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

Harlan Stenn stenn at ntp.org
Wed Nov 16 23:24:37 UTC 2011

A C wrote:
> > 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).

This doesn't make a lot of sense.  NMEA is known to be pretty much
useless without a PPS. So if your NMEA time data is "off" I gotta wonder
if your PPS is "off" too.

Does your GPS report any "health" information?

> 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).

Are you *certain* that your GPS is producing a "locked" PPS signal when
the NMEA time is wandering?


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