[ntp:questions] NTP, GPSD & PPS

Sander Smeenk ssmeenk at freshdot.net
Tue Dec 9 19:30:20 UTC 2014

Quoting William Unruh (unruh at invalid.ca):

> > 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.

I don't want to rely on only external timesources to get a correct time
sync. That's why i care (a little). I would be happier if i had a couple
of NTP-sources *and* the NMEA signal, i've seen it done with serial
attached GPS devices and i'm looking for tips to get it done myself.

> > 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?

It is 'untrue' because it is proxied by a piece of code while the kernel
has PPS support itself. That can only introduce more 'noise' in the signal.
This is what i've learnt on the internet, at least.

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