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mode_s.c
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mode_s.c
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// Part of dump1090, a Mode S message decoder for RTLSDR devices.
//
// mode_s.c: Mode S message decoding.
//
// Copyright (c) 2014-2016 Oliver Jowett <[email protected]>
//
// This file is free software: you may copy, redistribute and/or modify it
// under the terms of the GNU General Public License as published by the
// Free Software Foundation, either version 2 of the License, or (at your
// option) any later version.
//
// This file is distributed in the hope that it will be useful, but
// WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
// General Public License for more details.
//
// You should have received a copy of the GNU General Public License
// along with this program. If not, see <http:https://www.gnu.org/licenses/>.
// This file incorporates work covered by the following copyright and
// permission notice:
//
// Copyright (C) 2012 by Salvatore Sanfilippo <[email protected]>
//
// 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"
/* for PRIX64 */
#include <inttypes.h>
#include <assert.h>
//
// ===================== Mode S detection and decoding ===================
//
//
//
/* A timestamp that indicates the data is synthetic, created from a
* multilateration result
*/
#define MAGIC_MLAT_TIMESTAMP 0xFF004D4C4154ULL
//=========================================================================
//
// 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 ;
}
//
//=========================================================================
//
// 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:https://en.wikipedia.org/wiki/Gillham_code
//
static 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 UNIT_METERS or UNIT_FEET.
//
static int decodeAC13Field(int AC13Field, altitude_unit_t *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 = 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) {
return INVALID_ALTITUDE;
}
return (100 * n);
}
} else {
*unit = UNIT_METERS;
// TODO: Implement altitude when meter unit is selected
return INVALID_ALTITUDE;
}
}
//
//=========================================================================
//
// Decode the 12 bit AC altitude field (in DF 17 and others).
//
static int decodeAC12Field(int AC12Field, altitude_unit_t *unit) {
int q_bit = AC12Field & 0x10; // Bit 48 = Q
*unit = 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) {
return INVALID_ALTITUDE;
}
return (100 * n);
}
}
//
//=========================================================================
//
// Decode the 7 bit ground movement field PWL exponential style scale
//
static unsigned decodeMovementField(unsigned 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);
}
// Correct a decoded native-endian Address Announced field
// (from bits 8-31) if it is affected by the given error
// syndrome. Updates *addr and returns >0 if changed, 0 if
// it was unaffected.
static int correct_aa_field(uint32_t *addr, struct errorinfo *ei)
{
int i;
int addr_errors = 0;
if (!ei)
return 0;
for (i = 0; i < ei->errors; ++i) {
if (ei->bit[i] >= 8 && ei->bit[i] <= 31) {
*addr ^= 1 << (31 - ei->bit[i]);
++addr_errors;
}
}
return addr_errors;
}
// The first bit (MSB of the first byte) is numbered 1, for consistency
// with how the specs number them.
// Extract one bit from a message.
static inline __attribute__((always_inline)) unsigned getbit(unsigned char *data, unsigned bitnum)
{
unsigned bi = bitnum - 1;
unsigned by = bi >> 3;
unsigned mask = 1 << (7 - (bi & 7));
return (data[by] & mask) != 0;
}
// Extract some bits (firstbit .. lastbit inclusive) from a message.
static inline __attribute__((always_inline)) unsigned getbits(unsigned char *data, unsigned firstbit, unsigned lastbit)
{
unsigned fbi = firstbit - 1;
unsigned lbi = lastbit - 1;
unsigned nbi = (lastbit - firstbit + 1);
unsigned fby = fbi >> 3;
unsigned lby = lbi >> 3;
unsigned nby = (lby - fby) + 1;
unsigned shift = 7 - (lbi & 7);
unsigned topmask = 0xFF >> (fbi & 7);
assert (fbi <= lbi);
assert (nbi <= 32);
assert (nby <= 5);
if (nby == 5) {
return
((data[fby] & topmask) << (32 - shift)) |
(data[fby + 1] << (24 - shift)) |
(data[fby + 2] << (16 - shift)) |
(data[fby + 3] << (8 - shift)) |
(data[fby + 4] >> shift);
} else if (nby == 4) {
return
((data[fby] & topmask) << (24 - shift)) |
(data[fby + 1] << (16 - shift)) |
(data[fby + 2] << (8 - shift)) |
(data[fby + 3] >> shift);
} else if (nby == 3) {
return
((data[fby] & topmask) << (16 - shift)) |
(data[fby + 1] << (8 - shift)) |
(data[fby + 2] >> shift);
} else if (nby == 2) {
return
((data[fby] & topmask) << (8 - shift)) |
(data[fby + 1] >> shift);
} else if (nby == 1) {
return
(data[fby] & topmask) >> shift;
} else {
return 0;
}
}
// Score how plausible this ModeS message looks.
