[time-nuts] Leap Second quirk, Another hanging bridge
hmurray at megapathdsl.net
Tue Aug 12 14:36:57 EDT 2008
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:
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
If you were (un)lucky, you could get fooled into thinking that the offset was
This graph includes a few more units:
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.)
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