246 lines
9.7 KiB
C
246 lines
9.7 KiB
C
// dump1090, a Mode S messages decoder for RTLSDR devices.
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//
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// Copyright (C) 2012 by Salvatore Sanfilippo <antirez@gmail.com>
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//
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// All rights reserved.
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//
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// Redistribution and use in source and binary forms, with or without
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// modification, are permitted provided that the following conditions are
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// met:
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//
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// * Redistributions of source code must retain the above copyright
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// notice, this list of conditions and the following disclaimer.
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//
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// * Redistributions in binary form must reproduce the above copyright
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// notice, this list of conditions and the following disclaimer in the
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// documentation and/or other materials provided with the distribution.
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//
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// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
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// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
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// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
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// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
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// HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
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// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
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// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
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// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
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// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
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// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
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//
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#include "dump1090.h"
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// Parity table for MODE S Messages.
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// The table contains 112 elements, every element corresponds to a bit set
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// in the message, starting from the first bit of actual data after the
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// preamble.
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//
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// For messages of 112 bit, the whole table is used.
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// For messages of 56 bits only the last 56 elements are used.
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//
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// The algorithm is as simple as xoring all the elements in this table
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// for which the corresponding bit on the message is set to 1.
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//
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// The latest 24 elements in this table are set to 0 as the checksum at the
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// end of the message should not affect the computation.
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//
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// Note: this function can be used with DF11 and DF17, other modes have
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// the CRC xored with the sender address as they are reply to interrogations,
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// but a casual listener can't split the address from the checksum.
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//
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uint32_t modes_checksum_table[112] = {
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0x3935ea, 0x1c9af5, 0xf1b77e, 0x78dbbf, 0xc397db, 0x9e31e9, 0xb0e2f0, 0x587178,
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0x2c38bc, 0x161c5e, 0x0b0e2f, 0xfa7d13, 0x82c48d, 0xbe9842, 0x5f4c21, 0xd05c14,
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0x682e0a, 0x341705, 0xe5f186, 0x72f8c3, 0xc68665, 0x9cb936, 0x4e5c9b, 0xd8d449,
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0x939020, 0x49c810, 0x24e408, 0x127204, 0x093902, 0x049c81, 0xfdb444, 0x7eda22,
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0x3f6d11, 0xe04c8c, 0x702646, 0x381323, 0xe3f395, 0x8e03ce, 0x4701e7, 0xdc7af7,
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0x91c77f, 0xb719bb, 0xa476d9, 0xadc168, 0x56e0b4, 0x2b705a, 0x15b82d, 0xf52612,
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0x7a9309, 0xc2b380, 0x6159c0, 0x30ace0, 0x185670, 0x0c2b38, 0x06159c, 0x030ace,
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0x018567, 0xff38b7, 0x80665f, 0xbfc92b, 0xa01e91, 0xaff54c, 0x57faa6, 0x2bfd53,
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0xea04ad, 0x8af852, 0x457c29, 0xdd4410, 0x6ea208, 0x375104, 0x1ba882, 0x0dd441,
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0xf91024, 0x7c8812, 0x3e4409, 0xe0d800, 0x706c00, 0x383600, 0x1c1b00, 0x0e0d80,
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0x0706c0, 0x038360, 0x01c1b0, 0x00e0d8, 0x00706c, 0x003836, 0x001c1b, 0xfff409,
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0x000000, 0x000000, 0x000000, 0x000000, 0x000000, 0x000000, 0x000000, 0x000000,
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0x000000, 0x000000, 0x000000, 0x000000, 0x000000, 0x000000, 0x000000, 0x000000,
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0x000000, 0x000000, 0x000000, 0x000000, 0x000000, 0x000000, 0x000000, 0x000000
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};
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uint32_t modesChecksum(unsigned char *msg, int bits) {
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uint32_t crc = 0;
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uint32_t rem = 0;
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int offset = (bits == 112) ? 0 : (112-56);
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uint8_t theByte = *msg;
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uint32_t * pCRCTable = &modes_checksum_table[offset];
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int j;
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// We don't really need to include the checksum itself
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bits -= 24;
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for(j = 0; j < bits; j++) {
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if ((j & 7) == 0)
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theByte = *msg++;
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// If bit is set, xor with corresponding table entry.
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if (theByte & 0x80) {crc ^= *pCRCTable;}
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pCRCTable++;
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theByte = theByte << 1;
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}
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rem = (msg[0] << 16) | (msg[1] << 8) | msg[2]; // message checksum
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return ((crc ^ rem) & 0x00FFFFFF); // 24 bit checksum syndrome.
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}
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//=========================================================================
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//
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// Code for introducing a less CPU-intensive method of correcting
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// single bit errors.
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//
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// Makes use of the fact that the crc checksum is linear with respect to
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// the bitwise xor operation, i.e.
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// crc(m^e) = (crc(m)^crc(e)
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// where m and e are the message resp. error bit vectors.
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//
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// Call crc(e) the syndrome.
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//
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// The code below works by precomputing a table of (crc(e), e) for all
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// possible error vectors e (here only single bit and double bit errors),
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// search for the syndrome in the table, and correct the then known error.
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// The error vector e is represented by one or two bit positions that are
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// changed. If a second bit position is not used, it is -1.
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//
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// Run-time is binary search in a sorted table, plus some constant overhead,
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// instead of running through all possible bit positions (resp. pairs of
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// bit positions).
