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Improve the decoding for DF-11

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Three changes in this one

1) Change the checksum testing for DF-11
2) Recode the Checksum generation routine to use pointers.
3) Tidy up the appearance of some print debug statments
This commit is contained in:
Malcolm Robb 2013-04-11 19:01:03 +01:00
parent dc7e2b6a22
commit ff697dbb51
1 changed files with 33 additions and 29 deletions
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62
dump1090.c
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@ -218,6 +218,7 @@ struct modesMessage {
/* DF 11 */ /* DF 11 */
int ca; /* Responder capabilities. */ int ca; /* Responder capabilities. */
int iid;
/* DF 17 */ /* DF 17 */
int metype; /* Extended squitter message type. */ int metype; /* Extended squitter message type. */
@ -696,20 +697,22 @@ uint32_t modes_checksum_table[112] = {
}; };
uint32_t modesChecksum(unsigned char *msg, int bits) { uint32_t modesChecksum(unsigned char *msg, int bits) {
uint32_t crc = 0; uint32_t crc = 0;
int offset = (bits == 112) ? 0 : (112-56); int offset = (bits == 112) ? 0 : (112-56);
uint8_t theByte = *msg;
uint32_t * pCRCTable = &modes_checksum_table[offset];
int j; int j;
for(j = 0; j < bits; j++) { for(j = 0; j < bits; j++) {
int byte = j/8; if ((j & 7) == 0)
int bit = j%8; {theByte = *msg++;}
int bitmask = 1 << (7-bit);
/* If bit is set, xor with corresponding table entry. */ // If bit is set, xor with corresponding table entry.
if (msg[byte] & bitmask) if (theByte & 0x80) {crc ^= *pCRCTable;}
crc ^= modes_checksum_table[j+offset]; pCRCTable++;
theByte = theByte << 1;
} }
return crc; /* 24 bit checksum. */ return crc; // 24 bit checksum.
} }
/* Given the Downlink Format (DF) of the message, return the message length /* Given the Downlink Format (DF) of the message, return the message length
@ -991,23 +994,27 @@ void decodeModesMessage(struct modesMessage *mm, unsigned char *msg) {
char *ais_charset = "?ABCDEFGHIJKLMNOPQRSTUVWXYZ????? ???????????????0123456789??????"; char *ais_charset = "?ABCDEFGHIJKLMNOPQRSTUVWXYZ????? ???????????????0123456789??????";
/* Work on our local copy */ /* Work on our local copy */
memcpy(mm->msg,msg,MODES_LONG_MSG_BYTES); memcpy(mm->msg, msg, MODES_LONG_MSG_BYTES);
msg = mm->msg; msg = mm->msg;
/* Get the message type ASAP as other operations depend on this */ /* Get the message type ASAP as other operations depend on this */
mm->msgtype = msg[0]>>3; /* Downlink Format */ mm->msgtype = msg[0] >> 3; /* Downlink Format */
mm->msgbits = modesMessageLenByType(mm->msgtype); mm->msgbits = modesMessageLenByType(mm->msgtype);
/* CRC is always the last three bytes. */ /* CRC is always the last three bytes. */
mm->crc = ((uint32_t)msg[(mm->msgbits/8)-3] << 16) | mm->crc = ((uint32_t)msg[(mm->msgbits/8)-3] << 16) |
((uint32_t)msg[(mm->msgbits/8)-2] << 8) | ((uint32_t)msg[(mm->msgbits/8)-2] << 8) |
(uint32_t)msg[(mm->msgbits/8)-1]; (uint32_t)msg[(mm->msgbits/8)-1];
crc2 = modesChecksum(msg,mm->msgbits); crc2 = modesChecksum(msg, mm->msgbits);
mm->iid = (mm->crc ^ crc2);
/* Check CRC and fix single bit errors using the CRC when /* Check CRC and fix single bit errors using the CRC when
* possible (DF 11 and 17). */ * possible (DF 11 and 17). */
mm->errorbit = -1; /* No error */ mm->errorbit = -1; /* No error */
mm->crcok = (mm->crc == crc2); if (mm->msgtype == 11)
{mm->crcok = (mm->iid < 80);}
else
{mm->crcok = (mm->iid == 0);}
if (!mm->crcok && Modes.fix_errors && if (!mm->crcok && Modes.fix_errors &&
(mm->msgtype == 11 || mm->msgtype == 17)) (mm->msgtype == 11 || mm->msgtype == 17))
@ -1561,6 +1568,7 @@ void detectModeS(uint16_t *m, uint32_t mlen) {
/* Pass data to the next layer */ /* Pass data to the next layer */
useModesMessage(&mm); useModesMessage(&mm);
} else { } else {
if (Modes.debug & MODES_DEBUG_DEMODERR && use_correction) { if (Modes.debug & MODES_DEBUG_DEMODERR && use_correction) {
printf("The following message has %d demod errors\n", errors); printf("The following message has %d demod errors\n", errors);
@ -1568,12 +1576,11 @@ void detectModeS(uint16_t *m, uint32_t mlen) {
} }
} }
/* Retry with phase correction if possible. */ // Retry with phase correction if possible.
if (!good_message && !use_correction && j && detectOutOfPhase(pPreamble)) { if (!good_message && !use_correction && j && detectOutOfPhase(pPreamble)) {
j--; use_correction = 1; j--;
use_correction = 1;
} else { } else {
use_correction = 0; use_correction = 0;
} }
} }
} }
@ -2655,18 +2662,15 @@ int main(int argc, char **argv) {
/* If --ifile and --stats were given, print statistics. */ /* If --ifile and --stats were given, print statistics. */
if (Modes.stats && Modes.filename) { if (Modes.stats && Modes.filename) {
printf("%lld valid preambles\n", Modes.stat_valid_preamble); printf("%lld valid preambles\n", Modes.stat_valid_preamble);
printf("%lld demodulated again after phase correction\n", printf("%lld demodulated again after phase correction\n", Modes.stat_out_of_phase);
Modes.stat_out_of_phase); printf("%lld demodulated with zero errors\n", Modes.stat_demodulated);
printf("%lld demodulated with zero errors\n", printf("%lld with good crc\n", Modes.stat_goodcrc);
Modes.stat_demodulated); printf("%lld with bad crc\n", Modes.stat_badcrc);
printf("%lld with good crc\n", Modes.stat_goodcrc); printf("%lld errors corrected\n", Modes.stat_fixed);
printf("%lld with bad crc\n", Modes.stat_badcrc); printf("%lld single bit errors\n", Modes.stat_single_bit_fix);
printf("%lld errors corrected\n", Modes.stat_fixed); printf("%lld two bits errors\n", Modes.stat_two_bits_fix);
printf("%lld single bit errors\n", Modes.stat_single_bit_fix); printf("%lld total usable messages\n", Modes.stat_goodcrc + Modes.stat_fixed);
printf("%lld two bits errors\n", Modes.stat_two_bits_fix);
printf("%lld total usable messages\n",
Modes.stat_goodcrc + Modes.stat_fixed);
} }
rtlsdr_close(Modes.dev); rtlsdr_close(Modes.dev);