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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) 2014,2015 Oliver Jowett <oliver@mutability.co.uk>
//
// 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://www.gnu.org/licenses/>.
// This file incorporates work covered by the following copyright and
// permission notice:
//
// 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 ===================
//
//
//
//=========================================================================
//
// 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://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 MODES_UNIT_METERS or MDOES_UNIT_FEETS.
//
static 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).
//
static 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
//
static 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
static const char *ca_str[8] = {
/* 0 */ "Level 1",
/* 1 */ "reserved",
/* 2 */ "reserved",
/* 3 */ "reserved",
/* 4 */ "Level 2+, ground",
/* 5 */ "Level 2+, airborne",
/* 6 */ "Level 2+",
/* 7 */ "DR/Alert/SPI active"
};
// DF 18 Control field table.
static const 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
static const 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 */ "Reserved",
/* 7 */ "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 */ "Reserved",
/* 7 */ "Reserved"
};
//
//=========================================================================
//
static 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;
}
// 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;
}
// 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
int scoreModesMessage(unsigned char *msg, int validbits)
{
int msgtype, msgbits, crc, iid;
uint32_t addr;
struct errorinfo *ei;
if (validbits < 56)
return -2;
msgtype = msg[0] >> 3; // Downlink Format
msgbits = modesMessageLenByType(msgtype);
if (validbits < msgbits)
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)
return icaoFilterTest(crc) ? 1000 : -1;
case 11: // All-call reply
iid = crc & 0x7f;
crc = crc & 0xffff80;
addr = (msg[1] << 16) | (msg[2] << 8) | (msg[3]);
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 = (msg[1] << 16) | (msg[2] << 8) | (msg[3]);
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)
{
// 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;
// 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);
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)
// 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->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
}
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 = (msg[1] << 16) | (msg[2] << 8) | (msg[3]);
if (!icaoFilterTest(addr)) {
return -1;
}
}
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)
break; // all good
ei = modesChecksumDiagnose(mm->crc, mm->msgbits);
if (!ei) {
return -2; // couldn't fix it
}
addr1 = (msg[1] << 16) | (msg[2] << 8) | (msg[3]);
mm->correctedbits = ei->errors;
modesChecksumFix(msg, ei);
addr2 = (msg[1] << 16) | (msg[2] << 8) | (msg[3]);
// we are conservative here: only accept corrected messages that
// match an existing aircraft.
if (addr1 != addr2 && !icaoFilterTest(addr2)) {
return -1;
}
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->addr = mm->crc;
mm->bds = 0; // unknown
break;
}
#if 0
// This doesn't seem useful, as we mistake a lot of CRC errors
// for overlay control
// Try a fuzzy match
if ( (mm->addr = icaoFilterTestFuzzy(mm->crc)) != 0) {
// We have an address that would match, assume it's correct
mm->bds = (mm->crc ^ mm->addr) >> 16; // derive the BDS value based on what we think the address is
break;
}
#endif
return -1; // no good
default:
// All other message types, we don't know how to handle their CRCs, give up
return -2;
}
// decode the bulk of the message
mm->bFlags = 0;
// AA (Address announced)
