I've got a couple of GPS units that use the SiRF chips feeding NTP. I was looking for low cost units for time keeping. They don't work very well. The time offset of the NMEA message wanders/jitters by about 100 ms. I can easily correct for a constant offset, but I can't dance around an offset that won't hold still. (Oh, well. I tried.)
But I still have a couple of them collecting data. They seem to get confused by leap seconds. At least that's the only thing I can think of that changed recently. Here is a graph: http://www.megapathdsl.net/~hmurray/ntp/leap-gps3.gif It started, midnight, July 31. Does anybody know when the leap second announcement hit the satellites? I assume it's a software bug. Looks like it repeats on a weekly pattern. The red line on the top is a sane unit used as a reference to show that the time on the local system time isn't bouncing around. Here is a longer time span that shows a hanging bridge type pattern on the offset: http://www.megapathdsl.net/~hmurray/ntp/leap-gps2.gif If you were (un)lucky, you could get fooled into thinking that the offset was reasonably stable. This graph includes a few more units: http://www.megapathdsl.net/~hmurray/ntp/leap-gps.gif The spikes on the blue Garmin GPS 18 LVC usually happen when it is recovering from not-enough satellites. (I haven't checked these particular samples. That's what I found the last time I investigated.) -- These are my opinions, not necessarily my employer's. I hate spam. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
