Re: AW: Decreasing number of used satellites

[Greg Troxel]

"Wielant, Ulrich" <U.Wielant@leonardogermany.com> writes:

> I updated to gpsd 3.23.1 (I hope 3.23.1 is current enough...)

3.24 is current, but that's close enough for me to not give you a hard
time about it.

> I still have problems with the number of used satellites.
>
>  I have attached the output of cgps and gpsmon captured at rather the same 
> time.
>
> The GPGSV and GNGSA NMEA sentences describe a large number of satellites.
> The SKY JSON object in cgps indicate that none of them are used.
>
> Greg, can you tell me, why the gpsd indicates that none of the satellites are 
> used?

I suggested that you read the raw NMEA coming out of the receiver and
understand it.  That suggestion stands.  What you are providing is
output from gpsd infested with curses.

Your GGA sentence shows 28 used satellites:

(70) $GPGGA,122212,5107.8619,N,00644.1752,E,2,28,0.60,0.15,M,45.81,M,,*73

In your GSA sentences the GSID field is missing.

(52) $GNGSA,A,3,08,10,21,23,27,,,,,,,,1.07,0.60,0.89*13
(56) $GNGSA,A,3,67,68,69,74,75,83,84,,,,,,1.07,0.60,0.89*1B
(52) $GNGSA,A,3,01,05,13,15,21,,,,,,,,1.07,0.60,0.89*1C

The NMEA spec is costly to obtain, but it didn't take long to find:

  https://receiverhelp.trimble.com/alloy-gnss/en-us/NMEA-0183messages_GSA.html
  
https://www.hemispheregnss.com/technical-resource-manual/Import_Folder/GNGSA_Message.htm

See the man page about -N and -D.  It may be that GNGSA sentences really
are out of spec and gpsd is declining to process them.  Certainly
looking at them I can't see how to reliably parse them, other than
guessing that the three messages are GPS, GLONASS, Galileo in that order
(and the >64 NMEA ids aer a clue for GLONASS).  I would think that GPGSA
might omit GSID, as the P is a clue, but it doesn't make sense for GNGSA
to omit it.

I would suggest looking in your manual about GSA, and if not present
asking the manufacturer about the fields, and whether there is or isn't
a GSID field at the end.

You could also see if your receiver is set to NMEA 4.

After that, I suspect you're going to have to go down this rabbit hole
yourself.

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