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dump1090/mode_s.c
MalcolmRobb bff92c4ad7 Merge pull request #36 from mutability/improve-phase-enhancement 
Rearrange phase enhancement so that it handles phase errors in both directions
2014-10-31 13:01:59 +00:00

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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 (Surveillance 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"
};
// Emergency state table
// from https://www.ll.mit.edu/mission/aviation/publications/publication-files/atc-reports/Grappel_2007_ATC-334_WW-15318.pdf
// and 1090-DO-260B_FRAC
char *es_str[8] = {
/* 0 */ "No emergency",
/* 1 */ "General emergency (squawk 7700)",
/* 2 */ "Lifeguard/Medical",
/* 3 */ "Minimum fuel",
/* 4 */ "No communications (squawk 7600)",
/* 5 */ "Unlawful interference (squawk 7500)",
/* 6 */ "Downed Aircraft",
/* 7 */ "Reserved"
};
//
//=========================================================================
//
char *getMEDescription(int metype, int mesub) {
char *mename = "Unknown";
if (metype >= 1 && metype <= 4)
mename = "Aircraft Identification and Category";
else if (metype >= 5 && metype <= 8)
mename = "Surface Position";
else if (metype >= 9 && metype <= 18)
mename = "Airborne Position (Baro Altitude)";
else if (metype == 19 && mesub >=1 && mesub <= 4)
mename = "Airborne Velocity";
else if (metype >= 20 && metype <= 22)
mename = "Airborne Position (GNSS Height)";
else if (metype == 23 && mesub == 0)
mename = "Test Message";
else if (metype == 23 && mesub == 7)
mename = "Test Message -- Squawk";
else if (metype == 24 && mesub == 1)
mename = "Surface System Status";
else if (metype == 28 && mesub == 1)
mename = "Extended Squitter Aircraft Status (Emergency)";
else if (metype == 28 && mesub == 2)
mename = "Extended Squitter Aircraft Status (1090ES TCAS RA)";
else if (metype == 29 && (mesub == 0 || mesub == 1))
mename = "Target State and Status Message";
else if (metype == 31 && (mesub == 0 || mesub == 1))
mename = "Aircraft Operational Status Message";
return mename;
}
//
//=========================================================================
//
// 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);
}
// If we're checking CRC and the CRC is invalid, then we can't trust any
// of the data contents, so save time and give up now.
if ((Modes.check_crc) && (!mm->crcok) && (!mm->correctedbits)) { return;}
// 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 = (metype == 29 ? ((msg[4]&6)>>1) : (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 == 19) { // Airborne Velocity Message
// Presumably airborne if we get an Airborne Velocity Message
mm->bFlags |= MODES_ACFLAGS_AOG_VALID;
if ( (mesub >= 1) && (mesub <= 4) ) {
int vert_rate = ((msg[8] & 0x07) << 6) | (msg[9] >> 2);
if (vert_rate) {
--vert_rate;
if (msg[8] & 0x08)
{vert_rate = 0 - vert_rate;}
mm->vert_rate = vert_rate * 64;
mm->bFlags |= MODES_ACFLAGS_VERTRATE_VALID;
}
}
if ((mesub == 1) || (mesub == 2)) {
int ew_raw = ((msg[5] & 0x03) << 8) | msg[6];
int ew_vel = ew_raw - 1;
int ns_raw = ((msg[7] & 0x7F) << 3) | (msg[8] >> 5);
int ns_vel = ns_raw - 1;
if (mesub == 2) { // If (supersonic) unit is 4 kts
ns_vel = ns_vel << 2;
ew_vel = ew_vel << 2;
}
if (ew_raw) { // Do East/West
mm->bFlags |= MODES_ACFLAGS_EWSPEED_VALID;
if (msg[5] & 0x04)
{ew_vel = 0 - ew_vel;}
mm->ew_velocity = ew_vel;
}
if (ns_raw) { // Do North/South
mm->bFlags |= MODES_ACFLAGS_NSSPEED_VALID;
if (msg[7] & 0x80)
{ns_vel = 0 - ns_vel;}
mm->ns_velocity = ns_vel;
}
if (ew_raw && ns_raw) {
// Compute velocity and angle from the two speed components
mm->bFlags |= (MODES_ACFLAGS_SPEED_VALID | MODES_ACFLAGS_HEADING_VALID | MODES_ACFLAGS_NSEWSPD_VALID);
