DF Error detection and Correction
Added additional error detection and correction to the DF field of the first byte in every ModeS frame.
This commit is contained in:
Malcolm Robb
parent
7cf79580a4
commit
e161b7662c
109
dump1090.c
109
dump1090.c
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@ -243,7 +243,8 @@ struct {
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unsigned int stat_http_requests;
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unsigned int stat_sbs_connections;
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unsigned int stat_out_of_phase;
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unsigned int stat_DF_Corrected;
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unsigned int stat_DF_Len_Corrected;
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unsigned int stat_DF_Type_Corrected;
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unsigned int stat_ModeAC;
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} Modes;
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@ -462,7 +463,8 @@ void modesInit(void) {
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Modes.stat_http_requests = 0;
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Modes.stat_sbs_connections = 0;
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Modes.stat_out_of_phase = 0;
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Modes.stat_DF_Corrected = 0;
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Modes.stat_DF_Len_Corrected = 0;
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Modes.stat_DF_Type_Corrected = 0;
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Modes.stat_ModeAC = 0;
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Modes.exit = 0;
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}
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@ -1892,7 +1894,7 @@ void detectModeS(uint16_t *m, uint32_t mlen) {
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int good_message = 0;
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uint16_t *pPreamble, *pPayload, *pPtr;
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uint8_t theByte, theErrs;
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int msglen, sigStrength;
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int msglen, scanlen, sigStrength;
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pPreamble = &m[j];
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pPayload = &m[j+MODES_PREAMBLE_SAMPLES];
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@ -1997,69 +1999,97 @@ void detectModeS(uint16_t *m, uint32_t mlen) {
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+ (pPreamble[7]-pPreamble[6])
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+ (pPreamble[9]-pPreamble[8]);
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msglen = MODES_LONG_MSG_BITS;
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for (i = 0; i < msglen; i++) {
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msglen = scanlen = MODES_LONG_MSG_BITS;
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for (i = 0; i < scanlen; i++) {
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uint32_t a = *pPtr++;
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uint32_t b = *pPtr++;
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if (a > b)
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{sigStrength += (a-b); theByte |= 1;}
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{theByte |= 1; if (i < 56) {sigStrength += (a-b);}}
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else if (a < b)
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{sigStrength += (b-a); /*theByte |= 0;*/}
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{/*theByte |= 0;*/ if (i < 56) {sigStrength += (b-a);}}
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else if (i >= MODES_SHORT_MSG_BITS) //(a == b), and we're in the long part of a frame
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{errors++; /*theByte |= 0;*/}
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else if (i >= 5) //(a == b), and we're in the short part of a frame
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{errors56 = ++errors;/*theByte |= 0;*/}
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else //(a == b), and we're in the message type part of a frame
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{scanlen = MODES_LONG_MSG_BITS; errors56 = ++errors;/*theByte |= 0;*/}
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else if (i) //(a == b), and we're in the message type part of a frame
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{errorsTy = errors56 = ++errors; theErrs |= 1; /*theByte |= 0;*/}
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else //(a == b), and we're in the first bit of the message type part of a frame
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{errorsTy = errors56 = ++errors; theErrs |= 1; theByte |= 1;}
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if ((i & 7) == 7)
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{*pMsg++ = theByte;}
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else if ((i == 4) && (errors == 0))
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{msglen = modesMessageLenByType(theByte);}
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else if (i == 4) {
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msglen = modesMessageLenByType(theByte);
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if (errors == 0)
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{scanlen = msglen;}
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}
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theByte = theByte << 1;
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if (i < 8)
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if (i < 7)
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{theErrs = theErrs << 1;}
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// If we've exceeded the permissible number of encoding errors, abandon ship now
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if (errors > MODES_MSG_ENCODER_ERRS)
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{
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// If we're in the long frame when it went to pot, but it was still ok-ish when we
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// were in the short part of the frame, then try for a mis-identified short frame
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// we must believe that this should've been a long frame to get this far.
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if (i >= MODES_SHORT_MSG_BITS)
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{
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// If we did see some errors in the first byte of the frame, then it's possible
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// we guessed wrongly about the value of the bit. If we only saw one error, we may
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// be able to correct it by guessing the other way.
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if (errorsTy == 1)
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{
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// See if inverting the bit we guessed at would change the message type from a
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// long to a short. If it would, invert the bit, cross your fingers and carry on.
