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Add some testing code that looks for ambiguous syndromes that

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could confuse the partial correction used in DF11.

That code shows that yes, there are ambiguous syndromes in the
2-bit correction case only, so disable corrections of more than
1 bit in DF11.
This commit is contained in:
Oliver Jowett 2015-01-23 21:59:31 +00:00
parent 5ac90e30a8
commit 26feb1d078
2 changed files with 127 additions and 6 deletions
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9
mode_s.c
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@ -301,13 +301,10 @@ static int correct_aa_field(uint32_t *addr, struct errorinfo *ei)
// 1600: DF11 with IID==0, good CRC and an address matching a known aircraft
// 800: DF11 with IID==0, 1-bit error and an address matching a known aircraft
// 750: DF11 with IID==0, good CRC and an address not matching a known aircraft
// 400: DF11 with IID==0, 2-bit error and an address matching a known aircraft
// 375: DF11 with IID==0, 1-bit error and an address not matching a known aircraft
// 187: DF11 with IID==0, 2-bit error and an address not matching a known aircraft
// 1000: DF11 with IID!=0, good CRC and an address matching a known aircraft
// 500: DF11 with IID!=0, 1-bit error and an address matching a known aircraft
// 250: DF11 with IID!=0, 2-bit error and an address matching a known aircraft
// 1000: DF20/21 with a CRC-derived address matching a known aircraft
// 500: DF20/21 with a CRC-derived address matching a known aircraft (bottom 16 bits only - overlay control in use)
@ -349,6 +346,12 @@ int scoreModesMessage(unsigned char *msg, int validbits)
if (!ei)
return -2; // can't correct errors
// see crc.c comments: we do not attempt to fix
// more than single-bit errors, as two-bit
// errors are ambiguous in DF11.
if (ei->errors > 1)
return -2; // can't correct errors
// fix any errors in the address field
correct_aa_field(&addr, ei);