// The more positive, the more reliable the message is
// 1000: DF 0/4/5/16/24 with a CRC-derived address matching a known aircraft
// 1800: DF17/18 with good CRC and an address matching a known aircraft
// 1400: DF17/18 with good CRC and an address not matching a known aircraft
// 900: DF17/18 with 1-bit error and an address matching a known aircraft
// 700: DF17/18 with 1-bit error and an address not matching a known aircraft
// 450: DF17/18 with 2-bit error and an address matching a known aircraft
// 350: DF17/18 with 2-bit error and an address not matching a known aircraft
// 1600: DF11 with IID==0, good CRC and an address matching a known aircraft
// 800: DF11 with IID==0, 1-bit error and an address matching a known aircraft
// 750: DF11 with IID==0, good CRC and an address not matching a known aircraft
// 375: DF11 with IID==0, 1-bit error and an address not matching a known aircraft
// 1000: DF11 with IID!=0, good CRC and an address matching a known aircraft
// 500: DF11 with IID!=0, 1-bit error and an address matching a known aircraft
// 1000: DF20/21 with a CRC-derived address matching a known aircraft
// 500: DF20/21 with a CRC-derived address matching a known aircraft (bottom 16 bits only - overlay control in use)
// -1: message might be valid, but we couldn't validate the CRC against a known ICAO
// -2: bad message or unrepairable CRC error
static unsigned char all_zeros[14] = { 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 };
int scoreModesMessage(unsigned char *msg, int validbits)
{
int msgtype, msgbits, crc, iid;
uint32_t addr;
struct errorinfo *ei;
if (validbits < 56)
return -2;
msgtype = getbits(msg, 1, 5); // Downlink Format
msgbits = modesMessageLenByType(msgtype);
if (validbits < msgbits)
return -2;
if (!memcmp(all_zeros, msg, msgbits/8))
return -2;
crc = modesChecksum(msg, msgbits);
switch (msgtype) {
case 0: // short air-air surveillance
case 4: // surveillance, altitude reply
case 5: // surveillance, altitude reply
case 16: // long air-air surveillance
case 24: // Comm-D (ELM)
case 25: // Comm-D (ELM)
case 26: // Comm-D (ELM)
case 27: // Comm-D (ELM)
case 28: // Comm-D (ELM)
case 29: // Comm-D (ELM)
case 30: // Comm-D (ELM)
case 31: // Comm-D (ELM)
return icaoFilterTest(crc) ? 1000 : -1;
case 11: // All-call reply
iid = crc & 0x7f;
crc = crc & 0xffff80;
addr = getbits(msg, 9, 32);
ei = modesChecksumDiagnose(crc, msgbits);
if (!ei)
return -2; // can't correct errors
// see crc.c comments: we do not attempt to fix
// more than single-bit errors, as two-bit
// errors are ambiguous in DF11.
