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mode_s.c
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mode_s.c
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// dump1090, a Mode S messages decoder for RTLSDR devices.
//
// Copyright (C) 2012 by Salvatore Sanfilippo <antirez@gmail.com>
//
// All rights reserved.
//
// Redistribution and use in source and binary forms, with or without
// modification, are permitted provided that the following conditions are
// met:
//
// * Redistributions of source code must retain the above copyright
// notice, this list of conditions and the following disclaimer.
//
// * Redistributions in binary form must reproduce the above copyright
// notice, this list of conditions and the following disclaimer in the
// documentation and/or other materials provided with the distribution.
//
// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
// HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
//
#include "dump1090.h"
//
// ===================== Mode S detection and decoding ===================
//
// Parity table for MODE S Messages.
// The table contains 112 elements, every element corresponds to a bit set
// in the message, starting from the first bit of actual data after the
// preamble.
//
// For messages of 112 bit, the whole table is used.
// For messages of 56 bits only the last 56 elements are used.
//
// The algorithm is as simple as xoring all the elements in this table
// for which the corresponding bit on the message is set to 1.
//
// The latest 24 elements in this table are set to 0 as the checksum at the
// end of the message should not affect the computation.
//
// Note: this function can be used with DF11 and DF17, other modes have
// the CRC xored with the sender address as they are reply to interrogations,
// but a casual listener can't split the address from the checksum.
//
uint32_t modes_checksum_table[112] = {
0x3935ea, 0x1c9af5, 0xf1b77e, 0x78dbbf, 0xc397db, 0x9e31e9, 0xb0e2f0, 0x587178,
0x2c38bc, 0x161c5e, 0x0b0e2f, 0xfa7d13, 0x82c48d, 0xbe9842, 0x5f4c21, 0xd05c14,
0x682e0a, 0x341705, 0xe5f186, 0x72f8c3, 0xc68665, 0x9cb936, 0x4e5c9b, 0xd8d449,
0x939020, 0x49c810, 0x24e408, 0x127204, 0x093902, 0x049c81, 0xfdb444, 0x7eda22,
0x3f6d11, 0xe04c8c, 0x702646, 0x381323, 0xe3f395, 0x8e03ce, 0x4701e7, 0xdc7af7,
0x91c77f, 0xb719bb, 0xa476d9, 0xadc168, 0x56e0b4, 0x2b705a, 0x15b82d, 0xf52612,
0x7a9309, 0xc2b380, 0x6159c0, 0x30ace0, 0x185670, 0x0c2b38, 0x06159c, 0x030ace,
0x018567, 0xff38b7, 0x80665f, 0xbfc92b, 0xa01e91, 0xaff54c, 0x57faa6, 0x2bfd53,
0xea04ad, 0x8af852, 0x457c29, 0xdd4410, 0x6ea208, 0x375104, 0x1ba882, 0x0dd441,
0xf91024, 0x7c8812, 0x3e4409, 0xe0d800, 0x706c00, 0x383600, 0x1c1b00, 0x0e0d80,
0x0706c0, 0x038360, 0x01c1b0, 0x00e0d8, 0x00706c, 0x003836, 0x001c1b, 0xfff409,
0x000000, 0x000000, 0x000000, 0x000000, 0x000000, 0x000000, 0x000000, 0x000000,
0x000000, 0x000000, 0x000000, 0x000000, 0x000000, 0x000000, 0x000000, 0x000000,
0x000000, 0x000000, 0x000000, 0x000000, 0x000000, 0x000000, 0x000000, 0x000000
};
uint32_t modesChecksum(unsigned char *msg, int bits) {
uint32_t crc = 0;
uint32_t rem = 0;
int offset = (bits == 112) ? 0 : (112-56);
uint8_t theByte = *msg;
uint32_t * pCRCTable = &modes_checksum_table[offset];
int j;
// We don't really need to include the checksum itself
bits -= 24;
for(j = 0; j < bits; j++) {
if ((j & 7) == 0)
theByte = *msg++;
// If bit is set, xor with corresponding table entry.
if (theByte & 0x80) {crc ^= *pCRCTable;}
pCRCTable++;
theByte = theByte << 1;
}
rem = (msg[0] << 16) | (msg[1] << 8) | msg[2]; // message checksum
return ((crc ^ rem) & 0x00FFFFFF); // 24 bit checksum syndrome.
}
//
//=========================================================================
//
// Given the Downlink Format (DF) of the message, return the message length in bits.
//
// All known DF's 16 or greater are long. All known DF's 15 or less are short.
// There are lots of unused codes in both category, so we can assume ICAO will stick to
// these rules, meaning that the most significant bit of the DF indicates the length.
//
int modesMessageLenByType(int type) {
return (type & 0x10) ? MODES_LONG_MSG_BITS : MODES_SHORT_MSG_BITS ;
}
//
//=========================================================================
//
// Try to fix single bit errors using the checksum. On success modifies
// the original buffer with the fixed version, and returns the position
// of the error bit. Otherwise if fixing failed -1 is returned.
