Re: [gpsd-users] count of satellites_used is incorrect

[Gary E. Miller]

Yo David!

On Thu, 26 Apr 2018 19:45:26 +0100
"David J Taylor" <address@hidden> wrote:

> when I try to get the count of "satellites_used" by the following
> little program:
> []
> the count of "satellites_used" decreases to one after about half an 
> hour. The count of "satellites_used" in the program "gpsmon" stays 
> meanwhile nearly constant at eigth to ten. The other values from the 
> gpsdata structure correspond to the values on "gpsmon".

So you think gpsd is correct, but your program is incorrect?  Or?

Got some raw data to share?  Like a gpspipe -R > tmp.log

There are several ways to get the 'used' and 'seen' counts.  They are
often different.  Some GPS emit those as simple data items along with
other data.  Other GPS emit skyview data on each saellite, then gpsd
just adds up the number of sats marked used.

This depends on the GPS model/firmware and on how the GPS is configured.

You may very well get a count of 10 used sats, then a skyview with
11 sats in use.  Some GPS limit the numnber of sats in the skyview,
maybe to 12, but report the real in-use number elsewhere.  Some GPS
may put some sats in one type of skyview in one message, and others in
another skyview message.  This is a problem with newer GPS that get GPS,
GALILEO, GLONASS, BeiDou, etc.  Since each constellation starts their
PRN a 1, there is no agreed encoding for multiple constellations.

> I use a similar program on my Raspberry Pi, so in case it helps:
> 
>   http://www.satsignal.eu/raspberry-pi/monitoring.html#gps

That page shows some failure to understand what is going on.

"I have played with a Python script for this, but the snag is that
you need to wait for the gpsd to emit a message with the required
information.  I therefore also tried writing my own two programs to
retrieve the information directly from gpsd,


gpsd emits the SKY JSON whenever it receives skyview data.  Going
'directly' to gpsd does not change this.  If the client wants more
frequent SKY JSON, then the GPS must be set to output skyview data
more often.

RGDS
GARY
---------------------------------------------------------------------------
Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
        address@hidden  Tel:+1 541 382 8588

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