On Tue, Aug 12, 2008 at 11:36 AM, Hal Murray <[EMAIL PROTECTED]> wrote: > > 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.)
Yep, that's to be expected, given the iterative solver they're using. Try turning on the ZDA message, apparently the $ sign is aligned to the start of the second ... if its implemented in your version of firmware. Since you're running a BU-353 you could have all kinds of broken things in your firmware (not impressed by Globalsat). > But I still have a couple of them collecting data. As an aside, how and what are you collecting? I'm starting to think about setting up different loggers to catch the leap second... > 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? first notice of this in the archives is on the 28th... so probably 0000h UTC on the 29th. > 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. I've used the LVC w/ PPS as the reference clock... not sure that I trust the delay/jitter characteristics of USB enough to give me better time than a wrist-watch. > 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.) it's "nice" to see all 3 SiRF receivers failing in the same way. that does make me think it's firmware rather than busted hardware. -- GDB has a 'break' feature; why doesn't it have 'fix' too? _______________________________________________ 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.