/*
int fixSingleBitErrors(unsigned char *msg, int bits) {
int j;
unsigned char aux[MODES_LONG_MSG_BYTES];
memcpy(aux, msg, bits/8);
// Do not attempt to error correct Bits 0-4. These contain the DF, and must
// be correct because we can only error correct DF17
for (j = 5; j < bits; j++) {
int byte = j/8;
int bitmask = 1 << (7 - (j & 7));
aux[byte] ^= bitmask; // Flip j-th bit
if (0 == modesChecksum(aux, bits)) {
// The error is fixed. Overwrite the original buffer with the
// corrected sequence, and returns the error bit position
msg[byte] = aux[byte];
return (j);
}
aux[byte] ^= bitmask; // Flip j-th bit back again
}
return (-1);
}
*/
//=========================================================================
//
// Similar to fixSingleBitErrors() but try every possible two bit combination.
// This is very slow and should be tried only against DF17 messages that
// don't pass the checksum, and only in Aggressive Mode.
/*
int fixTwoBitsErrors(unsigned char *msg, int bits) {
int j, i;
unsigned char aux[MODES_LONG_MSG_BYTES];
memcpy(aux, msg, bits/8);
// Do not attempt to error correct Bits 0-4. These contain the DF, and must
// be correct because we can only error correct DF17
for (j = 5; j < bits; j++) {
int byte1 = j/8;
int bitmask1 = 1 << (7 - (j & 7));
aux[byte1] ^= bitmask1; // Flip j-th bit
// Don't check the same pairs multiple times, so i starts from j+1
for (i = j+1; i < bits; i++) {
int byte2 = i/8;
int bitmask2 = 1 << (7 - (i & 7));
aux[byte2] ^= bitmask2; // Flip i-th bit
if (0 == modesChecksum(aux, bits)) {
// The error is fixed. Overwrite the original buffer with
// the corrected sequence, and returns the error bit position
msg[byte1] = aux[byte1];
msg[byte2] = aux[byte2];
// We return the two bits as a 16 bit integer by shifting
// 'i' on the left. This is possible since 'i' will always
// be non-zero because i starts from j+1
return (j | (i << 8));
aux[byte2] ^= bitmask2; // Flip i-th bit back
}
aux[byte1] ^= bitmask1; // Flip j-th bit back
}
}
return (-1);
}
*/
//
//=========================================================================
//
// Code for introducing a less CPU-intensive method of correcting
// single bit errors.
//
// Makes use of the fact that the crc checksum is linear with respect to
// the bitwise xor operation, i.e.
// crc(m^e) = (crc(m)^crc(e)
// where m and e are the message resp. error bit vectors.
//
// Call crc(e) the syndrome.
//
// The code below works by precomputing a table of (crc(e), e) for all
// possible error vectors e (here only single bit and double bit errors),
// search for the syndrome in the table, and correct the then known error.
// The error vector e is represented by one or two bit positions that are
// changed. If a second bit position is not used, it is -1.
//
// Run-time is binary search in a sorted table, plus some constant overhead,
// instead of running through all possible bit positions (resp. pairs of
// bit positions).
//
struct errorinfo {
uint32_t syndrome; // CRC syndrome
int bits; // Number of bit positions to fix
int pos[MODES_MAX_BITERRORS]; // Bit positions corrected by this syndrome
};
#define NERRORINFO \
(MODES_LONG_MSG_BITS+MODES_LONG_MSG_BITS*(MODES_LONG_MSG_BITS-1)/2)
struct errorinfo bitErrorTable[NERRORINFO];
// Compare function as needed for stdlib's qsort and bsearch functions
int cmpErrorInfo(const void *p0, const void *p1) {
struct errorinfo *e0 = (struct errorinfo*)p0;
struct errorinfo *e1 = (struct errorinfo*)p1;
if (e0->syndrome == e1->syndrome) {
return 0;
} else if (e0->syndrome < e1->syndrome) {
return -1;
} else {
return 1;
}
}
//
//=========================================================================
//
// Compute the table of all syndromes for 1-bit and 2-bit error vectors
void modesInitErrorInfo() {
unsigned char msg[MODES_LONG_MSG_BYTES];
int i, j, n;
uint32_t crc;
n = 0;
memset(bitErrorTable, 0, sizeof(bitErrorTable));
memset(msg, 0, MODES_LONG_MSG_BYTES);
// Add all possible single and double bit errors
// don't include errors in first 5 bits (DF type)
for (i = 5; i < MODES_LONG_MSG_BITS; i++) {
int bytepos0 = (i >> 3);
int mask0 = 1 << (7 - (i & 7));
msg[bytepos0] ^= mask0; // create error0
crc = modesChecksum(msg, MODES_LONG_MSG_BITS);
bitErrorTable[n].syndrome = crc; // single bit error case
bitErrorTable[n].bits = 1;
bitErrorTable[n].pos[0] = i;
bitErrorTable[n].pos[1] = -1;
n += 1;
if (Modes.nfix_crc > 1) {
for (j = i+1; j < MODES_LONG_MSG_BITS; j++) {
int bytepos1 = (j >> 3);
int mask1 = 1 << (7 - (j & 7));
msg[bytepos1] ^= mask1; // create error1
crc = modesChecksum(msg, MODES_LONG_MSG_BITS);
if (n >= NERRORINFO) {
//fprintf(stderr, "Internal error, too many entries, fix NERRORINFO\n");
break;
}
bitErrorTable[n].syndrome = crc; // two bit error case
bitErrorTable[n].bits = 2;
bitErrorTable[n].pos[0] = i;
bitErrorTable[n].pos[1] = j;
n += 1;
msg[bytepos1] ^= mask1; // revert error1
}
}
msg[bytepos0] ^= mask0; // revert error0
}
qsort(bitErrorTable, NERRORINFO, sizeof(struct errorinfo), cmpErrorInfo);
// Test code: report if any syndrome appears at least twice. In this
// case the correction cannot be done without ambiguity.
