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Re: Definition of terms requested.

From: Gary E. Miller
Subject: Re: Definition of terms requested.
Date: Sat, 27 Nov 2021 10:57:06 -0800

Yo David!

On Sat, 27 Nov 2021 09:03:43 +0000
David Taylor <> wrote:

> Where can I find a definition of these terms as used in cgps?

The problem is, cgps has no control over what the terms mean.  They
come from your receiver, and gpsd and cgps just pass them on.

So the best place to look is in the documentation for your receiver.

Then you can look at "man gpsd_json" for how the variables are sent
from gpsd to cgps.

> Or
> what do the terms mean?
> - number of satellites visible?

Easy, the number of satellites that are over your receivers "elevation
mask", that the reciver has at least one of: ephhemeris, almanac or
has signal lock.

> - number of satellites used?

The number of satellites that youe reciver used in the current fix

receivers differ, so best to check your receiver doc first.

> I don't know whether these values are computed inside gpsd, or
> whether they are simply direct copies of the data sent from the GPS
> receiver itself.

From the receiver.

> The more I think of it perhaps the GPS receiver is
> responsible for the numbers...

gpsd knows nothing of epherii, almanacs or elevation mask, so the receiver
has to do that.  ubxtool, gpssubframe acan peak at those, but they are
debug tools, not in the users receiver -> gpsd -> cgps path.

> I ask because I recently moved an antenna about one foot higher to a
> slightly less shielded location (both indoors), and noticed a
> decrease in the number of satellites used, but with an increase of
> the number of satellites visible, accompanied by an increase of the
> average SNR.

That is normal.  Your receiver "sees" more sats, then it was able to
use a smaller set of them for best fix.

>    There are graphs at the bottom of this page:

I recommend ntpviz,  it gives similar graphs, and also gives you a 
lot of statistical summaries.  The statistics make it easier to see
smaller changes.

> I had (incorrectly) expected that the number "visible" would simply
> depend on the latitude and longitude of the observer, and should
> therefore be independent of antenna height or local obstructions.

Depends on your receiver.  Some just report above the "elevation mask",
others want to have recently has a signal lock.

>  So
> does "visible" include some SNR threshold somewhere?

Many.  Depending on your receiver there are many knobs you can adjust.

> Given that the location is a little better I expected an increase in
> SNR.

I'm unclear why you woud think that.
> I don't understand the decrease in the number of satellites "used".

The algorithms the receivers use are secret, but was can make guesses.
You will often get a better fix from four sats with high SNR and low
multipath, that from a large group of sats that have high multipath.

> It's a MAX-M8Q-0-01.

That you have a number of things you can tune.  RTFM.

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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