mm->velocity = (int) sqrt((ns_vel * ns_vel) + (ew_vel * ew_vel));
if (mm->velocity) {
mm->heading = (int) (atan2(ew_vel, ns_vel) * 180.0 / M_PI);
// We don't want negative values but a 0-360 scale
if (mm->heading < 0) mm->heading += 360;
}
}
} else if (mesub == 3 || mesub == 4) {
int airspeed = ((msg[7] & 0x7f) << 3) | (msg[8] >> 5);
if (airspeed) {
mm->bFlags |= MODES_ACFLAGS_SPEED_VALID;
--airspeed;
if (mesub == 4) // If (supersonic) unit is 4 kts
{airspeed = airspeed << 2;}
mm->velocity = airspeed;
}
if (msg[5] & 0x04) {
mm->bFlags |= MODES_ACFLAGS_HEADING_VALID;
mm->heading = ((((msg[5] & 0x03) << 8) | msg[6]) * 45) >> 7;
}
}
} else if (metype >= 5 && metype <= 22) { // 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 == 23) { // Test metype squawk field
if (mesub == 7) { // (see 1090-WP-15-20)
int ID13Field = (((msg[5] << 8) | msg[6]) & 0xFFF1)>>3;
if (ID13Field) {
mm->bFlags |= MODES_ACFLAGS_SQUAWK_VALID;
mm->modeA = decodeID13Field(ID13Field);
}
}
} else if (metype == 24) { // Reserved for Surface System Status
} else if (metype == 28) { // Extended Squitter Aircraft Status
if (mesub == 1) { // Emergency status squawk field
int ID13Field = (((msg[5] << 8) | msg[6]) & 0x1FFF);
if (ID13Field) {
mm->bFlags |= MODES_ACFLAGS_SQUAWK_VALID;
mm->modeA = decodeID13Field(ID13Field);
}
}
} else if (metype == 29) { // Aircraft Trajectory Intent
} else if (metype == 30) { // Aircraft Operational Coordination
} else if (metype == 31) { // Aircraft Operational Status
} else { // Other metypes
}
}
// Fields for DF20, DF21 Comm-B
if ((mm->msgtype == 20) || (mm->msgtype == 21)){
if (msg[4] == 0x20) { // Aircraft Identification
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 {
}
}
}
//
//=========================================================================
//
// This function gets a decoded Mode S Message and prints it on the screen
// in a human readable format.
//
void displayModesMessage(struct modesMessage *mm) {
int j;
unsigned char * pTimeStamp;
// Handle only addresses mode first.
if (Modes.onlyaddr) {
printf("%06x\n", mm->addr);
return; // Enough for --onlyaddr mode
}
// Show the raw message.
if (Modes.mlat && mm->timestampMsg) {
printf("@");
pTimeStamp = (unsigned char *) &mm->timestampMsg;
for (j=5; j>=0;j--) {
printf("%02X",pTimeStamp[j]);
}
} else
printf("*");
for (j = 0; j < mm->msgbits/8; j++) printf("%02x", mm->msg[j]);
printf(";\n");
if (Modes.raw) {
fflush(stdout); // Provide data to the reader ASAP
return; // Enough for --raw mode
}
if (mm->msgtype < 32)
printf("CRC: %06x (%s)\n", (int)mm->crc, mm->crcok ? "ok" : "wrong");
if (mm->correctedbits != 0)
printf("No. of bit errors fixed: %d\n", mm->correctedbits);
if (mm->msgtype == 0) { // DF 0
printf("DF 0: Short Air-Air Surveillance.\n");
printf(" VS : %s\n", (mm->msg[0] & 0x04) ? "Ground" : "Airborne");
printf(" CC : %d\n", ((mm->msg[0] & 0x02) >> 1));
printf(" SL : %d\n", ((mm->msg[1] & 0xE0) >> 5));
printf(" Altitude : %d %s\n", mm->altitude,
(mm->unit == MODES_UNIT_METERS) ? "meters" : "feet");
printf(" ICAO Address : %06x\n", mm->addr);
} else if (mm->msgtype == 4 || mm->msgtype == 20) {
printf("DF %d: %s, Altitude Reply.\n", mm->msgtype,
(mm->msgtype == 4) ? "Surveillance" : "Comm-B");
printf(" Flight Status : %s\n", fs_str[mm->fs]);
printf(" DR : %d\n", ((mm->msg[1] >> 3) & 0x1F));
printf(" UM : %d\n", (((mm->msg[1] & 7) << 3) | (mm->msg[2] >> 5)));
printf(" Altitude : %d %s\n", mm->altitude,
(mm->unit == MODES_UNIT_METERS) ? "meters" : "feet");
printf(" ICAO Address : %06x\n", mm->addr);
if (mm->msgtype == 20) {
printf(" Comm-B BDS : %x\n", mm->msg[4]);
// Decode the extended squitter message
if ( mm->msg[4] == 0x20) { // BDS 2,0 Aircraft identification
printf(" BDS 2,0 Aircraft Identification : %s\n", mm->flight);
