[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
Re: ✘NTRIP and RTCM
From: |
Gary E. Miller |
Subject: |
Re: ✘NTRIP and RTCM |
Date: |
Mon, 6 Dec 2021 14:35:43 -0800 |
Yo Greg!
On Fri, 03 Dec 2021 15:39:19 -0500
Greg Troxel <gdt@lexort.com> wrote:
> "Gary E. Miller" <gem@rellim.com> writes:
>
> >> > All from the Oregon ORGN network. Many thanks to them for
> >> > allowing me cess.
> >>
> >> Indeed, it's very nice of the states to allow public access.
> >> MassDOT runs MaCORS which anyone can use.
> >
> > Still not as easy as many other countries.
>
> Yes, I guess they could run a caster that does not require login.
And a lot of the USA has no nearby casters. Login or not, nothing
available.
> >> I am fuzzy but I gather that RTCM2.3 typically is pseudorange
> >> corrections, and GPS only. It may be L1 only.
> >
> > Yes, in this case, GPS and L1 only.
>
> So you lose 3 of 4 constellations, and you lose the abiltiy to
> estimate iono delay from L1/L2. Interesting that it makes things
> worse, but I am not super surprised.
ONe would hope the firmware would recognize the degradation and
stop using the RTCM2.3. Just like it stops using any satellite
that provides bad data.
> >> Do you have SBAS disabled, so it is really "no corrections".
> >
> > I do not have SBAS disabled, but that F9P rarely uses it. SNAS is
> > almost exactly RTCM 2.3
>
> So no L2C, no GLONASS even? I suppose someone(tm) should get the SBAS
> stream and decode it.
Thera are so few L2C GPS satellites, not much use. RTCM 2.3 does support
GLONASS, but not available to me nearby.
> >> Indeed, and you can see the resolution of the coordinates which
> >> looks like most of 1 cm. I wonder if this quantization is in the
> >> raw logs or if it could be in the analysis chain.
> >
> > I'm running in normal mode, with none of the "High Precision"
> > messages. Moany of the UBX-NAV-PVT data items are in mm. Worth a
> > test.
>
> I've been using high-precision NMEA, because my real goal is to log
> data in Vespucci for mapping/surveying, and I think I'm seeing well
> sub cm quantization.
What is "high-precision NMEA"? NMEA does not specify any resolutions.
u-blox HP mode gets to 0.1 mm. UBX-NAV-PVT (default) is 1 mm.
I'll recheck gpsd JSON path to confirm it can hadle 0.1mm.
> >> What I would find really useful is:
> >>
> >> record fix mode with each point as metadata
> >
> > I have all the raw data. Nothing more can, or could, be recorded.
> > And it is all 3D.
>
> Great, so you should be able to associate with each position what kind
> of fix it is.
They are all 3D differential. Nothing more to say. u-blox does not
report RTK float of RTK fix. AFAIK they never really meant anything.
> >> carry that through analysis and color the dots based on
> >> autonomous/differential/RTK-fixed/RTK-float
> >
> > u-blox is bad at reporting that.
>
> Do you mean there are no messages that say if the solution is from RTK
> FLOAT, RTK FIX, just nav satellites, differential etc
Correct.
> like you get in
> the mode field of the main NMEA message?
Uh, no. The NMEA mode field can also report RTK fix, RTK Float, and
other variants. Badly defined and inconsistently implemented.
> >> be able to use an input position in the analysis instead of the
> >> average. I bet your tight value in fix is pretty much right.
> >
> > Lost me. Explain?
>
> I think the analysis is computing an average over the hour, and then
> the CEP is the distance such that half the points are closer to the
> average. But that's not really "error" because you don't know the
> right answer.
I could go to a geodetic benchmrk, and repeat. But even they are not
accurate to 0.1mm.
> I am suggesting, more concretely:
>
> have the analysis program output the computed average in geojson or
> something into a file, maybe optionally
gpsprof does that.
> have an optional argument to read a "correct answer" from a file in
> that same format
If I could get a "correct answer", but I have none. So not useful. And
the math to add a constant error is trivial, if I knew what that was.
> when given a position this way, compute in terms of that position
> being the truth, rather than assuming that the average of *this
> session* is correct.
See above.
> What you would do is use the output from the MAX which appears to be
> RTK FIX, to process the rest.
It is the only output I have, so I dont care what it appears to be.
> That would show you biases among them,
> even if you aren't still really sure whihc is right.
Lost me again. What "them".
> >> I am curious what antenna you are using.
> >
> > The u-blox one, in a window sill.
>
> Do you mean the slightly larger mouse-style magmount one that is about
> $60? I have one and need to try it.
They onlye sell one:
https://www.u-blox.com/en/product/ann-mb1-antenna
I find it works well.
> >> What I don't understand is why you aren't able to get into FIX
> >> with a single reference a mile away.
> >
> > I don't know if I can or can't. But that is my long term
> > experience, and other see simiilar.
>
> I would love to see data with something like the survey or calibrated
> from this line. I am guessing you have the MB.
> https://www.ardusimple.com/simpleant2b/
Unclear what models they are selling. I have one of the mushrooms. It
has a 40db LNA, so good for feeding a power splitter. I have not tried
it with the F9P.
> So it would be really interesting to see data from your receiver/ant
> with the antenna fully outside in a repeatable postion.
Givne the data cluster at 1cm, I would prolly not be able to see any
improvement.
> I have a
> particular deck railing nail I place the stand over.
Ditto. Now if any 2 GPS ever gave me the same position of that nail...
I am running into one big problem. When I use the iMAX data, the
F9P CPU gets overloaded. It stopps sending all but the most basic
messages, the txbuf is at 99 to 100% full:
UBX-MON-TXBUF:
pending 0 12031 0 0 0 0
usage 0 99 0 0 0 0
peakUsage 0 99 0 11 0 0
tUsage 99 tPeakUsage 100 errors xc0 reserved1 0
errors (mem alloc) limit (0)
and constantly sending error messages:
$GNTXT,01,01,00,txbuf alloc*61
So the extra precision comes at great cost to other available data.
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
pgpik4E38s5h5.pgp
Description: OpenPGP digital signature