[ntp:questions] GPS driver failure mode question
Steve Kostecke
kostecke at ntp.org
Sun Jan 11 02:15:08 UTC 2009
On 2009-01-08, Charles Brown <charles.brown at sensis.com> wrote:
> I have a processor with a serial GPS on a closed private network. If
> the GPS is not locked, or the GPS inputs are missing, I still want the
> ntpd to serve *some* time, perhaps from the undisciplined local clock.
ntpd _never_ serves time "from" the Undisciplined Local Clock. All that
the Undisciplined Local Clock driver does is allow ntpd to claim to be
synced in the absence of a real time source.
> I.e., the server can't just do nothing. Does the GPS driver do that
> automatically, or do I have to configure two drivers in the ntpd.conf?
> Perhaps something like;
>
> prefer peer 127.127.20.u
> peer 127.127.1.u
That's all wrong. Use something like this:
# GPS
# see the type-20 driver page for the mode value
server 127.127.20.0 mode X minpoll 4 maxpoll 4
# Undisciplined Local Clock
server 127.127.1.0
fudge 127.127.1.0 stratum 10
> Suppose I have a second server, with a higher precision oscillator,
> on this same network. How would I configure that? Perhaps;
>
> prefer peer morePreciseServer # should be best source
> peer 127.127.20.u # next is the GPS
> peer 127.127.1.u # if all else fails...
The best thing to do is let NTP choose the best time source. But keep in
mind that NTP needs at least 3 time sources to choose the one that is
the best. When NTP has only two time sources it has no way of deciding
which one is correct.
>What would the configuration on the other processor look like? Perhaps;
>
> prefer peer 127.127.22.u # always best
> peer 127.127.1.u # if GPS fails, still better for a
> # couple of days
> peer lessPreciseServer # GPS missing a long time,
> # lessPreciseServer might have GPS
I don't understand this configuration example.
Unless the "better" system is using an _extremely_ high quality
oscillator is will most likely not be materially "better" than the fist
system.
--
Steve Kostecke <kostecke at ntp.org>
NTP Public Services Project - http://support.ntp.org/
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