NTP Microserver How-To

[Gary E. Miller]

Yo Mathew!

On Sun, 8 Jan 2017 20:04:25 -0500
Mathew Crane <mathew.crane at gmail.com> wrote:

> Hit send too early. It was my understanding that PPS was the
> canonical time source, and GPS was just a reference. Taken from here:
> http://www.catb.org/gpsd/gpsd-time-service-howto.html#_performance_tuning

We need to define our terms better.  The PPS comes from the GPS!
By GPS I was actually referring to the NMEA time from the GPS that is
used to match the closest second to the PPS to form the SHM(0) which
is a combinaaaaation of the NMEA and PPS from the GPS.

> "The asterisk in this example indicates that ntpd has correctly
> preferred .PPS. over .GPS., as it should. If for some reason it locks
> on to GPS time as a preferred source, you can add an overlarge fudge
> to the NMEA time to discourage it. Or add the suffix noselect so GPS
> time is never used, just monitored"

Yup, I wrote that.  You can do that, or use noselect if you have
other sources of coarse time.

> My versions and configs:
> 
> gpsd: 3.17~dev (revision release-3.16-535-ga5bbf14)
> ntpd ntpsec-0.9.6+149 2017-01-07T12:44:49-0800

Very good.

> /etc/ntp.conf:
> 
> # GPS Serial data reference (NTP0)
> *refclock shm unit 0 time1 0.120 refid GPS*

You want that LAST in the file.  On startup ntpd prefers reflocks
at the top of the file, and you do not want to prefer SHM(0) it is
your worst source.

And make the time1 a lot bigger, or add noselect. 0.120 is way too small
for the reasons noted above.

> # GPS PPS reference (NTP1)
> refclock shm unit 1 refid PPS prefer

Should be at the top of the file so it gets tried first.  For the RasPi
you also want:  minpoll 2 maxpoll 2

> server 10.3.123.1 minpoll 5
> pool us.pool.ntp.org iburst prefer

I'm not a fan of the pool for fixed locations.  Find some good public
servers near you and hard code them.

> # Drift file etc.
> driftfile /var/lib/ntp/ntp.drift

Good.

RGDS
GARY
---------------------------------------------------------------------------
Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
	gem at rellim.com  Tel:+1 541 382 8588

	    Veritas liberabit vos. -- Quid est veritas?
    "If you can’t measure it, you can’t improve it." - Lord Kelvin
-------------- next part --------------
A non-text attachment was scrubbed...
Name: not available
Type: application/pgp-signature
Size: 455 bytes
Desc: OpenPGP digital signature
URL: <https://lists.ntpsec.org/pipermail/devel/attachments/20170108/deb0ab7e/attachment.bin>