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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 <> 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.


> > > 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

> 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...

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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