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theByte = pMsg[0] ^ theErrs;
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if (MODES_SHORT_MSG_BITS == modesMessageLenByType(theByte))
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{
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pMsg[0] = theByte; // write the modified type back to the msg buffer
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errors = errors56; // revert to the number of errors prior to bit 56
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if (errors > MODES_MSG_ENCODER_ERRS) {
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if (i < MODES_SHORT_MSG_BITS) {
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msglen = 0;
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} else if ((errorsTy == 1) && (theErrs == 0x80)) {
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// If we only saw one error in the first bit of the byte of the frame, then it's possible
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// we guessed wrongly about the value of the bit. We may be able to correct it by guessing
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// the other way.
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//
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// We guessed a '1' at bit 7, which is the DF length bit == 112 Bits.
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// Inverting bit 7 will change the message type from a long to a short.
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// Invert the bit, cross your fingers and carry on.
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msglen = MODES_SHORT_MSG_BITS;
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i--; // this latest sample was zero, so we can ignore it.
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Modes.stat_DF_Corrected++;
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}
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}
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msg[0] ^= theErrs; errorsTy = 0;
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errors = errors56; // revert to the number of errors prior to bit 56
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Modes.stat_DF_Len_Corrected++;
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} else if (i < MODES_LONG_MSG_BITS) {
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msglen = MODES_SHORT_MSG_BITS;
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errors = errors56;
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} else {
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msglen = MODES_LONG_MSG_BITS;
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}
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break;
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}
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}
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if (msglen == modesMessageLenByType(theByte = msg[0])) {
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// The msglen is consistent with the DF type
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if ((errorsTy == 1) && (theErrs & 0x78)) {
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// We guessed at one of the message type bits. See if our guess is "likely"
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// to be correct by comparing the DF against a list of known good DF's
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int DF = ((theByte = msg[0]) >> 3) & 0x1f;
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if ( (DF != 0) && (DF != 4) && (DF != 5) && (DF != 11)
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&& (DF != 16) && (DF != 17) && (DF != 18) && (DF != 19) && (DF != 20) && (DF != 21) && (DF != 22) && (DF != 24) ) {
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// Other DF values are probably errors. Toggle the bit we guessed at and see if the resultant DF is more likely
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theByte ^= theErrs;
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DF = (theByte >> 3) & 0x1f;
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// if this DF any more likely??
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if ( (DF == 0) || (DF == 4) || (DF == 5) || (DF == 11)
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|| (DF == 16) || (DF == 17) || (DF == 18) || (DF == 19) || (DF == 20) || (DF == 21) || (DF == 22) || (DF == 24) ) {
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// Yep, more likely, so update the main message
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msg[0] = theByte;
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Modes.stat_DF_Type_Corrected++;
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errors--; // decrease the error count so we attempt to use the modified DF.
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}
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}
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}
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}
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// Don't forget to add 4 for the preamble samples. This also removes any risk of dividing by zero.
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sigStrength /= (msglen+4);
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sigStrength /= 60;
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/* If we reached this point, and error is zero, we are very likely
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* with a Mode S message in our hands, but it may still be broken
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* and CRC may not be correct. This is handled by the next layer. */
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if ( (sigStrength > MODES_MSG_SQUELCH_LEVEL) && (errors <= MODES_MSG_ENCODER_ERRS) )
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if ((msglen) && (sigStrength > MODES_MSG_SQUELCH_LEVEL) && (errors <= MODES_MSG_ENCODER_ERRS) )
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{
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struct modesMessage mm;
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@ -3580,7 +3610,8 @@ int main(int argc, char **argv) {
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if (Modes.stats && Modes.filename) {
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printf("%d ModeA/C detected\n", Modes.stat_ModeAC);
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printf("%d valid preambles\n", Modes.stat_valid_preamble);
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printf("%d DF-?? fields corrected for length\n", Modes.stat_DF_Corrected);
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printf("%d DF-?? fields corrected for length\n", Modes.stat_DF_Len_Corrected);
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printf("%d DF-?? fields corrected for type\n", Modes.stat_DF_Type_Corrected);
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printf("%d demodulated again after phase correction\n", Modes.stat_out_of_phase);
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printf("%d demodulated with zero errors\n", Modes.stat_demodulated);
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printf("%d with good crc\n", Modes.stat_goodcrc);
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