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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 10:49:04 -0700

Yo Curtis!

On Wed, 27 May 2020 12:01:41 -0500
Curtis Olson <address@hidden> wrote:

> Anyway, after all of this I now am actually getting SUBFRAME messages
> out of gpspipe -w


> I would be excited except I have no idea if the decoded orbital
> parameter values are plausible.

That is real easy to check.  Just compare to an online copy of the
empheris.  Not high on my list, but I'll get to it if someone does not
do it first.

> Is there interest in my code changes to support UBX-RXM-SFRBX?  The
> updates to driver_ubx.c are super straight forward, and then there is
> the one change to the preamble check in subframe.c that we would have
> to arm wrestle over which way is right:

> -- a/subframe.c
> +++ b/subframe.c
> @@ -173,7 +173,7 @@ gps_mask_t gpsd_interpret_subframe(struct
> gps_device_t *session,
>               tSVID, words[0], words[1], words[2], words[3], words[4],
>               words[5], words[6], words[7], words[8], words[9]);
> -    preamble = (uint8_t)((words[0] >> 16) & 0x0FF);
> +    preamble = (uint8_t)((words[0] >> 22) & 0xFF);

We'll need to handle both >>16 and >> 22.  Each is right in different

Where is the rest of your patch?

> Again, the only way I can make sense of the existing check is (a) it
> doesn't really work,

It most cerrtainly works now.  I have hand checked some output a while

> or (b) the receiver it works with is doing some
> pre-parsing/reduction (by shifting away 8 of the original bits for
> some reason) of the "telemetry word" (TLM) which is the first word of
> the subframe message.

Yes, each receiver is "helpful" in different ways.

> Let me know and I can generate a merge request (or easier) send you a
> git diff -c?  Or I can just carry the changes here locally and
> probably be happy enough ... presuming I've put the values in a form
> that are actually getting decoded correctly.

MR's are preferred.

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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