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//
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struct errorinfo {
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uint32_t syndrome; // CRC syndrome
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int bits; // Number of bit positions to fix
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int pos[MODES_MAX_BITERRORS]; // Bit positions corrected by this syndrome
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};
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#define NERRORINFO \
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(MODES_LONG_MSG_BITS+MODES_LONG_MSG_BITS*(MODES_LONG_MSG_BITS-1)/2)
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struct errorinfo bitErrorTable[NERRORINFO];
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// Compare function as needed for stdlib's qsort and bsearch functions
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static int cmpErrorInfo(const void *p0, const void *p1) {
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struct errorinfo *e0 = (struct errorinfo*)p0;
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struct errorinfo *e1 = (struct errorinfo*)p1;
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if (e0->syndrome == e1->syndrome) {
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return 0;
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} else if (e0->syndrome < e1->syndrome) {
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return -1;
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} else {
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return 1;
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}
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}
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//
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//=========================================================================
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//
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// Compute the table of all syndromes for 1-bit and 2-bit error vectors
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void modesInitErrorInfo() {
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unsigned char msg[MODES_LONG_MSG_BYTES];
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int i, j, n;
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uint32_t crc;
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n = 0;
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memset(bitErrorTable, 0, sizeof(bitErrorTable));
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memset(msg, 0, MODES_LONG_MSG_BYTES);
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// Add all possible single and double bit errors
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// don't include errors in first 5 bits (DF type)
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for (i = 5; i < MODES_LONG_MSG_BITS; i++) {
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int bytepos0 = (i >> 3);
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int mask0 = 1 << (7 - (i & 7));
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msg[bytepos0] ^= mask0; // create error0
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crc = modesChecksum(msg, MODES_LONG_MSG_BITS);
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bitErrorTable[n].syndrome = crc; // single bit error case
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bitErrorTable[n].bits = 1;
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bitErrorTable[n].pos[0] = i;
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bitErrorTable[n].pos[1] = -1;
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n += 1;
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if (Modes.nfix_crc > 1) {
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for (j = i+1; j < MODES_LONG_MSG_BITS; j++) {
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int bytepos1 = (j >> 3);
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int mask1 = 1 << (7 - (j & 7));
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msg[bytepos1] ^= mask1; // create error1
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crc = modesChecksum(msg, MODES_LONG_MSG_BITS);
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if (n >= NERRORINFO) {
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//fprintf(stderr, "Internal error, too many entries, fix NERRORINFO\n");
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break;
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}
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bitErrorTable[n].syndrome = crc; // two bit error case
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bitErrorTable[n].bits = 2;
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bitErrorTable[n].pos[0] = i;
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bitErrorTable[n].pos[1] = j;
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n += 1;
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msg[bytepos1] ^= mask1; // revert error1
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}
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}
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msg[bytepos0] ^= mask0; // revert error0
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}
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qsort(bitErrorTable, NERRORINFO, sizeof(struct errorinfo), cmpErrorInfo);
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// Test code: report if any syndrome appears at least twice. In this
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// case the correction cannot be done without ambiguity.
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// Tried it, does not happen for 1- and 2-bit errors.
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/*
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for (i = 1; i < NERRORINFO; i++) {
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if (bitErrorTable[i-1].syndrome == bitErrorTable[i].syndrome) {
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fprintf(stderr, "modesInitErrorInfo: Collision for syndrome %06x\n",
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(int)bitErrorTable[i].syndrome);
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}
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}
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for (i = 0; i < NERRORINFO; i++) {
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printf("syndrome %06x bit0 %3d bit1 %3d\n",
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bitErrorTable[i].syndrome,
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bitErrorTable[i].pos0, bitErrorTable[i].pos1);
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}
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*/
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}
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//
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//=========================================================================
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//
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// Search for syndrome in table and if an entry is found, flip the necessary
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// bits. Make sure the indices fit into the array
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// Additional parameter: fix only less than maxcorrected bits, and record
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// fixed bit positions in corrected[]. This array can be NULL, otherwise
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// must be of length at least maxcorrected.
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// Return number of fixed bits.
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//
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int fixBitErrors(unsigned char *msg, int bits, int maxfix, char *fixedbits) {
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struct errorinfo *pei;
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struct errorinfo ei;
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int bitpos, offset, res, i;
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memset(&ei, 0, sizeof(struct errorinfo));
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ei.syndrome = modesChecksum(msg, bits);
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pei = bsearch(&ei, bitErrorTable, NERRORINFO,
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sizeof(struct errorinfo), cmpErrorInfo);
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if (pei == NULL) {
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return 0; // No syndrome found
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}
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// Check if the syndrome fixes more bits than we allow
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if (maxfix < pei->bits) {
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return 0;
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}
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// Check that all bit positions lie inside the message length
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offset = MODES_LONG_MSG_BITS-bits;
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for (i = 0; i < pei->bits; i++) {
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bitpos = pei->pos[i] - offset;
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if ((bitpos < 0) || (bitpos >= bits)) {
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return 0;
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}
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}
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// Fix the bits
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for (i = res = 0; i < pei->bits; i++) {
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bitpos = pei->pos[i] - offset;
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msg[bitpos >> 3] ^= (1 << (7 - (bitpos & 7)));
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if (fixedbits) {
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fixedbits[res++] = bitpos;
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}
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}
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return res;
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}
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