[ntp:questions] Stick to PPS, even if the prefer server fails

alkopedia at googlemail.com alkopedia at googlemail.com
Thu Mar 26 00:51:20 UTC 2009


On Mar 26, 12:48 am, Unruh <unruh-s... at physics.ubc.ca> wrote:
> However if the only concern was losing PPS discipline due to prefer peer
> problems, then I agree this is tangential.

Yes, it was ;-)

> >I think he's monitoring the self-reported offsets between the NTP
> >disciplined clock and the local GPS receiver.  Network delay would not
> >be a factor if so.
>
> I got the impression that he was using this machine to monitor a whole
> bunch of other machine distributed over the USA to make sure that their
> clocks were well disciplined by their own on board GPS receivers.

You're both right:
When I was talking about the -5 us to +5 us offset, I was talking
about the self reported offset of the NTP disciplined clock and the
local and the local PPS signal.
But Unruh is right. The main task of the whole system is to monitor a
bunch of machines (called SSU) distributed over Germany to make sure,
that they _provide_ the right time to their clients. Of course it is
not possible to monitor them with microsecond resolution but that's
not needed. Just imagine one of these situations:
- SSUs have a firmware bug, which leads to wrong timings
- SSUs have a bug and don't notice a leap second
- SSUs have a problem at receiving the GPS signal
All these things may lead to wrong timings in the network and probably
nobody will notice fast enough.

And of course for my monitoring system it is very unlikely that GPS
and DCF77 time signals will fail at the same time, but it is possible.
Therefore was my initial question if it is possible to sync to the PPS
signal, even if both prefered peers disappear.
Furthermore the used cesium normale is part of the UTC calculation of
the BIPM in Paris. So the cesium timings are constantly measured and
ensured by Circular T: ftp://ftp2.bipm.fr/pub/tai/publication/cirt/cirt.254
;-)

Greetings,
Thorsten




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