if (ei->errors > 1)
return -2; // can't correct errors
// fix any errors in the address field
correct_aa_field(&addr, ei);
// validate address
if (iid == 0) {
if (icaoFilterTest(addr))
return 1600 / (ei->errors + 1);
else
return 750 / (ei->errors + 1);
} else {
if (icaoFilterTest(addr))
return 1000 / (ei->errors + 1);
else
return -1;
}
case 17: // Extended squitter
case 18: // Extended squitter/non-transponder
ei = modesChecksumDiagnose(crc, msgbits);
if (!ei)
return -2; // can't correct errors
// fix any errors in the address field
addr = getbits(msg, 9, 32);
correct_aa_field(&addr, ei);
if (icaoFilterTest(addr))
return 1800 / (ei->errors+1);
else
return 1400 / (ei->errors+1);
case 20: // Comm-B, altitude reply
case 21: // Comm-B, identity reply
if (icaoFilterTest(crc))
return 1000; // Address/Parity
#if 0
// This doesn't seem useful, as we mistake a lot of CRC errors
// for overlay control
if (icaoFilterTestFuzzy(crc))
return 500; // Data/Parity
#endif
return -2;
default:
// unknown message type
return -2;
}
}
//
//=========================================================================
//
// Decode a raw Mode S message demodulated as a stream of bytes by detectModeS(),
// and split it into fields populating a modesMessage structure.
//
static void decodeExtendedSquitter(struct modesMessage *mm);
static void decodeCommB(struct modesMessage *mm);
static char *ais_charset = "@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\\]^_ !\"#$%&'()*+,-./0123456789:;<=>?";
// return 0 if all OK
// -1: message might be valid, but we couldn't validate the CRC against a known ICAO
// -2: bad message or unrepairable CRC error
int decodeModesMessage(struct modesMessage *mm, unsigned char *msg)
{
int j;
FILE *fh;
// Work on our local copy.
memcpy(mm->msg, msg, MODES_LONG_MSG_BYTES);
if (Modes.net_verbatim) {
// Preserve the original uncorrected copy for later forwarding
memcpy(mm->verbatim, msg, MODES_LONG_MSG_BYTES);
}
msg = mm->msg;
// don't accept all-zeros messages
if (!memcmp(all_zeros, msg, 7))
return -2;
// Get the message type ASAP as other operations depend on this
mm->msgtype = getbits(msg, 1, 5); // Downlink Format
mm->msgbits = modesMessageLenByType(mm->msgtype);
mm->crc = modesChecksum(msg, mm->msgbits);
mm->correctedbits = 0;
mm->addr = 0;
// Do checksum work and set fields that depend on the CRC
switch (mm->msgtype) {
case 0: // short air-air surveillance
case 4: // surveillance, altitude reply
case 5: // surveillance, altitude reply
case 16: // long air-air surveillance
case 24: // Comm-D (ELM)
case 25: // Comm-D (ELM)
case 26: // Comm-D (ELM)
case 27: // Comm-D (ELM)
case 28: // Comm-D (ELM)
case 29: // Comm-D (ELM)
case 30: // Comm-D (ELM)
case 31: // Comm-D (ELM)
// These message types use Address/Parity, i.e. our CRC syndrome is the sender's ICAO address.
// We can't tell if the CRC is correct or not as we don't know the correct address.
// Accept the message if it appears to be from a previously-seen aircraft
if (!icaoFilterTest(mm->crc)) {
return -1;
}
mm->source = SOURCE_MODE_S;
mm->addr = mm->crc;
break;
case 11: // All-call reply
// This message type uses Parity/Interrogator, i.e. our CRC syndrome is CL + IC from the uplink message
// which we can't see. So we don't know if the CRC is correct or not.
//
// however! CL + IC only occupy the lower 7 bits of the CRC. So if we ignore those bits when testing
// the CRC we can still try to detect/correct errors.
mm->IID = mm->crc & 0x7f;
if (mm->crc & 0xffff80) {
int addr;
struct errorinfo *ei = modesChecksumDiagnose(mm->crc & 0xffff80, mm->msgbits);
if (!ei) {
return -2; // couldn't fix it
}
// see crc.c comments: we do not attempt to fix
// more than single-bit errors, as two-bit
// errors are ambiguous in DF11.
if (ei->errors > 1)
return -2; // can't correct errors
mm->correctedbits = ei->errors;
modesChecksumFix(msg, ei);
// check whether the corrected message looks sensible
// we are conservative here: only accept corrected messages that
// match an existing aircraft.
