[ntp:questions] Ignore one server except in extreme conditions?

A C agcarver+ntp at acarver.net
Thu Nov 17 06:41:51 UTC 2011

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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