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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: Wed, 7 Dec 2022 11:27:21 -0800

Yo Nick!

On Wed, 7 Dec 2022 10:50:03 +0000
Nick Taylor <> wrote:

> Yo Gary
> On 05/12/2022 23:04, Gary E. Miller wrote:
> > Yo Nick!
> >
> > I added your sample to the regressions.  The GGA -> uSat seems to
> > work.
> >
> > $GPRMC,113938.50,A,3842.86006889,N,11705.43645510,W,0.011,46.405,231122,999.9000,E,D*2E
> > {"class":"TPV","status":2,"mode":3,"time":"2022-11-23T11:39:38.500Z","ept":0.005,"lat":38.714334482,"lon":-117.090607585,"altHAE":1819.8860,"altMSL":1864.1820,"alt":1864.1820,"track":46.4050,"magtrack":686.3050,"magvar":999.9,"speed":0.006,"geoidSep":-44.296,"eph":7.125,"dgpsAge":1.4,"dgpsSta":0}
> > $GPGGA,113939.00,3842.86006288,N,11705.43645704,W,2,09,1.5,1864.172,M,-44.296,M,1.0,0000*46
> > {"class":"SKY","hdop":1.50,"uSat":9}
> >
> > Please test.
> >  
> Seems you have fixed up the json but we use
> gps_data->satellites_visible from the gpslib - I believe cgps does
> too.

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

> My guy has suggested the attached patch which seems to fix things up
> - please review and let us know what you think?

Not good.  Like this:

+            gpsdata->satellites_visible = uSat;

visible is what the receiver has an ephermeris for, that is over
the "hozizon".  "used" is what is used in the fix calculation.

Very different data items.  Do not mix them.

You want daat that is not present in your NMEA feed.  And there is no
way to calculate it from the data feed aither.

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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