[K4RY] BAuburn packet

[Keith Warren]

Hello Kris and anyone interested in APRS,

If you want to understand the B in your APRS comment field please refer to 
the APRS specification related to compressed data format.

Here is a link so you can follow along :
http://www.scribd.com/doc/79359362/23/The-Compression-Type-T-Byte

Your K4RY packet looks normal, you have a compressed Lat/Lon field 
sandwiched between Icon symbol set characters , in your case the \ and -

which means Alt house icon.  The compressed data is between the \ and -

On page 37 of the referenced APRS document in the table called Compressed 
Position Data you see the last symbol code is followed by two bytes that 
could represent course/speed or Radio Range or Altitude.

You are not using those bytes and they are blank.  Following those two bytes 
is another byte called "Comp Type" in the table and that is the compression 
type used.

This is the location in the packet where your B resides, after that begins 
the comment field where you have Auburn University ARC igate

Now the B is a code that represents the Compression Type and is explained on 
page 39 in the table and subsequent text.

If you take the ASCII B which is a decimal 66 and subtract 33 to get the 
decimal value that created that ASCII scan code, 66 minus 33 equals 33.

That translated to binary is 100001 and if you put that into the table on 
page 39 we will see what it means.

GPS fix =1 current
NMEA source 00 = other
Compression origin 001 = TNC BText

So , apparently it is a correctly formed packet but a lot of APRS programs 
are just not doing anything with that byte and appending it to the comment 
text

Unfortunately , that may be a lot of the programs out there. I know my old 
version of WinAPRS shows BAuburn and possibly UI-View does as well.

My son & I are writing a new APRS program right now and we will try to 
decode this correctly in our program.

In order to remedy this there are a couple of things you could do. One would 
be to use regular non-compressed format, and your status text or comment 
field would not be preceded by a Comp Type character.

If you really must have compressed format, you could try to use Mic-E type 
data which is more common and decodes correctly on a majority of programs.

If you need further help with this please let me know.

73,
Keith Warren
AK4KO

-----Original Message----- 
From: Kris Kirby
Sent: Monday, April 02, 2012 11:05 PM
To: K4RY at eng.auburn.edu
Subject: Re: [K4RY] Meeting tomorrow, 12:30 Broun Conference Room

On Mon, 2 Apr 2012, Kyle Owen wrote:
> That's right...it's that time again! We'll be in the conference room
> and can discuss some more software defined radios, as that did seem to
> spark some interest last week. Hopefully we can talk about what it
> will take for such a unit to receive on some other bands, particular
> in the HF spectrum.

I think I figured out the "B" from the APRS text. In MIC-E mode, the
stations transmit a shortened, compressed status. The B was part of the
packet. Some TNCs can't handle unprintable characters.

--
Kris Kirby, KE4AHR
Disinformation Analyst
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