addr = getbits(msg, 9, 32);
if (!icaoFilterTest(addr)) {
return -1;
}
}
mm->source = SOURCE_MODE_S_CHECKED;
break;
case 17: // Extended squitter
case 18: { // Extended squitter/non-transponder
struct errorinfo *ei;
int addr1, addr2;
// These message types use Parity/Interrogator, but are specified to set II=0
if (mm->crc != 0) {
ei = modesChecksumDiagnose(mm->crc, mm->msgbits);
if (!ei) {
return -2; // couldn't fix it
}
addr1 = getbits(msg, 9, 32);
mm->correctedbits = ei->errors;
modesChecksumFix(msg, ei);
addr2 = getbits(msg, 9, 32);
// we are conservative here: only accept corrected messages that
// match an existing aircraft.
if (addr1 != addr2 && !icaoFilterTest(addr2)) {
return -1;
}
}
mm->source = SOURCE_ADSB; // TIS-B decoding will override this if needed
break;
}
case 20: // Comm-B, altitude reply
case 21: // Comm-B, identity reply
// These message types either use Address/Parity (see DF0 etc)
// or Data Parity where the requested BDS is also xored into the top byte.
// So not only do we not know whether the CRC is right, we also don't know if
// the ICAO is right! Ow.
// Try an exact match
if (icaoFilterTest(mm->crc)) {
// OK.
mm->source = SOURCE_MODE_S;
mm->addr = mm->crc;
break;
}
// BDS / overlay control just doesn't work out.
return -1; // no good
/* Log DF19 and DF22 military to file */
case 19:
case 22:{
fh = fopen("/var/dump1090/dump1090_DF19_22.log", "a");
if(fh != NULL) {
if (Modes.mlat && mm->timestampMsg) {
fprintf(fh,"@%012" PRIX64, mm->timestampMsg);
} else
fprintf(fh,"*");
for (j = 0; j < mm->msgbits/8; j++) fprintf(fh,"%02x", mm->msg[j]);
fprintf(fh,";\n");
fclose(fh);
}
break;
}
default:
// All other message types, we don't know how to handle their CRCs, give up
return -2;
}
// decode the bulk of the message
// AA (Address announced)
if (mm->msgtype == 11 || mm->msgtype == 17 || mm->msgtype == 18) {
mm->AA = mm->addr = getbits(msg, 9, 32);
}
// AC (Altitude Code)
if (mm->msgtype == 0 || mm->msgtype == 4 || mm->msgtype == 16 || mm->msgtype == 20) {
mm->AC = getbits(msg, 20, 32);
if (mm->AC) { // Only attempt to decode if a valid (non zero) altitude is present
mm->altitude = decodeAC13Field(mm->AC, &mm->altitude_unit);
if (mm->altitude != INVALID_ALTITUDE)
mm->altitude_valid = 1;
mm->altitude_source = ALTITUDE_BARO;
}
}
// AF (DF19 Application Field) not decoded
// CA (Capability)
if (mm->msgtype == 11 || mm->msgtype == 17) {
mm->CA = getbits(msg, 6, 8);
switch (mm->CA) {
case 0:
mm->airground = AG_UNCERTAIN;
break;
case 4:
mm->airground = AG_GROUND;
break;
case 5:
mm->airground = AG_AIRBORNE;
break;
case 6:
mm->airground = AG_UNCERTAIN;
break;
case 7:
mm->airground = AG_UNCERTAIN;
break;
}
}
// CC (Cross-link capability)
