AFSK decoder enhancement?

[Dennis Eckert--KB7ST]

A friend lent me a Kenwood TM-D710GA dual-band radio with impressive features -- built-in GPS and APRS. But I ran into frustrating hours trying to figure out why I could not be seen on aprs.fi.

GQRX AFSK1200 decoder showed this beacon output --

"09:20:53$ fm KB7ST-9 to T7QXYR-0 via WIDE1-1,WIDE2-1 UIv PID=F0
          `/=Nl .-/]"8.}444.900MHz t103 +500146.52 V-Alert="

(As you can see, there is no GPS data so that suggested I had the wrong settings on the radio to output GPS data that would put me on aprs.fi or findu.)

After several frustrating days of reading and re-reading Kenwood docs and watching YouTube videos, for some reason I decided to try Direwolf packet decoder software as mentioned in the GQRX help document.  And voila!

Here is Direwolf output ~

"KB7ST-9 audio level = 22(8/10)   [NONE]   |||||||__
[0.3] KB7ST-9>T7QXYR,WIDE1-1,WIDE2-1:`/=Nl <0x1c>-/]"7{}444.900MHz T103 +500146.52 V-Alert=<0x0d>
MIC-E, House, Kenwood TM-D710, In Service
N 47 18.9200, W 119 33.5000, 0 MPH, alt 1224 ft, 444.900 MHz, +5M, PL 103.5
146.52 V-Alert"

I had no other way to figure out what was being transmitted other than to use GQRX's AFSK decoder.  I searched through group posts but didn't see a message that discussed this particular issue.  After all that that Mr. Csete has done for us, I'm somewhat reluctant for him to address.  Is there another programmer here who could help out?

The 'rest of the story' is when I discovered that no other station could hear me to make me visible on aprs.fi.  The digi that used to hear me (WAHITIS) is down for the winter.

Dennis -- KB7ST in central WA State

[David Ranch]

The Gqrx AFSK1200 packet decoder is just a basic AX.25 packet decoder.  It doesn't have any additional protocol stack decoders such as APRS in it.  That's perfectly fine in my book and I feel it serves it's basic functionality very well.  If you want to fully decode APRS with compressed POSITs (GPS position) like what was in your original example (the "`/=Nl .-/]"8.}" string), then Direwolf is an excellent choice.  You can also try other APRS programs as well if you want maps, etc.

--David
KI6ZHD

[Alexandru Csete]

Hi Dennis,

I'm not sure I understand what you are asking for... Is gqrx not
decoding all the packets?
It should print position data when received, just not formatted as
nicely as direwolf does.

Alex

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[Robin Gape]

Dennis,

adding to what Alex has said, and slightly intrigued by the differences in output, leads one to the APRS specification (to be found at http://www.aprs.org/doc/APRS101.PDF, and with subsequent revisions, not really relevant to this question). Page 20 of that document, towards the bottom, reads:

Two APRS data formats use base-91 notation: lat/long coordinates in
compressed format (see Chapter 9) and the altitude in Mic-E format (see
Chapter 10).
Base-91 data is compressed into a short string of characters. All the
characters are printable ASCII, with character codes in the range 33–124
decimal (i.e. ! through |).

Now all becomes clear! GQRX is presenting the Base-91 data as is, but Direwolf is parsing and decoding the APRS data back to its original form. In short, they are both correct!

HTH, 73,

Robin, G8DQX

[Alexandru Csete]

Okay guys, I'm getting seriously confused here :)

Gqrx decodes and prints position information in plain text, but it
will not pretty-print it as direwolf does. Just google "gqrx aprs" and
look at the videos and images. Look for characters N, S, W, E - the
numbers in front of those will be the coordinates.

It's hard to say why gqrx didn't decode the TM-D710GA. Perhaps the GPS
was not locked yet. Perhaps the signal was too weak. Perhaps it was
too strong. We have practically no information about the setup and
settings other than it didn't work.

Alex

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[Alexander Fasching]

Is it possible that the 0x0d character (carriage return) causes
problems during printing?

[Robin Gape]

Alex, Alexander,

perhaps neither myself nor David made ourselves clear. The APRS frame which Dennis quoted has encoded geographic co-ordinates. The encoding is Base-91. The APRS specification also allows for co-ordinates to be sent en-clair. Both are correct.

As to why one would choose encoding (which the specification refers to as compression) as opposed to en-clair transmission, that is quite another question, way off-topic for GQRX!

If one really wished to use GQRX as a front-end for APRS, then feeding UDP audio from GQRX to Direwolf might be a good way to go.

Robin, G8DQX

[Alexandru Csete]

Thanks for the clarification, Robin.

Alex

On Wed, Mar 20, 2019 at 12:44 AM 'Robin Gape' via Gqrx SDR

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