Re: how to forward RTCM data to gpsd?

[Gary E. Miller]

Yo Greg!

On Fri, 11 Dec 2020 20:19:44 -0500
Greg Troxel <gdt@lexort.com> wrote:

> I wonder why you are only getting GPS from the local station, but
> presumabbly it's old and not sending Galileo and GLONASS corrections.

RTCM 3 is GPS and GLONASS only.

> > RTCM3 can perform better.  
> 
> If by that, you mean "receiver does RTK based on carrier phase data
> carried in RTCM3", then agreed.

Plus Galileo, Beidou, etc.

> Repeating something I said earlier my local RTCM3 provider is now
> sending data on all 4 constellations.

Not all 6?

> > EGNOS is also basically RTCM2, so the limitations of the protocol
> > are similar.  One would think local corrections are better than area
> > corrections, but the real limitation is the receiver itself.  
> 
> There are two things going on at once: the error in pseudorange
> measurement, which is the same, and the errors due to pseudorange
> correction information or the standard model vs the actual delays.
> As I understand it SBAS has a fairly coarse grid so local DGPS if
> quality should be pretty good.

Depends on how you define "good".  I have never seen much improvement
without the local being very close (< 500m)

>  But it may be also that with 3
> constellations and the resulting high number of satellites, the SBAS
> approach is already good enough this doesn't matter.

I have run tests with GPS+SBAS and GPS only.  Both using SBAS correction.
Adding SBAS to the fix decreases the fix quality.  YMMMV.

> >> (**) Something strange:
> >> When not using dgps (so all three 3 + sbas enabled), xgps did say
> >> "3D FIX DGPS", but all three egnos satellites where marked as
> >> "used: no".  
> >
> > GPS sats orbit a lot lower than SBAS ones.  The accuracy of their
> > signals is also lower.  Thus they are only used in a fix when there
> > are not enough GPS in sight.  Some modern (u-blox 9) dont even
> > bother listening to SBAS at all.  
> 
> It would be nice if receivers were clear about "using corrections from
> this SBAS" vs "using this SBAS's signal to observe pseudoranges and
> including those in the position estimate".  It's very natural for
> someone to expect the using: field of an SBAS satellite to mean that
> corrections are used.

Some do.  Depending on your u-blox model ubxtool will tell your the
difference. Plain "used" means the ranges were used.  The DGPS flag
is set if SBAS or RTCM corrections applies.  So if you have no
local RTCM, the DGPS flag tells you if the SBAS corrections were used.
Indepene nt of the "used" flag.

> >> So what 'DGPS' does xgps refer to when all the egnos/sbas
> >> satellites are marked as 'not in use'?  
> >
> > Every chip/firmware is different.  Broadly DGPS means some
> > corrections have been used.  The RTCM2 from a SBAS may be used to
> > improve the fix even when ranging from the SBAS is not.  
> 
> Operationally there is a mode in the data from the receiver which has
> a small number of values and gpsd makes it available.
> 
> See gpsd_json(5).

Specifically "status" in the TPV message.

Also see the u-blox doc.  Things are also a bit different in NMEA vs.
binary mode.

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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