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Re: Small problem with NMEA feed - satellites_used

From: Gary E. Miller
Subject: Re: Small problem with NMEA feed - satellites_used
Date: Fri, 9 Dec 2022 11:07:18 -0800

Yo Nick!

On Fri, 9 Dec 2022 10:35:48 +0000
Nick Taylor <> wrote:

> Yo Gary
> > gps_data->satellites_visible is useless.  No two vendors compute it
> > the same way, and tells you more about the state of the local
> > ephemeris than the quality of the sky view.  It is the wrong thing
> > for what you say you want.
> >
> >  
> Sorry for confusion - my guy Orzo was battling to get any data at all 
> and switching between sats avail and sats used hence how he got mixed
> up...

I know how that happens.

> Basically we are after reporting sats used but using var in gpslib
> it's not there using this partial NMEA feed.

Uh, lost me.  Git head is now reporting uSat when given your Caterpillar
sample.  That Caterpillar sample is now part of the regressions tests.

You can see them here:


> Orzo is suggesting the attached patch but he's not 100% happy with
> it...

Oh, you men libgps, I could not figure out what gpslib was.  I do
see that libgps/libgps_json.c is not reading uSat.  Easy to fix.

But, this is wrong:

+            gpsdata->set &= ~SATELLITE_SET;

SATELLTIE_SET means there is individual sat data, which is not true in
this case.

"-1 == uSat"  tells you it is not set.

> Maybe an option would be to add new variable like gga_sats_used and
> then application code could choose to use this if normal sats used=0

No need for any new variable.  All the vars we need are these, just not
hooked up in your case.

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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