was milli not micro
> seconds. I do not believe that one could get the nmea down to
> microseconds. As I understand it the gps device sends out the nmea when
> it has time. Ie, much of the fluctuation is internal rather then
> transmission, although of course transmission is variable as well
> (consiidering that the length of the sentences fluctuate). 
> >
> > Few questions:
> >
> > 1) Can i get a 'true PPS sync' with this setup?
> > Eliminating gpsd so 'ntpq -p' shows 'oSHM(1)' instead of '*SHM(1)' ?
> What is untrue about the gpsd pps?
> >
> > 2) What could i possibly do to get NTP to sync/accept the NMEA data,
> > other than set it as a truechimer in the configuration?
> nmea will always have very bad fluctuations. You want to use the PPS. 

That is what he is trying to do.

You can't use the PPS without something to name the seconds. This can
either be the NMEA sentences provided by the ATOM driver. Or it can be
some other time server.

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