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Re: Questions about decoding subframe messages (ublox F9P)

From: Curtis Olson
Subject: Re: Questions about decoding subframe messages (ublox F9P)
Date: Tue, 26 May 2020 11:12:03 -0500

Hey Gary!

I am back with a quick update.  I uninstalled the system gpsd (fedora 32, version 3.19), then downloaded, compiled, and installed version 3.20 from the web site.  [ scons && scons check && sudo scons udev-install ]

I started up gpsd with:

gpsd /dev/ttyACM0

ACM0 is the port that the ublox F9P shows up on.  I can run gpspipe -w and tons of correct looking info is spit out.  TPV, RAW, and SKY messages.

However, I'm still not able to run the ubxtool command you suggested:

$ ubxtool -p STATUS -p CONFIG
ubxtool: poll CONFIG
ubxtool: poll CONFIG not found

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?

Also, gpspipe -w is not outputting any SUBFRAME messages.  If I filter out all the TPV, RAW, and SKY messages this is all that is displayed no matter how long I wait:

$ gpspipe -w | grep -v TPV | grep -v RAW | grep -v SKY
{"class":"DEVICES","devices":[{"class":"DEVICE","path":"/dev/ttyACM0","driver":"u-blox","subtype":"SW EXT CORE 1.00 (94e56e),HW 00190000","subtype1":",ROM BASE 0x118B2060,FWVER=HPG 1.11,PROTVER=27.10,MOD=ZED-F9P,GPS;GLO;GAL;BDS,QZSS","activated":"2020-05-26T15:35:24.437Z","flags":1,"native":1,"bps":57600,"parity":"N","stopbits":1,"cycle":1.00,"mincycle":0.25}]}

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.

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, so again, I believe the receiver is config'd to send the messages correctly.  Still gpspipe doesn't seem to recognize or display the subframe messages? 

For example:

$ ubxtool -p MODEL
ubxtool: poll MODEL

 mask 0x1 dynModel 0 fixmode 0 fixedAlt 0 FixedAltVar 0
 minElev 0 drLimit 0 pDop 0 tDop 0 pAcc 0 tAcc 0
 staticHoldThresh 0 dgpsTimeOut 0 cnoThreshNumSVs 0
 cnoThresh 0 res 0 staticHoldMaxDist 0 utcStandard 0
 reserved x0 0

  ACK to Class x06 (CFG) ID x24 (NAV5)

. [ snipped out nav-pvt and nav-raw messages ]

 gnssId 0 svId  30 reserved1 0 freqId 0 numWords 10
  reserved2 3 version 2 reserved3 0
    dwrd 22c03b02 92b62998 03b4001e 86f8dcab 2289f3fe 97eecd12 a823022a 860e0ff0 003fee69 b883ce74

 gnssId 0 svId  17 reserved1 0 freqId 0 numWords 10
  reserved2 19 version 2 reserved3 0
    dwrd 22c03b02 92b62998 03b4001e 86f8dcab 2289f3fe 97eecd12 a8233a74 13ce103d 80000db2 88e0ae18

This tells me that gpsd is indeed seeing the subframe messages when they come in and recognizing them as such.  It's parsing the packet header (figured out svId, numWords, etc.) and is displaying the actual subframe bytes (via ubxtool at least.)  However gpspipe -w does not display any of these subframe messages for me.

A few tiny steps forward ... ?

I am still interested in:
  • figuring out a client (or proper gpspipe options?) so I can see the subframe messages from my ublox f9p?
  • evaluating if gpsd is able to parse the actual subframe bytes, at least for the USA gps constellation.
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.)

Thanks again !!!! for all your help and patiently sharing your knowledge.  I realize none of us know all the things, but It's really quite amazing to stumble onto someone that has so much in depth knowledge of this field and the patience to share with us dummies who are just trying to come up to speed on some of this.

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

On Fri, May 22, 2020 at 2:43 PM Gary E. Miller <address@hidden> wrote:
Yo Curtis!

On Fri, 22 May 2020 14:00:30 -0500
Curtis Olson <address@hidden> wrote:

> > This works and gives me "TPV" messages (I'll assume you won't call
> > in an airstrike with this information!) 
> Look at the RAW messages, there is the orbital data.

Sorry, typo.  Look in the SUBFRAME messages.

They come in very infrequenntly, you will need to look at a lot
of gpspipe JSON before you see them.

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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