Re: PPS fix determination vs. cgps?

[Gary E. Miller]

Yo Steve!

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?

What version ntpd?

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

> 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

RGDS
GARY
---------------------------------------------------------------------------
Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
        gem@rellim.com  Tel:+1 541 382 8588

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