if (mm->msgtype == 0) {
mm->CC = getbit(msg, 7);
}
// CF (Control field)
if (mm->msgtype == 18) {
mm->CF = getbits(msg, 5, 8);
}
// DR (Downlink Request)
if (mm->msgtype == 4 || mm->msgtype == 5 || mm->msgtype == 20 || mm->msgtype == 21) {
mm->DR = getbits(msg, 9, 13);
}
// FS (Flight Status)
if (mm->msgtype == 4 || mm->msgtype == 5 || mm->msgtype == 20 || mm->msgtype == 21) {
mm->FS = getbits(msg, 6, 8);
mm->alert_valid = 1;
mm->spi_valid = 1;
switch (mm->FS) {
case 0:
mm->airground = AG_UNCERTAIN;
break;
case 1:
mm->airground = AG_GROUND;
break;
case 2:
mm->airground = AG_UNCERTAIN;
mm->alert = 1;
break;
case 3:
mm->airground = AG_GROUND;
mm->alert = 1;
break;
case 4:
mm->airground = AG_UNCERTAIN;
mm->alert = 1;
mm->spi = 1;
break;
case 5:
mm->airground = AG_UNCERTAIN;
mm->spi = 1;
break;
default:
mm->spi_valid = 0;
mm->alert_valid = 0;
break;
}
}
// ID (Identity)
if (mm->msgtype == 5 || mm->msgtype == 21) {
// Gillham encoded Squawk
mm->ID = getbits(msg, 20, 32);
if (mm->ID) {
mm->squawk = decodeID13Field(mm->ID);
mm->squawk_valid = 1;
}
}
// KE (Control, ELM)
if (mm->msgtype >= 24 && mm->msgtype <= 31) {
mm->KE = getbit(msg, 4);
}
// MB (messsage, Comm-B)
if (mm->msgtype == 20 || mm->msgtype == 21) {
memcpy(mm->MB, &msg[4], 7);
decodeCommB(mm);
}
// MD (message, Comm-D)
if (mm->msgtype >= 24 && mm->msgtype <= 31) {
memcpy(mm->MD, &msg[1], 10);
}
// ME (message, extended squitter)
if (mm->msgtype == 17 || mm->msgtype == 18) {
memcpy(mm->ME, &msg[4], 7);
decodeExtendedSquitter(mm);
}
// MV (message, ACAS)
if (mm->msgtype == 16) {
memcpy(mm->MV, &msg[4], 7);
}
// ND (number of D-segment, Comm-D)
if (mm->msgtype >= 24 && mm->msgtype <= 31) {
mm->ND = getbits(msg, 5, 8);
}
// RI (Reply information, ACAS)
if (mm->msgtype == 0 || mm->msgtype == 16) {
mm->RI = getbits(msg, 14, 17);
}
// SL (Sensitivity level, ACAS)
if (mm->msgtype == 0 || mm->msgtype == 16) {
mm->SL = getbits(msg, 9, 11);
}
// UM (Utility Message)
if (mm->msgtype == 4 || mm->msgtype == 5 || mm->msgtype == 20 || mm->msgtype == 21) {
mm->UM = getbits(msg, 14, 19);
}
// VS (Vertical Status)
if (mm->msgtype == 0 || mm->msgtype == 16) {
mm->VS = getbit(msg, 6);
if (mm->VS)
mm->airground = AG_GROUND;
else
mm->airground = AG_UNCERTAIN;
}
if (!mm->correctedbits && (mm->msgtype == 17 || mm->msgtype == 18 || (mm->msgtype == 11 && mm->IID == 0))) {
// No CRC errors seen, and either it was an DF17/18 extended squitter
// or a DF11 acquisition squitter with II = 0. We probably have the right address.
// We wait until here to do this as we may have needed to decode an ES to note
// the type of address in DF18 messages.
// NB this is the only place that adds addresses!