/*
} else if ( mm->msg[4] == 0x10) { // BDS 1,0 Datalink Capability report
printf(" BDS 1,0 Datalink Capability report\n");
} else if ( mm->msg[4] == 0x30) { // BDS 3,0 ACAS Active Resolution Advisory
printf(" BDS 3,0 ACAS Active Resolution Advisory\n");
} else if ((mm->msg[4] >> 3) == 28) { // BDS 6,1 Extended Squitter Emergency/Priority Status
printf(" BDS 6,1 Emergency/Priority Status\n");
} else if ((mm->msg[4] >> 3) == 29) { // BDS 6,2 Target State and Status
printf(" BDS 6,2 Target State and Status\n");
} else if ((mm->msg[4] >> 3) == 31) { // BDS 6,5 Extended Squitter Aircraft Operational Status
printf(" BDS 6,5 Aircraft Operational Status\n");
*/
}
}
} else if (mm->msgtype == 5 || mm->msgtype == 21) {
printf("DF %d: %s, Identity Reply.\n", mm->msgtype,
(mm->msgtype == 5) ? "Surveillance" : "Comm-B");
printf(" Flight Status : %s\n", fs_str[mm->fs]);
printf(" DR : %d\n", ((mm->msg[1] >> 3) & 0x1F));
printf(" UM : %d\n", (((mm->msg[1] & 7) << 3) | (mm->msg[2] >> 5)));
printf(" Squawk : %04x\n", mm->modeA);
printf(" ICAO Address : %06x\n", mm->addr);
if (mm->msgtype == 21) {
printf(" Comm-B BDS : %x\n", mm->msg[4]);
// Decode the extended squitter message
if ( mm->msg[4] == 0x20) { // BDS 2,0 Aircraft identification
printf(" BDS 2,0 Aircraft Identification : %s\n", mm->flight);
/*
} else if ( mm->msg[4] == 0x10) { // BDS 1,0 Datalink Capability report
printf(" BDS 1,0 Datalink Capability report\n");
} else if ( mm->msg[4] == 0x30) { // BDS 3,0 ACAS Active Resolution Advisory
printf(" BDS 3,0 ACAS Active Resolution Advisory\n");
} else if ((mm->msg[4] >> 3) == 28) { // BDS 6,1 Extended Squitter Emergency/Priority Status
printf(" BDS 6,1 Emergency/Priority Status\n");
} else if ((mm->msg[4] >> 3) == 29) { // BDS 6,2 Target State and Status
printf(" BDS 6,2 Target State and Status\n");
} else if ((mm->msg[4] >> 3) == 31) { // BDS 6,5 Extended Squitter Aircraft Operational Status
printf(" BDS 6,5 Aircraft Operational Status\n");
*/
}
}
} else if (mm->msgtype == 11) { // DF 11
printf("DF 11: All Call Reply.\n");
printf(" Capability : %d (%s)\n", mm->ca, ca_str[mm->ca]);
printf(" ICAO Address: %06x\n", mm->addr);
if (mm->iid > 16)
{printf(" IID : SI-%02d\n", mm->iid-16);}
else
{printf(" IID : II-%02d\n", mm->iid);}
} else if (mm->msgtype == 16) { // DF 16
printf("DF 16: Long Air to Air ACAS\n");
printf(" VS : %s\n", (mm->msg[0] & 0x04) ? "Ground" : "Airborne");
printf(" CC : %d\n", ((mm->msg[0] & 0x02) >> 1));
printf(" SL : %d\n", ((mm->msg[1] & 0xE0) >> 5));
printf(" Altitude : %d %s\n", mm->altitude,
(mm->unit == MODES_UNIT_METERS) ? "meters" : "feet");
printf(" ICAO Address : %06x\n", mm->addr);
} else if (mm->msgtype == 17) { // DF 17
printf("DF 17: ADS-B message.\n");
printf(" Capability : %d (%s)\n", mm->ca, ca_str[mm->ca]);
printf(" ICAO Address : %06x\n", mm->addr);
printf(" Extended Squitter Type: %d\n", mm->metype);
printf(" Extended Squitter Sub : %d\n", mm->mesub);
printf(" Extended Squitter Name: %s\n", getMEDescription(mm->metype, mm->mesub));
// Decode the extended squitter message
if (mm->metype >= 1 && mm->metype <= 4) { // Aircraft identification
printf(" Aircraft Type : %c%d\n", ('A' + 4 - mm->metype), mm->mesub);
printf(" Identification : %s\n", mm->flight);
} else if (mm->metype == 19) { // Airborne Velocity
if (mm->mesub == 1 || mm->mesub == 2) {
printf(" EW status : %s\n", (mm->bFlags & MODES_ACFLAGS_EWSPEED_VALID) ? "Valid" : "Unavailable");
printf(" EW velocity : %d\n", mm->ew_velocity);
printf(" NS status : %s\n", (mm->bFlags & MODES_ACFLAGS_NSSPEED_VALID) ? "Valid" : "Unavailable");
printf(" NS velocity : %d\n", mm->ns_velocity);
printf(" Vertical status : %s\n", (mm->bFlags & MODES_ACFLAGS_VERTRATE_VALID) ? "Valid" : "Unavailable");
