[gpsd-dev] refclock 28 gone wacky on me

Mike bellyacres at gmail.com
Sun Jun 19 21:46:48 UTC 2016

On 06/19/2016 04:23 PM, Gary E. Miller wrote:
> Yo Mike!
> `
> On Sun, 19 Jun 2016 15:59:16 -0400
> Mike <bellyacres at gmail.com> wrote:
>> On 06/10/2016 03:17 PM, Gary E. Miller wrote:
>>> Yo Eric!
>>> On Fri, 10 Jun 2016 03:49:22 -0400
>>> "Eric S. Raymond" <esr at thyrsus.com> wrote:
>>>> That's... very weird.  I've never seen it happen.
>>> But it would explain some old complaints.
>>> Now that we know which Skytraq option it is, I recall always
>>> selecting the workaround.
>>>> It sounds as though for some crazy reason the GPS is delivering the
>>>> PPS pulse late.
>>> Not late, as such.  Late in the second, but the time stamp of the
>>> fix (xxX.940) is pretty close to when actually sent.
>>>> I'm hard put to imagine how gpsmon could screw this up.
>>> Or Hal with his O-scope.
>>> RGDS
>>> GARY
>> I'm seeing this happen now, PPS is good NMEA is off nearly half a
>> sec...  NMEA delivery viewed in gpsmon is 0.011 after top of second.
>> *SHM(1)          .PPS.            0 l   10   16  377    0.000 -0.002   0.004
>> -SHM(0)          .GPS.            0 l    9   16 377    0.000 -496.59 6.661
> Looking good.  Now add a 0.500 fudge to your ntp.conf.  Like this:
>      server
>      fudge time1 0.500  refid GPS
> What do your other chimers in 'ntpq -p show'?  Youu still have not confirmmed
> which PPS edge you are on.  The leading or trailing.

I'll likely put an arrow through this module before I'm able to get a 
scope hooked up too it!  So leading or trailing is a coin toss at this 

I should have elaborated more I guess.  I had everything sailing along 
nicely.  Went off on a tangent with the rpi and left the module 
unpowered for a few days.  Its battery was still plugged in and came 
backup as expected when I powered it all on again.  At that point NMEA 
deliver was really late, as in > .940 that I was seeing earlier.  I 
wasn't real pleased and figured I'd have to reset the setting to get the 
NMEA delivery back to top of second.  Left is alone for several hours, 
come back and gpsmon shows that I was back to getting delivery close to 
the top of second again.

At that point PPS looks good, NMEA is way out, where before I had 
powered the module down is was looking pretty good.  I'll probably have 
to pull that info from the statistics if it's really relevant. I get 
that I can fudge out the difference shown above.  One shouldn't have to 
do that every time something is powered off though.

I believe that the ntpshmmon output was showing that PPS is picking up 
the first seen and NMEA is picking up the second.  There is 4 per second 
in the output that I posted.  And the time is off by approximately the 
offset above.  Or am I mis-reading the output that bad?


More information about the devel mailing list