[time-nuts] GPSDO and holdover

Tom Van Baak tvb at LeapSecond.com
Sat Apr 26 21:09:43 UTC 2014

>> I have noticed skipped 1PPS on the Adafruit GPS also.
> I've always assumed this could happen but as a result of RF signal loss not
> a glitch in the gps.  So I've started recording event timestamp deltas

To be clear, we are not talking about a system-wide GPS problem here; the satellites are fine. All my other GPS receivers are fine. It's just one particular GPS receiver board that Bob, and now Bob and I, are questioning.

> using the Linux kernel PPS interface.  I read assert events (e.g.
> 1398449188.001000242#1741672) and compute  timestamp and event deltas.  If
> the t delta is < .9 something horrible must have happened and if it's > 1
> some didn't happen assuming the event count delta is always 1.
> I wonder if this is a reasonable approach or if I'm being lazily
> optimistic.  I just started (and I haven't added a join with the valid fix
> indicator yet) but I've had two missing pulses in the last 24 hours.

Sure, that's reasonable. I'm using a 53132 TIC to compare my house atomic 1PPS (start) against the Adafruit GPS 1PPS (stop) and so no timestamping is even necessary: the readings themselves tell you if there is a missed pulse. For example, you get TI readings like 0.0000000nn for hours or days and then once in a while you get a 1.0000000nn or 2.0000000nn, indicating a missed pulse.

After Bob Stewart's first mention of the Adafruit Ultimate GPS board, I dug out mine and collected 30 days of data to see if I could duplicate or understand his measurements. As usual, a simple and quick test has turned into something more complicated and perplexing. I'll share some results maybe next week. The test is now multiple GPS boards, antennas, and counters.


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