printf(" Vertical rate src : %d\n", ((mm->msg[8] >> 4) & 1));
printf(" Vertical rate : %d\n", mm->vert_rate);
} else if (mm->mesub == 3 || mm->mesub == 4) {
printf(" Heading status : %s\n", (mm->bFlags & MODES_ACFLAGS_HEADING_VALID) ? "Valid" : "Unavailable");
printf(" Heading : %d\n", mm->heading);
printf(" Airspeed status : %s\n", (mm->bFlags & MODES_ACFLAGS_SPEED_VALID) ? "Valid" : "Unavailable");
printf(" Airspeed : %d\n", mm->velocity);
printf(" Vertical status : %s\n", (mm->bFlags & MODES_ACFLAGS_VERTRATE_VALID) ? "Valid" : "Unavailable");
printf(" Vertical rate src : %d\n", ((mm->msg[8] >> 4) & 1));
printf(" Vertical rate : %d\n", mm->vert_rate);
} else {
printf(" Unrecognized ME subtype: %d subtype: %d\n", mm->metype, mm->mesub);
}
} else if (mm->metype >= 5 && mm->metype <= 22) { // Airborne position Baro
printf(" F flag : %s\n", (mm->msg[6] & 0x04) ? "odd" : "even");
printf(" T flag : %s\n", (mm->msg[6] & 0x08) ? "UTC" : "non-UTC");
printf(" Altitude : %d feet\n", mm->altitude);
if (mm->bFlags & MODES_ACFLAGS_LATLON_VALID) {
printf(" Latitude : %f\n", mm->fLat);
printf(" Longitude: %f\n", mm->fLon);
} else {
printf(" Latitude : %d (not decoded)\n", mm->raw_latitude);
printf(" Longitude: %d (not decoded)\n", mm->raw_longitude);
}
} else if (mm->metype == 28) { // Extended Squitter Aircraft Status
if (mm->mesub == 1) {
printf(" Emergency State: %s\n", es_str[(mm->msg[5] & 0xE0) >> 5]);
printf(" Squawk: %04x\n", mm->modeA);
} else {
printf(" Unrecognized ME subtype: %d subtype: %d\n", mm->metype, mm->mesub);
}
} else if (mm->metype == 23) { // Test Message
if (mm->mesub == 7) {
printf(" Squawk: %04x\n", mm->modeA);
} else {
printf(" Unrecognized ME subtype: %d subtype: %d\n", mm->metype, mm->mesub);
}
} else {
printf(" Unrecognized ME type: %d subtype: %d\n", mm->metype, mm->mesub);
}
} else if (mm->msgtype == 18) { // DF 18
printf("DF 18: Extended Squitter.\n");
printf(" Control Field : %d (%s)\n", mm->ca, cf_str[mm->ca]);
if ((mm->ca == 0) || (mm->ca == 1) || (mm->ca == 6)) {
if (mm->ca == 1) {
printf(" Other Address : %06x\n", mm->addr);
} else {
printf(" ICAO Address : %06x\n", mm->addr);
}
printf(" Extended Squitter Type: %d\n", mm->metype);
printf(" Extended Squitter Sub : %d\n", mm->mesub);
printf(" Extended Squitter Name: %s\n", getMEDescription(mm->metype, mm->mesub));
// Decode the extended squitter message
if (mm->metype >= 1 && mm->metype <= 4) { // Aircraft identification
printf(" Aircraft Type : %c%d\n", ('A' + 4 - mm->metype), mm->mesub);
printf(" Identification : %s\n", mm->flight);
} else if (mm->metype == 19) { // Airborne Velocity
if (mm->mesub == 1 || mm->mesub == 2) {
printf(" EW status : %s\n", (mm->bFlags & MODES_ACFLAGS_EWSPEED_VALID) ? "Valid" : "Unavailable");
printf(" EW velocity : %d\n", mm->ew_velocity);
printf(" NS status : %s\n", (mm->bFlags & MODES_ACFLAGS_NSSPEED_VALID) ? "Valid" : "Unavailable");
printf(" NS velocity : %d\n", mm->ns_velocity);
printf(" Vertical status : %s\n", (mm->bFlags & MODES_ACFLAGS_VERTRATE_VALID) ? "Valid" : "Unavailable");
printf(" Vertical rate src : %d\n", ((mm->msg[8] >> 4) & 1));
printf(" Vertical rate : %d\n", mm->vert_rate);
} else if (mm->mesub == 3 || mm->mesub == 4) {
printf(" Heading status : %s\n", (mm->bFlags & MODES_ACFLAGS_HEADING_VALID) ? "Valid" : "Unavailable");
printf(" Heading : %d\n", mm->heading);
printf(" Airspeed status : %s\n", (mm->bFlags & MODES_ACFLAGS_SPEED_VALID) ? "Valid" : "Unavailable");
printf(" Airspeed : %d\n", mm->velocity);
printf(" Vertical status : %s\n", (mm->bFlags & MODES_ACFLAGS_VERTRATE_VALID) ? "Valid" : "Unavailable");
printf(" Vertical rate src : %d\n", ((mm->msg[8] >> 4) & 1));
printf(" Vertical rate : %d\n", mm->vert_rate);
} else {
