[time-nuts] Leap Second quirk, Another hanging bridge

Hal Murray 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 
reasonably stable.

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.)

These are my opinions, not necessarily my employer's.  I hate spam.

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