// Tried it, does not happen for 1- and 2-bit errors.
/*
for (i = 1; i < NERRORINFO; i++) {
if (bitErrorTable[i-1].syndrome == bitErrorTable[i].syndrome) {
fprintf(stderr, "modesInitErrorInfo: Collision for syndrome %06x\n",
(int)bitErrorTable[i].syndrome);
}
}
for (i = 0; i < NERRORINFO; i++) {
printf("syndrome %06x bit0 %3d bit1 %3d\n",
bitErrorTable[i].syndrome,
bitErrorTable[i].pos0, bitErrorTable[i].pos1);
}
*/
}
//
//=========================================================================
//
// Search for syndrome in table and if an entry is found, flip the necessary
// bits. Make sure the indices fit into the array
// Additional parameter: fix only less than maxcorrected bits, and record
// fixed bit positions in corrected[]. This array can be NULL, otherwise
// must be of length at least maxcorrected.
// Return number of fixed bits.
//
int fixBitErrors(unsigned char *msg, int bits, int maxfix, char *fixedbits) {
struct errorinfo *pei;
struct errorinfo ei;
int bitpos, offset, res, i;
memset(&ei, 0, sizeof(struct errorinfo));
ei.syndrome = modesChecksum(msg, bits);
pei = bsearch(&ei, bitErrorTable, NERRORINFO,
sizeof(struct errorinfo), cmpErrorInfo);
if (pei == NULL) {
return 0; // No syndrome found
}
// Check if the syndrome fixes more bits than we allow
if (maxfix < pei->bits) {
return 0;
}
// Check that all bit positions lie inside the message length
offset = MODES_LONG_MSG_BITS-bits;
for (i = 0; i < pei->bits; i++) {
bitpos = pei->pos[i] - offset;
if ((bitpos < 0) || (bitpos >= bits)) {
return 0;
}
}
// Fix the bits
for (i = res = 0; i < pei->bits; i++) {
bitpos = pei->pos[i] - offset;
msg[bitpos >> 3] ^= (1 << (7 - (bitpos & 7)));
if (fixedbits) {
fixedbits[res++] = bitpos;
}
}
return res;
}
//
// ============================== Debugging =================================
//
// Helper function for dumpMagnitudeVector().
// It prints a single bar used to display raw signals.
//
// Since every magnitude sample is between 0-255, the function uses
// up to 63 characters for every bar. Every character represents
// a length of 4, 3, 2, 1, specifically:
//
// "O" is 4
// "o" is 3
// "-" is 2
// "." is 1
//
void dumpMagnitudeBar(int index, int magnitude) {
char *set = " .-o";
char buf[256];
int div = magnitude / 256 / 4;
int rem = magnitude / 256 % 4;
memset(buf,'O',div);
buf[div] = set[rem];
buf[div+1] = '\0';
if (index >= 0)
printf("[%.3d] |%-66s 0x%04X\n", index, buf, magnitude);
else
printf("[%.2d] |%-66s 0x%04X\n", index, buf, magnitude);
}
//
//=========================================================================
//
// Display an ASCII-art alike graphical representation of the undecoded
// message as a magnitude signal.
//
// The message starts at the specified offset in the "m" buffer.
// The function will display enough data to cover a short 56 bit message.
//
// If possible a few samples before the start of the messsage are included
// for context.
//
void dumpMagnitudeVector(uint16_t *m, uint32_t offset) {
uint32_t padding = 5; // Show a few samples before the actual start.
uint32_t start = (offset < padding) ? 0 : offset-padding;
uint32_t end = offset + (MODES_PREAMBLE_SAMPLES)+(MODES_SHORT_MSG_SAMPLES) - 1;
uint32_t j;
for (j = start; j <= end; j++) {
dumpMagnitudeBar(j-offset, m[j]);
}
}
//
//=========================================================================
//
// Produce a raw representation of the message as a Javascript file
// loadable by debug.html.
