[ntp:questions] PPS indication in ntpq

Miguel Gonçalves mail at miguelgoncalves.com
Tue Aug 2 23:02:00 UTC 2011


Hi Dave!

Thanks for your support!

2011/8/2 Dave Hart <hart at ntp.org>:
> Actually both were seeing the PPS signal from the start, or else you
> would not have been seeing offsets in the low microseconds on both.
> ntpd 4.2.4 never uses the 'o' tally code, PPS or not, you will see *
> for the controlling peer.  With 4.2.6, you will see tally code 'o' for
> the controlling PPS, and you can also see '*' at the same time on
> another refclock that is "numbering the seconds" for the PPS, though
> not in your original configuration where a single NMEA driver is both
> numbering the seconds and providing the PPS, for that configuration
> you will see 'o' on NMEA once the PPS is engaged.

Understood.

> You do not need to configure a separate PPS driver, and are probably
> best off to not.  If you do, change the NMEA flag1 to 0 so it will not
> also attempt to process the PPS.

Will use only the NMEA driver then.

> Note the 'o' tally only appears when ntpd is talking directly to the
> PPSAPI interface.  A common configuration on Linux systems puts gpsd
> in the middle, and provides the GPS and PPS information via a shared
> memory driver to ntpd.  In that case, the selected refclock never will
> show 'o' regardless of ntpd version, because ntpd is not using PPSAPI.
>  The gpsd approach has the upside of letting you use the GPS from
> other programs at the same time as ntpd, and also does not require
> PPSAPI support, which mainstream Linux hasn't had until very recently.
>  The downside is the PPS is being timestamped in
> preemptively-scheduled gpsd code, rather than in an interrupt handler,
> so the PPS timestamps are noisier with gpsd compared to direct PPSAPI
> from ntpd.

I am using FreeBSD. This is a pair of stratum 1 servers for a mid-size
LAN. I am considering opening the NTP port to the World for one of the
servers. I am thinking about security issues this might involve.

BTW, I am using a Garmin 18 LVC and a Sure GPS board. The Sure is
great. It is working on indors with exterior blinds completely closed
and it is picking up 8-9 satellites regularly.

I was looking at the Trimble Resolution T to try it out. Can you
suggest a cheap and easy to assemble unit that has a detachable
antenna like the Sure?

Thanks!

Cheers,
Miguel



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