[OZAPRS] Decoding HF APRS

[Robert Bruninga]

>>> rob.thirkettle at canterbury.ac.nz 07/06/06 11:40 PM >>>
>Another problem is that with AX-25 there is no forward error 
>correction just a check sum. If 1 bit is corrupted then whole 
>packet is discarded.

Back in early APRS on HF in about the 1993 time frame I wanted
to write a decoder that simply took advantge of the inherrent
overcoding in APRS packets to recover HF packets even with
some bit errors.

It was simple.  Just set the TNC PASSALL command to ON, so
that it passes all APRS packets to the PC even with errors.
Then compare the pcket against previous packets and sum
out the errors.  The LAT/LONG field for example is only using
4 bits of each character out of each 8 bit byte.  By looking 
at the new posit with errors, and comparing it to the last
posit, a little heuristics can eliminate many errors and arrive
at the right solution.

But once the internet came along and gave APRS the longhaul
without having to use HF, I never got arond to it...

de WB4APR, Bob


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