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gr-satnogs
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Improve AX.25 filtering to reduce false positives 

The CRC16 of the AX.25 seems that is too weak for the new
decoding approach and we get many false positives.

Now the AX.25 decoder, makes waits for at least two
occurances of the AX.25 SYNC flag.
This commit is contained in:
Manolis Surligas 2019-12-27 19:51:54 +02:00
parent 157e1c163b
commit d48e066a66
2 changed files with 3 additions and 8 deletions
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2
include/satnogs/ax25.h
View File

@ -31,7 +31,7 @@ namespace gr {
namespace satnogs {
const size_t AX25_MIN_ADDR_LEN = 14;
const size_t AX25_MAX_ADDR_LEN = 28;
const size_t AX25_MAX_ADDR_LEN = (2 * 7 + 8 * 7);
const size_t AX25_MIN_CTRL_LEN = 1;
const size_t AX25_MAX_CTRL_LEN = 2;
const size_t AX25_MAX_FRAME_LEN = 256;

9
lib/ax25_decoder.cc
View File

@ -120,13 +120,8 @@ ax25_decoder::_decode(decoder_status_t &status)
* AX.25 flag. We believe that such transmissions are yet rare.
*/
bool have_sync = false;
if (d_descramble) {
have_sync = (d_shift_reg >> 8) == AX25_SYNC_FLAG;
}
else {
have_sync = ((d_shift_reg & 0xFF) == AX25_SYNC_FLAG)
&& (d_shift_reg >> 8) == AX25_SYNC_FLAG;
}
have_sync = ((d_shift_reg & 0xFF) == AX25_SYNC_FLAG)
&& (d_shift_reg >> 8) == AX25_SYNC_FLAG;
if (have_sync) {
d_bitstream.erase(d_bitstream.begin(),
d_bitstream.begin() + i + 1);