// dump1090, a Mode S messages decoder for RTLSDR devices. // // Copyright (C) 2012 by Salvatore Sanfilippo // // All rights reserved. // // Redistribution and use in source and binary forms, with or without // modification, are permitted provided that the following conditions are // met: // // * Redistributions of source code must retain the above copyright // notice, this list of conditions and the following disclaimer. // // * Redistributions in binary form must reproduce the above copyright // notice, this list of conditions and the following disclaimer in the // documentation and/or other materials provided with the distribution. // // THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS // "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT // LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR // A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT // HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, // SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT // LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, // DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY // THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT // (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. // #include "dump1090.h" // // ===================== Mode S detection and decoding =================== // // Parity table for MODE S Messages. // The table contains 112 elements, every element corresponds to a bit set // in the message, starting from the first bit of actual data after the // preamble. // // For messages of 112 bit, the whole table is used. // For messages of 56 bits only the last 56 elements are used. // // The algorithm is as simple as xoring all the elements in this table // for which the corresponding bit on the message is set to 1. // // The latest 24 elements in this table are set to 0 as the checksum at the // end of the message should not affect the computation. // // Note: this function can be used with DF11 and DF17, other modes have // the CRC xored with the sender address as they are reply to interrogations, // but a casual listener can't split the address from the checksum. // uint32_t modes_checksum_table[112] = { 0x3935ea, 0x1c9af5, 0xf1b77e, 0x78dbbf, 0xc397db, 0x9e31e9, 0xb0e2f0, 0x587178, 0x2c38bc, 0x161c5e, 0x0b0e2f, 0xfa7d13, 0x82c48d, 0xbe9842, 0x5f4c21, 0xd05c14, 0x682e0a, 0x341705, 0xe5f186, 0x72f8c3, 0xc68665, 0x9cb936, 0x4e5c9b, 0xd8d449, 0x939020, 0x49c810, 0x24e408, 0x127204, 0x093902, 0x049c81, 0xfdb444, 0x7eda22, 0x3f6d11, 0xe04c8c, 0x702646, 0x381323, 0xe3f395, 0x8e03ce, 0x4701e7, 0xdc7af7, 0x91c77f, 0xb719bb, 0xa476d9, 0xadc168, 0x56e0b4, 0x2b705a, 0x15b82d, 0xf52612, 0x7a9309, 0xc2b380, 0x6159c0, 0x30ace0, 0x185670, 0x0c2b38, 0x06159c, 0x030ace, 0x018567, 0xff38b7, 0x80665f, 0xbfc92b, 0xa01e91, 0xaff54c, 0x57faa6, 0x2bfd53, 0xea04ad, 0x8af852, 0x457c29, 0xdd4410, 0x6ea208, 0x375104, 0x1ba882, 0x0dd441, 0xf91024, 0x7c8812, 0x3e4409, 0xe0d800, 0x706c00, 0x383600, 0x1c1b00, 0x0e0d80, 0x0706c0, 0x038360, 0x01c1b0, 0x00e0d8, 0x00706c, 0x003836, 0x001c1b, 0xfff409, 0x000000, 0x000000, 0x000000, 0x000000, 0x000000, 0x000000, 0x000000, 0x000000, 0x000000, 0x000000, 0x000000, 0x000000, 0x000000, 0x000000, 0x000000, 0x000000, 0x000000, 0x000000, 0x000000, 0x000000, 0x000000, 0x000000, 0x000000, 0x000000 }; uint32_t modesChecksum(unsigned char *msg, int bits) { uint32_t crc = 0; uint32_t rem = 0; int offset = (bits == 112) ? 0 : (112-56); uint8_t theByte = *msg; uint32_t * pCRCTable = &modes_checksum_table[offset]; int j; // We don't really need to include the checksum itself bits -= 24; for(j = 0; j < bits; j++) { if ((j & 7) == 0) theByte = *msg++; // If bit is set, xor with corresponding table entry. if (theByte & 0x80) {crc ^= *pCRCTable;} pCRCTable++; theByte = theByte << 1; } rem = (msg[0] << 16) | (msg[1] << 8) | msg[2]; // message checksum return ((crc ^ rem) & 0x00FFFFFF); // 24 bit checksum syndrome. } // //========================================================================= // // Given the Downlink Format (DF) of the message, return the message length in bits. // // All known DF's 16 or greater are long. All known DF's 15 or less are short. // There are lots of unused codes in both category, so we can assume ICAO will stick to // these rules, meaning that the most significant bit of the DF indicates the length. // int modesMessageLenByType(int type) { return (type & 0x10) ? MODES_LONG_MSG_BITS : MODES_SHORT_MSG_BITS ; } // //========================================================================= // // Try to fix single bit errors using the checksum. On success modifies // the original buffer with the fixed version, and returns the position // of the error bit. Otherwise if fixing failed -1 is returned. /* int fixSingleBitErrors(unsigned char *msg, int bits) { int j; unsigned char aux[MODES_LONG_MSG_BYTES]; memcpy(aux, msg, bits/8); // Do not attempt to error correct Bits 0-4. These contain the DF, and must // be correct because we can only error correct DF17 for (j = 5; j < bits; j++) { int byte = j/8; int bitmask = 1 << (7 - (j & 7)); aux[byte] ^= bitmask; // Flip j-th bit if (0 == modesChecksum(aux, bits)) { // The error is fixed. Overwrite the original buffer with the // corrected sequence, and returns the error bit position msg[byte] = aux[byte]; return (j); } aux[byte] ^= bitmask; // Flip j-th bit back again } return (-1); } */ //========================================================================= // // Similar to fixSingleBitErrors() but try every possible two bit combination. // This is very slow and should be tried only against DF17 messages that // don't pass the checksum, and only in Aggressive Mode. /* int fixTwoBitsErrors(unsigned char *msg, int bits) { int j, i; unsigned char aux[MODES_LONG_MSG_BYTES]; memcpy(aux, msg, bits/8); // Do not attempt to error correct Bits 0-4. These contain the DF, and must // be correct because we can only error correct DF17 for (j = 5; j < bits; j++) { int byte1 = j/8; int bitmask1 = 1 << (7 - (j & 7)); aux[byte1] ^= bitmask1; // Flip j-th bit // Don't check the same pairs multiple times, so i starts from j+1 for (i = j+1; i < bits; i++) { int byte2 = i/8; int bitmask2 = 1 << (7 - (i & 7)); aux[byte2] ^= bitmask2; // Flip i-th bit if (0 == modesChecksum(aux, bits)) { // The error is fixed. Overwrite the original buffer with // the corrected sequence, and returns the error bit position msg[byte1] = aux[byte1]; msg[byte2] = aux[byte2]; // We return the two bits as a 16 bit integer by shifting // 'i' on the left. This is possible since 'i' will always // be non-zero because i starts from j+1 return (j | (i << 8)); aux[byte2] ^= bitmask2; // Flip i-th bit back } aux[byte1] ^= bitmask1; // Flip j-th bit back } } return (-1); } */ // //========================================================================= // // Code for introducing a less CPU-intensive method of correcting // single bit errors. // // Makes use of the fact that the crc checksum is linear with respect to // the bitwise xor operation, i.e. // crc(m^e) = (crc(m)^crc(e) // where m and e are the message resp. error bit vectors. // // Call crc(e) the syndrome. // // The code below works by precomputing a table of (crc(e), e) for all // possible error vectors e (here only single bit and double bit errors), // search for the syndrome in the table, and correct the then known error. // The error vector e is represented by one or two bit positions that are // changed. If a second bit position is not used, it is -1. // // Run-time is binary search in a sorted table, plus some constant overhead, // instead of running through all possible bit positions (resp. pairs of // bit positions). // struct errorinfo { uint32_t syndrome; // CRC syndrome int bits; // Number of bit positions to fix int pos[MODES_MAX_BITERRORS]; // Bit positions corrected by this syndrome }; #define NERRORINFO \ (MODES_LONG_MSG_BITS+MODES_LONG_MSG_BITS*(MODES_LONG_MSG_BITS-1)/2) struct errorinfo bitErrorTable[NERRORINFO]; // Compare function as needed for stdlib's qsort and bsearch functions int cmpErrorInfo(const void *p0, const void *p1) { struct errorinfo *e0 = (struct errorinfo*)p0; struct errorinfo *e1 = (struct errorinfo*)p1; if (e0->syndrome == e1->syndrome) { return 0; } else if (e0->syndrome < e1->syndrome) { return -1; } else { return 1; } } // //========================================================================= // // Compute the table of all syndromes for 1-bit and 2-bit error vectors void modesInitErrorInfo() { unsigned char msg[MODES_LONG_MSG_BYTES]; int i, j, n; uint32_t crc; n = 0; memset(bitErrorTable, 0, sizeof(bitErrorTable)); memset(msg, 0, MODES_LONG_MSG_BYTES); // Add all possible single and double bit errors // don't include errors in first 5 bits (DF type) for (i = 5; i < MODES_LONG_MSG_BITS; i++) { int bytepos0 = (i >> 3); int mask0 = 1 << (7 - (i & 7)); msg[bytepos0] ^= mask0; // create error0 crc = modesChecksum(msg, MODES_LONG_MSG_BITS); bitErrorTable[n].syndrome = crc; // single bit error case bitErrorTable[n].bits = 1; bitErrorTable[n].pos[0] = i; bitErrorTable[n].pos[1] = -1; n += 1; if (Modes.nfix_crc > 1) { for (j = i+1; j < MODES_LONG_MSG_BITS; j++) { int bytepos1 = (j >> 3); int mask1 = 1 << (7 - (j & 7)); msg[bytepos1] ^= mask1; // create error1 crc = modesChecksum(msg, MODES_LONG_MSG_BITS); if (n >= NERRORINFO) { //fprintf(stderr, "Internal error, too many entries, fix NERRORINFO\n"); break; } bitErrorTable[n].syndrome = crc; // two bit error case bitErrorTable[n].bits = 2; bitErrorTable[n].pos[0] = i; bitErrorTable[n].pos[1] = j; n += 1; msg[bytepos1] ^= mask1; // revert error1 } } msg[bytepos0] ^= mask0; // revert error0 } qsort(bitErrorTable, NERRORINFO, sizeof(struct errorinfo), cmpErrorInfo); // Test code: report if any syndrome appears at least twice. In this // case the correction cannot be done without ambiguity. // Tried it, does not happen for 1- and 2-bit errors. /* for (i = 1; i < NERRORINFO; i++) { if (bitErrorTable[i-1].syndrome == bitErrorTable[i].syndrome) { fprintf(stderr, "modesInitErrorInfo: Collision for syndrome %06x\n", (int)bitErrorTable[i].syndrome); } } for (i = 0; i < NERRORINFO; i++) { printf("syndrome %06x bit0 %3d bit1 %3d\n", bitErrorTable[i].syndrome, bitErrorTable[i].pos0, bitErrorTable[i].pos1); } */ } // //========================================================================= // // Search for syndrome in table and if an entry is found, flip the necessary // bits. Make sure the indices fit into the array // Additional parameter: fix only less than maxcorrected bits, and record // fixed bit positions in corrected[]. This array can be NULL, otherwise // must be of length at least maxcorrected. // Return number of fixed bits. // int fixBitErrors(unsigned char *msg, int bits, int maxfix, char *fixedbits) { struct errorinfo *pei; struct errorinfo ei; int bitpos, offset, res, i; memset(&ei, 0, sizeof(struct errorinfo)); ei.syndrome = modesChecksum(msg, bits); pei = bsearch(&ei, bitErrorTable, NERRORINFO, sizeof(struct errorinfo), cmpErrorInfo); if (pei == NULL) { return 0; // No syndrome found } // Check if the syndrome fixes more bits than we allow if (maxfix < pei->bits) { return 0; } // Check that all bit positions lie inside the message length offset = MODES_LONG_MSG_BITS-bits; for (i = 0; i < pei->bits; i++) { bitpos = pei->pos[i] - offset; if ((bitpos < 0) || (bitpos >= bits)) { return 0; } } // Fix the bits for (i = res = 0; i < pei->bits; i++) { bitpos = pei->pos[i] - offset; msg[bitpos >> 3] ^= (1 << (7 - (bitpos & 7))); if (fixedbits) { fixedbits[res++] = bitpos; } } return res; } // // ============================== Debugging ================================= // // Helper function for dumpMagnitudeVector(). // It prints a single bar used to display raw signals. // // Since every magnitude sample is between 0-255, the function uses // up to 63 characters for every bar. Every character represents // a length of 4, 3, 2, 1, specifically: // // "O" is 4 // "o" is 3 // "-" is 2 // "." is 1 // void dumpMagnitudeBar(int index, int magnitude) { char *set = " .