//
void dumpRawMessageJS(char *descr, unsigned char *msg,
uint16_t *m, uint32_t offset, int fixable, char *bitpos)
{
int padding = 5; // Show a few samples before the actual start.
int start = offset - padding;
int end = offset + (MODES_PREAMBLE_SAMPLES)+(MODES_LONG_MSG_SAMPLES) - 1;
FILE *fp;
int j;
MODES_NOTUSED(fixable);
if ((fp = fopen("frames.js","a")) == NULL) {
fprintf(stderr, "Error opening frames.js: %s\n", strerror(errno));
exit(1);
}
fprintf(fp,"frames.push({\"descr\": \"%s\", \"mag\": [", descr);
for (j = start; j <= end; j++) {
fprintf(fp,"%d", j < 0 ? 0 : m[j]);
if (j != end) fprintf(fp,",");
}
fprintf(fp,"], \"fix1\": %d, \"fix2\": %d, \"bits\": %d, \"hex\": \"",
bitpos[0], bitpos[1] , modesMessageLenByType(msg[0]>>3));
for (j = 0; j < MODES_LONG_MSG_BYTES; j++)
fprintf(fp,"\\x%02x",msg[j]);
fprintf(fp,"\"});\n");
fclose(fp);
}
//
//=========================================================================
//
// This is a wrapper for dumpMagnitudeVector() that also show the message
// in hex format with an additional description.
//
// descr is the additional message to show to describe the dump.
// msg points to the decoded message
// m is the original magnitude vector
// offset is the offset where the message starts
//
// The function also produces the Javascript file used by debug.html to
// display packets in a graphical format if the Javascript output was
// enabled.
//
void dumpRawMessage(char *descr, unsigned char *msg, uint16_t *m, uint32_t offset) {
int j;
int msgtype = msg[0] >> 3;
int fixable = 0;
char bitpos[MODES_MAX_BITERRORS];
for (j = 0; j < MODES_MAX_BITERRORS; j++) {
bitpos[j] = -1;
}
if (msgtype == 17) {
fixable = fixBitErrors(msg, MODES_LONG_MSG_BITS, MODES_MAX_BITERRORS, bitpos);
}
if (Modes.debug & MODES_DEBUG_JS) {
dumpRawMessageJS(descr, msg, m, offset, fixable, bitpos);
return;
}
printf("\n--- %s\n ", descr);
for (j = 0; j < MODES_LONG_MSG_BYTES; j++) {
printf("%02x",msg[j]);
if (j == MODES_SHORT_MSG_BYTES-1) printf(" ... ");
}
printf(" (DF %d, Fixable: %d)\n", msgtype, fixable);
dumpMagnitudeVector(m,offset);
printf("---\n\n");
}
//
//=========================================================================
//
// Code for testing the timing: run all possible 1- and 2-bit error
// the test message by all 1-bit errors. Run the old code against
// all of them, and new the code.
//
// Example measurements:
// Timing old vs. new crc correction code:
// Old code: 1-bit errors on 112 msgs: 3934 usecs
// New code: 1-bit errors on 112 msgs: 104 usecs
// Old code: 2-bit errors on 6216 msgs: 407743 usecs
// New code: 2-bit errors on 6216 msgs: 5176 usecs
// indicating a 37-fold resp. 78-fold improvement in speed for 1-bit resp.
// 2-bit error.
/*
unsigned char tmsg0[MODES_LONG_MSG_BYTES] = {
// Test data: first ADS-B message from testfiles/modes1.bin
0x8f, 0x4d, 0x20, 0x23, 0x58, 0x7f, 0x34, 0x5e,
0x35, 0x83, 0x7e, 0x22, 0x18, 0xb2
};
#define NTWOBITS (MODES_LONG_MSG_BITS*(MODES_LONG_MSG_BITS-1)/2)
unsigned char tmsg1[MODES_LONG_MSG_BITS][MODES_LONG_MSG_BYTES];
unsigned char tmsg2[NTWOBITS][MODES_LONG_MSG_BYTES];
// Init an array of cloned messages with all possible 1-bit errors present,
// applied to each message at the respective position
//
void inittmsg1() {
int i, bytepos, mask;
for (i = 0; i < MODES_LONG_MSG_BITS; i++) {
bytepos = i >> 3;
mask = 1 << (7 - (i & 7));
memcpy(&tmsg1[i][0], tmsg0, MODES_LONG_MSG_BYTES);
tmsg1[i][bytepos] ^= mask;
}
}
// Run sanity check on all but first 5 messages / bits, as those bits
// are not corrected.
