// Part of dump1090, a Mode S message decoder for RTLSDR devices. // // mode_s.c: Mode S message decoding. // // Copyright (c) 2014,2015 Oliver Jowett // // This file is free software: you may copy, redistribute and/or modify it // under the terms of the GNU General Public License as published by the // Free Software Foundation, either version 2 of the License, or (at your // option) any later version. // // This file is distributed in the hope that it will be useful, but // WITHOUT ANY WARRANTY; without even the implied warranty of // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU // General Public License for more details. // // You should have received a copy of the GNU General Public License // along with this program. If not, see . // This file incorporates work covered by the following copyright and // permission notice: // // 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 =================== // // // //========================================================================= // // 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 ; } // //========================================================================= // // 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 // static 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. // static 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). // static 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 // static 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 static const char *ca_str[8] = { /* 0 */ "Level 1", /* 1 */ "reserved", /* 2 */ "reserved", /* 3 */ "reserved", /* 4 */ "Level 2+, ground", /* 5 */ "Level 2+, airborne", /* 6 */ "Level 2+", /* 7 */ "DR/Alert/SPI active" }; // DF 18 Control field table. static const 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 static const 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 */ "Reserved", /* 7 */ "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 */ "Reserved", /* 7 */ "Reserved" }; // //========================================================================= // static 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; } // Correct a decoded native-endian Address Announced field // (from bits 8-31) if it is affected by the given error // syndrome. Updates *addr and returns >0 if changed, 0 if // it was unaffected. static int correct_aa_field(uint32_t *addr, struct errorinfo *ei) { int i; int addr_errors = 0; if (!ei) return 0; for (i = 0; i < ei->errors; ++i) { if (ei->bit[i] >= 8 && ei->bit[i] <= 31) { *addr ^= 1 << (31 - ei->bit[i]); ++addr_errors; } } return addr_errors; } // Score how plausible this ModeS message looks. // The more positive, the more reliable the message is // 1000: DF 0/4/5/16/24 with a CRC-derived address matching a known aircraft // 1800: DF17/18 with good CRC and an address matching a known aircraft // 1400: DF17/18 with good CRC and an address not matching a known aircraft // 900: DF17/18 with 1-bit error and an address matching a known aircraft // 700: DF17/18 with 1-bit error and an address not matching a known aircraft // 450: DF17/18 with 2-bit error and an address matching a known aircraft // 350: DF17/18 with 2-bit error and an address not matching a known aircraft // 1600: DF11 with IID==0, good CRC and an address matching a known aircraft // 800: DF11 with IID==0, 1-bit error and an address matching a known aircraft // 750: DF11 with IID==0, good CRC and an address not matching a known aircraft // 375: DF11 with IID==0, 1-bit error and an address not matching a known aircraft // 1000: DF11 with IID!=0, good CRC and an address matching a known aircraft // 500: DF11 with IID!=0, 1-bit error and an address matching a known aircraft // 1000: DF20/21 with a CRC-derived address matching a known aircraft // 500: DF20/21 with a CRC-derived address matching a known aircraft (bottom 16 bits only - overlay control in use) // -1: message might be valid, but we couldn't validate the CRC against a known ICAO // -2: bad message or unrepairable CRC error int scoreModesMessage(unsigned char *msg, int validbits) { int msgtype, msgbits, crc, iid; uint32_t addr; struct errorinfo *ei; if (validbits < 56) return -2; msgtype = msg[0] >> 3; // Downlink Format msgbits = modesMessageLenByType(msgtype); if (validbits < msgbits) return -2; crc = modesChecksum(msg, msgbits); switch (msgtype) { case 0: // short air-air surveillance case 4: // surveillance, altitude reply case 5: // surveillance, altitude reply case 16: // long air-air surveillance case 24: // Comm-D (ELM) return icaoFilterTest(crc) ? 