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Questions about decoding subframe messages (ublox F9P)

From: Curtis Olson
Subject: Questions about decoding subframe messages (ublox F9P)
Date: Fri, 22 May 2020 10:08:33 -0500

Hi, this is my first time posting to this list, hopefully someone can point me in the right direction, or tell me if I have to do things the hard way.

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

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 UBX-RXM-SFRBX.  I have verified these are coming through, and verified the gpsd is reporting the basic TPV message and also the individual satellite pseudorange, etc. values from the RXM-RAWX message.

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.

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.

But so far, I haven't figured out if there is a mechanism to connect this all up in gpsd?  Is there a way to coax gpsd into parsing my subframes and extracting all the values for me?  How would I ask for the values back from gpsd?

If gpsd already has this capability, I would love to be able to use it.  

Thanks in advance,

Curtis Olson
University of Minnesota, Aerospace Engineering and Mechanics, UAS Lab

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