if (mm->msgtype == 11 || mm->msgtype == 17 || mm->msgtype == 18) {
mm->addr = (msg[1] << 16) | (msg[2] << 8) | (msg[3]);
}
// AC (Altitude Code)
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);
}
}
// AF (DF19 Application Field) not decoded
// CA (Capability)
if (mm->msgtype == 11 || mm->msgtype == 17) {
mm->ca = (msg[0] & 0x07);
if (mm->ca == 4) {
mm->bFlags |= MODES_ACFLAGS_AOG_VALID | MODES_ACFLAGS_AOG;
} else if (mm->ca == 5) {
mm->bFlags |= MODES_ACFLAGS_AOG_VALID;
}
}
// CC (Cross-link capability) not decoded
// CF (Control field)
if (mm->msgtype == 18) {
mm->cf = msg[0] & 7;
}
// DR (Downlink Request) not decoded
// FS (Flight Status)
if (mm->msgtype == 4 || mm->msgtype == 5 || mm->msgtype == 20 || mm->msgtype == 21) {
mm->bFlags |= MODES_ACFLAGS_FS_VALID;
mm->fs = msg[0] & 7;
if (mm->fs <= 3) {
mm->bFlags |= MODES_ACFLAGS_AOG_VALID;
if (mm->fs & 1)
mm->bFlags |= MODES_ACFLAGS_AOG;
}
}
// ID (Identity)
if (mm->msgtype == 5 || mm->msgtype == 21) {
// Gillham encoded Squawk
int ID13Field = ((msg[2] << 8) | msg[3]) & 0x1FFF;
if (ID13Field) {
mm->bFlags |= MODES_ACFLAGS_SQUAWK_VALID;
mm->modeA = decodeID13Field(ID13Field);
}
}
// KE (Control, ELM) not decoded
// MB (messsage, Comm-B)
if (mm->msgtype == 20 || mm->msgtype == 21) {
decodeCommB(mm);
}
// MD (message, Comm-D) not decoded
// ME (message, extended squitter)
if (mm->msgtype == 17 || // Extended squitter
mm->msgtype == 18) { // Extended squitter/non-transponder:
decodeExtendedSquitter(mm);
}
// MV (message, ACAS) not decoded
// ND (number of D-segment) not decoded
// RI (Reply information) not decoded
// SL (Sensitivity level, ACAS) not decoded
// UM (Utility Message) not decoded
// VS (Vertical Status)
if (mm->msgtype == 0 || mm->msgtype == 16) {
mm->bFlags |= MODES_ACFLAGS_AOG_VALID;
if (msg[0] & 0x04)
mm->bFlags |= MODES_ACFLAGS_AOG;
}
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);
}
// all done
return 0;
}
// Decode BDS2,0 carried in Comm-B or ES
static void decodeBDS20(struct modesMessage *mm)
{
uint32_t chars1, chars2;
unsigned char *msg = mm->msg;
chars1 = (msg[5] << 16) | (msg[6] << 8) | (msg[7]);
chars2 = (msg[8] << 16) | (msg[9] << 8) | (msg[10]);
// A common failure mode seems to be to intermittently send
// all zeros. Catch that here.
if (chars1 == 0 && chars2 == 0)
return;
mm->bFlags |= MODES_ACFLAGS_CALLSIGN_VALID;
mm->flight[3] = ais_charset[chars1 & 0x3F]; chars1 = chars1 >> 6;
mm->flight[2] = ais_charset[chars1 & 0x3F]; chars1 = chars1 >> 6;
mm->flight[1] = ais_charset[chars1 & 0x3F]; chars1 = chars1 >> 6;
mm->flight[0] = ais_charset[chars1 & 0x3F];
mm->flight[7] = ais_charset[chars2 & 0x3F]; chars2 = chars2 >> 6;
mm->flight[6] = ais_charset[chars2 & 0x3F]; chars2 = chars2 >> 6;
mm->flight[5] = ais_charset[chars2 & 0x3F]; chars2 = chars2 >> 6;
mm->flight[4] = ais_charset[chars2 & 0x3F];
mm->flight[8] = '\0';
}
static void decodeExtendedSquitter(struct modesMessage *mm)
{
unsigned char *msg = mm->msg;
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
int check_imf = 0;
// Check CF on DF18 to work out the format of the ES and whether we need to look for an IMF bit
if (mm->msgtype == 18) {
switch (mm->cf) {
case 0: // ADS-B ES/NT devices that report the ICAO 24-bit address in the AA field
break;
case 1: // Reserved for ADS-B for ES/NT devices that use other addressing techniques in the AA field
case 5: // TIS-B messages that relay ADS-B Messages using anonymous 24-bit addresses (format not explicitly defined, but it seems to follow DF17)
mm->addr |= MODES_NON_ICAO_ADDRESS;
break;
case 2: // Fine TIS-B message (formats are close enough to DF17 for our purposes)
case 6: // ADS-B rebroadcast using the same type codes and message formats as defined for DF = 17 ADS-B messages
check_imf = 1;
break;
case 3: // Coarse TIS-B airborne position and velocity.
// TODO: decode me.
