Re: PPS fix determination vs. cgps?

[Steve Bourland]

Yo Gary!

> Date: Thu, 21 Jan 2021 12:27:03 -0800
> From: "Gary E. Miller" <gem@rellim.com>
> Subject: Re: PPS fix determination vs. cgps? (fwd)
> Message-ID: <20210121122703.2866349d@spidey.rellim.com>
>
> Yo Steve!
>
> On Thu, 21 Jan 2021 13:42:15 -0600
> Steve Bourland <sbourland@swri.org> wrote:
>
> Looks like you are sending GPRMC, GPGSA and GPGSV every cycle.  That
> is not normal.  The Garmin, and most other receivers, do not compute
> GSV every fix, so sending it every fix is a waste.  Maybe that is
> confusing the cycle ender.
>
> BTW, what is your serial port speed?

4800 baud....happy to see if I can speed that up if the Garmin will do 
it...but feel like years ago I looked into it and in NMEA mode, 4800 is 
the max output speed?  Will investigate.

> Can you reset your device to gpsd normal NMEA sentences and see if that
> changes anything?  At least try GSV every 5 fixes, not every one.

Reading through the Garmin docs to see how to manage this....assuming I'll 
need to do something along the lines of:

# cat command.dos > /dev/ttyS0

Where command.dos is just a simple text file with the appropriate ASCII 
magic in it from the docs.....terminated with <CR><LF> (hence the .dos 
suffix)?  Can I use gpsctl to send commands to the unit (checking the man 
page...looks like '-x' might be my friend there), or will it struggle 
since it is using the "NMEA0183" driver?

Also....do these lines worry you as they worry me?
gpsd:PROG: => Probing for Garmin NMEA
gpsd:IO: SER: => GPS: $PGRMCE*0E\x0d\x0a
gpsd:IO: <= GPS: $GPGSA,A,3,01,26,,31,32,,,,,,,,1.9,1.7,0.9*33^M
gpsd:PROG: NMEA0183: xxGSA sets mode 3
gpsd:DATA: NMEA0183: xxGSA: clear sats_used

My reading of those indicate to me that gpsd tried to send 
$PGRMCE*0E\x0d\x0a to the unit, which according to the docs should result 
in a response with the values for PGRMC currently being used....but I 
don't see that reply in the log :(

Not sure when I will get around to this but didn't want to leave this 
thread dangling.

> > > Correct.  But, as you said, xgps and xgps are working as you
> > > expect, so no point looking at the working part. 
> >
> > My thinking was to see why they thought they had a good fix (what
> > they were keying off of)
>
> That is right there in the log you sent:
>
> gpsd:IO: <= GPS: $GPGSA,A,3,01,26,,31,32,,,,,,,,1.9,1.7,0.9*33
> gpsd:PROG: NMEA0183: xxGSA sets mode 3
>
> > to see if PPS was looking for a different
> > key...
>
> Nope.  Same thing, the mode byte.  You see it in include/gps.h as
> gps_fix_t -> mode.  The issue here is always WHEN, not WHERE.  Nothing
> is the "same time" in gpsd.

So the GPGSA is indicating we have a 3D fix...but not in time for the PPS 
to see it?  Sorry, trying to understand what's going on...that problem 
indicated by the line:
gpsd:PROG: NMEA0183: GPGSA is just after a cycle ender.
So cgps and xgps are "seeing" the GPGSA lines, but the PPS thread isn't 
getting it before the cycle ends...and therefore doesn't know we have the 
fix?  Am I anywhere close to understanding what we think is happening?
                                        Thanks,
                                          Steve