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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 <oliver@mutability.co.uk>
// Copyright (c) 2021 FlightAware LLC
//
// 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"
#include "ais_charset.h"
/* for PRIX64 */
#include <inttypes.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://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 (ADS-B v2)
//
static float decodeMovementFieldV2(unsigned movement) {
// Note : movement codes 0,125,126,127 are all invalid, but they are
// trapped for before this function is called.
// Each movement value is a range of speeds;
// we return the midpoint of the range (rounded to the nearest integer)
if (movement >= 125) return 0; // invalid
else if (movement == 124) return 180; // gs > 175kt, pick a value..
else if (movement >= 109) return 100 + (movement - 109 + 0.5) * 5; // 100 < gs <= 175 in 5kt steps
else if (movement >= 94) return 70 + (movement - 94 + 0.5) * 2; // 70 < gs <= 100 in 2kt steps
else if (movement >= 39) return 15 + (movement - 39 + 0.5) * 1; // 15 < gs <= 70 in 1kt steps
else if (movement >= 13) return 2 + (movement - 13 + 0.5) * 0.50; // 2 < gs <= 15 in 0.5kt steps
else if (movement >= 9) return 1 + (movement - 9 + 0.5) * 0.25; // 1 < gs <= 2 in 0.25kt steps
else if (movement >= 3) return 0.125 + (movement - 3 + 0.5) * 0.875 / 6; // 0.125 < gs <= 1 in 0.875/6 kt step
else if (movement >= 2) return 0.125 / 2; // 0 < gs <= 0.125
// 1: stopped, gs = 0
// 0: no data
else return 0;
}
//
//=========================================================================
//
// Decode the 7 bit ground movement field PWL exponential style scale (ADS-B v0)
//
static float decodeMovementFieldV0(unsigned movement) {
// Note : movement codes 0,125,126,127 are all invalid, but they are
// trapped for before this function is called.
// Each movement value is a range of speeds;
// we return the midpoint of the range
if (movement >= 125) return 0; // invalid
else if (movement == 124) return 180; // gs >= 175kt, pick a value..
else if (movement >= 109) return 100 + (movement - 109 + 0.5) * 5; // 100 < gs <= 175 in 5kt steps
else if (movement >= 94) return 70 + (movement - 94 + 0.5) * 2; // 70 < gs <= 100 in 2kt steps
else if (movement >= 39) return 15 + (movement - 39 + 0.5) * 1; // 15 < gs <= 70 in 1kt steps
else if (movement >= 13) return 2 + (movement - 13 + 0.5) * 0.50; // 2 < gs <= 15 in 0.5kt steps
else if (movement >= 9) return 1 + (movement - 9 + 0.5) * 0.25; // 1 < gs <= 2 in 0.25kt steps
else if (movement >= 2) return 0.125 + (movement - 2 + 0.5) * 0.125; // 0.125 < gs <= 1 in 0.125kt step
// 1: stopped, gs < 0.125kt
// 0: no data
else return 0;
}
// Apply possible corrections to the 14-byte message in "in", storing the result in "out"
//
// If the message has a correct CRC, copies in to out unchanged and returns 0
// If the message has correctable errors, applies the corrections to out and returns the number of corrected errors
// If the message is uncorrectable (this may mean the message type does not have CRC coverage), returns -1
// is this message a long-form message with a DF that uses Parity/Interrogator?
static bool isLongPIMessage(const unsigned char *msg)
{
const unsigned df = getbits(msg, 1, 5);
if (df == 17 || df == 18)
return true;
return false;
}
// is this message a short-form message with a DF that uses Parity/Interrogator?
static bool isShortPIMessage(const unsigned char *msg)
{
const unsigned df = getbits(msg, 1, 5);
return (df == 11); // assume IID==0
}
#define UNCHECKED_SYNDROME 0xFFFFFFFFU
static int correctMessage(const unsigned char *in, unsigned char *out, uint32_t *short_syndrome, uint32_t *long_syndrome)
{
// Possible DF values of the first byte of a message that could be a valid DF11/17/18
// message after correction. See tools/df-correction-arrays.py for generator code.
