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Re: Questions about decoding subframe messages (ublox F9P)
From: |
Gary E. Miller |
Subject: |
Re: Questions about decoding subframe messages (ublox F9P) |
Date: |
Wed, 27 May 2020 21:09:50 -0700 |
Yo Curtis!
On Wed, 27 May 2020 22:50:00 -0500
Curtis Olson <curtolson@flightgear.org> wrote:
> > I would like some doc on that "send GGA" protocol. ORGN wants the
> > same thing, but has no idea what to send or how.
> I'll try to dig something up for you, I remember it making sense when
> I did it, but I think I was looking at a working example at the time.
Thanks.
> > > For our lab's purposes, one of our core strengths (ours, not
> > > necessarily mine) is a 15-state EKF (kalman filter) that estimates
> > > attitude and location using gyro, accelerometer, and gps.
> >
> > What? No magnetometer? 9-axis works a lot better than 6-axis.
> > Until you fly by a magnetic source...
> >
>
> This a fun discussion, but probably outside the scope of gpsd, but
> let me say a couple thoughts.
This was my son's project at OSU, So I heard a lot on the subject.
The gyro and accelerometers can be much higher rate than the GNSS
receiver, but when turning there is ambiguity, which the magnetomer
can resolve.
> If you wrap all that
> up into a kalman filter (I'm being extremely hand wavey) and iterate
> over time as you fly, magic starts happening.
More data (magnetometer) leads to better magic.
> Magnetometers are never perfectly calibrated, so adding those to the
> system is really just adding another thing that pulls your solution
> away from the truth. But they can help you start out with a better
> initial guess ... so it s a love/hate relationship.
Easy to calibrate. And gpsd gives you a good start on the Magnetic
Variation. With a snall change it could give you dip as well. There
are external programs that can take lat/lon and give even better
resultes.
> We do have a version of our EKF with mags incorporated into the
> measurement update (we can compare the sensed mag vector against the
> estimated ideal mag vector from the WMM.) This works pretty well
> when the mags are well calibrated, but if the calibration is not
> great, then it's a constant source of error driving the system away
> from truth.
Which is why you don't use it for that, you use it to disambiguate the
gyro and accelerometers.
> That stupid HUD
> visualization has caused me a ton of work!
Long time past when a decent HUD is open source.
> > Is your alt MSL or HAE? No ball?
> >
>
> This is MSL (basically GPS altitude after the EKF has smoothed it.)
Ugh, gps altitude != MSL. gps altitude == HAE.
Pilots use MSL.
> The ekf altitude still chases gps error, but I get a lot of the more
> subtle variations out of the INS and it's not as noisy as the baro.
Sadly, you need to be bug compatible with the baro.
> Sorry for all the typos, it's late here (for me). :-)
Blursday here...
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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- Re: Questions about decoding subframe messages (ublox F9P), (continued)
- Re: Questions about decoding subframe messages (ublox F9P), Curtis Olson, 2020/05/26
- Re: Questions about decoding subframe messages (ublox F9P), Gary E. Miller, 2020/05/26
- Re: Questions about decoding subframe messages (ublox F9P), Curtis Olson, 2020/05/27
- Re: Questions about decoding subframe messages (ublox F9P), Gary E. Miller, 2020/05/27
- Re: Questions about decoding subframe messages (ublox F9P), Gary E. Miller, 2020/05/27
- Re: Questions about decoding subframe messages (ublox F9P), Curtis Olson, 2020/05/27
- Re: Questions about decoding subframe messages (ublox F9P), Gary E. Miller, 2020/05/27
- Re: Questions about decoding subframe messages (ublox F9P), Curtis Olson, 2020/05/27
- Re: Questions about decoding subframe messages (ublox F9P),
Gary E. Miller <=