//
void checktmsg1(FILE *out) {
int i, k;
uint32_t crc;
for (i = 5; i < MODES_LONG_MSG_BITS; i++) {
crc = modesChecksum(&tmsg1[i][0], MODES_LONG_MSG_BITS);
if (crc != 0) {
fprintf(out, "CRC not fixed for "
"positon %d\n", i);
fprintf(out, " MSG ");
for (k = 0; k < MODES_LONG_MSG_BYTES; k++) {
fprintf(out, "%02x", tmsg1[i][k]);
}
fprintf(out, "\n");
}
}
}
void inittmsg2() {
int i, j, n, bytepos0, bytepos1, mask0, mask1;
n = 0;
for (i = 0; i < MODES_LONG_MSG_BITS; i++) {
bytepos0 = i >> 3;
mask0 = 1 << (7 - (i & 7));
for (j = i+1; j < MODES_LONG_MSG_BITS; j++) {
bytepos1 = j >> 3;
mask1 = 1 << (7 - (j & 7));
memcpy(&tmsg2[n][0], tmsg0, MODES_LONG_MSG_BYTES);
tmsg2[n][bytepos0] ^= mask0;
tmsg2[n][bytepos1] ^= mask1;
n += 1;
}
}
}
long difftvusec(struct timeval *t0, struct timeval *t1) {
long res = 0;
res = t1->tv_usec-t0->tv_usec;
res += (t1->tv_sec-t0->tv_sec)*1000000L;
return res;
}
// the actual test code
void testAndTimeBitCorrection() {
struct timeval starttv, endtv;
int i;
// Run timing on 1-bit errors
printf("Timing old vs. new crc correction code:\n");
inittmsg1();
gettimeofday(&starttv, NULL);
for (i = 0; i < MODES_LONG_MSG_BITS; i++) {
fixSingleBitErrors(&tmsg1[i][0], MODES_LONG_MSG_BITS);
}
gettimeofday(&endtv, NULL);
printf(" Old code: 1-bit errors on %d msgs: %ld usecs\n",
MODES_LONG_MSG_BITS, difftvusec(&starttv, &endtv));
checktmsg1(stdout);
// Re-init
inittmsg1();
gettimeofday(&starttv, NULL);
for (i = 0; i < MODES_LONG_MSG_BITS; i++) {
fixBitErrors(&tmsg1[i][0], MODES_LONG_MSG_BITS, MODES_MAX_BITERRORS, NULL);
}
gettimeofday(&endtv, NULL);
printf(" New code: 1-bit errors on %d msgs: %ld usecs\n",
MODES_LONG_MSG_BITS, difftvusec(&starttv, &endtv));
checktmsg1(stdout);
// Run timing on 2-bit errors
inittmsg2();
gettimeofday(&starttv, NULL);
for (i = 0; i < NTWOBITS; i++) {
fixSingleBitErrors(&tmsg2[i][0], MODES_LONG_MSG_BITS);
}
gettimeofday(&endtv, NULL);
printf(" Old code: 2-bit errors on %d msgs: %ld usecs\n",
NTWOBITS, difftvusec(&starttv, &endtv));
// Re-init
inittmsg2();
gettimeofday(&starttv, NULL);
for (i = 0; i < NTWOBITS; i++) {
fixBitErrors(&tmsg2[i][0], MODES_LONG_MSG_BITS, MODES_MAX_BITERRORS, NULL);
}
gettimeofday(&endtv, NULL);
printf(" New code: 2-bit errors on %d msgs: %ld usecs\n",
NTWOBITS, difftvusec(&starttv, &endtv));
}
*/
//=========================================================================
//
// Hash the ICAO address to index our cache of MODES_ICAO_CACHE_LEN
// elements, that is assumed to be a power of two
//
uint32_t ICAOCacheHashAddress(uint32_t a) {
// The following three rounds wil make sure that every bit affects
// every output bit with ~ 50% of probability.
a = ((a >> 16) ^ a) * 0x45d9f3b;
a = ((a >> 16) ^ a) * 0x45d9f3b;
a = ((a >> 16) ^ a);
return a & (MODES_ICAO_CACHE_LEN-1);
}
//
//=========================================================================
//
// Add the specified entry to the cache of recently seen ICAO addresses.
// Note that we also add a timestamp so that we can make sure that the
// entry is only valid for MODES_ICAO_CACHE_TTL seconds.
//
void addRecentlySeenICAOAddr(uint32_t addr) {
uint32_t h = ICAOCacheHashAddress(addr);
Modes.icao_cache[h*2] = addr;
Modes.icao_cache[h*2+1] = (uint32_t) time(NULL);
}
//
//=========================================================================
//
// Returns 1 if the specified ICAO address was seen in a DF format with
// proper checksum (not xored with address) no more than * MODES_ICAO_CACHE_TTL
// seconds ago. Otherwise returns 0.
//
int ICAOAddressWasRecentlySeen(uint32_t addr) {
uint32_t h = ICAOCacheHashAddress(addr);
uint32_t a = Modes.icao_cache[h*2];
uint32_t t = Modes.icao_cache[h*2+1];
uint64_t tn = time(NULL);
return ( (a) && (a == addr) && ( (tn - t) <= MODES_ICAO_CACHE_TTL) );
}
//
//=========================================================================
//
// In the squawk (identity) field bits are interleaved as follows in
// (message bit 20 to bit 32):
//
// C1-A1-C2-A2-C4-A4-ZERO-B1-D1-B2-D2-B4-D4
//
// So every group of three bits A, B, C, D represent an integer from 0 to 7.
//
// The actual meaning is just 4 octal numbers, but we convert it into a hex
// number tha happens to represent the four octal numbers.
