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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: Fri, 22 May 2020 10:45:06 -0700

Yo Curtis!

On Fri, 22 May 2020 10:08:33 -0500
Curtis Olson <address@hidden> wrote:

> First; what am I trying to do?  The long term target is to develop a
> tightly integrated kalman filter for attitude and location
> estimation.

Good luck with that, certainly the highway you the hard way.

> We would like to run multiple instances of our filter on
> different subsets of satellites.  All the work we do in our lab is
> published under the MIT open-source license.


> By my understanding,
> this project will require knowing satellite orbital information,
> locations, etc., and (by my understanding) this requires reading and
> parsing subframe messages.

Require the orbital info?  Yes.  From subframe messages?  No, many
sources for that info.

Without details no way to sugggest options.

> We have a ublox F9P receiver up and running here.  I have it setup to
> output the following messages: NAV-PVT, UBX-RXM-RAWX, and

Send here the output of this, so we can see what you have:
        ubxtool --p STATUS -p CONFIG

> I telnet'd to the gpsd daemon and ran a manual WATCH command with raw
> enabled and I am seeing the raw subframe messages (class=0x02,
> id=0x13) in the stream, so gpsd is at least seeing/receiving the
> RXM-SRFBX messages from the ublox F9P.

Good.  BUt only first step in a long journey.

> I see the gpsd code has subframe.c which looks like it knows how to
> parse the subframe messages.  I see lots of patches in the gpsd-dev
> list related to subframe parsing fixes.

Yeah, much of it.  We dont know of many users of those messages
so there will be bungs in there.

> But so far, I haven't figured out if there is a mechanism to connect
> this all up in gpsd?

You don't.  Your gpsd is doing exactly what it needs to do as a daemon.
You effort will be as a client.  Look at the gpsd clients, obviously
influenced by your programming language of choice.

> Is there a way to coax gpsd into parsing my
> subframes and extracting all the values for me?

As you say, it already does that.

>  How would I ask for
> the values back from gpsd?

Easy way to start:
        gpspipe -w

Have you looked at RTKLIB?  Or some of the other "soft gnss" projects?
They take the raw + orbital and generate fixes.

This may also be interesting to you:

Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
        address@hidden  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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