icaoFilterAdd(mm->addr);
}
// MLAT overrides all other sources
if (mm->remote && mm->timestampMsg == MAGIC_MLAT_TIMESTAMP)
mm->source = SOURCE_MLAT;
// all done
return 0;
}
// Decode BDS2,0 carried in Comm-B or ES
static void decodeBDS20(struct modesMessage *mm)
{
unsigned char *msg = mm->msg;
mm->callsign[0] = ais_charset[getbits(msg, 41, 46)];
mm->callsign[1] = ais_charset[getbits(msg, 47, 52)];
mm->callsign[2] = ais_charset[getbits(msg, 53, 58)];
mm->callsign[3] = ais_charset[getbits(msg, 59, 64)];
mm->callsign[4] = ais_charset[getbits(msg, 65, 70)];
mm->callsign[5] = ais_charset[getbits(msg, 71, 76)];
mm->callsign[6] = ais_charset[getbits(msg, 77, 82)];
mm->callsign[7] = ais_charset[getbits(msg, 83, 88)];
mm->callsign[8] = 0;
// Catch possible bad decodings since BDS2,0 is not
// 100% reliable: accept only alphanumeric data
mm->callsign_valid = 1;
for (int i = 0; i < 8; ++i) {
if (! ((mm->callsign[i] >= 'A' && mm->callsign[i] <= 'Z') ||
(mm->callsign[i] >= '0' && mm->callsign[i] <= '9') ||
mm->callsign[i] == ' ') ) {
mm->callsign_valid = 0;
break;
}
}
}
static void decodeESIdentAndCategory(struct modesMessage *mm)
{
// Aircraft Identification and Category
unsigned char *me = mm->ME;
mm->mesub = getbits(me, 6, 8);
mm->callsign[0] = ais_charset[getbits(me, 9, 14)];
mm->callsign[1] = ais_charset[getbits(me, 15, 20)];
mm->callsign[2] = ais_charset[getbits(me, 21, 26)];
mm->callsign[3] = ais_charset[getbits(me, 27, 32)];
mm->callsign[4] = ais_charset[getbits(me, 33, 38)];
mm->callsign[5] = ais_charset[getbits(me, 39, 44)];
mm->callsign[6] = ais_charset[getbits(me, 45, 50)];
mm->callsign[7] = ais_charset[getbits(me, 51, 56)];
// A common failure mode seems to be to intermittently send
// all zeros. Catch that here.
mm->callsign_valid = (strcmp(mm->callsign, "@@@@@@@@") != 0);
mm->category = ((0x0E - mm->metype) << 4) | mm->mesub;
mm->category_valid = 1;
}
// Handle setting a non-ICAO address
static void setIMF(struct modesMessage *mm)
{
mm->addr |= MODES_NON_ICAO_ADDRESS;
switch (mm->addrtype) {
case ADDR_ADSB_ICAO:
case ADDR_ADSB_ICAO_NT:
// Shouldn't happen, but let's try to handle it
mm->addrtype = ADDR_ADSB_OTHER;
break;
case ADDR_TISB_ICAO:
mm->addrtype = ADDR_TISB_TRACKFILE;
break;
case ADDR_ADSR_ICAO:
mm->addrtype = ADDR_ADSR_OTHER;
break;
default:
// Nothing.
break;
}
}
static void decodeESAirborneVelocity(struct modesMessage *mm, int check_imf)
{
// Airborne Velocity Message
unsigned char *me = mm->ME;
mm->mesub = getbits(me, 6, 8);
if (check_imf && getbit(me, 9))
setIMF(mm);
if (mm->mesub < 1 || mm->mesub > 4)
return;
unsigned vert_rate = getbits(me, 38, 46);
if (vert_rate) {
mm->vert_rate = (vert_rate - 1) * (getbit(me, 37) ? -64 : 64);
mm->vert_rate_valid = 1;
}
mm->vert_rate_source = (getbit(me, 36) ? ALTITUDE_GNSS : ALTITUDE_BARO);
switch (mm->mesub) {
case 1: case 2:
{
unsigned ew_raw = getbits(me, 15, 24);
unsigned ns_raw = getbits(me, 26, 35);
if (ew_raw && ns_raw) {
int ew_vel = (ew_raw - 1) * (getbit(me, 14) ? -1 : 1) * ((mm->mesub == 2) ? 4 : 1);
int ns_vel = (ns_raw - 1) * (getbit(me, 25) ? -1 : 1) * ((mm->mesub == 2) ? 4 : 1);
// Compute velocity and angle from the two speed components
mm->speed = (unsigned) sqrt((ns_vel * ns_vel) + (ew_vel * ew_vel) + 0.5);
mm->speed_valid = 1;
if (mm->speed) {
int heading = (int) (atan2(ew_vel, ns_vel) * 180.0 / M_PI + 0.5);
// We don't want negative values but a 0-360 scale
if (heading < 0)
heading += 360;
mm->heading = (unsigned) heading;
mm->heading_source = HEADING_TRUE;
mm->heading_valid = 1;
}
mm->speed_source = SPEED_GROUNDSPEED;
}
break;
}
case 3: case 4:
{
unsigned airspeed = getbits(me, 26, 35);
if (airspeed) {
mm->speed = (airspeed - 1) * (mm->mesub == 4 ? 4 : 1);
mm->speed_source = getbit(me, 25) ? SPEED_TAS : SPEED_IAS;
mm->speed_valid = 1;
}
if (getbit(me, 14)) {
mm->heading = getbits(me, 15, 24);
mm->heading_source = HEADING_MAGNETIC;
mm->heading_valid = 1;
}
break;
}
}
unsigned raw_delta = getbits(me, 50, 56);
if (raw_delta) {
mm->gnss_delta_valid = 1;
mm->gnss_delta = (raw_delta - 1) * (getbit(me, 49) ? -25 : 25);
}
}
static void decodeESSurfacePosition(struct modesMessage *mm, int check_imf)
{
// Surface position and movement
unsigned char *me = mm->ME;
if (check_imf && getbit(me, 21))
setIMF(mm);
mm->airground = AG_GROUND; // definitely.
