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Re: Strangeness with speed

From: Gary E. Miller
Subject: Re: Strangeness with speed
Date: Tue, 15 Feb 2022 13:11:49 -0800

Yo Владимир!

On Tue, 15 Feb 2022 23:35:25 +0300
Владимир Калачихин <> 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

> 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

> 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.

Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703  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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