printf(" Unrecognized ME subtype: %d subtype: %d\n", mm->metype, mm->mesub);
}
} else if (mm->metype >= 5 && mm->metype <= 22) { // Ground or Airborne position, Baro or GNSS
printf(" F flag : %s\n", (mm->msg[6] & 0x04) ? "odd" : "even");
printf(" T flag : %s\n", (mm->msg[6] & 0x08) ? "UTC" : "non-UTC");
printf(" Altitude : %d feet\n", mm->altitude);
if (mm->bFlags & MODES_ACFLAGS_LATLON_VALID) {
printf(" Latitude : %f\n", mm->fLat);
printf(" Longitude: %f\n", mm->fLon);
} else {
printf(" Latitude : %d (not decoded)\n", mm->raw_latitude);
printf(" Longitude: %d (not decoded)\n", mm->raw_longitude);
}
} else {
printf(" Unrecognized ME type: %d subtype: %d\n", mm->metype, mm->mesub);
}
}
} else if (mm->msgtype == 19) { // DF 19
printf("DF 19: Military Extended Squitter.\n");
} else if (mm->msgtype == 22) { // DF 22
printf("DF 22: Military Use.\n");
} else if (mm->msgtype == 24) { // DF 24
printf("DF 24: Comm D Extended Length Message.\n");
} else if (mm->msgtype == 32) { // DF 32 is special code we use for Mode A/C
printf("SSR : Mode A/C Reply.\n");
if (mm->fs & 0x0080) {
printf(" Mode A : %04x IDENT\n", mm->modeA);
} else {
printf(" Mode A : %04x\n", mm->modeA);
if (mm->bFlags & MODES_ACFLAGS_ALTITUDE_VALID)
{printf(" Mode C : %d feet\n", mm->altitude);}
}
} else {
printf("DF %d: Unknown DF Format.\n", mm->msgtype);
}
printf("\n");
}
//
//=========================================================================
//
// Turn I/Q samples pointed by Modes.data into the magnitude vector
// pointed by Modes.magnitude.
//
void computeMagnitudeVector(uint16_t *p) {
uint16_t *m = &Modes.magnitude[MODES_PREAMBLE_SAMPLES+MODES_LONG_MSG_SAMPLES];
uint32_t j;
memcpy(Modes.magnitude,&Modes.magnitude[MODES_ASYNC_BUF_SAMPLES], MODES_PREAMBLE_SIZE+MODES_LONG_MSG_SIZE);
// Compute the magnitudo vector. It's just SQRT(I^2 + Q^2), but
// we rescale to the 0-255 range to exploit the full resolution.
for (j = 0; j < MODES_ASYNC_BUF_SAMPLES; j ++) {
*m++ = Modes.maglut[*p++];
}
}
//
//=========================================================================
//
// 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
//
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;
}
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.
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 (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 detectModeS(uint16_t *m, uint32_t mlen) {
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;
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, sigStrength;
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.crcok =
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 = Modes.timestampBlk + ((j+1) * 6);
// Decode the received message
decodeModeAMessage(&mm, ModeA);
// Pass data to the next layer
useModesMessage(&mm);
j += MODEAC_MSG_SAMPLES;
Modes.stat_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.stat_valid_preamble++;
}
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]);
Modes.stat_out_of_phase++;
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
sigStrength = (pPreamble[0]-pPreamble[1])
+ (pPreamble[2]-pPreamble[3])
+ (pPreamble[7]-pPreamble[6])
+ (pPreamble[9]-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) {sigStrength += (a-b);}}
else if (a < b)
{/*theByte |= 0;*/ if (i < 56) {sigStrength += (b-a);}}
else 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
Modes.stat_DF_Len_Corrected++;
} 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
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;
Modes.stat_DF_Type_Corrected++;
errors--; // decrease the error count so we attempt to use the modified DF.
}
}
}
// We measured signal strength over the first 56 bits. Don't forget to add 4
// for the preamble samples, so round up and divide by 60.
