// 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 A/C detection and decoding =================== // // // This table is used to build the Mode A/C variable called ModeABits.Each // bit period is inspected, and if it's value exceeds the threshold limit, // then the value in this table is or-ed into ModeABits. // // At the end of message processing, ModeABits will be the decoded ModeA value. // // We can also flag noise in bits that should be zeros - the xx bits. Noise in // these bits cause bits (31-16) in ModeABits to be set. Then at the end of message // processing we can test for errors by looking at these bits. // uint32_t ModeABitTable[24] = { 0x00000000, // F1 = 1 0x00000010, // C1 0x00001000, // A1 0x00000020, // C2 0x00002000, // A2 0x00000040, // C4 0x00004000, // A4 0x40000000, // xx = 0 Set bit 30 if we see this high 0x00000100, // B1 0x00000001, // D1 0x00000200, // B2 0x00000002, // D2 0x00000400, // B4 0x00000004, // D4 0x00000000, // F2 = 1 0x08000000, // xx = 0 Set bit 27 if we see this high 0x04000000, // xx = 0 Set bit 26 if we see this high 0x00000080, // SPI 0x02000000, // xx = 0 Set bit 25 if we see this high 0x01000000, // xx = 0 Set bit 24 if we see this high 0x00800000, // xx = 0 Set bit 23 if we see this high 0x00400000, // xx = 0 Set bit 22 if we see this high 0x00200000, // xx = 0 Set bit 21 if we see this high 0x00100000, // xx = 0 Set bit 20 if we see this high }; // // This table is used to produce an error variable called ModeAErrs.Each // inter-bit period is inspected, and if it's value falls outside of the // expected range, then the value in this table is or-ed into ModeAErrs. // // At the end of message processing, ModeAErrs will indicate if we saw // any inter-bit anomolies, and the bits that are set will show which // bits had them. // uint32_t ModeAMidTable[24] = { 0x80000000, // F1 = 1 Set bit 31 if we see F1_C1 error 0x00000010, // C1 Set bit 4 if we see C1_A1 error 0x00001000, // A1 Set bit 12 if we see A1_C2 error 0x00000020, // C2 Set bit 5 if we see C2_A2 error 0x00002000, // A2 Set bit 13 if we see A2_C4 error 0x00000040, // C4 Set bit 6 if we see C3_A4 error 0x00004000, // A4 Set bit 14 if we see A4_xx error 0x40000000, // xx = 0 Set bit 30 if we see xx_B1 error 0x00000100, // B1 Set bit 8 if we see B1_D1 error 0x00000001, // D1 Set bit 0 if we see D1_B2 error 0x00000200, // B2 Set bit 9 if we see B2_D2 error 0x00000002, // D2 Set bit 1 if we see D2_B4 error 0x00000400, // B4 Set bit 10 if we see B4_D4 error 0x00000004, // D4 Set bit 2 if we see D4_F2 error 0x20000000, // F2 = 1 Set bit 29 if we see F2_xx error 0x08000000, // xx = 0 Set bit 27 if we see xx_xx error 0x04000000, // xx = 0 Set bit 26 if we see xx_SPI error 0x00000080, // SPI Set bit 15 if we see SPI_xx error 0x02000000, // xx = 0 Set bit 25 if we see xx_xx error 0x01000000, // xx = 0 Set bit 24 if we see xx_xx error 0x00800000, // xx = 0 Set bit 23 if we see xx_xx error 0x00400000, // xx = 0 Set bit 22 if we see xx_xx error 0x00200000, // xx = 0 Set bit 21 if we see xx_xx error 0x00100000, // xx = 0 Set bit 20 if we see xx_xx error }; // // The "off air" format is,, // _F1_C1_A1_C2_A2_C4_A4_xx_B1_D1_B2_D2_B4_D4_F2_xx_xx_SPI_ // // Bit spacing is 1.45uS, with 0.45uS high, and 1.00us low. This is a problem // because we ase sampling at 2Mhz (500nS) so we are below Nyquist. // // The bit spacings are.. // F1 : 0.00, // 1.45, 2.90, 4.35, 5.80, 7.25, 8.70, // X : 10.15, // : 11.60, 13.05, 14.50, 15.95, 17.40, 18.85, // F2 : 20.30, // X : 21.75, 23.20, 24.65 // // This equates to the following sample point centers at 2Mhz. // [ 0.0], // [ 2.9], [ 5.8], [ 8.7], [11.6], [14.5], [17.4], // [20.3], // [23.2], [26.1], [29.0], [31.9], [34.8], [37.7] // [40.6] // [43.5], [46.4], [49.3] // // We know that this is a supposed to be a binary stream, so the signal // should