-o"; char buf[256]; int div = magnitude / 256 / 4; int rem = magnitude / 256 % 4; memset(buf,'O',div); buf[div] = set[rem]; buf[div+1] = '\0'; if (index >= 0) printf("[%.3d] |%-66s 0x%04X\n", index, buf, magnitude); else printf("[%.2d] |%-66s 0x%04X\n", index, buf, magnitude); } // //========================================================================= // // Display an ASCII-art alike graphical representation of the undecoded // message as a magnitude signal. // // The message starts at the specified offset in the "m" buffer. // The function will display enough data to cover a short 56 bit message. // // If possible a few samples before the start of the messsage are included // for context. // void dumpMagnitudeVector(uint16_t *m, uint32_t offset) { uint32_t padding = 5; // Show a few samples before the actual start. uint32_t start = (offset < padding) ? 0 : offset-padding; uint32_t end = offset + (MODES_PREAMBLE_SAMPLES)+(MODES_SHORT_MSG_SAMPLES) - 1; uint32_t j; for (j = start; j <= end; j++) { dumpMagnitudeBar(j-offset, m[j]); } } // //========================================================================= // // Produce a raw representation of the message as a Javascript file // loadable by debug.html. // void dumpRawMessageJS(char *descr, unsigned char *msg, uint16_t *m, uint32_t offset, int fixable, char *bitpos) { int padding = 5; // Show a few samples before the actual start. int start = offset - padding; int end = offset + (MODES_PREAMBLE_SAMPLES)+(MODES_LONG_MSG_SAMPLES) - 1; FILE *fp; int j; MODES_NOTUSED(fixable); if ((fp = fopen("frames.js","a")) == NULL) { fprintf(stderr, "Error opening frames.js: %s\n", strerror(errno)); exit(1); } fprintf(fp,"frames.push({\"descr\": \"%s\", \"mag\": [", descr); for (j = start; j <= end; j++) { fprintf(fp,"%d", j < 0 ? 0 : m[j]); if (j != end) fprintf(fp,","); } fprintf(fp,"], \"fix1\": %d, \"fix2\": %d, \"bits\": %d, \"hex\": \"", bitpos[0], bitpos[1] , modesMessageLenByType(msg[0]>>3)); for (j = 0; j < MODES_LONG_MSG_BYTES; j++) fprintf(fp,"\\x%02x",msg[j]); fprintf(fp,"\"});\n"); fclose(fp); } // //========================================================================= // // This is a wrapper for dumpMagnitudeVector() that also show the message // in hex format with an additional description. // // descr is the additional message to show to describe the dump. // msg points to the decoded message // m is the original magnitude vector // offset is the offset where the message starts // // The function also produces the Javascript file used by debug.html to // display packets in a graphical format if the Javascript output was // enabled. // void dumpRawMessage(char *descr, unsigned char *msg, uint16_t *m, uint32_t offset) { int j; int msgtype = msg[0] >> 3; int fixable = 0; char bitpos[MODES_MAX_BITERRORS]; for (j = 0; j < MODES_MAX_BITERRORS; j++) { bitpos[j] = -1; } if (msgtype == 17) { fixable = fixBitErrors(msg, MODES_LONG_MSG_BITS, MODES_MAX_BITERRORS, bitpos); } if (Modes.debug & MODES_DEBUG_JS) { dumpRawMessageJS(descr, msg, m, offset, fixable, bitpos); return; } printf("\n--- %s\n ", descr); for (j = 0; j < MODES_LONG_MSG_BYTES; j++) { printf("%02x",msg[j]); if (j == MODES_SHORT_MSG_BYTES-1) printf(" ... "); } printf(" (DF %d, Fixable: %d)\n", msgtype, fixable); dumpMagnitudeVector(m,offset); printf("---\n\n"); } // //========================================================================= // // Code for testing the timing: run all possible 1- and 2-bit error // the test message by all 1-bit errors. Run the old code against // all of them, and new the code. // // Example measurements: // Timing old vs. new crc correction code: // Old code: 1-bit errors on 112 msgs: 3934 usecs // New code: 1-bit errors on 112 msgs: 104 usecs // Old code: 2-bit errors on 6216 msgs: 407743 usecs // New code: 2-bit errors on 6216 msgs: 5176 usecs // indicating a 37-fold resp. 78-fold improvement in speed for 1-bit resp. // 2-bit error. /* unsigned char tmsg0[MODES_LONG_MSG_BYTES] = { // Test data: first ADS-B message from testfiles/modes1.bin 0x8f, 0x4d, 0x20, 0x23, 0x58, 0x7f, 0x34, 0x5e, 0x35, 0x83, 0x7e, 0x22, 0x18, 0xb2 }; #define NTWOBITS (MODES_LONG_MSG_BITS*(MODES_LONG_MSG_BITS-1)/2) unsigned char tmsg1[MODES_LONG_MSG_BITS][MODES_LONG_MSG_BYTES]; unsigned char tmsg2[NTWOBITS][MODES_LONG_MSG_BYTES]; // Init an array of cloned messages with all possible 1-bit errors present, // applied to each message at the respective position // void inittmsg1() { int i, bytepos, mask; for (i = 0; i < MODES_LONG_MSG_BITS; i++) { bytepos = i >> 3; mask = 1 << (7 - (i & 7)); memcpy(&tmsg1[i][0], tmsg0, MODES_LONG_MSG_BYTES); tmsg1[i][bytepos] ^= mask; } } // Run sanity check on all but first 5 messages / bits, as those bits // are not corrected. // void checktmsg1(FILE *out) { int i, k; uint32_t crc; for (i = 5; i < MODES_LONG_MSG_BITS; i++) { crc = modesChecksum(&tmsg1[i][0], MODES_LONG_MSG_BITS); if (crc != 0) { fprintf(out, "CRC not fixed for " "positon %d\n", i); fprintf(out, " MSG "); for (k = 0; k < MODES_LONG_MSG_BYTES; k++) { fprintf(out, "%02x", tmsg1[i][k]); } fprintf(out, "\n"); } } } void inittmsg2() { int i, j, n, bytepos0, bytepos1, mask0, mask1; n = 0; for (i = 0; i < MODES_LONG_MSG_BITS; i++) { bytepos0 = i >> 3; mask0 = 1 << (7 - (i & 7)); for (j = i+1; j < MODES_LONG_MSG_BITS; j++) { bytepos1 = j >> 3; mask1 = 1 << (7 - (j & 7)); memcpy(&tmsg2[n][0], tmsg0, MODES_LONG_MSG_BYTES); tmsg2[n][bytepos0] ^= mask0; tmsg2[n][bytepos1] ^= mask1; n += 1; } } } long difftvusec(struct timeval *t0, struct timeval *t1) { long res = 0; res = t1->tv_usec-t0->tv_usec; res += (t1->tv_sec-t0->tv_sec)*1000000L; return res; } // the actual test code void testAndTimeBitCorrection() { struct timeval starttv, endtv; int i; // Run timing on 1-bit errors printf("Timing old vs. new crc correction code:\n"); inittmsg1(); gettimeofday(&starttv, NULL); for (i = 0; i < MODES_LONG_MSG_BITS; i++) { fixSingleBitErrors(&tmsg1[i][0], MODES_LONG_MSG_BITS); } gettimeofday(&endtv, NULL); printf(" Old code: 1-bit errors on %d msgs: %ld usecs\n", MODES_LONG_MSG_BITS, difftvusec(&starttv, &endtv)); checktmsg1(stdout); // Re-init inittmsg1(); gettimeofday(&starttv, NULL); for (i = 0; i < MODES_LONG_MSG_BITS; i++) { fixBitErrors(&tmsg1[i][0], MODES_LONG_MSG_BITS, MODES_MAX_BITERRORS, NULL); } gettimeofday(&endtv, NULL); printf(" New code: 1-bit errors on %d msgs: %ld usecs\n", MODES_LONG_MSG_BITS, difftvusec(&starttv, &endtv)); checktmsg1(stdout); // Run timing on 2-bit errors inittmsg2(); gettimeofday(&starttv, NULL); for (i = 0; i < NTWOBITS; i++) { fixSingleBitErrors(&tmsg2[i][0], MODES_LONG_MSG_BITS); } gettimeofday(&endtv, NULL); printf(" Old code: 2-bit errors on %d msgs: %ld usecs\n", NTWOBITS, difftvusec(&starttv, &endtv)); // Re-init inittmsg2(); gettimeofday(&starttv, NULL); for (i = 0; i < NTWOBITS; i++) { fixBitErrors(&tmsg2[i][0], MODES_LONG_MSG_BITS, MODES_MAX_BITERRORS, NULL); } gettimeofday(&endtv, NULL); printf(" New code: 2-bit errors on %d msgs: %ld usecs\n", NTWOBITS, difftvusec(&starttv, &endtv)); } */ //========================================================================= // // Hash the ICAO address to index our cache of MODES_ICAO_CACHE_LEN // elements, that is assumed to be a power of two // uint32_t ICAOCacheHashAddress(uint32_t a) { // The following three rounds wil make sure that every bit affects // every output bit with ~ 50% of probability. a = ((a >> 16) ^ a) * 0x45d9f3b; a = ((a >> 16) ^ a) * 0x45d9f3b; a = ((a >> 16) ^ a); return a & (MODES_ICAO_CACHE_LEN-1); } // //========================================================================= // // Add the specified entry to the cache of recently seen ICAO addresses. // Note that we also add a timestamp so that we can make sure that the // entry is only valid for MODES_ICAO_CACHE_TTL seconds. // void addRecentlySeenICAOAddr(uint32_t addr) { uint32_t h = ICAOCacheHashAddress(addr); Modes.icao_cache[h*2] = addr; Modes.icao_cache[h*2+1] = (uint32_t) time(NULL); } // //========================================================================= // // Returns 1 if the specified ICAO address was seen in a DF format with // proper checksum (not xored with address) no more than * MODES_ICAO_CACHE_TTL // seconds ago. Otherwise returns 0. // int ICAOAddressWasRecentlySeen(uint32_t addr) { uint32_t h = ICAOCacheHashAddress(addr); uint32_t a = Modes.icao_cache[h*2]; uint32_t t = Modes.icao_cache[h*2+1]; uint64_t tn = time(NULL); return ( (a) && (a == addr) && ( (tn - t) <= MODES_ICAO_CACHE_TTL) ); } // //========================================================================= // // In the squawk (identity) field bits are interleaved as follows in // (message bit 20 to bit 32): // // C1-A1-C2-A2-C4-A4-ZERO-B1-D1-B2-D2-B4-D4 // // So every group of three bits A, B, C, D represent an integer from 0 to 7. // // The actual meaning is just 4 octal numbers, but we convert it into a hex // number tha happens to represent the four octal numbers. // // For more info: http://en.wikipedia.org/wiki/Gillham_code // int decodeID13Field(int ID13Field) { int hexGillham = 0; if (ID13Field & 0x1000) {hexGillham |= 0x0010;} // Bit 12 = C1 if (ID13Field & 0x0800) {hexGillham |= 0x1000;} // Bit 11 = A1 if (ID13Field & 0x0400) {hexGillham |= 0x0020;} // Bit 10 = C2 if (ID13Field & 0x0200) {hexGillham |= 0x2000;} // Bit 9 = A2 if (ID13Field & 0x0100) {hexGillham |= 0x0040;} // Bit 8 = C4 if (ID13Field & 0x0080) {hexGillham |= 0x4000;} // Bit 7 = A4 //if (ID13Field & 0x0040) {hexGillham |= 0x0800;} // Bit 6 = X or M if (ID13Field & 0x0020) {hexGillham |= 0x0100;} // Bit 5 = B1 if (ID13Field & 0x0010) {hexGillham |= 0x0001;} // Bit 4 = D1 or Q if (ID13Field & 0x0008) {hexGillham |= 0x0200;} // Bit 3 = B2 if (ID13Field & 0x0004) {hexGillham |= 0x0002;} // Bit 2 = D2 if (ID13Field & 0x0002) {hexGillham |= 0x0400;} // Bit 1 = B4 if (ID13Field & 0x0001) {hexGillham |= 0x0004;} // Bit 0 = D4 