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Re: [gpsd-dev] [PATCH] [log.chk] [TPV timing] implement $GPVTG message,

From: Gary E. Miller
Subject: Re: [gpsd-dev] [PATCH] [log.chk] [TPV timing] implement $GPVTG message, and proper timing for output "TPV" messages
Date: Thu, 17 May 2018 13:31:06 -0700

Yo teyrana!

On Thu, 17 May 2018 16:13:03 -0400
teyrana <address@hidden> wrote:

> > Looks like you have more data in the GPVTG than you are putting in
> > the ATT.
> >
> > .log.chk pasted here:  
> 'VTG' turns out to not have much data--- JUST track and speed.
> (repeated with different units, oddly enough)

Well, you may not care about track in true AND magnetic heading, but a
lot of people do.  So report both.  Airplane pilots and boat pilots
want the magnetic.  gpsd has places in ATT for both.

> And the JSON//ATT message only has "heading", but not 'track'/CoG   We
> actually run into this distinction frequently.  Particularly at low
> speeds, when the two values can be different by 90+ deg!

Interesting.  Maybe instead of ATT heading= you should be using
TPV track?  ATT is for gyro data, you have GPS data.

GPVTG does say track, not heading, yet you are putting your data in 

> > Yeah, that's a tricky part.  You want to wait until you have all
> > the data of a cycle before outputting, but no longer.  REPORT_IS,
> > and friends, is the key to how that works.
> >  
> Right.  This patch sets "REPORT_IS", BUT NOT "CLEAR_IS".

Can't set just REPORT_IS.  or just REPORT_IS|CLEAR_IS.  Neither
alone will actually send a report.

> I'm okay with this current behavior, but I'm just not sure if other
> people are.

Tricky to get right.  A few GPS do report end of cycle, otherwise
you have to guess.

> Ah, good point.  Patch uploaded here:

Not right:

+  if (session->newdata.mode < MODE_2D) {
+      session->newdata.mode = MODE_2D;
+      mask |= MODE_SET;
+  }

Loss of mode (fix) is also worth reporting.  A common bug in gpsd
drivers, but users get upset when gpsd reports an old fix, when it should
report fix lost.

OK, since SPEED_SET, but maybe too soon?

+  // request to report an output message
+  mask |= REPORT_IS;

So, getting there.

Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
        address@hidden  Tel:+1 541 382 8588

            Veritas liberabit vos. -- Quid est veritas?
    "If you can’t measure it, you can’t improve it." - Lord Kelvin

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