//
// For more info: http://en.wikipedia.org/wiki/Gillham_code
//
int decodeID13Field(int ID13Field) {
int hexGillham = 0;
if (ID13Field & 0x1000) {hexGillham |= 0x0010;} // Bit 12 = C1
if (ID13Field & 0x0800) {hexGillham |= 0x1000;} // Bit 11 = A1
if (ID13Field & 0x0400) {hexGillham |= 0x0020;} // Bit 10 = C2
if (ID13Field & 0x0200) {hexGillham |= 0x2000;} // Bit 9 = A2
if (ID13Field & 0x0100) {hexGillham |= 0x0040;} // Bit 8 = C4
if (ID13Field & 0x0080) {hexGillham |= 0x4000;} // Bit 7 = A4
//if (ID13Field & 0x0040) {hexGillham |= 0x0800;} // Bit 6 = X or M
if (ID13Field & 0x0020) {hexGillham |= 0x0100;} // Bit 5 = B1
if (ID13Field & 0x0010) {hexGillham |= 0x0001;} // Bit 4 = D1 or Q
if (ID13Field & 0x0008) {hexGillham |= 0x0200;} // Bit 3 = B2
if (ID13Field & 0x0004) {hexGillham |= 0x0002;} // Bit 2 = D2
if (ID13Field & 0x0002) {hexGillham |= 0x0400;} // Bit 1 = B4
if (ID13Field & 0x0001) {hexGillham |= 0x0004;} // Bit 0 = D4
return (hexGillham);
}
//
//=========================================================================
//
// Decode the 13 bit AC altitude field (in DF 20 and others).
// Returns the altitude, and set 'unit' to either MODES_UNIT_METERS or MDOES_UNIT_FEETS.
//
int decodeAC13Field(int AC13Field, int *unit) {
int m_bit = AC13Field & 0x0040; // set = meters, clear = feet
int q_bit = AC13Field & 0x0010; // set = 25 ft encoding, clear = Gillham Mode C encoding
if (!m_bit) {
*unit = MODES_UNIT_FEET;
if (q_bit) {
// N is the 11 bit integer resulting from the removal of bit Q and M
int n = ((AC13Field & 0x1F80) >> 2) |
((AC13Field & 0x0020) >> 1) |
(AC13Field & 0x000F);
// The final altitude is resulting number multiplied by 25, minus 1000.
return ((n * 25) - 1000);
} else {
// N is an 11 bit Gillham coded altitude
int n = ModeAToModeC(decodeID13Field(AC13Field));
if (n < -12) {n = 0;}
return (100 * n);
}
} else {
*unit = MODES_UNIT_METERS;
// TODO: Implement altitude when meter unit is selected
}
return 0;
}
//
//=========================================================================
//
// Decode the 12 bit AC altitude field (in DF 17 and others).
//
int decodeAC12Field(int AC12Field, int *unit) {
int q_bit = AC12Field & 0x10; // Bit 48 = Q
*unit = MODES_UNIT_FEET;
if (q_bit) {
/// N is the 11 bit integer resulting from the removal of bit Q at bit 4
int n = ((AC12Field & 0x0FE0) >> 1) |
(AC12Field & 0x000F);
// The final altitude is the resulting number multiplied by 25, minus 1000.
return ((n * 25) - 1000);
} else {
// Make N a 13 bit Gillham coded altitude by inserting M=0 at bit 6
int n = ((AC12Field & 0x0FC0) << 1) |
(AC12Field & 0x003F);
n = ModeAToModeC(decodeID13Field(n));
if (n < -12) {n = 0;}
return (100 * n);
}
}
//
//=========================================================================
//
// Decode the 7 bit ground movement field PWL exponential style scale
//
int decodeMovementField(int movement) {
int gspeed;
// Note : movement codes 0,125,126,127 are all invalid, but they are
// trapped for before this function is called.
if (movement > 123) gspeed = 199; // > 175kt
else if (movement > 108) gspeed = ((movement - 108) * 5) + 100;
else if (movement > 93) gspeed = ((movement - 93) * 2) + 70;
else if (movement > 38) gspeed = ((movement - 38) ) + 15;
else if (movement > 12) gspeed = ((movement - 11) >> 1) + 2;
else if (movement > 8) gspeed = ((movement - 6) >> 2) + 1;
else gspeed = 0;
return (gspeed);
}
//
//=========================================================================
//
// Capability table
char *ca_str[8] = {
/* 0 */ "Level 1 (Survillance Only)",
/* 1 */ "Level 2 (DF0,4,5,11)",
/* 2 */ "Level 3 (DF0,4,5,11,20,21)",
/* 3 */ "Level 4 (DF0,4,5,11,20,21,24)",
/* 4 */ "Level 2+3+4 (DF0,4,5,11,20,21,24,code7 - is on ground)",
/* 5 */ "Level 2+3+4 (DF0,4,5,11,20,21,24,code7 - is airborne)",
/* 6 */ "Level 2+3+4 (DF0,4,5,11,20,21,24,code7)",
/* 7 */ "Level 7 ???"
