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

A C agcarver+ntp at acarver.net
Wed Nov 16 23:09:13 UTC 2011

On 11/16/2011 12:25, David Lord wrote:
> A C wrote:
>> 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.
> *GPS_NMEA(2) .GPSb. 0 l 29 64 377 0.000 -52.888 11.213
> offset: -0.000001 s
> So are you saying that the -52.888ms is making a significant
> contribution to the offset of -0.000001s ?

Yes, if I disable the NMEA source with a "noselect" and leave the 
Internet servers and the PPS clock (22) running, my overall system 
offset drops and holds at a few tens of microseconds.  If I leave it in, 
the system offset wanders around.  The magnitude of the wander appears 
to correlate with the magnitude of the NMEA offset.  For very large NMEA 
offsets (sometimes exceeding +/- 50ms) the system itself starts to drift 
away to large ms offsets.  Overall I seem to get better performance 
without the NMEA driver contributing than with it included hence the 
desire to make it accessible only when all the other Internet sources 
fail (but GPS is still working).

Both PPS and NMEA are coming from the same physical GPS just using two 
serial ports, one for the PPS and one for the serial data (this split is 
required due to serial port driver limitations).

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