return (hexGillham); } // //========================================================================= // // Decode the 13 bit AC altitude field (in DF 20 and others). // Returns the altitude, and set 'unit' to either MODES_UNIT_METERS or MDOES_UNIT_FEETS. // int decodeAC13Field(int AC13Field, int *unit) { int m_bit = AC13Field & 0x0040; // set = meters, clear = feet int q_bit = AC13Field & 0x0010; // set = 25 ft encoding, clear = Gillham Mode C encoding if (!m_bit) { *unit = MODES_UNIT_FEET; if (q_bit) { // N is the 11 bit integer resulting from the removal of bit Q and M int n = ((AC13Field & 0x1F80) >> 2) | ((AC13Field & 0x0020) >> 1) | (AC13Field & 0x000F); // The final altitude is resulting number multiplied by 25, minus 1000. return ((n * 25) - 1000); } else { // N is an 11 bit Gillham coded altitude int n = ModeAToModeC(decodeID13Field(AC13Field)); if (n < -12) {n = 0;} return (100 * n); } } else { *unit = MODES_UNIT_METERS; // TODO: Implement altitude when meter unit is selected } return 0; } // //========================================================================= // // Decode the 12 bit AC altitude field (in DF 17 and others). // int decodeAC12Field(int AC12Field, int *unit) { int q_bit = AC12Field & 0x10; // Bit 48 = Q *unit = MODES_UNIT_FEET; if (q_bit) { /// N is the 11 bit integer resulting from the removal of bit Q at bit 4 int n = ((AC12Field & 0x0FE0) >> 1) | (AC12Field & 0x000F); // The final altitude is the resulting number multiplied by 25, minus 1000. return ((n * 25) - 1000); } else { // Make N a 13 bit Gillham coded altitude by inserting M=0 at bit 6 int n = ((AC12Field & 0x0FC0) << 1) | (AC12Field & 0x003F); n = ModeAToModeC(decodeID13Field(n)); if (n < -12) {n = 0;} return (100 * n); } } // //========================================================================= // // Decode the 7 bit ground movement field PWL exponential style scale // int decodeMovementField(int movement) { int gspeed; // Note : movement codes 0,125,126,127 are all invalid, but they are // trapped for before this function is called. if (movement > 123) gspeed = 199; // > 175kt else if (movement > 108) gspeed = ((movement - 108) * 5) + 100; else if (movement > 93) gspeed = ((movement - 93) * 2) + 70; else if (movement > 38) gspeed = ((movement - 38) ) + 15; else if (movement > 12) gspeed = ((movement - 11) >> 1) + 2; else if (movement > 8) gspeed = ((movement - 6) >> 2) + 1; else gspeed = 0; return (gspeed); } // //========================================================================= // // Capability table char *ca_str[8] = { /* 0 */ "Level 1 (Surveillance Only)", /* 1 */ "Level 2 (DF0,4,5,11)", /* 2 */ "Level 3 (DF0,4,5,11,20,21)", /* 3 */ "Level 4 (DF0,4,5,11,20,21,24)", /* 4 */ "Level 2+3+4 (DF0,4,5,11,20,21,24,code7 - is on ground)", /* 5 */ "Level 2+3+4 (DF0,4,5,11,20,21,24,code7 - is airborne)", /* 6 */ "Level 2+3+4 (DF0,4,5,11,20,21,24,code7)", /* 7 */ "Level 7 ???" }; // DF 18 Control field table. char *cf_str[8] = { /* 0 */ "ADS-B ES/NT device with ICAO 24-bit address", /* 1 */ "ADS-B ES/NT device with other address", /* 2 */ "Fine format TIS-B", /* 3 */ "Coarse format TIS-B", /* 4 */ "TIS-B management message", /* 5 */ "TIS-B relay of ADS-B message with other address", /* 6 */ "ADS-B rebroadcast using DF-17 message format", /* 7 */ "Reserved" }; // Flight status table char *fs_str[8] = { /* 0 */ "Normal, Airborne", /* 1 */ "Normal, On the ground", /* 2 */ "ALERT, Airborne", /* 3 */ "ALERT, On the ground", /* 4 */ "ALERT & Special Position Identification. Airborne or Ground", /* 5 */ "Special Position Identification. Airborne or Ground", /* 6 */ "Value 6 is not assigned", /* 7 */ "Value 7 is not assigned" }; // Emergency state table // from https://www.ll.mit.edu/mission/aviation/publications/publication-files/atc-reports/Grappel_2007_ATC-334_WW-15318.pdf // and 1090-DO-260B_FRAC char *es_str[8] = { /* 0 */ "No emergency", /* 1 */ "General emergency (squawk 7700)", /* 2 */ "Lifeguard/Medical", /* 3 */ "Minimum fuel", /* 4 */ "No communications (squawk 7600)", /* 5 */ "Unlawful interference (squawk 7500)", /* 6 */ "Downed Aircraft", /* 7 */ "Reserved" }; // //========================================================================= // char *getMEDescription(int metype, int mesub) { char *mename = "Unknown"; if (metype >= 1 && metype <= 4) mename = "Aircraft Identification and Category"; else if (metype >= 5 && metype <= 8) mename = "Surface Position"; else if (metype >= 9 && metype <= 18) mename = "Airborne Position (Baro Altitude)"; else if (metype == 19 && mesub >=1 && mesub <= 4) mename = "Airborne Velocity"; else if (metype >= 20 && metype <= 22) mename = "Airborne Position (GNSS Height)"; else if (metype == 23 && mesub == 0) mename = "Test Message"; else if (metype == 23 && mesub == 7) mename = "Test Message -- Squawk"; else if (metype == 24 && mesub == 1) mename = "Surface System Status"; else if (metype == 28 && mesub == 1) mename = "Extended Squitter Aircraft Status (Emergency)"; else if (metype == 28 && mesub == 2) mename = "Extended Squitter Aircraft Status (1090ES TCAS RA)"; else if (metype == 29 && (mesub == 0 || mesub == 1)) mename = "Target State and Status Message"; else if (metype == 31 && (mesub == 0 || mesub == 1)) mename = "Aircraft Operational Status Message"; return mename; } // //========================================================================= // // Decode a raw Mode S message demodulated as a stream of bytes by detectModeS(), // and split it into fields populating a modesMessage structure. // void decodeModesMessage(struct modesMessage *mm, unsigned char *msg) { char *ais_charset = "?ABCDEFGHIJKLMNOPQRSTUVWXYZ????? ???????????????0123456789??????"; // Work on our local copy memcpy(mm->msg, msg, MODES_LONG_MSG_BYTES); msg = mm->msg; // Get the message type ASAP as other operations depend on this mm->msgtype = msg[0] >> 3; // Downlink Format mm->msgbits = modesMessageLenByType(mm->msgtype); mm->crc = modesChecksum(msg, mm->msgbits); if ((mm->crc) && (Modes.nfix_crc) && ((mm->msgtype == 17) || (mm->msgtype == 18))) { // if ((mm->crc) && (Modes.nfix_crc) && ((mm->msgtype == 11) || (mm->msgtype == 17))) { // // Fixing single bit errors in DF-11 is a bit dodgy because we have no way to // know for sure if the crc is supposed to be 0 or not - it could be any value // less than 80. Therefore, attempting to fix DF-11 errors can result in a // multitude of possible crc solutions, only one of which is correct. // // We should probably perform some sanity checks on corrected DF-11's before // using the results. Perhaps check the ICAO against known aircraft, and check // IID against known good IID's. That's a TODO. // mm->correctedbits = fixBitErrors(msg, mm->msgbits, Modes.nfix_crc, mm->corrected); // If we correct, validate ICAO addr to help filter birthday paradox solutions. if (mm->correctedbits) { uint32_t ulAddr = (msg[1] << 16) | (msg[2] << 8) | (msg[3]); if (!ICAOAddressWasRecentlySeen(ulAddr)) mm->correctedbits = 0; } } // // Note that most of the other computation happens *after* we fix the // single/two bit errors, otherwise we would need to recompute the fields again. // if (mm->msgtype == 11) { // DF 11 mm->iid = mm->crc; mm->addr = (msg[1] << 16) | (msg[2] << 8) | (msg[3]); mm->ca = (msg[0] & 0x07); // Responder capabilities if ((mm->crcok = (0 == mm->crc))) { // DF 11 : if crc == 0 try to populate our ICAO addresses whitelist. addRecentlySeenICAOAddr(mm->addr); } else if (mm->crc < 80) { mm->crcok = ICAOAddressWasRecentlySeen(mm->addr); if (mm->crcok) { addRecentlySeenICAOAddr(mm->addr); } } } else if (mm->msgtype == 17) { // DF 17 mm->addr = (msg[1] << 16) | (msg[2] << 8) | (msg[3]); mm->ca = (msg[0] & 0x07); // Responder capabilities if ((mm->crcok = (0 == mm->crc))) { // DF 17 : if crc == 0 try to populate our ICAO addresses whitelist. addRecentlySeenICAOAddr(mm->addr); } } else if (mm->msgtype == 18) { // DF 18 mm->addr = (msg[1] << 16) | (msg[2] << 8) | (msg[3]); mm->ca = (msg[0] & 0x07); // Control Field if ((mm->crcok = (0 == mm->crc))) { // DF 18 : if crc == 0 try to populate our ICAO addresses whitelist. addRecentlySeenICAOAddr(mm->addr); } } else { // All other DF's // Compare the checksum with the whitelist of recently seen ICAO // addresses. If it matches one, then declare the message as valid mm->crcok = ICAOAddressWasRecentlySeen(mm->addr = mm->crc); } // If we're checking CRC and the CRC is invalid, then we can't trust any // of the data contents, so save time and give up now. if ((Modes.check_crc) && (!mm->crcok) && (!mm->correctedbits)) { return;} // If decoding is disabled, this is as far as we go. if (Modes.no_decode) return; // Fields for DF0, DF16 if (mm->msgtype == 0 || mm->msgtype == 16) { if (msg[0] & 0x04) { // VS Bit mm->bFlags |= MODES_ACFLAGS_AOG_VALID | MODES_ACFLAGS_AOG; } else { mm->bFlags |= MODES_ACFLAGS_AOG_VALID; } } // Fields for DF11, DF17 if (mm->msgtype == 11 || mm->msgtype == 17) { if (mm->ca == 4) { mm->bFlags |= MODES_ACFLAGS_AOG_VALID | MODES_ACFLAGS_AOG; } else if (mm->ca == 5) { mm->bFlags |= MODES_ACFLAGS_AOG_VALID; } } // Fields for DF5, DF21 = Gillham encoded Squawk if (mm->msgtype == 5 || mm->msgtype == 21) { int ID13Field = ((msg[2] << 8) | msg[3]) & 0x1FFF; if (ID13Field) { mm->bFlags |= MODES_ACFLAGS_SQUAWK_VALID; mm->modeA = decodeID13Field(ID13Field); } } // Fields for DF0, DF4, DF16, DF20 13 bit altitude if (mm->msgtype == 0 || mm->msgtype == 4 || mm->msgtype == 16 || mm->msgtype == 20) { int AC13Field = ((msg[2] << 8) | msg[3]) & 0x1FFF; if (AC13Field) { // Only attempt to decode if a valid (non zero) altitude is present mm->bFlags |= MODES_ACFLAGS_ALTITUDE_VALID; mm->altitude = decodeAC13Field(AC13Field, &mm->unit); } } // Fields for DF4, DF5, DF20, DF21 if ((mm->msgtype == 4) || (mm->msgtype == 20) || (mm->msgtype == 5) || (mm->msgtype == 21)) { mm->bFlags |= MODES_ACFLAGS_FS_VALID; mm->fs = msg[0] & 7; // Flight status for DF4,5,20,21 if (mm->fs <= 3) { mm->bFlags |= MODES_ACFLAGS_AOG_VALID; if (mm->fs & 1) {mm->bFlags |= MODES_ACFLAGS_AOG;} } } // Fields for DF17, DF18_CF0, DF18_CF1, DF18_CF6 squitters if ( (mm->msgtype == 17) || ((mm->msgtype == 18) && ((mm->ca == 0) || (mm->ca == 1) || (mm->ca == 6)) )) { int metype = mm->metype = msg[4] >> 3; // Extended squitter message type int mesub = mm->mesub = (metype == 29 ? ((msg[4]&6)>>1) : (msg[4] & 7)); // Extended squitter message subtype // Decode the extended squitter message if (metype >= 1 && metype <= 4) { // Aircraft Identification and Category uint32_t chars; mm->bFlags |= MODES_ACFLAGS_CALLSIGN_VALID; chars = (msg[5] << 16) | (msg[6] << 8) | (msg[7]); mm->flight[3] = ais_charset[chars & 0x3F]; chars = chars >> 6; mm->flight[2] = ais_charset[chars & 0x3F]; chars = chars >> 6; mm->flight[1] = ais_charset[chars & 0x3F]; chars = chars >> 6; mm->flight[0] = ais_charset[chars & 0x3F]; chars = (msg[8] << 16) | (msg[9] << 8) | (msg[10]); mm->flight[7] = ais_charset[chars & 0x3F]; chars = chars >> 6; mm->flight[6] = ais_charset[chars & 0x3F]; chars = chars >> 6; mm->flight[5] = ais_charset[chars & 0x3F]; chars = chars >> 6; mm->flight[4] = ais_charset[chars & 0x3F]; mm->flight[8] = '\0'; } else if (metype == 19) { // Airborne Velocity Message // Presumably airborne if we get an Airborne Velocity Message mm->bFlags |= MODES_ACFLAGS_AOG_VALID; if ( (mesub >= 1) && (mesub <= 4) ) { int vert_rate = ((msg[8] & 0x07) << 6) | (msg[9] >> 2); if (vert_rate) { --vert_rate; if (msg[8] & 0x08) {vert_rate = 0 - vert_rate;} mm->vert_rate = vert_rate * 64; mm->bFlags |= MODES_ACFLAGS_VERTRATE_VALID; } } if ((mesub == 1) || (mesub == 2)) { int ew_raw = ((msg[5] & 0x03) << 8) | msg[6]; int ew_vel = ew_raw - 1; int ns_raw = ((msg[7] & 0x7F) << 3) | (msg[8] >> 5); int ns_vel = ns_raw - 1; if (mesub == 2) { // If (supersonic) unit is 4 kts ns_vel = ns_vel << 2; ew_vel = ew_vel << 2; } if (ew_raw) { // Do East/West mm->bFlags |= MODES_ACFLAGS_EWSPEED_VALID; if (msg[5] & 0x04) {ew_vel = 0 - ew_vel;} mm->ew_velocity = ew_vel; } if (ns_raw) { // Do North/South mm->bFlags |= MODES_ACFLAGS_NSSPEED_VALID; if (msg[7] & 0x80) {ns_vel = 0 - ns_vel;} mm->ns_velocity = ns_vel; } if (ew_raw && ns_raw) { // Compute velocity and angle from the two speed components mm->bFlags |= (MODES_ACFLAGS_SPEED_VALID | MODES_ACFLAGS_HEADING_VALID | MODES_ACFLAGS_NSEWSPD_VALID); mm->velocity = (int) sqrt((ns_vel * ns_vel) + (ew_vel * ew_vel)); if (mm->velocity) { mm->heading = (int) (atan2(ew_vel, ns_vel) * 180.0 / M_PI); // We don't want negative values but a 0-360 scale if (mm->heading < 0) mm->heading += 360; } } } else if (mesub == 3 || mesub == 4) { int airspeed = ((msg[7] & 0x7f) << 3) | (msg[8] >> 5); if (airspeed) { mm->bFlags |= MODES_ACFLAGS_SPEED_VALID; --airspeed; if (mesub == 4) // If (supersonic) unit is 4 kts {airspeed = airspeed << 2;} mm->velocity = airspeed; } if (msg[5] & 0x04) { mm->bFlags |= MODES_ACFLAGS_HEADING_VALID; mm->heading = ((((msg[5] & 0x03) << 8) | msg[6]) * 45) >> 7; } } } else if (metype >= 5 && metype <= 22) { // Position Message mm->raw_latitude = ((msg[6] & 3) << 15) | (msg[7] << 7) | (msg[8] >> 1); mm->raw_longitude = ((msg[8] & 1) << 16) | (msg[9] << 8) | (msg[10]); mm->bFlags |= (mm->msg[6] & 0x04) ? MODES_ACFLAGS_LLODD_VALID : MODES_ACFLAGS_LLEVEN_VALID; if (metype >= 9) { // Airborne int AC12Field = ((msg[5] << 4) | (msg[6] >> 4)) & 0x0FFF; mm->bFlags |= MODES_ACFLAGS_AOG_VALID; if (AC12Field) {// Only attempt to decode if a valid (non zero) altitude is present mm->bFlags |= MODES_ACFLAGS_ALTITUDE_VALID; mm->altitude = decodeAC12Field(AC12Field, &mm->unit); } } else { // Ground int movement = ((msg[4] << 4) | (msg[5] >> 4)) & 0x007F; mm->bFlags |= MODES_ACFLAGS_AOG_VALID | MODES_ACFLAGS_AOG; if ((movement) && (movement < 125)) { mm->bFlags |= MODES_ACFLAGS_SPEED_VALID; mm->velocity = decodeMovementField(movement); } if (msg[5] & 0x08) { mm->bFlags |= MODES_ACFLAGS_HEADING_VALID; mm->heading = ((((msg[5] << 4) | (msg[6] >> 4)) & 0x007F) * 45) >> 4; } } } else if (metype == 23) { // Test metype squawk field if (mesub == 7) { // (see 1090-WP-15-20) int ID13Field = (((msg[5] << 8) | msg[6]) & 0xFFF1)>>3; if (ID13Field) { mm->bFlags |= MODES_ACFLAGS_SQUAWK_VALID; mm->modeA = decodeID13Field(ID13Field); } } } else if (metype == 24) { // Reserved for Surface System Status } else if (metype == 28) { // Extended Squitter Aircraft Status if (mesub == 1) { // Emergency status squawk field int ID13Field = (((msg[5] << 8) | msg[6]) & 0x1FFF); if (ID13Field) { mm->bFlags |= MODES_ACFLAGS_SQUAWK_VALID; mm->modeA = decodeID13Field(ID13Field); } } } else if (metype == 29) { // Aircraft Trajectory Intent } else if (metype == 30) { // Aircraft Operational Coordination } else if (metype == 31) { // Aircraft Operational Status } else { // Other metypes } } // Fields for DF20, DF21 Comm-B if ((mm->msgtype == 20) || (mm->msgtype == 21)){ if (msg[4] == 0x20) { // Aircraft Identification uint32_t chars; mm->bFlags |= MODES_ACFLAGS_CALLSIGN_VALID; chars = (msg[5] << 16) | (msg[6] << 8) | (msg[7]); mm->flight[3] = ais_charset[chars & 0x3F]; chars = chars >> 6; mm->flight[2] = ais_charset[chars & 0x3F]; chars = chars >> 6; mm->flight[1] = ais_charset[chars & 0x3F]; chars = chars >> 6; mm->flight[0] = ais_charset[chars & 0x3F]; chars = (msg[8] << 16) | (msg[9] << 8) | (msg[10]); mm->flight[7] = ais_charset[chars & 0x3F]; chars = chars >> 6; mm->flight[6] = ais_charset[chars & 0x3F]; chars = chars >> 6; mm->flight[5] = ais_charset[chars & 0x3F]; chars = chars >> 6; mm->flight[4] = ais_charset[chars & 0x3F]; mm->flight[8] = '\0'; } else { } } } // //========================================================================= // // This function gets a decoded Mode S Message and prints it on the screen // in a human readable format. // void displayModesMessage(struct modesMessage *mm) { int j; unsigned char * pTimeStamp; // Handle only addresses mode first. if (Modes.onlyaddr) { printf("%06x\n", mm->addr); return; // Enough for --onlyaddr mode } // Show the raw message. if (Modes.mlat && mm->timestampMsg) { printf("@"); pTimeStamp = (unsigned char *) &mm->timestampMsg; for (j=5; j>=0;j--) { printf("%02X",pTimeStamp[j]); } } else printf("*"); for (j = 0; j < mm->msgbits/8; j++) printf("%02x", mm->msg[j]); printf(";\n"); if (Modes.raw) { fflush(stdout); // Provide data to the reader ASAP return; // Enough for --raw mode } if (mm->msgtype < 32) printf("CRC: %06x (%s)\n", (int)mm->crc, mm->crcok ? "ok" : "wrong"); if (mm->correctedbits != 0) printf("No. of bit errors fixed: %d\n", mm->correctedbits); printf("SNR: %d.%d dB\n", mm->signalLevel/5, 2*(mm->signalLevel%5)); if (Modes.no_decode) { // Show DF type and address only; the rest is not decoded. printf("DF %d; address: %06x\n", mm->msgtype, mm->addr); return; } if (mm->msgtype == 0) { // DF 0 printf("DF 0: Short Air-Air Surveillance.\n"); printf(" VS : %s\n", (mm->msg[0] & 0x04) ? "Ground" : "Airborne"); printf(" CC : %d\n", ((mm->msg[0] & 0x02) >> 1)); printf(" SL : %d\n", ((mm->msg[1] & 0xE0) >> 5)); printf(" Altitude : %d %s\n", mm->altitude, (mm->unit == MODES_UNIT_METERS) ? "meters" : "feet"); printf(" ICAO Address : %06x\n", mm->addr); } else if (mm->msgtype == 4 || mm->msgtype == 20) { printf("DF %d: %s, Altitude Reply.\n", mm->msgtype, (mm->msgtype == 4) ? "Surveillance" : "Comm-B"); printf(" Flight Status : %s\n", fs_str[mm->fs]); printf(" DR : %d\n", ((mm->msg[1] >> 3) & 0x1F)); printf(" UM : %d\n", (((mm->msg[1] & 7) << 3) | (mm->msg[2] >> 5))); printf(" Altitude : %d %s\n", mm->altitude, (mm->unit == MODES_UNIT_METERS) ? "meters" : "feet"); printf(" ICAO Address : %06x\n", mm->addr); if (mm->msgtype == 20) { printf(" Comm-B BDS : %x\n", mm->msg[4]); // Decode the extended squitter message if ( mm->msg[4] == 0x20) { // BDS 2,0 Aircraft identification printf(" BDS 2,0 Aircraft Identification : %s\n", mm->flight); /* } else if ( mm->msg[4] == 0x10) { // BDS 1,0 Datalink Capability report printf(" BDS 1,0 Datalink Capability report\n"); } else if ( mm->msg[4] == 0x30) { // BDS 3,0 ACAS Active Resolution Advisory printf(" BDS 3,0 ACAS Active Resolution Advisory\n"); } else if ((mm->msg[4] >> 3) == 28) { // BDS 6,1 Extended Squitter Emergency/Priority Status printf(" BDS 6,1 Emergency/Priority Status\n"); } else if ((mm->msg[4] >> 3) == 29) { // BDS 6,2 Target State and Status printf(" BDS 6,2 Target State and Status\n"); } else if ((mm->msg[4] >> 3) == 31) { // BDS 6,5 Extended Squitter Aircraft Operational Status printf(" BDS 6,5 Aircraft Operational Status\n"); */ } } } else if (mm->msgtype == 5 || mm->msgtype == 21) { printf("DF %d: %s, Identity Reply.\n", mm->msgtype, (mm->msgtype == 5) ? "Surveillance" : "Comm-B"); printf(" Flight Status : %s\n", fs_str[mm->fs]); printf(" DR : %d\n", ((mm->msg[1] >> 3) & 0x1F)); printf(" UM : %d\n", (((mm->msg[1] & 7) << 3) | (mm->msg[2] >> 5))); printf(" Squawk : %04x\n", mm->modeA); printf(" ICAO Address : %06x\n", mm->addr); if (mm->msgtype == 21) { printf(" Comm-B BDS : %x\n", mm->msg[4]); // Decode the extended squitter message if ( mm->msg[4] == 0x20) { // BDS 2,0 Aircraft identification printf(" BDS 2,0 Aircraft Identification : %s\n", mm->flight); /* } else if ( mm->msg[4] == 0x10) { // BDS 1,0 Datalink Capability report printf(" BDS 1,0 Datalink Capability report\n"); } else if ( mm->msg[4] == 0x30) { // BDS 3,0 ACAS Active Resolution Advisory printf(" BDS 3,0 ACAS Active Resolution Advisory\n"); } else if ((mm->msg[4] >> 3) == 28) { // BDS 6,1 Extended Squitter Emergency/Priority Status printf(" BDS 6,1 Emergency/Priority Status\n"); } else if ((mm->msg[4] >> 3) == 29) { // BDS 6,2 Target State and Status printf(" BDS 6,2 Target State and Status\n"); } else if ((mm->msg[4] >> 3) == 31) { // BDS 6,5 Extended Squitter Aircraft Operational Status printf(" BDS 6,5 Aircraft Operational Status\n"); */ } } } else if (mm->msgtype == 11) { // DF 11 printf("DF 11: All Call Reply.\n"); printf(" Capability : %d (%s)\n", mm->ca, ca_str[mm->ca]); printf(" ICAO Address: %06x\n", mm->addr); if (mm->iid > 16) {printf(" IID : SI-%02d\n", mm->iid-16);} else {printf(" IID : II-%02d\n", mm->iid);} } else if (mm->msgtype == 16) { // DF 16 printf("DF 16: Long Air to Air ACAS\n"); printf(" VS : %s\n", (mm->msg[0] & 0x04) ? "Ground" : "Airborne"); printf(" CC : %d\n", ((mm->msg[0] & 0x02) >> 1)); printf(" SL : %d\n", ((mm->msg[1] & 0xE0) >> 5)); printf(" Altitude : %d %s\n", mm->altitude, (mm->unit == MODES_UNIT_METERS) ? "meters" : "feet"); printf(" ICAO Address : %06x\n", mm->addr); } else if (mm->msgtype == 17) { // DF 17 printf("DF 17: ADS-B message.\n"); printf(" Capability : %d (%s)\n", mm->ca, ca_str[mm->ca]); printf(" ICAO Address : %06x\n", mm->addr); printf(" Extended Squitter Type: %d\n", mm->metype); printf(" Extended Squitter Sub : %d\n", mm->mesub); printf(" Extended Squitter Name: %s\n", getMEDescription(mm->metype, mm->mesub)); // Decode the extended squitter message if (mm->metype >= 1 && mm->metype <= 4) { // Aircraft identification printf(" Aircraft Type : %c%d\n", ('A' + 4 - mm->metype), mm->mesub); printf(" Identification : %s\n", mm->flight); } else if (mm->metype == 19) { // Airborne Velocity if (mm->mesub == 1 || mm->mesub == 2) { printf(" EW status : %s\n", (mm->bFlags & MODES_ACFLAGS_EWSPEED_VALID) ? "Valid" : "Unavailable"); printf(" EW velocity : %d\n", mm->ew_velocity); printf(" NS status : %s\n", (mm->bFlags & MODES_ACFLAGS_NSSPEED_VALID) ? "Valid" : "Unavailable"); printf(" NS velocity : %d\n", mm->ns_velocity); printf(" Vertical status : %s\n", (mm->bFlags & MODES_ACFLAGS_VERTRATE_VALID) ? "Valid" : "Unavailable"); printf(" Vertical rate src : %d\n", ((mm->msg[8] >> 4) & 1)); printf(" Vertical rate : %d\n", mm->vert_rate); } else if (mm->mesub == 3 || mm->mesub == 4) { printf(" Heading status : %s\n", (mm->bFlags & MODES_ACFLAGS_HEADING_VALID) ? "Valid" : "Unavailable"); printf(" Heading : %d\n", mm->heading); printf(" Airspeed status : %s\n", (mm->bFlags & MODES_ACFLAGS_SPEED_VALID) ? "Valid" : "Unavailable"); printf(" Airspeed : %d\n", mm->velocity); printf(" Vertical status : %s\n", (mm->bFlags & MODES_ACFLAGS_VERTRATE_VALID) ? "Valid" : "Unavailable"); printf(" Vertical rate src : %d\n", ((mm->msg[8] >> 4) & 1)); printf(" Vertical rate : %d\n", mm->vert_rate); } else { printf(" Unrecognized ME subtype: %d subtype: %d\n", mm->metype, mm->mesub); } } else if (mm->metype >= 5 && mm->metype <= 22) { // Airborne position Baro printf(" F flag : %s\n", (mm->msg[6] & 0x04) ? "odd" : "even"); printf(" T flag : %s\n", (mm->msg[6] & 0x08) ? "UTC" : "non-UTC"); printf(" Altitude : %d feet\n", mm->altitude); if (mm->bFlags & MODES_ACFLAGS_LATLON_VALID) { printf(" Latitude : %f\n", mm->fLat); printf(" Longitude: %f\n", mm->fLon); } else { printf(" Latitude : %d (not decoded)\n", mm->raw_latitude); printf(" Longitude: %d (not decoded)\n", mm->raw_longitude); } } else if (mm->metype == 28) { // Extended Squitter Aircraft Status if (mm->mesub == 1) { printf(" Emergency State: %s\n", es_str[(mm->msg[5] & 0xE0) >> 5]); printf(" Squawk: %04x\n", mm->modeA); } else { printf(" Unrecognized ME subtype: %d subtype: %d\n", mm->metype, mm->mesub); } } else if (mm->metype == 23) { // Test Message if (mm->mesub == 7) { printf(" Squawk: %04x\n", mm->modeA); } else { printf(" Unrecognized ME subtype: %d subtype: %d\n", mm->metype, mm->mesub); } } else { printf(" Unrecognized ME type: %d subtype: %d\n", mm->metype, mm->mesub); } } else if (mm->msgtype == 18) { // DF 18 printf("DF 18: Extended Squitter.