On 11/16/2011 15:24, Harlan Stenn wrote:
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?
Yes, I'm certain the PPS signal is not wandering. That's what this
particular GPS receiver was designed to do. It keeps the phase of PPS
steady but sacrifices the NMEA sentence timing. Health reports are
fine, it passes all the diagnostics and is locked on an average of seven
satellites. Most of the time it's more like nine or ten.
_______________________________________________
questions mailing list
[email protected]
http://lists.ntp.org/listinfo/questions