sigStrength = (sigStrength + 29) / 60;
// 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)
&& (sigStrength > MODES_MSG_SQUELCH_LEVEL)
&& (errors <= MODES_MSG_ENCODER_ERRS) ) {
// Set initial mm structure details
mm.timestampMsg = Modes.timestampBlk + (j*6);
sigStrength = (sigStrength + 0x7F) >> 8;
mm.signalLevel = ((sigStrength < 255) ? sigStrength : 255);
mm.phase_corrected = use_correction;
// Decode the received message
decodeModesMessage(&mm, msg);
// Update statistics
if (Modes.stats) {
if (mm.crcok || use_correction || mm.correctedbits) {
if (use_correction) {
switch (errors) {
case 0: {Modes.stat_ph_demodulated0++; break;}
case 1: {Modes.stat_ph_demodulated1++; break;}
case 2: {Modes.stat_ph_demodulated2++; break;}
default:{Modes.stat_ph_demodulated3++; break;}
}
} else {
switch (errors) {
case 0: {Modes.stat_demodulated0++; break;}
case 1: {Modes.stat_demodulated1++; break;}
case 2: {Modes.stat_demodulated2++; break;}
default:{Modes.stat_demodulated3++; break;}
}
}
if (mm.correctedbits == 0) {
if (use_correction) {
if (mm.crcok) {Modes.stat_ph_goodcrc++;}
else {Modes.stat_ph_badcrc++;}
} else {
if (mm.crcok) {Modes.stat_goodcrc++;}
else {Modes.stat_badcrc++;}
}
} else if (use_correction) {
Modes.stat_ph_badcrc++;
Modes.stat_ph_fixed++;
if ( (mm.correctedbits)
&& (mm.correctedbits <= MODES_MAX_BITERRORS) ) {
Modes.stat_ph_bit_fix[mm.correctedbits-1] += 1;
}
} else {
Modes.stat_badcrc++;
Modes.stat_fixed++;
if ( (mm.correctedbits)
&& (mm.correctedbits <= MODES_MAX_BITERRORS) ) {
Modes.stat_bit_fix[mm.correctedbits-1] += 1;
}
}
}
}
// 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 &&
(!mm.crcok || mm.correctedbits != 0))
dumpRawMessage("Decoded with bad CRC", msg, m, j);
else if (Modes.debug & MODES_DEBUG_GOODCRC && mm.crcok &&
mm.correctedbits == 0)
dumpRawMessage("Decoded with good CRC", msg, m, j);
}
// Skip this message if we are sure it's fine
if (mm.crcok) {
j += (MODES_PREAMBLE_US+msglen)*2 - 1;
}
// Pass data to the next layer
useModesMessage(&mm);
} else {
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 enabled, necessary and possible.
if (Modes.phase_enhance && !mm.crcok && !mm.correctedbits && !use_correction && j && detectOutOfPhase(pPreamble)) {
use_correction = 1; j--;
} else {
use_correction = 0;
}
}
//Send any remaining partial raw buffers now
if (Modes.rawOutUsed || Modes.beastOutUsed)
{
Modes.net_output_raw_rate_count++;
if (Modes.net_output_raw_rate_count > Modes.net_output_raw_rate)
{
if (Modes.rawOutUsed) {
modesSendAllClients(Modes.ros, Modes.rawOut, Modes.rawOutUsed);
Modes.rawOutUsed = 0;
}
if (Modes.beastOutUsed) {
modesSendAllClients(Modes.bos, Modes.beastOut, Modes.beastOutUsed);
Modes.beastOutUsed = 0;
}
Modes.net_output_raw_rate_count = 0;
}
}
else if ( (Modes.net)
&& (Modes.net_heartbeat_rate)
&& ((++Modes.net_heartbeat_count) > Modes.net_heartbeat_rate) ) {
//
// We haven't received any Mode A/C/S messages for some time. To try and keep any TCP
// links alive, send a null frame. This will help stop any routers discarding our TCP
// link which will cause an un-recoverable link error if/when a real frame arrives.
//
// Fudge up a null message
memset(&mm, 0, sizeof(mm));
mm.msgbits = MODES_SHORT_MSG_BITS;
mm.timestampMsg = Modes.timestampBlk;
// Feed output clients
modesQueueOutput(&mm);
// Reset the heartbeat counter
Modes.net_heartbeat_count = 0;
}
}
//
//=========================================================================
//
// When a new message is available, because it was decoded from the RTL device,
// file, or received in the TCP input port, or any other way we can receive a
// decoded message, we call this function in order to use the message.
//
// Basically this function passes a raw message to the upper layers for further
// processing and visualization
//
void useModesMessage(struct modesMessage *mm) {
if ((Modes.check_crc == 0) || (mm->crcok) || (mm->correctedbits)) { // not checking, ok or fixed
// Always track aircraft
interactiveReceiveData(mm);
// In non-interactive non-quiet mode, display messages on standard output
if (!Modes.interactive && !Modes.quiet) {
displayModesMessage(mm);
}
// Feed output clients
if (Modes.net) {modesQueueOutput(mm);}
// Heartbeat not required whilst we're seeing real messages
Modes.net_heartbeat_count = 0;
}
}
//
//=========================================================================
//
// Always positive MOD operation, used for CPR decoding.