\n"); printf(" Control Field : %d (%s)\n", mm->ca, cf_str[mm->ca]); if ((mm->ca == 0) || (mm->ca == 1) || (mm->ca == 6)) { if (mm->ca == 1) { printf(" Other Address : %06x\n", mm->addr); } else { printf(" ICAO Address : %06x\n", mm->addr); } printf(" Extended Squitter Type: %d\n", mm->metype); printf(" Extended Squitter Sub : %d\n", mm->mesub); printf(" Extended Squitter Name: %s\n", getMEDescription(mm->metype, mm->mesub)); // Decode the extended squitter message if (mm->metype >= 1 && mm->metype <= 4) { // Aircraft identification printf(" Aircraft Type : %c%d\n", ('A' + 4 - mm->metype), mm->mesub); printf(" Identification : %s\n", mm->flight); } else if (mm->metype == 19) { // Airborne Velocity if (mm->mesub == 1 || mm->mesub == 2) { printf(" EW status : %s\n", (mm->bFlags & MODES_ACFLAGS_EWSPEED_VALID) ? "Valid" : "Unavailable"); printf(" EW velocity : %d\n", mm->ew_velocity); printf(" NS status : %s\n", (mm->bFlags & MODES_ACFLAGS_NSSPEED_VALID) ? "Valid" : "Unavailable"); printf(" NS velocity : %d\n", mm->ns_velocity); printf(" Vertical status : %s\n", (mm->bFlags & MODES_ACFLAGS_VERTRATE_VALID) ? "Valid" : "Unavailable"); printf(" Vertical rate src : %d\n", ((mm->msg[8] >> 4) & 1)); printf(" Vertical rate : %d\n", mm->vert_rate); } else if (mm->mesub == 3 || mm->mesub == 4) { printf(" Heading status : %s\n", (mm->bFlags & MODES_ACFLAGS_HEADING_VALID) ? "Valid" : "Unavailable"); printf(" Heading : %d\n", mm->heading); printf(" Airspeed status : %s\n", (mm->bFlags & MODES_ACFLAGS_SPEED_VALID) ? "Valid" : "Unavailable"); printf(" Airspeed : %d\n", mm->velocity); printf(" Vertical status : %s\n", (mm->bFlags & MODES_ACFLAGS_VERTRATE_VALID) ? "Valid" : "Unavailable"); printf(" Vertical rate src : %d\n", ((mm->msg[8] >> 4) & 1)); printf(" Vertical rate : %d\n", mm->vert_rate); } else { printf(" Unrecognized ME subtype: %d subtype: %d\n", mm->metype, mm->mesub); } } else if (mm->metype >= 5 && mm->metype <= 22) { // Ground or Airborne position, Baro or GNSS printf(" F flag : %s\n", (mm->msg[6] & 0x04) ? "odd" : "even"); printf(" T flag : %s\n", (mm->msg[6] & 0x08) ? "UTC" : "non-UTC"); printf(" Altitude : %d feet\n", mm->altitude); if (mm->bFlags & MODES_ACFLAGS_LATLON_VALID) { printf(" Latitude : %f\n", mm->fLat); printf(" Longitude: %f\n", mm->fLon); } else { printf(" Latitude : %d (not decoded)\n", mm->raw_latitude); printf(" Longitude: %d (not decoded)\n", mm->raw_longitude); } } else { printf(" Unrecognized ME type: %d subtype: %d\n", mm->metype, mm->mesub); } } } else if (mm->msgtype == 19) { // DF 19 printf("DF 19: Military Extended Squitter.\n"); } else if (mm->msgtype == 22) { // DF 22 printf("DF 22: Military Use.\n"); } else if (mm->msgtype == 24) { // DF 24 printf("DF 24: Comm D Extended Length Message.\n"); } else if (mm->msgtype == 32) { // DF 32 is special code we use for Mode A/C printf("SSR : Mode A/C Reply.\n"); if (mm->fs & 0x0080) { printf(" Mode A : %04x IDENT\n", mm->modeA); } else { printf(" Mode A : %04x\n", mm->modeA); if (mm->bFlags & MODES_ACFLAGS_ALTITUDE_VALID) {printf(" Mode C : %d feet\n", mm->altitude);} } } else { printf("DF %d: Unknown DF Format.\n", mm->msgtype); } printf("\n"); } // //========================================================================= // // Turn I/Q samples pointed by Modes.data into the magnitude vector // pointed by Modes.magnitude. // void computeMagnitudeVector(uint16_t *p) { uint16_t *m = &Modes.magnitude[Modes.trailing_space]; uint32_t j; memcpy(Modes.magnitude,&Modes.magnitude[MODES_ASYNC_BUF_SAMPLES], Modes.trailing_space); // Compute the magnitudo vector. It's just SQRT(I^2 + Q^2), but // we rescale to the 0-255 range to exploit the full resolution. for (j = 0; j < MODES_ASYNC_BUF_SAMPLES; j ++) { *m++ = Modes.maglut[*p++]; } } // //========================================================================= // // Return -1 if the message is out of fase left-side // Return 1 if the message is out of fase right-size // Return 0 if the message is not particularly out of phase. // // Note: this function will access pPreamble[-1], so the caller should make sure to // call it only if we are not at the start of the current buffer // int detectOutOfPhase(uint16_t *pPreamble) { if (pPreamble[ 3] > pPreamble[2]/3) return 1; if (pPreamble[10] > pPreamble[9]/3) return 1; if (pPreamble[ 6] > pPreamble[7]/3) return -1; if (pPreamble[-1] > pPreamble[1]/3) return -1; return 0; } uint16_t clamped_scale(uint16_t v, uint16_t scale) { uint32_t scaled = (uint32_t)v * scale / 16384; if (scaled > 65535) return 65535; return (uint16_t) scaled; } // This function decides whether we are sampling early or late, // and by approximately how much, by looking at the energy in // preamble bits before and after the expected pulse locations. // // It then deals with one sample pair at a time, comparing samples // to make a decision about the bit value. Based on this decision it // modifies the sample value of the *adjacent* sample which will // contain some of the energy from the bit we just inspected. // // pPayload[0] should be the start of the preamble, // pPayload[-1 .. MODES_PREAMBLE_SAMPLES + MODES_LONG_MSG_SAMPLES - 1] should be accessible. // pPayload[MODES_PREAMBLE_SAMPLES .. MODES_PREAMBLE_SAMPLES + MODES_LONG_MSG_SAMPLES - 1] will be updated. void applyPhaseCorrection(uint16_t *pPayload) { int j; // we expect 1 bits at 0, 2, 7, 9 // and 0 bits at -1, 1, 3, 4, 5, 6, 8, 10, 11, 12, 13, 14 // use bits -1,6 for early detection (bit 0/7 arrived a little early, our sample period starts after the bit phase so we include some of the next bit) // use bits 3,10 for late detection (bit 2/9 arrived a little late, our sample period starts before the bit phase so we include some of the last bit) uint32_t onTime = (pPayload[0] + pPayload[2] + pPayload[7] + pPayload[9]); uint32_t early = (pPayload[-1] + pPayload[6]) << 1; uint32_t late = (pPayload[3] + pPayload[10]) << 1; if (early > late) { // Our sample period starts late and so includes some of the next bit. uint16_t scaleUp = 16384 + 16384 * early / (early + onTime); // 1 + early / (early+onTime) uint16_t scaleDown = 16384 - 16384 * early / (early + onTime); // 1 - early / (early+onTime) // trailing bits are 0; final data sample will be a bit low. pPayload[MODES_PREAMBLE_SAMPLES + MODES_LONG_MSG_SAMPLES - 1] = clamped_scale(pPayload[MODES_PREAMBLE_SAMPLES + MODES_LONG_MSG_SAMPLES - 1], scaleUp); for (j = MODES_PREAMBLE_SAMPLES + MODES_LONG_MSG_SAMPLES - 2; j > MODES_PREAMBLE_SAMPLES; j -= 2) { if (pPayload[j] > pPayload[j+1]) { // x [1 0] y // x overlapped with the "1" bit and is slightly high pPayload[j-1] = clamped_scale(pPayload[j-1], scaleDown); } else { // x [0 1] y // x overlapped with the "0" bit and is slightly low pPayload[j-1] = clamped_scale(pPayload[j-1], scaleUp); } } } else { // Our sample period starts early and so includes some of the previous bit. uint16_t scaleUp = 16384 + 16384 * late / (late + onTime); // 1 + late / (late+onTime) uint16_t scaleDown = 16384 - 16384 * late / (late + onTime); // 1 - late / (late+onTime) // leading bits are 0; first data sample will be a bit low. pPayload[MODES_PREAMBLE_SAMPLES] = clamped_scale(pPayload[MODES_PREAMBLE_SAMPLES], scaleUp); for (j = MODES_PREAMBLE_SAMPLES; j < MODES_PREAMBLE_SAMPLES + MODES_LONG_MSG_SAMPLES - 2; j += 2) { if (pPayload[j] > pPayload[j+1]) { // x [1 0] y // y overlapped with the "0" bit and is slightly low pPayload[j+2] = clamped_scale(pPayload[j+2], scaleUp); } else { // x [0 1] y // y overlapped with the "1" bit and is slightly high pPayload[j+2] = clamped_scale(pPayload[j+2], scaleDown); } } } } // //========================================================================= // // Detect a Mode S messages inside the magnitude buffer pointed by 'm' and of // size 'mlen' bytes. Every detected Mode S message is convert it into a // stream of bits and passed to the function to display it. // void detectModeS(uint16_t *m, uint32_t mlen) { struct modesMessage mm; unsigned char msg[MODES_LONG_MSG_BYTES], *pMsg; uint16_t aux[MODES_PREAMBLE_SAMPLES+MODES_LONG_MSG_SAMPLES+1]; uint32_t j; int use_correction = 0; memset(&mm, 0, sizeof(mm)); // The Mode S preamble is made of impulses of 0.5 microseconds at // the following time offsets: // // 0 - 0.5 usec: first impulse. // 1.0 - 1.5 usec: second impulse. // 3.5 - 4 usec: third impulse. // 4.5 - 5 usec: last impulse. // // Since we are sampling at 2 Mhz every sample in our magnitude vector // is 0.5 usec, so the preamble will look like this, assuming there is // an impulse at offset 0 in the array: // // 0 ----------------- // 1 - // 2 ------------------ // 3 -- // 4 - // 5 -- // 6 - // 7 ------------------ // 8 -- // 9 ------------------- // for (j = 0; j < mlen; j++) { int high, i, errors, errors56, errorsTy; uint16_t *pPreamble, *pPayload, *pPtr; uint8_t theByte, theErrs; int msglen, scanlen; uint32_t sigLevel, noiseLevel; uint16_t snr; pPreamble = &m[j]; pPayload = &m[j+MODES_PREAMBLE_SAMPLES]; // Rather than clear the whole mm structure, just clear the parts which are required. The clear // is required for every bit of the input stream, and we don't want to be memset-ing the whole // modesMessage structure two million times per second if we don't have to.. mm.bFlags = mm.crcok = mm.correctedbits = 0; if (!use_correction) // This is not a re-try with phase correction { // so try to find a new preamble if (Modes.mode_ac) { int ModeA = detectModeA(pPreamble, &mm); if (ModeA) // We have found a valid ModeA/C in the data { mm.timestampMsg = Modes.timestampBlk + ((j+1) * 6); // Decode the received message decodeModeAMessage(&mm, ModeA); // Pass data to the next layer useModesMessage(&mm); j += MODEAC_MSG_SAMPLES; Modes.stat_ModeAC++; continue; } } // First check of relations between the first 10 samples // representing a valid preamble. We don't even investigate further // if this simple test is not passed if (!