[time-nuts] GPSDO and holdover

[Bob Stewart]

I mentioned to Tom that I had seen the xgps program duplicate a lot of its satellites when I missed a PPS.  I noticed my GPSDO go into holdover so I quickly brought up xgsp and noticed it happening again.  This screen showed a few times intermixed with a normal screen.  I have no idea whether it's a bug in xgps or due to something coming from the Adafruit, but it's interesting, nonetheless.

http://www.evoria.net/Adafruit/Holdover.png

Bob - AE6RV



>________________________________
> From: Paul <tic-toc at bodosom.net>
>To: Discussion of precise time and frequency measurement <time-nuts at febo.com> 
>Sent: Friday, April 25, 2014 1:20 PM
>Subject: Re: [time-nuts] GPSDO and holdover
> 
>
>On Tue, Apr 22, 2014 at 9:57 PM, Tom Van Baak <tvb at leapsecond.com> wrote:
>
>> 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
>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.
>