Re: PPS fix determination vs. cgps?

[Steve Bourland]

Yo Gary!  (please be patient, take a deep breath....I tried to....but had 
a frustrating day and expect it might show here)

> On Tue, 19 Jan 2021 16:27:49 -0600
> Steve Bourland <sbourland@swri.org> wrote:
>
> > I have a very strange case running with one of my Garmin 18 LVC
> > units. Once it is locked in, both cgps and xgps that it has a 3D fix,
> > and all seems just fine, but won't feed the PPS signal to ntpd with
> > the following in the debug message:
> >
> > gpsd:INFO: PPS:/dev/ttyS0 Assert hooks called clock:
> > 1611094941.139482499 real:  1611094941.000000000: no fix
>
> What version gpsd?

3.22....freshly downloaded and installed.

> What version ntpd?

Does it really matter?  Right now I am asking about deviasion between 
cgps/xgps and gpsd's PPS reporting.  But to answer your question:
ntpd ntpsec-1.1.0+419 2018-03-14T12:03:57-0700

> How long did you let it run?  It can often take 15 to 30 mins from
> startup for a Garmin to send good PPS.

Long enough for cgps and xgps to report that they had a 3D fix.....so I'd 
say long enough?

> > I have tried to sift through the source code to figure out how the
> > PPS determines the fix status vs. how cgps and/or xgps determine the
> > fix status, to better understand why I am getting conflicting
> > results.  The device is sending NMEA sentences GPRMC, GPGGA, GPGSA,
> > and GPGSV.
>
> The best way is to capture a log of your session this way as root:
>
> # gpsd -nND 6 /dev/ttyS0 >& tmp.log
>
> Let that run a few minutes, then send tmp.log here so we can look at it.
>
> > Is it easy to point me to which files in the source I should be
> > looking at to get this figured out?
>
> That's eaasy:
>
> root gpsd # fgrep "Assert hooks called clock" * -r
> gpsd/ppsthread.c:  "PPS:%s %.10s hooks called clock: %s real: %s: %.20s\n",
>
> Look in gpsd/ppsthread.c

OK.....so it really looks to me like you are missing the key point of my 
question...

WTF am I getting told by cgps and xgps that I have 3D fix....but the debug 
log for PPS is telling me no fix?

Pointing me to ppsthread.c in no way shows me why cgps and xgps both think 
I have a 3D fix, no?  Sorry Gary....I know you handle about 20,000 
questions per day.  I'm trying really hard to point you to the point that 
is my confusion, why in the same code base...I'm being told I have a 3D 
fix and no fix at the same time?  This should really bother you too in my 
mind as a fellow developer, which is why I am poking at the bear.  I want 
an answer...I don't want to waste your time, but I feel like you're 
missing the point?

So here is the main issue in my mind:
  Both cgps and xgps show me "3D fix" (I can include logs if you need 
it....or you can just trust me, they report that, version 3.22....latest 
release...built expressly for troubleshooting this), the debug log from 
gpsd shows the quoted PPS line with "no fix."  Am I insane for thinking 
there is a disconnect somewhere?