(pPreamble[0] > pPreamble[1] && pPreamble[1] < pPreamble[2] && pPreamble[2] > pPreamble[3] && pPreamble[3] < pPreamble[0] && pPreamble[4] < pPreamble[0] && pPreamble[5] < pPreamble[0] && pPreamble[6] < pPreamble[0] && pPreamble[7] > pPreamble[8] && pPreamble[8] < pPreamble[9] && pPreamble[9] > pPreamble[6])) { if (Modes.debug & MODES_DEBUG_NOPREAMBLE && *pPreamble > MODES_DEBUG_NOPREAMBLE_LEVEL) dumpRawMessage("Unexpected ratio among first 10 samples", msg, m, j); continue; } // The samples between the two spikes must be < than the average // of the high spikes level. We don't test bits too near to // the high levels as signals can be out of phase so part of the // energy can be in the near samples high = (pPreamble[0] + pPreamble[2] + pPreamble[7] + pPreamble[9]) / 6; if (pPreamble[4] >= high || pPreamble[5] >= high) { if (Modes.debug & MODES_DEBUG_NOPREAMBLE && *pPreamble > MODES_DEBUG_NOPREAMBLE_LEVEL) dumpRawMessage("Too high level in samples between 3 and 6", msg, m, j); continue; } // Similarly samples in the range 11-14 must be low, as it is the // space between the preamble and real data. Again we don't test // bits too near to high levels, see above if (pPreamble[11] >= high || pPreamble[12] >= high || pPreamble[13] >= high || pPreamble[14] >= high) { if (Modes.debug & MODES_DEBUG_NOPREAMBLE && *pPreamble > MODES_DEBUG_NOPREAMBLE_LEVEL) dumpRawMessage("Too high level in samples between 10 and 15", msg, m, j); continue; } Modes.stat_valid_preamble++; } else { // If the previous attempt with this message failed, retry using // magnitude correction // Make a copy of the Payload, and phase correct the copy memcpy(aux, &pPreamble[-1], sizeof(aux)); applyPhaseCorrection(&aux[1]); Modes.stat_out_of_phase++; pPayload = &aux[1 + MODES_PREAMBLE_SAMPLES]; // TODO ... apply other kind of corrections } // Decode all the next 112 bits, regardless of the actual message // size. We'll check the actual message type later pMsg = &msg[0]; pPtr = pPayload; theByte = 0; theErrs = 0; errorsTy = 0; errors = 0; errors56 = 0; // We should have 4 'bits' of 0/1 and 1/0 samples in the preamble, // so include these in the signal strength sigLevel = pPreamble[0] + pPreamble[2] + pPreamble[7] + pPreamble[9]; noiseLevel = pPreamble[1] + pPreamble[3] + pPreamble[4] + pPreamble[6] + pPreamble[8]; msglen = scanlen = MODES_LONG_MSG_BITS; for (i = 0; i < scanlen; i++) { uint32_t a = *pPtr++; uint32_t b = *pPtr++; if (a > b) {theByte |= 1; if (i < 56) { sigLevel += a; noiseLevel += b; }} else if (a < b) {/*theByte |= 0;*/ if (i < 56) { sigLevel += b; noiseLevel += a; }} else { if (i < 56) { sigLevel += a; noiseLevel += a; } if (i >= MODES_SHORT_MSG_BITS) //(a == b), and we're in the long part of a frame {errors++; /*theByte |= 0;*/} else if (i >= 5) //(a == b), and we're in the short part of a frame {scanlen = MODES_LONG_MSG_BITS; errors56 = ++errors;/*theByte |= 0;*/} else if (i) //(a == b), and we're in the message type part of a frame {errorsTy = errors56 = ++errors; theErrs |= 1; /*theByte |= 0;*/} else //(a == b), and we're in the first bit of the message type part of a frame {errorsTy = errors56 = ++errors; theErrs |= 1; theByte |= 1;} } if ((i & 7) == 7) {*pMsg++ = theByte;} else if (i == 4) { msglen = modesMessageLenByType(theByte); if (errors == 0) {scanlen = msglen;} } theByte = theByte << 1; if (i < 7) {theErrs = theErrs << 1;} // If we've exceeded the permissible number of encoding errors, abandon ship now if (errors > MODES_MSG_ENCODER_ERRS) { if (i < MODES_SHORT_MSG_BITS) { msglen = 0; } else if ((errorsTy == 1) && (theErrs == 0x80)) { // If we only saw one error in the first bit of the byte of the frame, then it's possible // we guessed wrongly about the value of the bit. We may be able to correct it by guessing // the other way. // // We guessed a '1' at bit 7, which is the DF length bit == 112 Bits. // Inverting bit 7 will change the message type from a long to a short. // Invert the bit, cross your fingers and carry on. msglen = MODES_SHORT_MSG_BITS; msg[0] ^= theErrs; errorsTy = 0; errors = errors56; // revert to the number of errors prior to bit 56 Modes.stat_DF_Len_Corrected++; } else if (i < MODES_LONG_MSG_BITS) { msglen = MODES_SHORT_MSG_BITS; errors = errors56; } else { msglen = MODES_LONG_MSG_BITS; } break; } } // Ensure msglen is consistent with the DF type i = modesMessageLenByType(msg[0] >> 3); if (msglen > i) {msglen = i;} else if (msglen < i) {msglen = 0;} // // If we guessed at any of the bits in the DF type field, then look to see if our guess was sensible. // Do this by looking to see if the original guess results in the DF type being one of the ICAO defined // message types. If it isn't then toggle the guessed bit and see if this new value is ICAO defined. // if the new value is ICAO defined, then update it in our message. if ((msglen) && (errorsTy == 1) && (theErrs & 0x78)) { // We guessed at one (and only one) of the message type bits. See if our guess is "likely" // to be correct by comparing the DF against a list of known good DF's int thisDF = ((theByte = msg[0]) >> 3) & 0x1f; uint32_t validDFbits = 0x017F0831; // One bit per 32 possible DF's. Set bits 0,4,5,11,16.17.18.19,20,21,22,24 uint32_t thisDFbit = (1 << thisDF); if (0 == (validDFbits & thisDFbit)) { // The current DF is not ICAO defined, so is probably an errors. // Toggle the bit we guessed at and see if the resultant DF is more likely theByte ^= theErrs; thisDF = (theByte >> 3) & 0x1f; thisDFbit = (1 << thisDF); // if this DF any more likely? if (validDFbits & thisDFbit) { // Yep, more likely, so update the main message msg[0] = theByte; Modes.stat_DF_Type_Corrected++; errors--; // decrease the error count so we attempt to use the modified DF. } } } // snr = 5 * 20log10(sigLevel / noiseLevel) (in units of 0.2dB) // = 100log10(sigLevel) - 100log10(noiseLevel) while (sigLevel > 65535 || noiseLevel > 65535) { sigLevel >>= 1; noiseLevel >>= 1; } snr = Modes.log10lut[sigLevel] - Modes.log10lut[noiseLevel]; // When we reach this point, if error is small, and the signal strength is large enough // we may have a Mode S message on our hands. It may still be broken and the CRC may not // be correct, but this can be handled by the next layer. if ( (msglen) && ((2 * snr) > (int) (MODES_MSG_SQUELCH_DB * 10)) && (errors <= MODES_MSG_ENCODER_ERRS) ) { // Set initial mm structure details mm.timestampMsg = Modes.timestampBlk + (j*6); mm.signalLevel = (snr > 255 ? 255 : (uint8_t)snr); mm.phase_corrected = use_correction; // Decode the received message decodeModesMessage(&mm, msg); // Update statistics if (Modes.stats) { if (mm.crcok || use_correction || mm.correctedbits) { if (use_correction) { switch (errors) { case 0: {Modes.stat_ph_demodulated0++; break;} case 1: {Modes.stat_ph_demodulated1++; break;} case 2: {Modes.stat_ph_demodulated2++; break;} default:{Modes.stat_ph_demodulated3++; break;} } } else { switch (errors) { case 0: {Modes.stat_demodulated0++; break;} case 1: {Modes.stat_demodulated1++; break;} case 2: {Modes.stat_demodulated2++; break;} default:{Modes.stat_demodulated3++; break;} } } if (mm.correctedbits == 0) { if (use_correction) { if (mm.crcok) {Modes.stat_ph_goodcrc++;} else {Modes.stat_ph_badcrc++;} } else { if (mm.crcok) {Modes.stat_goodcrc++;} else {Modes.stat_badcrc++;} } } else if (use_correction) { Modes.stat_ph_badcrc++; Modes.stat_ph_fixed++; if ( (mm.correctedbits) && (mm.correctedbits <= MODES_MAX_BITERRORS) ) { Modes.stat_ph_bit_fix[mm.correctedbits-1] += 1; } } else { Modes.stat_badcrc++; Modes.stat_fixed++; if ( (mm.correctedbits) && (mm.correctedbits <= MODES_MAX_BITERRORS) ) { Modes.stat_bit_fix[mm.correctedbits-1] += 1; } } } } // Output debug mode info if needed if (use_correction) { if (Modes.debug & MODES_DEBUG_DEMOD) dumpRawMessage("Demodulated with 0 errors", msg, m, j); else if (Modes.debug & MODES_DEBUG_BADCRC && mm.msgtype == 17 && (!mm.crcok || mm.correctedbits != 0)) dumpRawMessage("Decoded with bad CRC", msg, m, j); else if (Modes.debug & MODES_DEBUG_GOODCRC && mm.crcok && mm.correctedbits == 0) dumpRawMessage("Decoded with good CRC", msg, m, j); } // Skip this message if we are sure it's fine if (mm.crcok) { j += (MODES_PREAMBLE_US+msglen)*2 - 1; } // Pass data to the next layer useModesMessage(&mm); } else { if (Modes.debug & MODES_DEBUG_DEMODERR && use_correction) { printf("The following message has %d demod errors\n", errors); dumpRawMessage("Demodulated with errors", msg, m, j); } } // Retry with phase correction if enabled, necessary and possible. if (Modes.phase_enhance && !mm.crcok && !mm.correctedbits && !use_correction && j && detectOutOfPhase(pPreamble)) { use_correction = 1; j--; } else { use_correction = 0; } } //Send any remaining partial raw buffers now if (Modes.rawOutUsed || Modes.beastOutUsed) { Modes.net_output_raw_rate_count++; if (Modes.net_output_raw_rate_count > Modes.net_output_raw_rate) { if (Modes.rawOutUsed) { modesSendAllClients(Modes.ros, Modes.rawOut, Modes.rawOutUsed); Modes.rawOutUsed = 0; } if (Modes.beastOutUsed) { modesSendAllClients(Modes.bos, Modes.beastOut, Modes.beastOutUsed); Modes.beastOutUsed = 0; } Modes.net_output_raw_rate_count = 0; } } else if ( (Modes.net) && (Modes.net_heartbeat_rate) && ((++Modes.net_heartbeat_count) > Modes.net_heartbeat_rate) ) { // // We haven't received any Mode A/C/S messages for some time. To try and keep any TCP // links alive, send a null frame. This will help stop any routers discarding our TCP // link which will cause an un-recoverable link error if/when a real frame arrives. // // Fudge up a null message memset(&mm, 0, sizeof(mm)); mm.msgbits = MODES_SHORT_MSG_BITS; mm.timestampMsg = Modes.timestampBlk; // Feed output clients modesQueueOutput(&mm); // Reset the heartbeat counter Modes.net_heartbeat_count = 0; } } // 2.4MHz sampling rate version // // When sampling at 2.4MHz we have exactly 6 samples per 5 symbols. // Each symbol is 500ns wide, each sample is 416.7ns wide // // We maintain a phase offset that is expressed in units of 1/5 of a sample i.e. 1/6 of a symbol, 83.333ns // Each symbol we process advances the phase offset by 6 i.e. 6/5 of a sample, 500ns // // The correlation functions below correlate a 1-0 pair of symbols (i.e. manchester encoded 1 bit) // starting at the given sample, and assuming that the symbol starts at a fixed 0-5 phase offset within // m[0]. They return a correlation value, generally interpreted as >0 = 1 bit, <0 = 0 bit // TODO check if there are better (or more balanced) correlation functions to use here // nb: the correlation functions sum to zero, so we do not need to adjust for the DC offset in the input signal // (adding any constant value to all of m[0..3] does not change the result) static inline