};
// DF 18 Control field table.
char *cf_str[8] = {
/* 0 */ "ADS-B ES/NT device with ICAO 24-bit address",
/* 1 */ "ADS-B ES/NT device with other address",
/* 2 */ "Fine format TIS-B",
/* 3 */ "Coarse format TIS-B",
/* 4 */ "TIS-B management message",
/* 5 */ "TIS-B relay of ADS-B message with other address",
/* 6 */ "ADS-B rebroadcast using DF-17 message format",
/* 7 */ "Reserved"
};
// Flight status table
char *fs_str[8] = {
/* 0 */ "Normal, Airborne",
/* 1 */ "Normal, On the ground",
/* 2 */ "ALERT, Airborne",
/* 3 */ "ALERT, On the ground",
/* 4 */ "ALERT & Special Position Identification. Airborne or Ground",
/* 5 */ "Special Position Identification. Airborne or Ground",
/* 6 */ "Value 6 is not assigned",
/* 7 */ "Value 7 is not assigned"
};
//
//=========================================================================
//
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 == 24 && mesub == 1)
mename = "Surface System Status";
else if (metype == 28 && mesub == 1)
mename = "Extended Squitter Aircraft Status (Emergency)";
else if (metype == 28 && mesub == 2)
mename = "Extended Squitter Aircraft Status (1090ES TCAS RA)";
else if (metype == 29 && (mesub == 0 || mesub == 1))
mename = "Target State and Status Message";
else if (metype == 31 && (mesub == 0 || mesub == 1))
mename = "Aircraft Operational Status Message";
return mename;
}
//
//=========================================================================
//
// Decode a raw Mode S message demodulated as a stream of bytes by detectModeS(),
// and split it into fields populating a modesMessage structure.
//
void decodeModesMessage(struct modesMessage *mm, unsigned char *msg) {
char *ais_charset = "?ABCDEFGHIJKLMNOPQRSTUVWXYZ????? ???????????????0123456789??????";
// Work on our local copy
memcpy(mm->msg, msg, MODES_LONG_MSG_BYTES);
msg = mm->msg;
// Get the message type ASAP as other operations depend on this
mm->msgtype = msg[0] >> 3; // Downlink Format
mm->msgbits = modesMessageLenByType(mm->msgtype);
mm->crc = modesChecksum(msg, mm->msgbits);
if ((mm->crc) && (Modes.nfix_crc) && ((mm->msgtype == 17) || (mm->msgtype == 18))) {
// if ((mm->crc) && (Modes.nfix_crc) && ((mm->msgtype == 11) || (mm->msgtype == 17))) {
//
// Fixing single bit errors in DF-11 is a bit dodgy because we have no way to
// know for sure if the crc is supposed to be 0 or not - it could be any value
// less than 80. Therefore, attempting to fix DF-11 errors can result in a
// multitude of possible crc solutions, only one of which is correct.
//
// We should probably perform some sanity checks on corrected DF-11's before
// using the results. Perhaps check the ICAO against known aircraft, and check
// IID against known good IID's. That's a TODO.
//
mm->correctedbits = fixBitErrors(msg, mm->msgbits, Modes.nfix_crc, mm->corrected);
// If we correct, validate ICAO addr to help filter birthday paradox solutions.
if (mm->correctedbits) {
uint32_t ulAddr = (msg[1] << 16) | (msg[2] << 8) | (msg[3]);
if (!ICAOAddressWasRecentlySeen(ulAddr))
mm->correctedbits = 0;
}
}
//
// Note that most of the other computation happens *after* we fix the
// single/two bit errors, otherwise we would need to recompute the fields again.
//
if (mm->msgtype == 11) { // DF 11
mm->iid = mm->crc;
mm->addr = (msg[1] << 16) | (msg[2] << 8) | (msg[3]);
mm->ca = (msg[0] & 0x07); // Responder capabilities
if ((mm->crcok = (0 == mm->crc))) {
// DF 11 : if crc == 0 try to populate our ICAO addresses whitelist.
addRecentlySeenICAOAddr(mm->addr);
} else if (mm->crc < 80) {
mm->crcok = ICAOAddressWasRecentlySeen(mm->addr);
if (mm->crcok) {
addRecentlySeenICAOAddr(mm->addr);
}
}
} else if (mm->msgtype == 17) { // DF 17
mm->addr = (msg[1] << 16) | (msg[2] << 8) | (msg[3]);
mm->ca = (msg[0] & 0x07); // Responder capabilities
if ((mm->crcok = (0 == mm->crc))) {
// DF 17 : if crc == 0 try to populate our ICAO addresses whitelist.
addRecentlySeenICAOAddr(mm->addr);
}
} else if (mm->msgtype == 18) { // DF 18
mm->addr = (msg[1] << 16) | (msg[2] << 8) | (msg[3]);
mm->ca = (msg[0] & 0x07); // Control Field
if ((mm->crcok = (0 == mm->crc))) {
// DF 18 : if crc == 0 try to populate our ICAO addresses whitelist.
