On Wed, Nov 27, 2013 at 04:06:58PM -0500, Battocchi, Scott L. wrote: > I ran the GPS while connected to a handful of ntp servers and saw that my gps > offset (originally 0.180) was too low, so I bumped it up to 0.530 for the > next two tests. I've attached plots of the offset as recorded in the > statistics.log file, if there are other metrics that would be useful I'm > happy to graph them and send them out. > ntp.png is with 5 pool servers and the GPS set to noselect (PPS is not locked > to anything, but is selectable) > gps.png is after the ntp test but back to just using the GPS and PPS, it > looks like sometimes GPS gets selected as the source forcing the PPS signal > to look like it is drifting relative to the system.
That looks similar to what I see with with a Garmin 18x LVC. This is a capture 30 hours long I did some time ago (the NMEA source's offset value was set to 0.5): http://mlichvar.fedorapeople.org/tmp/18x_nmea.png Since gpsd has added support for kernel PPS, I think it's better to use the SHM 1 or SOCK source instead of PPS. Let it handle the HW details and pair the PPS and NMEA samples. > I think a portion of my original confusion was that the chronyc sources > command was indicating that the pulse had never been seen, as opposed to it > being seen and ignored. I need to compare the GPS logs with the chrony logs > to see if the changing offset is a function of the number of satellites in > view, otherwise I don't have a great explanation for the wander seen in the > ntp plot. >From what I remember from other discussions about NMEA timing, it mainly depends on how is the firmware implemented and the number of visible satellites may have nothing to do with it. -- Miroslav Lichvar -- To unsubscribe email chrony-users-requ...@chrony.tuxfamily.org with "unsubscribe" in the subject. For help email chrony-users-requ...@chrony.tuxfamily.org with "help" in the subject. Trouble? Email listmas...@chrony.tuxfamily.org.