mm->cpr_lat = getbits(me, 23, 39);
mm->cpr_lon = getbits(me, 40, 56);
mm->cpr_odd = getbit(me, 22);
mm->cpr_nucp = (14 - mm->metype);
mm->cpr_valid = 1;
mm->cpr_type = CPR_SURFACE;
unsigned movement = getbits(me, 6, 12);
if (movement > 0 && movement < 125) {
mm->speed_valid = 1;
mm->speed = decodeMovementField(movement);
mm->speed_source = SPEED_GROUNDSPEED;
}
if (getbit(me, 13)) {
mm->heading_valid = 1;
mm->heading_source = HEADING_TRUE;
mm->heading = getbits(me, 14, 20) * 360 / 128;
}
}
static void decodeESAirbornePosition(struct modesMessage *mm, int check_imf)
{
// Airborne position and altitude
unsigned char *me = mm->ME;
if (check_imf && getbit(me, 8))
setIMF(mm);
unsigned AC12Field = getbits(me, 9, 20);
if (mm->metype == 0) {
mm->cpr_nucp = 0;
} else {
// Catch some common failure modes and don't mark them as valid
// (so they won't be used for positioning)
mm->cpr_lat = getbits(me, 23, 39);
mm->cpr_lon = getbits(me, 40, 56);
if (AC12Field == 0 && mm->cpr_lon == 0 && (mm->cpr_lat & 0x0fff) == 0 && mm->metype == 15) {
// Seen from at least:
// 400F3F (Eurocopter ECC155 B1) - Bristow Helicopters
// 4008F3 (BAE ATP) - Atlantic Airlines
// 400648 (BAE ATP) - Atlantic Airlines
// altitude == 0, longitude == 0, type == 15 and zeros in latitude LSB.
// Can alternate with valid reports having type == 14
Modes.stats_current.cpr_filtered++;
} else {
// Otherwise, assume it's valid.
mm->cpr_valid = 1;
mm->cpr_type = CPR_AIRBORNE;
mm->cpr_odd = getbit(me, 22);
if (mm->metype == 18 || mm->metype == 22)
mm->cpr_nucp = 0;
else if (mm->metype < 18)
mm->cpr_nucp = (18 - mm->metype);
else
mm->cpr_nucp = (29 - mm->metype);
}
}
if (AC12Field) {// Only attempt to decode if a valid (non zero) altitude is present
mm->altitude = decodeAC12Field(AC12Field, &mm->altitude_unit);
if (mm->altitude != INVALID_ALTITUDE) {
mm->altitude_valid = 1;
}
mm->altitude_source = (mm->metype == 20 || mm->metype == 21 || mm->metype == 22) ? ALTITUDE_GNSS : ALTITUDE_BARO;
}
}
static void decodeESTestMessage(struct modesMessage *mm)
{
unsigned char *me = mm->ME;
mm->mesub = getbits(me, 6, 8);
if (mm->mesub == 7) { // (see 1090-WP-15-20)
int ID13Field = getbits(me, 9, 21);