//
int cprModFunction(int a, int b) {
int res = a % b;
if (res < 0) res += b;
return res;
}
//
//=========================================================================
//
// The NL function uses the precomputed table from 1090-WP-9-14
//
int cprNLFunction(double lat) {
if (lat < 0) lat = -lat; // Table is simmetric about the equator
if (lat < 10.47047130) return 59;
if (lat < 14.82817437) return 58;
if (lat < 18.18626357) return 57;
if (lat < 21.02939493) return 56;
if (lat < 23.54504487) return 55;
if (lat < 25.82924707) return 54;
if (lat < 27.93898710) return 53;
if (lat < 29.91135686) return 52;
if (lat < 31.77209708) return 51;
if (lat < 33.53993436) return 50;
if (lat < 35.22899598) return 49;
if (lat < 36.85025108) return 48;
if (lat < 38.41241892) return 47;
if (lat < 39.92256684) return 46;
if (lat < 41.38651832) return 45;
if (lat < 42.80914012) return 44;
if (lat < 44.19454951) return 43;
if (lat < 45.54626723) return 42;
if (lat < 46.86733252) return 41;
if (lat < 48.16039128) return 40;
if (lat < 49.42776439) return 39;
if (lat < 50.67150166) return 38;
if (lat < 51.89342469) return 37;
if (lat < 53.09516153) return 36;
if (lat < 54.27817472) return 35;
if (lat < 55.44378444) return 34;
if (lat < 56.59318756) return 33;
if (lat < 57.72747354) return 32;
if (lat < 58.84763776) return 31;
if (lat < 59.95459277) return 30;
if (lat < 61.04917774) return 29;
if (lat < 62.13216659) return 28;
if (lat < 63.20427479) return 27;
if (lat < 64.26616523) return 26;
if (lat < 65.31845310) return 25;
if (lat < 66.36171008) return 24;
if (lat < 67.39646774) return 23;
if (lat < 68.42322022) return 22;
if (lat < 69.44242631) return 21;
if (lat < 70.45451075) return 20;
if (lat < 71.45986473) return 19;
if (lat < 72.45884545) return 18;
if (lat < 73.45177442) return 17;
if (lat < 74.43893416) return 16;
if (lat < 75.42056257) return 15;
if (lat < 76.39684391) return 14;
if (lat < 77.36789461) return 13;
if (lat < 78.33374083) return 12;
if (lat < 79.29428225) return 11;
if (lat < 80.24923213) return 10;
if (lat < 81.19801349) return 9;
if (lat < 82.13956981) return 8;
if (lat < 83.07199445) return 7;
if (lat < 83.99173563) return 6;
if (lat < 84.89166191) return 5;
if (lat < 85.75541621) return 4;
if (lat < 86.53536998) return 3;
if (lat < 87.00000000) return 2;
else return 1;
}
//
//=========================================================================
//
int cprNFunction(double lat, int fflag) {
int nl = cprNLFunction(lat) - (fflag ? 1 : 0);
if (nl < 1) nl = 1;
return nl;
}
//
//=========================================================================
//
double cprDlonFunction(double lat, int fflag, int surface) {
return (surface ? 90.0 : 360.0) / cprNFunction(lat, fflag);
}
//
//=========================================================================
//
// This algorithm comes from:
// http://www.lll.lu/~edward/edward/adsb/DecodingADSBposition.html.
//
// A few remarks:
// 1) 131072 is 2^17 since CPR latitude and longitude are encoded in 17 bits.
//
int decodeCPR(struct aircraft *a, int fflag, int surface) {
double AirDlat0 = (surface ? 90.0 : 360.0) / 60.0;
double AirDlat1 = (surface ? 90.0 : 360.0) / 59.0;
double lat0 = a->even_cprlat;
double lat1 = a->odd_cprlat;
double lon0 = a->even_cprlon;
double lon1 = a->odd_cprlon;
// Compute the Latitude Index "j"
int j = (int) floor(((59*lat0 - 60*lat1) / 131072) + 0.5);
double rlat0 = AirDlat0 * (cprModFunction(j,60) + lat0 / 131072);
double rlat1 = AirDlat1 * (cprModFunction(j,59) + lat1 / 131072);
time_t now = time(NULL);
double surface_rlat = MODES_USER_LATITUDE_DFLT;
double surface_rlon = MODES_USER_LONGITUDE_DFLT;
if (surface) {
// If we're on the ground, make sure we have a (likely) valid Lat/Lon
if ((a->bFlags & MODES_ACFLAGS_LATLON_VALID) && (((int)(now - a->seenLatLon)) < Modes.interactive_display_ttl)) {
surface_rlat = a->lat;
surface_rlon = a->lon;
} else if (Modes.bUserFlags & MODES_USER_LATLON_VALID) {
surface_rlat = Modes.fUserLat;
surface_rlon = Modes.fUserLon;
} else {
// No local reference, give up
return (-1);
}
rlat0 += floor(surface_rlat / 90.0) * 90.0; // Move from 1st quadrant to our quadrant
rlat1 += floor(surface_rlat / 90.0) * 90.0;
} else {
if (rlat0 >= 270) rlat0 -= 360;
if (rlat1 >= 270) rlat1 -= 360;
}
// Check to see that the latitude is in range: -90 .. +90
if (rlat0 < -90 || rlat0 > 90 || rlat1 < -90 || rlat1 > 90)
return (-1);
// Check that both are in the same latitude zone, or abort.