either be a 1 or a 0. Therefore, any energy above the noise level // in two adjacent samples must be from the same pulse, so we can simply // add the values together.. // int detectModeA(uint16_t *m, struct modesMessage *mm) { int j, lastBitWasOne; int ModeABits = 0; int ModeAErrs = 0; int byte, bit; int thisSample, lastBit, lastSpace = 0; int m0, m1, m2, m3, mPhase; int n0, n1, n2 ,n3; int F1_sig, F1_noise; int F2_sig, F2_noise; int fSig, fNoise, fLevel, fLoLo; int snr; // m[0] contains the energy from 0 -> 499 nS // m[1] contains the energy from 500 -> 999 nS // m[2] contains the energy from 1000 -> 1499 nS // m[3] contains the energy from 1500 -> 1999 nS // // We are looking for a Frame bit (F1) whose width is 450nS, followed by // 1000nS of quiet. // // The width of the frame bit is 450nS, which is 90% of our sample rate. // Therefore, in an ideal world, all the energy for the frame bit will be // in a single sample, preceeded by (at least) one zero, and followed by // two zeros, Best case we can look for ... // // 0 - 1 - 0 - 0 // // However, our samples are not phase aligned, so some of the energy from // each bit could be spread over two consecutive samples. Worst case is // that we sample half in one bit, and half in the next. In that case, // we're looking for // // 0 - 0.5 - 0.5 - 0. m0 = m[0]; m1 = m[1]; if (m0 >= m1) // m1 *must* be bigger than m0 for this to be F1 {return (0);} m2 = m[2]; m3 = m[3]; // // if (m2 <= m0), then assume the sample bob on (Phase == 0), so don't look at m3 if ((m2 <= m0) || (m2 < m3)) {m3 = m2; m2 = m0;} if ( (m3 >= m1) // m1 must be bigger than m3 || (m0 > m2) // m2 can be equal to m0 if ( 0,1,0,0 ) || (m3 > m2) ) // m2 can be equal to m3 if ( 0,1,0,0 ) {return (0);} // m0 = noise // m1 = noise + (signal * X)) // m2 = noise + (signal * (1-X)) // m3 = noise // // Hence, assuming all 4 samples have similar amounts of noise in them // signal = (m1 + m2) - ((m0 + m3) * 2) // noise = (m0 + m3) / 2 // F1_sig = (m1 + m2) - ((m0 + m3) << 1); F1_noise = (m0 + m3) >> 1; if ( (F1_sig < MODEAC_MSG_SQUELCH_LEVEL) // minimum required F1 signal amplitude || (F1_sig < (F1_noise << 2)) ) // minimum allowable Sig/Noise ratio 4:1 {return (0);} // If we get here then we have a potential F1, so look for an equally valid F2 20.3uS later // // Our F1 is centered somewhere between samples m[1] and m[2]. We can guestimate where F2 is // by comparing the ratio of m1 and m2, and adding on 20.3 uS (40.6 samples) // mPhase = ((m2 * 20) / (m1 + m2)); byte = (mPhase + 812) / 20; n0 = m[byte++]; n1 = m[byte++]; if (n0 >= n1) // n1 *must* be bigger than n0 for this to be F2 {return (0);} n2 = m[byte++]; // // if the sample bob on (Phase == 0), don't look at n3 // if ((mPhase + 812) % 20) {n3 = m[byte++];} else {n3 = n2; n2 = n0;} if ( (n3 >= n1) // n1 must be bigger than n3 || (n0 > n2) // n2 can be equal to n0 ( 0,1,0,0 ) || (n3 > n2) ) // n2 can be equal to n3 ( 0,1,0,0 ) {return (0);} F2_sig = (n1 + n2) - ((n0 + n3) << 1); F2_noise = (n0 + n3) >> 1; if ( (F2_sig < MODEAC_MSG_SQUELCH_LEVEL) // minimum required F2 signal amplitude || (F2_sig < (F2_noise << 2)) ) // maximum allowable Sig/Noise ratio 4:1 {return (0);} fSig = (F1_sig + F2_sig) >> 1; fNoise = (F1_noise + F2_noise) >> 1; fLoLo = fNoise + (fSig >> 2); // 1/2 fLevel = fNoise + (fSig >> 1); lastBitWasOne = 1; lastBit = F1_sig; // // Now step by a half ModeA bit, 0.725nS, which is 1.45 samples, which is 29/20 // No need to do bit 0 because we've already selected it as a valid F1 // Do several bits past the SPI to increase error rejection // for (j = 1, mPhase += 29; j < 48; mPhase += 29, j ++) { byte = 1 + (mPhase / 20); thisSample = m[byte] - fNoise; if (mPhase % 20) // If the bit is split over two samples... {thisSample += (m[byte+1] - fNoise);} // add in the second sample's energy // If we're calculating