// Part of dump1090, a Mode S message decoder for RTLSDR devices. // // demod_2000.c: 2MHz Mode S demodulator. // // 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 2.0MHz demodulator // // ===================== 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; // 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);} mm->signalLevel = (fSig + fNoise) * (fSig + fNoise) / MAX_POWER; return ModeABits; } // ============================== 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 // static 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. // static 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. // static void dumpRawMessageJS(char *descr, unsigned char *msg, uint16_t *m, uint32_t offset, struct errorinfo *ei) { 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; 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, "], "); for (j = 0; j < MODES_MAX_BITERRORS; ++j) fprintf(fp,"\"fix%d\": %d, ", j, ei->bit[j]); fprintf(fp, "\"bits\": %d, \"hex\": \"", 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. // static void dumpRawMessage(char *descr, unsigned char *msg, uint16_t *m, uint32_t offset) { int j; int msgtype = msg[0] >> 3; struct errorinfo *ei = NULL; if (msgtype == 17) { int len = modesMessageLenByType(msgtype); uint32_t csum = modesChecksum(msg, len); ei = modesChecksumDiagnose(csum, len); } if (Modes.debug & MODES_DEBUG_JS) { dumpRawMessageJS(descr, msg, m, offset, ei); 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, ei ? ei->errors : 0); dumpMagnitudeVector(m,offset); printf("---\n\n"); } // //========================================================================= // // 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 // static 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; } static 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. static 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 (onTime == 0 && early == 0 && late == 0) { // Blah, can't do anything with this, avoid a divide-by-zero return; } 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 demodulate2000(struct mag_buf *mag) { 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; unsigned mlen = mag->length; uint16_t *m = mag->data; 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; int message_ok; 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.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 = mag->sampleTimestamp + ((j+1) * 6); // compute message receive time as block-start-time + difference in the 12MHz clock mm.sysTimestampMsg = mag->sysTimestamp; // start of block time mm.sysTimestampMsg.tv_nsec += receiveclock_ns_elapsed(mag->sampleTimestamp, mm.timestampMsg); normalize_timespec(&mm.sysTimestampMsg); // Decode the received message decodeModeAMessage(&mm, ModeA); // Pass data to the next layer useModesMessage(&mm); j += MODEAC_MSG_SAMPLES; Modes.stats_current.demod_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.stats_current.demod_preambles++; } 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]); 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 } 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 if (msglen > 0) { 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; 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) ) { int result; // Set initial mm structure details mm.timestampMsg = mag->sampleTimestamp + (j*6); // compute message receive time as block-start-time + difference in the 12MHz clock mm.sysTimestampMsg = mag->sysTimestamp; // start of block time mm.sysTimestampMsg.tv_nsec += receiveclock_ns_elapsed(mag->sampleTimestamp, mm.timestampMsg); normalize_timespec(&mm.sysTimestampMsg); mm.signalLevel = (365.0*60 + sigLevel + noiseLevel) * (365.0*60 + sigLevel + noiseLevel) / MAX_POWER / 60 / 60; // Decode the received message result = decodeModesMessage(&mm, msg); if (result < 0) { message_ok = 0; if (result == -1) Modes.stats_current.demod_rejected_unknown_icao++; else Modes.stats_current.demod_rejected_bad++; } else { message_ok = 1; Modes.stats_current.demod_accepted[mm.correctedbits]++; } // Update statistics // 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 && (!message_ok || mm.correctedbits > 0)) dumpRawMessage("Decoded with bad CRC", msg, m, j); else if (Modes.debug & MODES_DEBUG_GOODCRC && message_ok && mm.correctedbits == 0) dumpRawMessage("Decoded with good CRC", msg, m, j); } // Skip this message if we are sure it's fine if (message_ok) { j += (MODES_PREAMBLE_US+msglen)*2 - 1; // Pass data to the next layer useModesMessage(&mm); } } else { message_ok = 0; 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 necessary and possible. if ((!message_ok || mm.correctedbits > 0) && !use_correction && j && detectOutOfPhase(pPreamble)) { use_correction = 1; j--; } else { use_correction = 0; } } }