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