On 11/16/2011 17:41, Richard B. Gilbert wrote:
On 11/16/2011 12:47 AM, A C wrote:
Is there a way to configure ntpd to ignore a particular clock unless
there is no other choice? What I'm thinking is to ignore the GPS
receiver NMEA data and use only the PPS plus Internet servers for most
of the synchronization. But if the Internet servers go down, accept the
GPS NMEA data for seconds numbering and let the tick be controlled by
PPS.
I guess I'm asking for something opposite to the "prefer" flag, a "not
unless you have to" flag. The GPS does get selected among the various
clock sources but it's the worst of the bunch with respect to jitter and
offset. Those two wander all over while the rest of the Internet servers
and the PPS are stable. It is almost never rejected as an outlier or
false ticker even if it's way out of line with the other sources.
Which GPS receiver are you using? I have an old Motorola that has been
ticking away flawlessly for the last six or seven years.
This is a Globalsat ET212 OEM board. It's running just fine it just
doesn't give high priority to ensuring that the NMEA sentences are
emitted in a timely manner. It spends most of its CPU cycles ensuring
the PPS is on time and stable in phase. I can watch the NMEA sentence
wander around relative to the PPS but the PPS doesn't move a bit.
I found that getting time over the internet was not very satisfactory.
From local midnight until about 7:00 AM it was pretty good but once
people woke up and started using the internet, NTP wasn't working very
well.
Internet time with local PPS seems to be doing quite well. I may just
give up and drop the NMEA entirely and use external force to switch
configurations if a net connection drops but that means a restart of ntpd.
I'd offer to serve time but my contract with Comcast forbids operating a
server of any kind!
The idea is to provide a local reference clock to the rest of my
machines on the home network (including IP cameras, the various routers,
the computers, and an IP clock). I just want all those to work even if
the network drops.
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