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

A C agcarver+ntp at acarver.net
Wed Nov 16 16:14:08 UTC 2011

On 11/16/2011 03:20, David Lord wrote:
> 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.
> Doesn't ntp do that already?
> If you have nmea + pps the pps will be used for sync.
> If your gps + pps is down your internet servers will be used for sync.

Not if PPS and NMEA are independent clock sources.  Two lines in the 
config file, one is and the other is (not 
including the Internet servers).  The PPS is synced almost all the time 
but the choice of clock sources moves around from Internet to NMEA.  In 
my case the NMEA sentences wander but the NMEA clock is always listed as 
a member of the accepted clocks (the "+" indicator) even if its own data 
is so bad that another clock with similar offset and jitter has been 
labeled an outlier/false ticker.

Note I'm not talking at all about the GPS going away.  I'm talking about 
the NMEA sentence wandering around dragging the clock with it because 
it's part of the overall computed solution.  By way of example, one of 
the Internet servers in my peer list has an offset of 1.887 and jitter 
of 0.586 and is labeled an outlier.  The NMEA data at that same moment 
has an offset of 21.863 and a jitter of 10.290 and it's labeled as accepted.

What I'm looking to do is to ignore the NMEA in favor of the Internet 
clocks as long as they are available then fall back to NMEA in case the 
network drops out.

Before anyone asks, the GPS itself is fine and functioning normally.  It 
was designed as a PPS reference for a fixed location telemetry network 
over GSM.  The PPS portion is very accurate but it sacrifices some 
stability in the NMEA data to achieve the PPS stability (PPS pulse 
generation is higher priority for the CPU than NMEA generation).  Since 
the telemetry device was intended to be in a fixed location it wasn't 
considered important to have very precise NMEA data, only PPS pulses to 
control timing and slotting.  Jitter on the PPS signal is reported by 
ntpd to be 0.061 ms (ntpd never shows jitter numbers lower than this for 
any source, must be something in the floating point calculations) and 
doesn't wander away from that (I can see the same thing on an 
oscilloscope, the pulse is quite stable).

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