if (cprNLFunction(rlat0) != cprNLFunction(rlat1))
return (-1);
// Compute ni and the Longitude Index "m"
if (fflag) { // Use odd packet.
int ni = cprNFunction(rlat1,1);
int m = (int) floor((((lon0 * (cprNLFunction(rlat1)-1)) -
(lon1 * cprNLFunction(rlat1))) / 131072.0) + 0.5);
a->lon = cprDlonFunction(rlat1, 1, surface) * (cprModFunction(m, ni)+lon1/131072);
a->lat = rlat1;
} else { // Use even packet.
int ni = cprNFunction(rlat0,0);
int m = (int) floor((((lon0 * (cprNLFunction(rlat0)-1)) -
(lon1 * cprNLFunction(rlat0))) / 131072) + 0.5);
a->lon = cprDlonFunction(rlat0, 0, surface) * (cprModFunction(m, ni)+lon0/131072);
a->lat = rlat0;
}
if (surface) {
a->lon += floor(surface_rlon / 90.0) * 90.0; // Move from 1st quadrant to our quadrant
} else if (a->lon > 180) {
a->lon -= 360;
}
a->seenLatLon = a->seen;
a->timestampLatLon = a->timestamp;
a->bFlags |= (MODES_ACFLAGS_LATLON_VALID | MODES_ACFLAGS_LATLON_REL_OK);
return 0;
}
//
//=========================================================================
//
// This algorithm comes from:
// 1090-WP29-07-Draft_CPR101 (which also defines decodeCPR() )
//
// There is an error in this document related to CPR relative decode.
// Should use trunc() rather than the floor() function in Eq 38 and related for deltaZI.
// floor() returns integer less than argument
// trunc() returns integer closer to zero than argument.
// Note: text of document describes trunc() functionality for deltaZI calculation
// but the formulae use floor().
//
int decodeCPRrelative(struct aircraft *a, int fflag, int surface) {
double AirDlat;
double AirDlon;
double lat;
double lon;
double lonr, latr;
double rlon, rlat;
int j,m;
if (a->bFlags & MODES_ACFLAGS_LATLON_REL_OK) { // Ok to try aircraft relative first
latr = a->lat;
lonr = a->lon;
} else if (Modes.bUserFlags & MODES_USER_LATLON_VALID) { // Try ground station relative next
latr = Modes.fUserLat;
lonr = Modes.fUserLon;
} else {
return (-1); // Exit with error - can't do relative if we don't have ref.
}
if (fflag) { // odd
AirDlat = (surface ? 90.0 : 360.0) / 59.0;
lat = a->odd_cprlat;
lon = a->odd_cprlon;
} else { // even
AirDlat = (surface ? 90.0 : 360.0) / 60.0;
lat = a->even_cprlat;
lon = a->even_cprlon;
}
// Compute the Latitude Index "j"
j = (int) (floor(latr/AirDlat) +
trunc(0.5 + cprModFunction((int)latr, (int)AirDlat)/AirDlat - lat/131072));
rlat = AirDlat * (j + lat/131072);
if (rlat >= 270) rlat -= 360;
// Check to see that the latitude is in range: -90 .. +90
if (rlat < -90 || rlat > 90) {
a->bFlags &= ~MODES_ACFLAGS_LATLON_REL_OK; // This will cause a quick exit next time if no global has been done
return (-1); // Time to give up - Latitude error
}
// Check to see that answer is reasonable - ie no more than 1/2 cell away
if (fabs(rlat - a->lat) > (AirDlat/2)) {
a->bFlags &= ~MODES_ACFLAGS_LATLON_REL_OK; // This will cause a quick exit next time if no global has been done
return (-1); // Time to give up - Latitude error
}
// Compute the Longitude Index "m"
AirDlon = cprDlonFunction(rlat, fflag, surface);
m = (int) (floor(lonr/AirDlon) +
trunc(0.5 + cprModFunction((int)lonr, (int)AirDlon)/AirDlon - lon/131072));
rlon = AirDlon * (m + lon/131072);
if (rlon > 180) rlon -= 360;
// Check to see that answer is reasonable - ie no more than 1/2 cell away
if (fabs(rlon - a->lon) > (AirDlon/2)) {
a->bFlags &= ~MODES_ACFLAGS_LATLON_REL_OK; // This will cause a quick exit next time if no global has been done
return (-1); // Time to give up - Longitude error
}
a->lat = rlat;
a->lon = rlon;
a->seenLatLon = a->seen;
a->timestampLatLon = a->timestamp;
a->bFlags |= (MODES_ACFLAGS_LATLON_VALID | MODES_ACFLAGS_LATLON_REL_OK);
return (0);
}
//
// ===================== Mode S detection and decoding ===================
//
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