Re: DGPS / SBAS

[Gary E. Miller]

Yo Nick!

On Tue, 5 Dec 2023 10:11:11 +0000
Nick Taylor <nicktaylor@dataskill.uk> wrote:

> Apologies for screenshots -

You had no way to know.

> have now managed to freeze cgps so that I 
> can cut and paste from it...

^S

> We don't have xgps on our devices.

You don;t need xgps on your devices, as long as you have xon xgps
somewhere.  The start gpsd with -G, and connect from a remote
host with xgps.

> Just to clarify, I don't think there's anything wrong with gpsd, but
> my observations just seem strange:

We don't know, until we know.

> My window is single glazed but old and thick - I found references to
> old glass having metallic traces that obv aren't good for distant
> radio signals.

Yes, the faint 1.5GHz signals do not pass trough solids well.

> With antenna inside the glass GPS works fine but no SBAS satellites - 
> and obv reduced signals, if I poke the antenna outside then signals 
> increase & I then get SBAS signals so that gps can obtain DGPS fix.

SBAS are geostationary. From your cgps:

    SB123    36  26.0 150.0

That is an elevation of 26 degrees and an azimuth of 150.  Be sure the
path to there is clear.

Remember all GPS and SBAS have the exact same frequency, modulation, and
power on the surface.  The only difference is Gold code (PRN), elevation
and azimuth.

> The weirdness is that once I get DGPS - I can move antenna inside and 
> the DGPS stays - even for several days.

Not weird.  SBAS broadcast the well over 20 year old RTCM2 corrections.
Never very accurate and they are valid for a while.  I don't recall how
long.

> SBAS shows SNR of 0.0 and if
> I power cycle the gps then fix reverts to std 3D GPS and never gets
> DGPS again.

Once your receiver has downloaded the current corrections, it is a 
waste of a channel to use ot for SBA, until the next corrections are
out.

> I'm attaching cgps1.txt & cgps2.txt where gps still has DGPS but no
> SBAS seen - and cgps3.txt after power cycle. Along with this gps5.log
> which corresponds with cgps1 & 2.

And what do you think they show?

> The only explanations that I can think of are:
> 
> 1. gps device has a bug

Several here have looked at the NMEA and think it is good.

OTOH, the MTK-3301 is pretty old.  A newer GPS will work better.

> 2. There is some low power SBAS signal that's enough to maintain DGPS 
> but not enough to initiate it?? (Is this even possible??)

No. The receiver is honest on what it is listenting to.

> In any case none of this is a deal breaker - just a curiosity...

Curiosity killed the cat.  Satisfaction brought him back.

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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