// For now we only look at the IMF bit.
if (msg[4] & 0x80)
mm->addr |= MODES_NON_ICAO_ADDRESS;
return;
default: // All others, we don't know the format.
mm->addr |= MODES_NON_ICAO_ADDRESS; // assume non-ICAO
return;
}
}
switch (metype) {
case 1: case 2: case 3: case 4: {
// Aircraft Identification and Category
uint32_t chars1, chars2;
chars1 = (msg[5] << 16) | (msg[6] << 8) | (msg[7]);
chars2 = (msg[8] << 16) | (msg[9] << 8) | (msg[10]);
// A common failure mode seems to be to intermittently send
// all zeros. Catch that here.
if (chars1 != 0 || chars2 != 0) {
mm->bFlags |= MODES_ACFLAGS_CALLSIGN_VALID;
mm->flight[3] = ais_charset[chars1 & 0x3F]; chars1 = chars1 >> 6;
mm->flight[2] = ais_charset[chars1 & 0x3F]; chars1 = chars1 >> 6;
mm->flight[1] = ais_charset[chars1 & 0x3F]; chars1 = chars1 >> 6;
mm->flight[0] = ais_charset[chars1 & 0x3F];
mm->flight[7] = ais_charset[chars2 & 0x3F]; chars2 = chars2 >> 6;
mm->flight[6] = ais_charset[chars2 & 0x3F]; chars2 = chars2 >> 6;
mm->flight[5] = ais_charset[chars2 & 0x3F]; chars2 = chars2 >> 6;
mm->flight[4] = ais_charset[chars2 & 0x3F];
mm->flight[8] = '\0';
}
break;
}
case 19: { // Airborne Velocity Message
if (check_imf && (msg[5] & 0x80))
mm->addr |= MODES_NON_ICAO_ADDRESS;
// 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;
}
}
break;
}
case 5: case 6: case 7: case 8: {
// Ground position
int movement;
if (check_imf && (msg[6] & 0x08))
mm->addr |= MODES_NON_ICAO_ADDRESS;
mm->bFlags |= MODES_ACFLAGS_AOG_VALID | MODES_ACFLAGS_AOG;
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;
movement = ((msg[4] << 4) | (msg[5] >> 4)) & 0x007F;
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;
}
break;
}
case 0: // Airborne position, baro altitude only
case 9: case 10: case 11: case 12: case 13: case 14: case 15: case 16: case 17: case 18: // Airborne position, baro
case 20: case 21: case 22: { // Airborne position, GNSS HAE
int AC12Field = ((msg[5] << 4) | (msg[6] >> 4)) & 0x0FFF;
if (check_imf && (msg[4] & 0x01))
mm->addr |= MODES_NON_ICAO_ADDRESS;
mm->bFlags |= MODES_ACFLAGS_AOG_VALID;
if (metype != 0) {
// Catch some common failure modes and don't mark them as valid
// (so they won't be used for positioning)
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]);
if (AC12Field == 0 && mm->raw_longitude == 0 && (mm->raw_latitude & 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->bFlags |= (mm->msg[6] & 0x04) ? MODES_ACFLAGS_LLODD_VALID
: MODES_ACFLAGS_LLEVEN_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);
}
break;
}
case 23: { // Test message
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);
}
}
break;
}
case 24: // Reserved for Surface System Status
break;
case 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);
}
if (check_imf && (msg[10] & 0x01))
mm->addr |= MODES_NON_ICAO_ADDRESS;
}
break;
}
case 29: // Aircraft Trajectory Intent
break;
case 30: // Aircraft Operational Coordination
break;
case 31: // Aircraft Operational Status
if (check_imf && (msg[10] & 0x01))
mm->addr |= MODES_NON_ICAO_ADDRESS;
break;
default:
break;
}
}
static void decodeCommB(struct modesMessage *mm)
{
unsigned char *msg = mm->msg;
// This is a bit hairy as we don't know what the requested register was
if (msg[4] == 0x20) { // BDS 2,0 Aircraft Identification
decodeBDS20(mm);
}
}
//
//=========================================================================
//
// These functions gets a decoded Mode S Message and prints it on the screen
// in a human readable format.
//
static void displayExtendedSquitter(struct modesMessage *mm) {
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) {
if (mm->bFlags & MODES_ACFLAGS_REL_CPR_USED)
printf(" Local CPR decoding used.\n");
else
printf(" Global CPR decoding used.\n");
printf(" Latitude : %f (%d)\n", mm->fLat, mm->raw_latitude);
printf(" Longitude: %f (%d)\n", mm->fLon, mm->raw_longitude);
} else {
if (!(mm->bFlags & MODES_ACFLAGS_LLEITHER_VALID))
printf(" Bad position data, not decoded.\n");
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) {