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[aprssig] "Best" packet decoder solution

Scott Miller scott at opentrac.org
Wed Dec 29 18:42:58 UTC 2010


I really wouldn't encourage users running IGates to just turn on 
PASSALL.  Seems like it would be a very bad idea.

I think the heuristics are going to get complicated when you have to 
distinguish mangled lat/lon packets from compressed mode packets and 
everything else that might be there.

Now, if you encoded lat and lon each as 24-bit integers in Gray code, a 
single-bit error anywhere in your position would only result in a 
maximum 2.4 meter error, while needing only 6 bytes of payload! ;)

Scott
N1VG

On 12/29/2010 10:22 AM, Robert Bruninga wrote:
>>  I think if we're going to see any FEC scheme
>>  succeed for APRS, it needs to be either
>>  simple to implement or ...
>
> There is HUGE code redundancy already built into APRS that could
> be exploited.  Looking at the following standard APRS oosit"
>
> !DDMM.MMN/DDDMM.MMW$...
>
> Most all of those digits are decimal digits,(only 4 bits) yet
> they are encoded in 8 bit ascii.  So a bit error in any digit
> only has a 50% chance of messing up the 4 bits needed.
>
> Further, For the LOWER order bytes, every posit cannot be any
> further away from the previuos one by the SPEED times TIME.
> Further, even if the error is in a given BYTE, you can at least
> convert it to an ambiguous position within the degree of
> ambiguity of the messed up byte.
>
> Further, for the upper order bytes, you can infer the position
> on RF because your digipeater has to be within a degree of LAT
> or LONG to even hear it.
>
> So, simply run your TNC with PASSALL ON, and you will then see
> packets even with errors.  Then use the above huristics to
> decide on the best estimated posit.  The give it your best shot
> and give it the QUESTIONMARK symbol so that you can see that it
> is estimated until a solid good packet comes through.
>
> I would think this can greatly improve the receipt of weak
> signals.
> Of course, it has been shown that AX.25 errors are rarely single
> bit errors but runs of bits and so it is not quite so
> simplistic, but it sure is a first step and can be done on ALL
> EXISTING PACKETS on the air...
>
> Bob, WB4APR
>
>
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