Re: $GPGGA / increased number of satellites

[Gary E. Miller]

Yo Hans!

On Mon, 13 May 2024 20:45:56 +0200
Hans Mayer via <gpsd-users@nongnu.org> wrote:

> Yo Gary !
> 
> > Uh, no.  I much prefer: gpspipe -R -x 30 > raw.log  
> 
> Attached you can find such a file as .gz compressed.

Got it.

> > The sat data is not just in GPGGA, and you might be getting some
> > binary messages as well.

As I expected, your receiver is sending binary, not NMEA, 
That is why the gpspipe --nmea is misleading.  The GPGGA is psuedo.

> >> Now I realized that the logs before the upgrade had 86400 lines +/-
> >> 1. This is one line per second. Now the files have about 10800
> >> lines. Each night I do a logrotate.  
> > One line a seconds???  No way that is enough!  
> 
> Big sorry, my mistake. Before the update there are 86400 lines per
> day, but after the update I see now 108000 lines per day ( I missed a
> zero previously )

My comment still stands: 86400 lines per day of sat data is WAY too
lettle.

Exactly how are you "counting lines per day"??

I'm totally lost.

> >>> By signal, I mean: L1, L2, L5, B2, etc.
> >>>
> >>> You can see this using cgps, or xgps.
> >>>     
> >> Hmm ? Not sure what you mean. When I start xgps I don't see the
> >> bands.  
> > Yes, you do, but not obvious.  Hover your mouse over a sat icon in
> > the skyview.  
> 
> Yes, when I move the mouse over a sat I see a lot of additional 
> information. But what is the interesting value ?

Well, all of it, but in this case, most important is the signal: L1, L2,
etc.

Now if you look at the sat list, to the right of the "svid" column, is the
"sig" column.

> Do you mean the SIGID. Some of the satellites have one SIGID, same
> have 2 values.

Yes.  Those are counted, at least in some NMEA versions, as two
satellites, not one satellite with two signals.

> >> What I see is a flickering screen which changes about twice per
> >> second.  
> > And that fkilering screen is chock full of data.  
> 
> I tried to attach a 3 second video. But this is too big for a mailing 
> list with a limit of 400 KB.

No need, the flickering is normal. Your receiver dribbles the data to
gpsd, which dribbles it to xgps.

> >> On the bottom right corner in one screen there is sat seen: 50 and
> >> used 31, the other one say seen: 78 and used 71  
> > And both can be true.  
> 
> Why ? Independent "what is a satellite" it should not change twice 
> within a second.

It does not "change" it "adds".

> > NMEA uses both definitions.  Very confusing.  
> 
> But colloquially I would say a satelite is that what I can see on the
> sky.

Sadly, GNSS does not use colloquial language.  Complain nto NMEA, they
keep changing the definition, to exxtend an old secret standard to a
new world.

> >> This is the way I have done it:
> >>
> >> mayer# gpscsv -n 10 -c SAT --header 0 | awk -F, '
> >> BEGIN {
> >>     GNSSID=99999 ;
> >>     SVID=99999 ;
> >>     PRN=99999 ;
> >> }  
> > You are ignoring the sigid.  So you will count PRN 20/L1 and PRN
> > 20/Las as one "satellite", not two.  
> 
> Fine for me. I want to know the number of satellites and not signals.

Then consider your "satellties" and "NMEA satellites" are NOT the
same thing.

> And how do I get the SIGID ? Obviously not with gpscsv.

Sure you can.  But, I just tried part of your example:

$  gpscsv -n 10 -c SAT
time,gnssid,svid,PRN,az,el,ss,used,health
2024-05-13T19:18:51.000Z,0,5,5,152.0,34.0,36.0,True,1

Ah, I see sig is not in the defaults for SAT, you would need to ask for
it directly.  With the -f option, but that gets messy. I just pushed a
fix to gpscsv to add sigid to the defaults.

It is gonna be years before this sigid thing is resolved, if ever.  The
industry gas yet to agree on a consistent naming schemes for sigid, so
portability is zero, and shifts monthly.

> If I am correct the OID for the number of satellites is 
> .1.3.6.1.2.1.25.1.32

That is NOT gpscsv, that is gpssnmp.  SNMP is even more messed up
by the progress in GNSS than NMEA.

The way SNMP count satellites is yet again a different way than NMEA,
UBX, etc.  If you want to retain your sanity, stay away from SNMP.

> This is also jumping between 2 values:

Because the SNMP spec does not allow for what your F9P is sending.  The
F9P sends one set of gnssid:svid:sigid tripples for one sigid. gpsd
sends that on, then the F9P send a second set of gnssid:svid:sigid
triples for another sigid. gpsd sends that on.  Potentially up to 12
sets!

There is no way for gpsd to know when another set is coming, so it passes
on each set as it receives it.

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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