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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 20:12:48 -0700

Yo Curtis!

On Wed, 27 May 2020 21:26:09 -0500
Curtis Olson <> wrote:

> In MN we have something called MnCORS for which an account is needed.

In Oregon we have ORGN:

The only way gpsd works with it now is to specifically request
a nearby station.

>  My understanding is that MN has setup a number of base stations
> across the state all feeding into the same central system.  I can
> send a request to the server (essentially "here I am") and it will
> craft custom/virtual RTCM messages for my location.  I don't know
> exactly how this works under the hood but I am imagining some sort of
> interpolation between base stations based on the location I report
> myself at?

ORGN does similar.  No one knows how it works.  Not even well enough
to document the required protocol.  No doc, no gpsd support.

> For our system we have this little cell network xbee that runs
> micropython.  The xbee is connected (2-way) to UART2 of the u-blox
> F9P.

Common.  Works well.  Which is why gpsd has not needed to support
base/rover, just use the second serial ports.  There is one twist, the
transit time can not exceed 700 ms or the rover will reject the RTCM.

> The GGA message has lon/lat in clear text.  When the GGA messages
> start flowing, the xbee grabs the first one and crafts a custom
> request message from the MnCORS server (also by this time, hopefully
> the xbee has managed to connect up to the cell phone based interweb.)

I would like some doc on that "send GGA" protocol.  ORGN wants the
same thing, but has no idea what to send or how.

> 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...

>  We are
> hoping to investigate how much attitude accuracy improvements we
> could achieve with an RTK fix'd gps versus your stock ublox8.

Attitude?  RTK does not fix your attitude, unless you have two
on the same rover.

> Ideally for us this will give us better attitude (roll, pitch, yaw)
> estimates and that will help some of our projects.  But if we have
> sit around for an hour waiting for fix that may or may not happen,
> that's not good.

The 9-series is fast to initial fix, and then fast to get into RTK mode.


Looks nice, I've never flown HITS.  I'd have the next waypoint,
course, distance and time to next waypoint, plus baro altitude.

Is your alt MSL or HAE?  No ball?

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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