a space value if (j & 1) {lastSpace = thisSample;} else {// We're calculating a new bit value bit = j >> 1; if (thisSample >= fLevel) {// We're calculating a new bit value, and its a one ModeABits |= ModeABitTable[bit--]; // or in the correct bit if (lastBitWasOne) { // This bit is one, last bit was one, so check the last space is somewhere less than one if ( (lastSpace >= (thisSample>>1)) || (lastSpace >= lastBit) ) {ModeAErrs |= ModeAMidTable[bit];} } else {// This bit,is one, last bit was zero, so check the last space is somewhere less than one if (lastSpace >= (thisSample >> 1)) {ModeAErrs |= ModeAMidTable[bit];} } lastBitWasOne = 1; } else {// We're calculating a new bit value, and its a zero if (lastBitWasOne) { // This bit is zero, last bit was one, so check the last space is somewhere in between if (lastSpace >= lastBit) {ModeAErrs |= ModeAMidTable[bit];} } else {// This bit,is zero, last bit was zero, so check the last space is zero too if (lastSpace >= fLoLo) {ModeAErrs |= ModeAMidTable[bit];} } lastBitWasOne = 0; } lastBit = (thisSample >> 1); } } // // Output format is : 00:A4:A2:A1:00:B4:B2:B1:00:C4:C2:C1:00:D4:D2:D1 // if ((ModeABits < 3) || (ModeABits & 0xFFFF8808) || (ModeAErrs) ) {return (ModeABits = 0);} // snr = 5 * 20log10(fSig / (fSig+fNoise)) (in units of 0.2dB) // = 100log10(fSig) - 100log10(fSig+fNoise) while (fSig > 65535 || (fSig + fNoise) > 65535) { fSig >>= 1; fNoise >>= 1; } snr = Modes.log10lut[fSig] - Modes.log10lut[fSig + fNoise]; mm->signalLevel = ((snr < 255) ? (uint8_t)snr : 255); return ModeABits; } // //========================================================================= // // Input format is : 00:A4:A2:A1:00:B4:B2:B1:00:C4:C2:C1:00:D4:D2:D1 // int ModeAToModeC(unsigned int ModeA) { unsigned int FiveHundreds = 0; unsigned int OneHundreds = 0; if ( (ModeA & 0xFFFF888B) // D1 set is illegal. D2 set is > 62700ft which is unlikely || ((ModeA & 0x000000F0) == 0) ) // C1,,C4 cannot be Zero {return -9999;} if (ModeA & 0x0010) {OneHundreds ^= 0x007;} // C1 if (ModeA & 0x0020) {OneHundreds ^= 0x003;} // C2 if (ModeA & 0x0040) {OneHundreds ^= 0x001;} // C4 // Remove 7s from OneHundreds (Make 7->5, snd 5->7). if ((OneHundreds & 5) == 5) {OneHundreds ^= 2;} // Check for invalid codes, only 1 to 5 are valid if (OneHundreds > 5) {return -9999;} //if (ModeA & 0x0001) {FiveHundreds ^= 0x1FF;} // D1 never used for altitude if (ModeA & 0x0002) {FiveHundreds ^= 0x0FF;} // D2 if (ModeA & 0x0004) {FiveHundreds ^= 0x07F;} // D4 if (ModeA & 0x1000) {FiveHundreds ^= 0x03F;} // A1 if (ModeA & 0x2000) {FiveHundreds ^= 0x01F;} // A2 if (ModeA & 0x4000) {FiveHundreds ^= 0x00F;} // A4 if (ModeA & 0x0100) {FiveHundreds ^= 0x007;} // B1 if (ModeA & 0x0200) {FiveHundreds ^= 0x003;} // B2 if (ModeA & 0x0400) {FiveHundreds ^= 0x001;} // B4 // Correct order of OneHundreds. if (FiveHundreds & 1) {OneHundreds = 6 - OneHundreds;} return ((FiveHundreds * 5) + OneHundreds - 13); } // //========================================================================= // void decodeModeAMessage(struct modesMessage *mm, int ModeA) { mm->msgtype = 32; // Valid Mode S DF's are DF-00 to DF-31. // so use 32 to indicate Mode A/C mm->msgbits = 16; // Fudge up a Mode S style data stream mm->msg[0] = (ModeA >> 8); mm->msg[1] = (ModeA); // Fudge an ICAO address based on Mode A (remove the Ident bit) // Use an upper address byte of FF, since this is ICAO unallocated mm->addr = 0x00FF0000 | (ModeA & 0x0000FF7F); // Set the Identity field to ModeA mm->modeA = ModeA & 0x7777; mm->bFlags |= MODES_ACFLAGS_SQUAWK_VALID; // Flag ident in flight status mm->fs = ModeA & 0x0080; // Not much else we can tell from a Mode A/C reply. // Just fudge up a few bits to keep other code happy mm->crcok = 1; mm->correctedbits = 0; } // // ===================== Mode A/C detection and decoding =================== //