int correlate_phase0(uint16_t *m) { return (5 * m[0] - 3 * m[1] - 2 * m[2]) * 10 / 19; } static inline int correlate_phase1(uint16_t *m) { return (4 * m[0] - 1 * m[1] - 3 * m[2]) * 10 / 13; } static inline int correlate_phase2(uint16_t *m) { return (3 * m[0] + 1 * m[1] - 4 * m[2]) * 10 / 13; } static inline int correlate_phase3(uint16_t *m) { return (2 * m[0] + 3 * m[1] - 5 * m[2]) * 10 / 14; } static inline int correlate_phase4(uint16_t *m) { return (1 * m[0] + 5 * m[1] - 5 * m[2] - 1 * m[3]) * 10 / 28; } // // These functions work out the correlation quality for the 10 symbols (5 bits) starting at m[0] + given phase offset. // This is used to find the right phase offset to use for decoding. // static inline int correlate_check_0(uint16_t *m) { return abs(correlate_phase0(&m[0])) + abs(correlate_phase2(&m[2])) + abs(correlate_phase4(&m[4])) + abs(correlate_phase1(&m[7])) + abs(correlate_phase3(&m[9])); } static inline int correlate_check_1(uint16_t *m) { return abs(correlate_phase1(&m[0])) + abs(correlate_phase3(&m[2])) + abs(correlate_phase0(&m[5])) + abs(correlate_phase2(&m[7])) + abs(correlate_phase4(&m[9])); } static inline int correlate_check_2(uint16_t *m) { return abs(correlate_phase2(&m[0])) + abs(correlate_phase4(&m[2])) + abs(correlate_phase1(&m[5])) + abs(correlate_phase3(&m[7])) + abs(correlate_phase0(&m[10])); } static inline int correlate_check_3(uint16_t *m) { return abs(correlate_phase3(&m[0])) + abs(correlate_phase0(&m[3])) + abs(correlate_phase2(&m[5])) + abs(correlate_phase4(&m[7])) + abs(correlate_phase1(&m[10])); } static inline int correlate_check_4(uint16_t *m) { return abs(correlate_phase4(&m[0])) + abs(correlate_phase1(&m[3])) + abs(correlate_phase3(&m[5])) + abs(correlate_phase0(&m[8])) + abs(correlate_phase2(&m[10])); } // Work out the best phase offset to use for the given message. static int best_phase(uint16_t *m) { int test; int best = -1; int bestval = 50; // minimum correlation quality we will accept // empirical testing suggests that 3..8 is the best range to test for here // (testing a wider range runs the danger of picking the wrong phase for // a message that would otherwise be successfully decoded - the correlation // functions can match well with a one symbol / half bit offset) test = correlate_check_3(&m[0]); if (test > bestval) { bestval = test; best = 3; } test = correlate_check_4(&m[0]); if (test > bestval) { bestval = test; best = 4; } test = correlate_check_0(&m[1]); if (test > bestval) { bestval = test; best = 5; } test = correlate_check_1(&m[1]); if (test > bestval) { bestval = test; best = 6; } test = correlate_check_2(&m[1]); if (test > bestval) { bestval = test; best = 7; } test = correlate_check_3(&m[1]); if (test > bestval) { bestval = test; best = 8; } return best; } // //========================================================================= // // Detect a Mode S messages inside the magnitude buffer pointed by 'm' and of // size 'mlen' bytes. Every detected Mode S message is convert it into a // stream of bits and passed to the function to display it. // void detectModeS_oversample(uint16_t *m, uint32_t mlen) { struct modesMessage mm; unsigned char msg[MODES_LONG_MSG_BYTES], *pMsg; uint32_t j; memset(&mm, 0, sizeof(mm)); for (j = 0; j < mlen; j++) { uint16_t *preamble = &m[j]; int high, i, initial_phase, phase, errors, errors56, errorsTy; int msglen, scanlen; uint16_t *pPtr; uint8_t theByte, theErrs; // Look for a message starting at around sample 0 with phase offset 3..7 // Ideal sample values for preambles with different phase // Xn is the first data symbol with phase offset N // // sample#: 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 // phase 3: 2/4\0/5\1 0 0 0 0/5\1/3 3\0 0 0 0 0 0 X4 // phase 4: 1/5\0/4\2 0 0 0 0/4\2 2/4\0 0 0 0 0 0 0 X0 // phase 5: 0/5\1/3 3\0 0 0 0/3 3\1/5\0 0 0 0 0 0 0 X1 // phase 6: 0/4\2 2/4\0 0 0 0 2/4\0/5\1 0 0 0 0 0 0 X2 // phase 7: 0/3 3\1/5\0 0 0 0 1/5\0/4\2 0 0 0 0 0 0 X3 // // quick check: we must have a rising edge 0->1 and a falling edge 12->13 if (! (preamble[0] < preamble[1] && preamble[12] > preamble[13]) ) continue; if (preamble[1] > preamble[2] && // 1 preamble[2] < preamble[3] && preamble[3] > preamble[4] && // 3 preamble[8] < preamble[9] && preamble[9] > preamble[10] && // 9 preamble[10] < preamble[11]) { // 11-12 // peaks at 1,3,9,11-12: phase 3 high = (preamble[1] + preamble[3] + preamble[9] + preamble[11] + preamble[12]) / 4; } else if (preamble[1] > preamble[2] && // 1 preamble[2] < preamble[3] && preamble[3] > preamble[4] && // 3 preamble[8] < preamble[9] && preamble[9] > preamble[10] && // 9 preamble[11] < preamble[12]) { // 12 // peaks at 1,3,9,12: phase 4 high = (preamble[1] + preamble[3] + preamble[9] + preamble[12]) / 4; } else if (preamble[1] > preamble[2] && // 1 preamble[2] < preamble[3] && preamble[4] > preamble[5] && // 3-4 preamble[8] < preamble[9] && preamble[10] > preamble[11] && // 9-10 preamble[11] < preamble[12]) { // 12 // peaks at 1,3-4,9-10,12: phase 5 high = (preamble[1] + preamble[3] + preamble[4] + preamble[9] + preamble[10] + preamble[12]) / 4; } else if (preamble[1] > preamble[2] && // 1 preamble[3] < preamble[4] && preamble[4] > preamble[5] && // 4 preamble[9] < preamble[10] && preamble[10] > preamble[11] && // 10 preamble[11] < preamble[12]) { // 12 // peaks at 1,4,10,12: phase 6 high = (preamble[1] + preamble[4] + preamble[10] + preamble[12]) / 4; } else if (preamble[2] > preamble[3] && // 1-2 preamble[3] < preamble[4] && preamble[4] > preamble[5] && // 4 preamble[9] < preamble[10] && preamble[10] > preamble[11] && // 10 preamble[11] < preamble[12]) { // 12 // peaks at 1-2,4,10,12: phase 7 high = (preamble[1] + preamble[2] + preamble[4] + preamble[10] + preamble[12]) / 4; } else { // no suitable peaks continue; } // Check that the "quiet" bits 6,7,15,16,17 are actually quiet if (preamble[6] >= high || preamble[7] >= high || preamble[14] >= high || preamble[15] >= high || preamble[16] >= high || preamble[17] >= high) { ++Modes.stat_preamble_not_quiet; continue; } // Crosscorrelate against the first few bits to find a likely phase offset initial_phase = best_phase(&preamble[19]); if (initial_phase < 0) { ++Modes.stat_preamble_no_correlation; continue; // nothing satisfactory } Modes.stat_valid_preamble++; Modes.stat_preamble_phase[initial_phase%MODES_MAX_PHASE_STATS]++; // Rather than clear the whole mm structure, just clear the parts which are required. The clear // is required for every possible preamble, and we don't want to be memset-ing the whole // modesMessage structure if we don't have to.. mm.bFlags = mm.crcok = mm.correctedbits = 0; // Decode all the next 112 bits, regardless of the actual message // size. We'll check the actual message type later pMsg = &msg[0]; pPtr = &m[j+19] + (initial_phase/5); phase = initial_phase % 5; theByte = 0; theErrs = 0; errorsTy = 0; errors = 0; errors56 = 0; msglen = scanlen = MODES_LONG_MSG_BITS; for (i = 0; i < scanlen; i++) { int test; switch (phase) { case 0: test = correlate_phase0(pPtr); phase = 2; pPtr += 2; break; case 1: test = correlate_phase1(pPtr); phase = 3; pPtr += 2; break; case 2: test = correlate_phase2(pPtr); phase = 4; pPtr += 2; break; case 3: test = correlate_phase3(pPtr); phase = 0; pPtr += 3; break; case 4: test = correlate_phase4(pPtr); phase = 1; pPtr += 3; break; default: test = 0; break; } if (test > 0) theByte |= 1; /* else if (test < 0) theByte |= 0; */ else if (test == 0) { if (i >= MODES_SHORT_MSG_BITS) { // poor correlation, and we're in the long part of a frame errors++; } else if (i >= 5) { // poor correlation, and we're in the short part of a frame scanlen = MODES_LONG_MSG_BITS; errors56 = ++errors; } else if (i) { // poor correlation, and we're in the message type part of a frame errorsTy = errors56 = ++errors; theErrs |= 1; } else { // poor correlation, and we're in the first bit of the message type part of a frame errorsTy = errors56 = ++errors; theErrs |= 1; } } if ((i & 7) == 7) *pMsg++ = theByte; theByte = theByte << 1; if (i < 7) {theErrs = theErrs << 1;} // If we've exceeded the permissible number of encoding errors, abandon ship now if (errors > MODES_MSG_ENCODER_ERRS) { if (i < MODES_SHORT_MSG_BITS) { msglen = 0; } else if ((errorsTy == 1) && (theErrs == 0x80)) { // If we only saw one error in the first bit of the byte of the frame, then it's possible // we guessed wrongly about the value of the bit. We may be able to correct it by guessing // the other way. // // We guessed a '1' at bit 7, which is the DF length bit == 112 Bits. // Inverting bit 7 will change the message type from a long to a short. // Invert the bit, cross your fingers and carry on. msglen = MODES_SHORT_MSG_BITS; msg[0] ^= theErrs; errorsTy = 0; errors = errors56; // revert to the number of errors prior to bit 56 Modes.stat_DF_Len_Corrected++; } else if (i < MODES_LONG_MSG_BITS) { msglen = MODES_SHORT_MSG_BITS; errors = errors56; } else { msglen = MODES_LONG_MSG_BITS; } break; } } // Ensure msglen is consistent with the DF type i = modesMessageLenByType(msg[0] >> 3); if (msglen > i) {msglen = i;} else if (msglen < i) {msglen = 0;} // // If we guessed at any of the bits in the DF type field, then look to see if our guess was sensible. // Do this by looking to see if the original guess results in the DF type being one of the ICAO defined // message types. If it isn't then toggle the guessed bit and see if this new value is ICAO defined. // if the new value is ICAO defined, then update it in our message. if ((msglen) && (errorsTy == 1) && (theErrs & 0x78)) { // We guessed at one (and only one) of the message type bits. See if our guess is "likely" // to be correct by comparing the DF against a list of known good DF's int thisDF = ((theByte = msg[0]) >> 3) & 0x1f; uint32_t validDFbits = 0x017F0831; // One bit per 32 possible DF's. Set bits 0,4,5,11,16.17.18.19,20,21,22,24 uint32_t thisDFbit = (1 << thisDF); if (0 == (validDFbits & thisDFbit)) { // The current DF is not ICAO defined, so is probably an errors. // Toggle the bit we guessed at and see if the resultant DF is more likely theByte ^= theErrs; thisDF = (theByte >> 3) & 0x1f; thisDFbit = (1 << thisDF); // if this DF any more likely? if (validDFbits & thisDFbit) { // Yep, more likely, so update the main message msg[0] = theByte; Modes.stat_DF_Type_Corrected++; errors--; // decrease the error count so we attempt to use the modified DF. } } } // When we reach this point, if error is small, and the signal strength is large enough // we may have a Mode S message on our hands. It may still be broken and the CRC may not // be correct, but this can be handled by the next layer. if ( (msglen > 0) && (errors <= MODES_MSG_ENCODER_ERRS) ) { // Set initial mm structure details mm.timestampMsg = Modes.timestampBlk + (j*5); mm.signalLevel = 0; mm.phase_corrected = 0; //dumpRawMessage("decoded with oversampling", msg, m, j); // Decode the received message decodeModesMessage(&mm, msg); // Update statistics if (Modes.stats) { switch (errors) { case 0: {Modes.stat_demodulated0++; break;} case 