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dump1090/mode_s.c
Oliver Jowett 26feb1d078 Add some testing code that looks for ambiguous syndromes that 
could confuse the partial correction used in DF11.

That code shows that yes, there are ambiguous syndromes in the
2-bit correction case only, so disable corrections of more than
1 bit in DF11.
2015-01-23 21:59:31 +00:00

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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;
// CF (Control field)
// done first so we can use it in AA.
if (mm->msgtype == 18) {
mm->cf = msg[0] & 7;
}
// AA (Address announced)
if (mm->msgtype == 11 || mm->msgtype == 17 || mm->msgtype == 18) {
mm->addr = (msg[1] << 16) | (msg[2] << 8) | (msg[3]);
if (mm->msgtype == 18 && (mm->cf != 0 && mm->cf != 6))
mm->addr |= MODES_NON_ICAO_ADDRESS; // don't confuse this with any ICAO address
if (!mm->correctedbits && (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.
// NB this is the only place that adds addresses!
icaoFilterAdd(mm->addr);
}
}
// 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
// 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:
(mm->cf == 0 || // ADS-B ES/NT devices that report the ICAO 24-bit address in the AA field
mm->cf == 1 || // Reserved for ADS-B for ES/NT devices that use other addressing techniques in the AA field
mm->cf == 5 || // TIS-B messages that relay ADS-B Messages using anonymous 24-bit addresses
mm->cf == 6))) { // ADS-B rebroadcast using the same type codes and message formats as defined for DF = 17 ADS-B messages
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;
}
// 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
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
// 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;
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;
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);
}
}
break;
}
case 29: // Aircraft Trajectory Intent
break;
case 30: // Aircraft Operational Coordination
break;
case 31: // Aircraft Operational Status
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) {
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);
}
}
static void displayCommB(struct modesMessage *mm)
{
if (mm->bds != 0)
printf(" Comm-B BDS : %02x (maybe)\n", mm->bds);
// Decode the Comm-B message
if (mm->msg[4] == 0x20 && (mm->bds == 0 || mm->bds == 0x20)) { // BDS 2,0 Aircraft identification
printf(" BDS 2,0 Aircraft Identification : %s\n", mm->flight);
} else {
int i;
printf(" Comm-B MB : ");
for (i = 4; i < 11; ++i)
printf("%02x", mm->msg[i]);
printf("\n");
}
}
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\n", mm->crc);
if (mm->correctedbits != 0)
printf("No. of bit errors fixed: %d\n", mm->correctedbits);
if (mm->signalLevel > 0)
printf("RSSI: %.1f dBFS\n", 10 * log10(mm->signalLevel));
if (mm->score)
printf("Score: %d\n", mm->score);
if (mm->timestampMsg)
printf("Time: %.2fus (phase: %d)\n", mm->timestampMsg / 12.0, (unsigned int) (360 * (mm->timestampMsg % 6) / 6));
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) {
displayCommB(mm);
}
} 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) {
displayCommB(mm);
}
} 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);
displayExtendedSquitter(mm);
} else if (mm->msgtype == 18) { // DF 18
printf("DF 18: Extended Squitter.\n");
printf(" Control Field : %d (%s)\n", mm->cf, cf_str[mm->cf]);
if ((mm->cf == 0) || (mm->cf == 1) || (mm->cf == 5) || (mm->cf == 6)) {
if (mm->cf == 1 || mm->cf == 5) {
printf(" Other Address : %06x\n", mm->addr);
} else {
printf(" ICAO Address : %06x\n", mm->addr);
}
displayExtendedSquitter(mm);
}
} 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.trailing_samples];
uint32_t j;
memcpy(Modes.magnitude,&Modes.magnitude[MODES_ASYNC_BUF_SAMPLES], Modes.trailing_samples * 2);
// 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++];
}
}
//
//=========================================================================
//
// 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) {
++Modes.stats_current.messages_total;
// If we are decoding, 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);}
}
//
// ===================== Mode S detection and decoding ===================
//
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