addRecentlySeenICAOAddr(mm->addr);
}
} else { // All other DF's
// Compare the checksum with the whitelist of recently seen ICAO
// addresses. If it matches one, then declare the message as valid
mm->crcok = ICAOAddressWasRecentlySeen(mm->addr = mm->crc);
}
// Fields for DF0, DF16
if (mm->msgtype == 0 || mm->msgtype == 16) {
if (msg[0] & 0x04) { // VS Bit
mm->bFlags |= MODES_ACFLAGS_AOG_VALID | MODES_ACFLAGS_AOG;
} else {
mm->bFlags |= MODES_ACFLAGS_AOG_VALID;
}
}
// Fields for DF11, DF17
if (mm->msgtype == 11 || mm->msgtype == 17) {
if (mm->ca == 4) {
mm->bFlags |= MODES_ACFLAGS_AOG_VALID | MODES_ACFLAGS_AOG;
} else if (mm->ca == 5) {
mm->bFlags |= MODES_ACFLAGS_AOG_VALID;
}
}
// Fields for DF5, DF21 = Gillham encoded Squawk
if (mm->msgtype == 5 || mm->msgtype == 21) {
int ID13Field = ((msg[2] << 8) | msg[3]) & 0x1FFF;
if (ID13Field) {
mm->bFlags |= MODES_ACFLAGS_SQUAWK_VALID;
mm->modeA = decodeID13Field(ID13Field);
}
}
// Fields for DF0, DF4, DF16, DF20 13 bit altitude
if (mm->msgtype == 0 || mm->msgtype == 4 ||
mm->msgtype == 16 || mm->msgtype == 20) {
int AC13Field = ((msg[2] << 8) | msg[3]) & 0x1FFF;
if (AC13Field) { // Only attempt to decode if a valid (non zero) altitude is present
mm->bFlags |= MODES_ACFLAGS_ALTITUDE_VALID;
mm->altitude = decodeAC13Field(AC13Field, &mm->unit);
}
}
// Fields for DF4, DF5, DF20, DF21
if ((mm->msgtype == 4) || (mm->msgtype == 20) ||
(mm->msgtype == 5) || (mm->msgtype == 21)) {
mm->bFlags |= MODES_ACFLAGS_FS_VALID;
mm->fs = msg[0] & 7; // Flight status for DF4,5,20,21
if (mm->fs <= 3) {
mm->bFlags |= MODES_ACFLAGS_AOG_VALID;
if (mm->fs & 1)
{mm->bFlags |= MODES_ACFLAGS_AOG;}
}
}
// Fields for DF17, DF18_CF0, DF18_CF1, DF18_CF6 squitters
if ( (mm->msgtype == 17)
|| ((mm->msgtype == 18) && ((mm->ca == 0) || (mm->ca == 1) || (mm->ca == 6)) )) {
int metype = mm->metype = msg[4] >> 3; // Extended squitter message type
int mesub = mm->mesub = msg[4] & 7; // Extended squitter message subtype
// Decode the extended squitter message
if (metype >= 1 && metype <= 4) { // Aircraft Identification and Category
uint32_t chars;
mm->bFlags |= MODES_ACFLAGS_CALLSIGN_VALID;
chars = (msg[5] << 16) | (msg[6] << 8) | (msg[7]);
mm->flight[3] = ais_charset[chars & 0x3F]; chars = chars >> 6;
mm->flight[2] = ais_charset[chars & 0x3F]; chars = chars >> 6;
mm->flight[1] = ais_charset[chars & 0x3F]; chars = chars >> 6;
mm->flight[0] = ais_charset[chars & 0x3F];
chars = (msg[8] << 16) | (msg[9] << 8) | (msg[10]);
mm->flight[7] = ais_charset[chars & 0x3F]; chars = chars >> 6;
mm->flight[6] = ais_charset[chars & 0x3F]; chars = chars >> 6;
mm->flight[5] = ais_charset[chars & 0x3F]; chars = chars >> 6;
mm->flight[4] = ais_charset[chars & 0x3F];
mm->flight[8] = '\0';
} else if (metype >= 5 && metype <= 18) { // Position Message
mm->raw_latitude = ((msg[6] & 3) << 15) | (msg[7] << 7) | (msg[8] >> 1);
mm->raw_longitude = ((msg[8] & 1) << 16) | (msg[9] << 8) | (msg[10]);
mm->bFlags |= (mm->msg[6] & 0x04) ? MODES_ACFLAGS_LLODD_VALID
: MODES_ACFLAGS_LLEVEN_VALID;
if (metype >= 9) { // Airborne
int AC12Field = ((msg[5] << 4) | (msg[6] >> 4)) & 0x0FFF;
mm->bFlags |= MODES_ACFLAGS_AOG_VALID;
if (AC12Field) {// Only attempt to decode if a valid (non zero) altitude is present
mm->bFlags |= MODES_ACFLAGS_ALTITUDE_VALID;
mm->altitude = decodeAC12Field(AC12Field, &mm->unit);
}
} else { // Ground
int movement = ((msg[4] << 4) | (msg[5] >> 4)) & 0x007F;
mm->bFlags |= MODES_ACFLAGS_AOG_VALID | MODES_ACFLAGS_AOG;
if ((movement) && (movement < 125)) {
mm->bFlags |= MODES_ACFLAGS_SPEED_VALID;
mm->velocity = decodeMovementField(movement);
}
if (msg[5] & 0x08) {
mm->bFlags |= MODES_ACFLAGS_HEADING_VALID;
mm->heading = ((((msg[5] << 4) | (msg[6] >> 4)) & 0x007F) * 45) >> 4;
}
}
} else if (metype == 19) { // Airborne Velocity Message
// Presumably airborne if we get an Airborne Velocity Message
mm->bFlags |= MODES_ACFLAGS_AOG_VALID;
if ( (mesub >= 1) && (mesub <= 4) ) {