1: {Modes.stat_demodulated1++; break;} case 2: {Modes.stat_demodulated2++; break;} default:{Modes.stat_demodulated3++; break;} } if (mm.correctedbits == 0) { if (mm.crcok) { Modes.stat_goodcrc++; Modes.stat_goodcrc_phase[initial_phase%MODES_MAX_PHASE_STATS]++; } else {Modes.stat_badcrc++;} } else { Modes.stat_badcrc++; Modes.stat_fixed++; if ( (mm.correctedbits) && (mm.correctedbits <= MODES_MAX_BITERRORS) ) { Modes.stat_bit_fix[mm.correctedbits-1] += 1; } } } // Skip this message if we are sure it's fine if (mm.crcok) { j += (16+msglen)*6/5 - 1; } // Pass data to the next layer useModesMessage(&mm); } } //Send any remaining partial raw buffers now if (Modes.rawOutUsed || Modes.beastOutUsed) { Modes.net_output_raw_rate_count++; if (Modes.net_output_raw_rate_count > Modes.net_output_raw_rate) { if (Modes.rawOutUsed) { modesSendAllClients(Modes.ros, Modes.rawOut, Modes.rawOutUsed); Modes.rawOutUsed = 0; } if (Modes.beastOutUsed) { modesSendAllClients(Modes.bos, Modes.beastOut, Modes.beastOutUsed); Modes.beastOutUsed = 0; } Modes.net_output_raw_rate_count = 0; } } else if ( (Modes.net) && (Modes.net_heartbeat_rate) && ((++Modes.net_heartbeat_count) > Modes.net_heartbeat_rate) ) { // // We haven't received any Mode A/C/S messages for some time. To try and keep any TCP // links alive, send a null frame. This will help stop any routers discarding our TCP // link which will cause an un-recoverable link error if/when a real frame arrives. // // Fudge up a null message memset(&mm, 0, sizeof(mm)); mm.msgbits = MODES_SHORT_MSG_BITS; mm.timestampMsg = Modes.timestampBlk; // Feed output clients modesQueueOutput(&mm); // Reset the heartbeat counter Modes.net_heartbeat_count = 0; } } // //========================================================================= // // When a new message is available, because it was decoded from the RTL device, // file, or received in the TCP input port, or any other way we can receive a // decoded message, we call this function in order to use the message. // // Basically this function passes a raw message to the upper layers for further // processing and visualization // void useModesMessage(struct modesMessage *mm) { if ((Modes.check_crc == 0) || (mm->crcok) || (mm->correctedbits)) { // not checking, ok or fixed // If we are decoding, track aircraft if (!Modes.no_decode) { interactiveReceiveData(mm); } // In non-interactive non-quiet mode, display messages on standard output if (!Modes.interactive && !Modes.quiet) { displayModesMessage(mm); } // Feed output clients if (Modes.net) {modesQueueOutput(mm);} // Heartbeat not required whilst we're seeing real messages Modes.net_heartbeat_count = 0; } } // //========================================================================= // // Always positive MOD operation, used for CPR decoding. // int cprModFunction(int a, int b) { int res = a % b; if (res < 0) res += b; return res; } // //========================================================================= // // The NL function uses the precomputed table from 1090-WP-9-14 // int cprNLFunction(double lat) { if (lat < 0) lat = -lat; // Table is simmetric about the equator if (lat < 10.47047130) return 59; if (lat < 14.82817437) return 58; if (lat < 18.18626357) return 57; if (lat < 21.02939493) return 56; if (lat < 23.54504487) return 55; if (lat < 25.82924707) return 54; if (lat < 27.93898710) return 53; if (lat < 29.91135686) return 52; if (lat < 31.77209708) return 51; if (lat < 33.53993436) return 50; if (lat < 35.22899598) return 49; if (lat < 36.85025108) return 48; if (lat < 38.41241892) return 47; if (lat < 39.92256684) return 46; if (lat < 41.38651832) return 45; if (lat < 42.80914012) return 44; if (lat < 44.19454951) return 43; if (lat < 45.54626723) return 42; if (lat < 46.86733252) return 41; if (lat < 48.16039128) return 40; if (lat < 49.42776439) return 39; if (lat < 50.67150166) return 38; if (lat < 51.89342469) return 37; if (lat < 53.09516153) return 36; if (lat < 54.27817472) return 35; if (lat < 55.44378444) return 34; if (lat < 56.59318756) return 33; if (lat < 57.72747354) return 32; if (lat < 58.84763776) return 31; if (lat < 59.95459277) return 30; if (lat < 61.04917774) return 29; if (lat < 62.13216659) return 28; if (lat < 63.20427479) return 27; if (lat < 64.26616523) return 26; if (lat < 65.31845310) return 25; if (lat < 66.36171008) return 24; if (lat < 67.39646774) return 23; if (lat < 68.42322022) return 22; if (lat < 69.44242631) return 21; if (lat < 70.45451075) return 20; if (lat < 71.45986473) return 19; if (lat < 72.45884545) return 18; if (lat < 73.45177442) return 17; if (lat < 74.43893416) return 16; if (lat < 75.42056257) return 15; if (lat < 76.39684391) return 14; if (lat < 77.36789461) return 13; if (lat < 78.33374083) return 12; if (lat < 79.29428225) return 11; if (lat < 80.24923213) return 10; if (lat < 81.19801349) return 9; if (lat < 82.13956981) return 8; if (lat < 83.07199445) return 7; if (lat < 83.99173563) return 6; if (lat < 84.89166191) return 5; if (lat < 85.75541621) return 4; if (lat < 86.53536998) return 3; if (lat < 87.00000000) return 2; else return 1; } // //========================================================================= // int cprNFunction(double lat, int fflag) { int nl = cprNLFunction(lat) - (fflag ? 1 : 0); if (nl < 1) nl = 1; return nl; } // //========================================================================= // double cprDlonFunction(double lat, int fflag, int surface) { return (surface ? 90.0 : 360.0) / cprNFunction(lat, fflag); } // //========================================================================= // // This algorithm comes from: // http://www.lll.lu/~edward/edward/adsb/DecodingADSBposition.html. // // A few remarks: // 1) 131072 is 2^17 since CPR latitude and longitude are encoded in 17 bits. // int decodeCPR(struct aircraft *a, int fflag, int surface) { double AirDlat0 = (surface ? 90.0 : 360.0) / 60.0; double AirDlat1 = (surface ? 90.0 : 360.0) / 59.0; double lat0 = a->even_cprlat; double lat1 = a->odd_cprlat; double lon0 = a->even_cprlon; double lon1 = a->odd_cprlon; // Compute the Latitude Index "j" int j = (int) floor(((59*lat0 - 60*lat1) / 131072) + 0.5); double rlat0 = AirDlat0 * (cprModFunction(j,60) + lat0 / 131072); double rlat1 = AirDlat1 * (cprModFunction(j,59) + lat1 / 131072); time_t now = time(NULL); double surface_rlat = MODES_USER_LATITUDE_DFLT; double surface_rlon = MODES_USER_LONGITUDE_DFLT; if (surface) { // If we're on the ground, make sure we have a (likely) valid Lat/Lon if ((a->bFlags & MODES_ACFLAGS_LATLON_VALID) && (((int)(now - a->seenLatLon)) < Modes.interactive_display_ttl)) { surface_rlat = a->lat; surface_rlon = a->lon; } else if (Modes.bUserFlags & MODES_USER_LATLON_VALID) { surface_rlat = Modes.fUserLat; surface_rlon = Modes.fUserLon; } else { // No local reference, give up return (-1); } rlat0 += floor(surface_rlat / 90.0) * 90.0; // Move from 1st quadrant to our quadrant rlat1 += floor(surface_rlat / 90.0) * 90.0; } else { if (rlat0 >= 270) rlat0 -= 360; if (rlat1 >= 270) rlat1 -= 360; } // Check to see that the latitude is in range: -90 .. +90 if (rlat0 < -90 || rlat0 > 90 || rlat1 < -90 || rlat1 > 90) return (-1); // Check that both are in the same latitude zone, or abort. if (cprNLFunction(rlat0) != cprNLFunction(rlat1)) return (-1); // Compute ni and the Longitude Index "m" if (fflag) { // Use odd packet. int ni = cprNFunction(rlat1,1); int m = (int) floor((((lon0 * (cprNLFunction(rlat1)-1)) - (lon1 * cprNLFunction(rlat1))) / 131072.0) + 0.5); a->lon = cprDlonFunction(rlat1, 1, surface) * (cprModFunction(m, ni)+lon1/131072); a->lat = rlat1; } else { // Use even packet. int ni = cprNFunction(rlat0,0); int m = (int) floor((((lon0 * (cprNLFunction(rlat0)-1)) - (lon1 * cprNLFunction(rlat0))) / 131072) + 0.5); a->lon = cprDlonFunction(rlat0, 0, surface) * (cprModFunction(m, ni)+lon0/131072); a->lat = rlat0; } if (surface) { a->lon += floor(surface_rlon / 90.0) * 90.0; // Move from 1st quadrant to our quadrant } else if (a->lon > 180) { a->lon -= 360; } a->seenLatLon = a->seen; a->timestampLatLon = a->timestamp; a->bFlags |= (MODES_ACFLAGS_LATLON_VALID | MODES_ACFLAGS_LATLON_REL_OK); return 0; } // //========================================================================= // // This algorithm comes from: // 1090-WP29-07-Draft_CPR101 (which also defines decodeCPR() ) // // There is an error in this document related to CPR relative decode. // Should use trunc() rather than the floor() function in Eq 38 and related for deltaZI. // floor() returns integer less than argument // trunc() returns integer closer to zero than argument. // Note: text of document describes trunc() functionality for deltaZI calculation // but the formulae use floor(). // int decodeCPRrelative(struct aircraft *a, int fflag, int surface) { double AirDlat; double AirDlon; double lat; double lon; double lonr, latr; double rlon, rlat; int j,m; if (a->bFlags & MODES_ACFLAGS_LATLON_REL_OK) { // Ok to try aircraft relative first latr = a->lat; lonr = a->lon; } else if (Modes.bUserFlags & MODES_USER_LATLON_VALID) { // Try ground station relative next latr = Modes.fUserLat; lonr = Modes.fUserLon; } else { return (-1); // Exit with error - can't do relative if we don't have ref. } if (fflag) { // odd AirDlat = (surface ? 90.0 : 360.0) / 59.0; lat = a->odd_cprlat; lon = a->odd_cprlon; } else { // even AirDlat = (surface ? 90.0 : 360.0) / 60.0; lat = a->even_cprlat; lon = a->even_cprlon; } // Compute the Latitude Index "j" j = (int) (floor(latr/AirDlat) + trunc(0.5 + cprModFunction((int)latr, (int)AirDlat)/AirDlat - lat/131072)); rlat = AirDlat * (j + lat/131072); if (rlat >= 270) rlat -= 360; // Check to see that the latitude is in range: -90 .. +90 if (rlat < -90 || rlat > 90) { a->bFlags &= ~MODES_ACFLAGS_LATLON_REL_OK; // This will cause a quick exit next time if no global has been done return (-1); // Time to give up - Latitude error } // Check to see that answer is reasonable - ie no more than 1/2 cell away if (fabs(rlat - a->lat) > (AirDlat/2)) { a->bFlags &= ~MODES_ACFLAGS_LATLON_REL_OK; // This will cause a quick exit next time if no global has been done return (-1); // Time to give up - Latitude error } // Compute the Longitude Index "m" AirDlon = cprDlonFunction(rlat, fflag, surface); m = (int) (floor(lonr/AirDlon) + trunc(0.5 + cprModFunction((int)lonr, (int)AirDlon)/AirDlon - lon/131072)); rlon = AirDlon * (m + lon/131072); if (rlon > 180) rlon -= 360; // Check to see that answer is reasonable - ie no more than 1/2 cell away if (fabs(rlon - a->lon) > (AirDlon/2)) { a->bFlags &= ~MODES_ACFLAGS_LATLON_REL_OK; // This will cause a quick exit next time if no global has been done return (-1); // Time to give up - Longitude error } a->lat = rlat; a->lon = rlon; a->seenLatLon = a->seen; a->timestampLatLon = a->timestamp; a->bFlags |= (MODES_ACFLAGS_LATLON_VALID | MODES_ACFLAGS_LATLON_REL_OK); return (0); } // // ===================== Mode S detection and decoding =================== //