Re: Strangeness with speed

[Gary E. Miller]

Yo Владимир!

On Tue, 15 Feb 2022 23:35:25 +0300
Владимир Калачихин <v.kalachikhin@gmail.com> wrote:

> > So instead of this:
> > 
> > 
> > $GPRMC,114618.00, ...
> > $GPGGA,114618.00, ...
> > $GPRMC,114618.00, ...
> > $GPGGA,114618.00, ...
> > 
> > This:
> > 
> > $GPRMC,114618.01, ...
> > $GPGGA,114618.01, ...
> > $GPRMC,114618.02, ...
> > $GPGGA,114618.02, ...
> > 
> > That way gpsd knows which GPRMC belongs with which GPGGA.  
> 
> I'm sorry, Gary, but it doesn't make any difference.

Facts not in evidnce.  Send me a proper NMEA file and I'll look at it.

> First: the time on this file is not constant - it changes every
> second.

But different data in multiple consecutive GPRMC.  That confuses
gpsd.

> Second: this file is understood by gpsd fine.

And yet you complain about how gpsd understands it?

Pick one: gpsd understands it (thus no problem in JSON), gpsd fails to
understand it (thus a problem in JSON).

> Strangeness occurs only if several GGA sentences have the same time 
> within the current second.

Yes!  Now build on that thought!

> If time in the past - no problem with same
> times.

And yet, you report a problem?

Pick one: gpsd understands it (thus no problem in JSON), gpsd fails to
understand it (thus a problem in JSON).

> But now I understand that speed == 0 if two consecutive GGA sentences 
> have the same time is normal, not strangeness.

I do not agree that is notmal.  Stange (garbage input) yeilds sstring
(garbage) output.

> Strangeness is that
> speed == null

Facts not in eveidence.  Speed can never be a bad pointer!

> if gpsd receives a few GGA sentences with consecutive
> timestamps.

Facts not in eveidence.  The NMEA you send has consecutive GGA sentences
with the exact same tim.

> Did you say that the speed should be calculated?

Nope.  But everything from a GNSS receiver is calculated.  How it is
calculated is hard to say.  The receiver may calculate from dP/dT, or
from doppler, or IMU..  Or gpsd could calcualte from dP/dT.  Hard to
know.

> This is
> not happening at least for gpsd 3.20

Lots has changed since 3.20.  Since you have garbage in, gpsd does not
care about the matching garbage ou.  Just like the C language itself,
there are many things that are undefined bahavior.  Your NMEA is one
of them.

Fix your NMEA.

RGDS
GARY
---------------------------------------------------------------------------
Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
        gem@rellim.com  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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