1000 : -1; case 11: // All-call reply iid = crc & 0x7f; crc = crc & 0xffff80; addr = (msg[1] << 16) | (msg[2] << 8) | (msg[3]); ei = modesChecksumDiagnose(crc, msgbits); if (!ei) return -2; // can't correct errors // see crc.c comments: we do not attempt to fix // more than single-bit errors, as two-bit // errors are ambiguous in DF11. if (ei->errors > 1) return -2; // can't correct errors // fix any errors in the address field correct_aa_field(&addr, ei); // validate address if (iid == 0) { if (icaoFilterTest(addr)) return 1600 / (ei->errors + 1); else return 750 / (ei->errors + 1); } else { if (icaoFilterTest(addr)) return 1000 / (ei->errors + 1); else return -1; } case 17: // Extended squitter case 18: // Extended squitter/non-transponder ei = modesChecksumDiagnose(crc, msgbits); if (!ei) return -2; // can't correct errors // fix any errors in the address field addr = (msg[1] << 16) | (msg[2] << 8) | (msg[3]); correct_aa_field(&addr, ei); if (icaoFilterTest(addr)) return 1800 / (ei->errors+1); else return 1400 / (ei->errors+1); case 20: // Comm-B, altitude reply case 21: // Comm-B, identity reply if (icaoFilterTest(crc)) return 1000; // Address/Parity #if 0 // This doesn't seem useful, as we mistake a lot of CRC errors // for overlay control if (icaoFilterTestFuzzy(crc)) return 500; // Data/Parity #endif return -2; default: // unknown message type return -2; } } // //========================================================================= // // Decode a raw Mode S message demodulated as a stream of bytes by detectModeS(), // and split it into fields populating a modesMessage structure. // static void decodeExtendedSquitter(struct modesMessage *mm); static void decodeCommB(struct modesMessage *mm); static char *ais_charset = "@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\\]^_ !\"#$%&'()*+,-./0123456789:;<=>?"; // return 0 if all OK // -1: message might be valid, but we couldn't validate the CRC against a known ICAO // -2: bad message or unrepairable CRC error int decodeModesMessage(struct modesMessage *mm, unsigned char *msg) { // Work on our local copy. memcpy(mm->msg, msg, MODES_LONG_MSG_BYTES); if (Modes.net_verbatim) { // Preserve the original uncorrected copy for later forwarding memcpy(mm->verbatim, 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); mm->correctedbits = 0; mm->addr = 0; // Do checksum work and set fields that depend on the CRC switch (mm->msgtype) { case 0: // short air-air surveillance case 4: // surveillance, altitude reply case 5: // surveillance, altitude reply case 16: // long air-air surveillance case 24: // Comm-D (ELM) // These message types use Address/Parity, i.e. our CRC syndrome is the sender's ICAO address. // We can't tell if the CRC is correct or not as we don't know the correct address. // Accept the message if it appears to be from a previously-seen aircraft if (!icaoFilterTest(mm->crc)) { return -1; } mm->addr = mm->crc; break; case 11: // All-call reply // This message type uses Parity/Interrogator, i.e. our CRC syndrome is CL + IC from the uplink message // which we can't see. So we don't know if the CRC is correct or not. // // however! CL + IC only occupy the lower 7 bits of the CRC. So if we ignore those bits when testing // the CRC we can still try to detect/correct errors. mm->iid = mm->crc & 0x7f; if (mm->crc & 0xffff80) { int addr; struct errorinfo *ei = modesChecksumDiagnose(mm->crc & 0xffff80, mm->msgbits); if (!ei) { return -2; // couldn't fix it } mm->correctedbits = ei->errors; modesChecksumFix(msg, ei); // check whether the corrected message looks sensible // we are conservative here: only accept corrected messages that // match an existing aircraft. addr = (msg[1] << 16) | (msg[2] << 8) | (msg[3]); if (!icaoFilterTest(addr)) { return -1; } } break; case 17: // Extended squitter case 18: { // Extended squitter/non-transponder struct errorinfo *ei; int addr1, addr2; // These message types use Parity/Interrogator, but are specified to set II=0 if (mm->crc == 0) break; // all good ei = modesChecksumDiagnose(mm->crc, mm->msgbits); if (!ei) { return -2; // couldn't fix it } addr1 = (msg[1] << 16) | (msg[2] << 8) | (msg[3]); mm->correctedbits = ei->errors; modesChecksumFix(msg, ei); addr2 = (msg[1] << 16) | (msg[2] << 8) | (msg[3]); // we are conservative here: only accept corrected messages that // match an existing aircraft. if (addr1 != addr2 && !icaoFilterTest(addr2)) { return -1; } break; } case 20: // Comm-B, altitude reply case 21: // Comm-B, identity reply // These message types either use Address/Parity (see DF0 etc) // or Data Parity where the requested BDS is also xored into the top byte. // So not only do we not know whether the CRC is right, we also don't know if // the ICAO is right! Ow. // Try an exact match if (icaoFilterTest(mm->crc)) { // OK. mm->addr = mm->crc; mm->bds = 0; // unknown break; } #if 0 // This doesn't seem useful, as we mistake a lot of CRC errors // for overlay control // Try a fuzzy match if ( (mm->addr = icaoFilterTestFuzzy(mm->crc)) != 0) { // We have an address that would match, assume it's correct mm->bds = (mm->crc ^ mm->addr) >> 16; // derive the BDS value based on what we think the address is break; } #endif return -1; // no good default: // All other message types, we don't know how to handle their CRCs, give up return -2; } // decode the bulk of the message mm->bFlags = 0; // AA (Address announced) if (mm->msgtype == 11 || mm->msgtype == 17 || mm->msgtype == 18) { mm->addr = (msg[1] << 16) | (msg[2] << 8) | (msg[3]); } // AC (Altitude Code) 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); } } // AF (DF19 Application Field) not decoded // CA (Capability) if (mm->msgtype == 11 || mm->msgtype == 17) { mm->ca = (msg[0] & 0x07); if (mm->ca == 4) { mm->bFlags |= MODES_ACFLAGS_AOG_VALID | MODES_ACFLAGS_AOG; } else if (mm->ca == 5) { mm->bFlags |= MODES_ACFLAGS_AOG_VALID; } } // CC (Cross-link capability) not decoded // CF (Control field) if (mm->msgtype == 18) { mm->cf = msg[0] & 7; } // DR (Downlink Request) not decoded // FS (Flight Status) if (mm->msgtype == 4 || mm->msgtype == 5 || mm->msgtype == 20 || mm->msgtype == 21) { mm->bFlags |= MODES_ACFLAGS_FS_VALID; mm->fs = msg[0] & 7; if (mm->fs <= 3) { mm->bFlags |= MODES_ACFLAGS_AOG_VALID; if (mm->fs & 1) mm->bFlags |= MODES_ACFLAGS_AOG; } } // ID (Identity) if (mm->msgtype == 5 || mm->msgtype == 21) { // Gillham encoded Squawk int ID13Field = ((msg[2] << 8) | msg[3]) & 0x1FFF; if (ID13Field) { mm->bFlags |= MODES_ACFLAGS_SQUAWK_VALID; mm->modeA = decodeID13Field(ID13Field); } } // KE (Control, ELM) not decoded // MB (messsage, Comm-B) if (mm->msgtype == 20 || mm->msgtype == 21) { decodeCommB(mm); } // MD (message, Comm-D) not decoded // ME (message, extended squitter) if (mm->msgtype == 17 || // Extended squitter mm->msgtype == 18) { // Extended squitter/non-transponder: decodeExtendedSquitter(mm); } // MV (message, ACAS) not decoded // ND (number of D-segment) not decoded // RI (Reply information) not decoded // SL (Sensitivity level, ACAS) not decoded // UM (Utility Message) not decoded // VS (Vertical Status) if (mm->msgtype == 0 || mm->msgtype == 16) { mm->bFlags |= MODES_ACFLAGS_AOG_VALID; if (msg[0] & 0x04) mm->bFlags |= MODES_ACFLAGS_AOG; } if (!mm->correctedbits && (mm->msgtype == 17 || mm->msgtype == 18 || (mm->msgtype != 11 || mm->iid == 0))) { // No CRC errors seen, and either it was an DF17/18 extended squitter // or a DF11 acquisition squitter with II = 0. We probably have the right address. // We wait until here to do this as we may have needed to decode an ES to note // the type of address in DF18 messages. // NB this is the only place that adds addresses! icaoFilterAdd(mm->addr); } // all done return 0; } // Decode BDS2,0 carried in Comm-B or ES static void decodeBDS20(struct modesMessage *mm) { uint32_t chars1, chars2; unsigned char *msg = mm->msg; chars1 = (msg[5] << 16) | (msg[6] << 8) | (msg[7]); chars2 = (msg[8] << 16) | (msg[9] << 8) | (msg[10]); // A common failure mode seems to be to intermittently send // all zeros. Catch that here. if (chars1 == 0 && chars2 == 0) return; mm->bFlags |= MODES_ACFLAGS_CALLSIGN_VALID; mm->flight[3] = ais_charset[chars1 & 0x3F]; chars1 = chars1 >> 6; mm->flight[2] = ais_charset[chars1 & 0x3F]; chars1 = chars1 >> 6; mm->flight[1] = ais_charset[chars1 & 0x3F]; chars1 = chars1 >> 6; mm->flight[0] = ais_charset[chars1 & 0x3F]; mm->flight[7] = ais_charset[chars2 & 0x3F]; chars2 = chars2 >> 6; mm->flight[6] = ais_charset[chars2 & 0x3F]; chars2 = chars2 >> 6; mm->flight[5] = ais_charset[chars2 & 0x3F]; chars2 = chars2 >> 6; mm->flight[4] = ais_charset[chars2 & 0x3F]; mm->flight[8] = '\0'; } static void decodeExtendedSquitter(struct modesMessage *mm) { unsigned char *msg = mm->msg; 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 int check_imf = 0; // Check CF on DF18 to work out the format of the ES and whether we need to look for an IMF bit if (mm->msgtype == 18) { switch (mm->cf) { case 0: // ADS-B ES/NT devices that report the ICAO 24-bit address in the AA field break; case 1: // Reserved for ADS-B for ES/NT devices that use other addressing techniques in the AA field case 5: // TIS-B messages that relay ADS-B Messages using anonymous 24-bit addresses (format not explicitly defined, but it seems to follow DF17) mm->addr |= MODES_NON_ICAO_ADDRESS; break; case 2: // Fine TIS-B message (formats are close enough to DF17 for our purposes) case 6: // ADS-B rebroadcast using the same type codes and message formats as defined for DF = 17 ADS-B messages check_imf = 1; break; case 3: // Coarse TIS-B airborne position and velocity. // TODO: decode me. // For now we only look at the IMF bit. if (msg[4] & 0x80) mm->addr |= MODES_NON_ICAO_ADDRESS; return; default: // All others, we don't know the format. mm->addr |= MODES_NON_ICAO_ADDRESS; // assume non-ICAO return; } } switch (metype) { case 1: case 2: case 3: case 4: { // Aircraft Identification and Category uint32_t chars1, chars2; chars1 = (msg[5] << 16) | (msg[6] << 8) | (msg[7]); chars2 = (msg[8] << 16) | (msg[9] << 8) | (msg[10]); // A common failure mode seems to be to intermittently send // all zeros. Catch that here. if (chars1 != 0 || chars2 != 0) { mm->bFlags |= MODES_ACFLAGS_CALLSIGN_VALID; mm->flight[3] = ais_charset[chars1 & 0x3F]; chars1 = chars1 >> 6; mm->flight[2] = ais_charset[chars1 & 0x3F]; chars1 = chars1 >> 6; mm->flight[1] = ais_charset[chars1 & 0x3F]; chars1 = chars1 >> 6; mm->flight[0] = ais_charset[chars1 & 0x3F]; mm->flight[7] = ais_charset[chars2 & 0x3F]; chars2 = chars2 >> 6; mm->flight[6] = ais_charset[chars2 & 0x3F]; chars2 = chars2 >> 6; mm->flight[5] = ais_charset[chars2 & 0x3F]; chars2 = chars2 >> 6; mm->flight[4] = ais_charset[chars2 & 0x3F]; mm->flight[8] = '\0'; } break; } case 19: { // Airborne Velocity Message if (check_imf && (msg[5] & 0x80)) mm->addr |= MODES_NON_ICAO_ADDRESS; // 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; } } break; } case 5: case 6: case 7: case 8: { // Ground position int movement; if (check_imf && (msg[6] & 0x08)) mm->addr |= MODES_NON_ICAO_ADDRESS; mm->bFlags |= MODES_ACFLAGS_AOG_VALID | MODES_ACFLAGS_AOG; 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; movement = ((msg[4] << 4) | (msg[5] >> 4)) & 0x007F; 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; } break; } case 0: // Airborne position, baro altitude only case 9: case 10: case 11: case 12: case 13: case 14: case 15: case 16: case 17: case 18: // Airborne position, baro case 20: case 21: case 22: { // Airborne position, GNSS HAE int AC12Field = ((msg[5] << 4) | (msg[6] >> 4)) & 0x0FFF; if (check_imf && (msg[4] & 0x01)) mm->addr |= MODES_NON_ICAO_ADDRESS; mm->bFlags |= MODES_ACFLAGS_AOG_VALID; if (metype != 0) { // Catch some common failure modes and don't mark them as valid // (so they won't be used for positioning) 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]); if (AC12Field == 0 && mm->raw_longitude == 0 && (mm->raw_latitude & 0x0fff) == 0 && mm->metype == 15) { // Seen from at least: // 400F3F (Eurocopter ECC155 B1) - Bristow Helicopters // 4008F3 (BAE ATP) - Atlantic Airlines // 400648 (BAE ATP) - Atlantic Airlines // altitude == 0, longitude == 0, type == 15 and zeros in latitude LSB. // Can alternate with valid reports having type == 14 Modes.stats_current.cpr_filtered++; } else { // Otherwise, assume it's valid. mm->bFlags |= (mm->msg[6] & 0x04) ? MODES_ACFLAGS_LLODD_VALID : MODES_ACFLAGS_LLEVEN_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); } break; } case 23: { // Test message 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); } } break; } case 24: // Reserved for Surface System Status break; case 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); } if (check_imf && (msg[10] & 0x01)) mm->addr |= MODES_NON_ICAO_ADDRESS; } break; } case 29: // Aircraft Trajectory Intent break; case 30: // Aircraft Operational Coordination break; case 31: // Aircraft Operational Status if (check_imf && (msg[10] & 0x01)) mm->addr |= MODES_NON_ICAO_ADDRESS; break; default: break; } } static void decodeCommB(struct modesMessage *mm) { unsigned char *msg = mm->msg; // This is a bit hairy as we don't know what the requested register was if (msg[4] == 0x20) { // BDS 2,0 Aircraft Identification decodeBDS20(mm); } } // //========================================================================= // // These functions gets a decoded Mode S Message and prints it on the screen // in a human readable format. // static void displayExtendedSquitter(struct modesMessage *mm) { 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) { if (mm->bFlags & MODES_ACFLAGS_REL_CPR_USED) printf(" Local CPR decoding used.\n"); else printf(" Global CPR decoding used.\n"); printf(" Latitude : %f (%d)\n", mm->fLat, mm->raw_latitude); printf(" Longitude: %f (%d)\n", mm->fLon, mm->raw_longitude); } else { if (!(mm->bFlags & MODES_ACFLAGS_LLEITHER_VALID)) printf(" Bad position data, not decoded.\n"); 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); } } static void displayCommB(struct modesMessage *mm) { if (mm->bds != 0) printf(" Comm-B BDS : %02x (maybe)\n", mm->bds); // Decode the Comm-B message if (mm->msg[4] == 0x20 && (mm->bds == 0 || mm->bds == 0x20)) { // BDS 2,0 Aircraft identification printf(" BDS 2,0 Aircraft Identification : %s\n", mm->flight); } else { int i; printf(" Comm-B MB : "); for (i = 4; i < 11; ++i) printf("%02x", mm->msg[i]); printf("\n"); } } 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\n", mm->crc); if (mm->correctedbits != 0) printf("No. of bit errors fixed: %d\n", mm->correctedbits); if (mm->signalLevel > 0) printf("RSSI: %.1f dBFS\n", 10 * log10(mm->signalLevel)); if (mm->score) printf("Score: %d\n", mm->score); if (mm->timestampMsg) printf("Time: %.2fus (phase: %d)\n", mm->timestampMsg / 12.0, (unsigned int) (360 * (mm->timestampMsg % 6) / 6)); 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) { displayCommB(mm); } } 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) { displayCommB(mm); } } 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); displayExtendedSquitter(mm); } else if (mm->msgtype == 18) { // DF 18 printf("DF 18: Extended Squitter.\n"); printf(" Control Field : %d (%s)\n", mm->cf, cf_str[mm->cf]); if ((mm->cf == 0) || (mm->cf == 1) || (mm->cf == 5) || (mm->cf == 6)) { if (mm->cf == 1 || mm->cf == 5) { printf(" Other Address : %06x\n", mm->addr); } else { printf(" ICAO Address : %06x\n", mm->addr); } displayExtendedSquitter(mm); } } 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_samples]; uint32_t j; memcpy(Modes.magnitude,&Modes.magnitude[MODES_ASYNC_BUF_SAMPLES], Modes.trailing_samples * 2); // 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++]; } } // //========================================================================= // // 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) { ++Modes.stats_current.messages_total; // If we are decoding, track aircraft 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);} } // // ===================== Mode S detection and decoding =================== //