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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: Tue, 26 May 2020 10:08:32 -0700

Yo Curtis!

On Tue, 26 May 2020 11:12:03 -0500
Curtis Olson <address@hidden> wrote:

> I started up gpsd with:
> gpsd /dev/ttyACM0

You forgot the -n.

> However, I'm still not able to run the ubxtool command you suggested:
> $ ubxtool -p STATUS -p CONFIG

Because you installed 3.20 and not git head.  A lot of 9-series was
still not finished in 3.20.

First, you need to read and follow the Initial Setup here:

Then try:

$ ubxtool -p STATUS -P 27.10 -v 2
$ ubxtool -p CONFIG -P 27.10 -v 2

> I can run "ubxtool -h -v 2" and see a whole lot of other -p
> parameters and they seem to do things when I try them.  CONFIG and
> STATUS don't seem to be supported for this gps unit?  Would that make
> sense?

Nope.  What is missing is the option for two -p on the same command line.

> I know the gps is sending the subframe messages ... I've validated
> that several ways -- including watching the messages come in on
> u-center after a complete power-cycle of the gps.  They are still
> coming in today.

Not so fast.  When you switch from gpsd to u-center, and back, things

> It turns out that ubxtool mostly dumps out nav pvt and rxm-rawx, but
> once in a while it dumps an rxm-sfrbx message as they come through,

Of course.  All depending on how you configured your receiver to send
different messages, and how fast the receiver get them.

> so again, I believe the receiver is config'd to send the messages
> correctly.

Belief is for church.  Not here.

> Still gpspipe doesn't seem to recognize or display the
> subframe messages?


> This tells me that gpsd is indeed seeing the subframe messages when
> they come in and recognizing them as such.

RAWX subframes are not decoded by gpsd, not not relevant to your issue.
The JSON SUBFRAME are different than RAWX.

>    - figuring out a client (or proper gpspipe options?) so I can see
> the subframe messages from my ublox f9p?

First you need to get your device sending them.

>    - evaluating if gpsd is able to parse the actual subframe bytes, at
>    least for the USA gps constellation.

It is, at least most of the documented ones.  A lot of subframe stuff
is still CLASSIFIED.

> On our side I am checking if our project could proceed with just
> downloading the ephemeris data off the interweb.  (I suspect the
> answer will be a hesitant maybe, but longer term it would be super
> nice to not have an internet dependency when we test in the field.)

The advantage of downloaded ephererides is that they are more accurate
than the ones from the sats.  At least the ones computed after the sats
have completed their orbits and the actual orbits calcualted.  This
is the basis for PPP.

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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