// This is used to shortcut message correction so that we don't bother computing a CRC over
// messages that couldn't possibly become one of those message types.
// These are bitsets, where the bit with value 1<<N represents a match for DF N
static const uint32_t df_correctable_short[MODES_MAX_BITERRORS + 1] = {
0x00000800, 0x08008e08, 0x08008e08
};
static const uint32_t df_correctable_long[MODES_MAX_BITERRORS + 1] = {
0x00060000, 0x066f0006, 0x6fff066f
};
*short_syndrome = UNCHECKED_SYNDROME;
*long_syndrome = UNCHECKED_SYNDROME;
// Try to correct, including corrections to the initial 5 bit DF field
// that determines message format
const unsigned uncorrected_df = getbits(in, 1, 5);
const uint32_t df_bit = 1 << uncorrected_df;
// Select the right bitset based on the maximum number of bit errors in the DF field that we could correct.
// nb: strictly speaking, --no-fix-df doesn't _entirely_ disable correction of the DF field when nfix_crc == 2
// (DF17 could be corrected to DF18 or vice versa), but it does disable the CPU hungry part of it.
const unsigned fix_df_bits = (Modes.fix_df ? Modes.nfix_crc : 0);
struct errorinfo *long_ei = NULL;
if (df_correctable_long[fix_df_bits] & df_bit) {
*long_syndrome = modesChecksum(in, MODES_LONG_MSG_BITS);
if (isLongPIMessage(in) && *long_syndrome == 0) {
// DF17/18 message with correct checksum
memcpy(out, in, MODES_LONG_MSG_BYTES);
return 0;
}
long_ei = modesChecksumDiagnose(*long_syndrome, MODES_LONG_MSG_BITS);
}
struct errorinfo *short_ei = NULL;
if (df_correctable_short[fix_df_bits] & df_bit) {
*short_syndrome = modesChecksum(in, MODES_SHORT_MSG_BITS);
if (isShortPIMessage(in) && (*short_syndrome & 0xFFFF80) == 0) {
// DF11 message with correct checksum
// (low 7 bits may be IID)
memcpy(out, in, MODES_SHORT_MSG_BYTES);
return 0;
}
short_ei = modesChecksumDiagnose(*short_syndrome, MODES_SHORT_MSG_BITS); // assume IID == 0
}
// Might be a damaged DF11/17/18, or might be another message type that doesn't have a full CRC
unsigned long_errors = (long_ei ? long_ei->errors : 999);
unsigned short_errors = (short_ei ? short_ei->errors : 999);
// If both 56-bit and 112-bit corrections are possible:
// try the correction with fewer error bits first
// if there's a tie, try the 112-bit version first
if (long_ei && long_errors <= short_errors) {
memcpy(out, in, MODES_LONG_MSG_BYTES);
modesChecksumFix(out, long_ei);
if (isLongPIMessage(out)) {
// valid DF17/18 message after corrections
return long_errors;
}
}
// Don't try to correct >1 error in DF11 (see crc.c)
if (short_ei && short_errors == 1) {
memcpy(out, in, MODES_SHORT_MSG_BYTES);
modesChecksumFix(out, short_ei);
if (isShortPIMessage(out)) {
// valid DF11 message after corrections
return short_errors;
}
}
if (long_ei && long_errors > short_errors) {
memcpy(out, in, MODES_LONG_MSG_BYTES);
modesChecksumFix(out, long_ei);
if (isLongPIMessage(out)) {
// valid DF17/18 message after corrections
return long_errors;
}
}
// Nothing more to try, we can't correct this one further
memcpy(out, in, MODES_LONG_MSG_BYTES);
return -1;
}
// Score how plausible this ModeS message looks.
// The more positive, the more reliable the message is.
score_rank scoreModesMessage(const unsigned char *uncorrected)
{
// This is a "valid" DF0 message, but it's not useful; we discard these messages
static const unsigned char all_zeros[MODES_SHORT_MSG_BYTES] = { 0, 0, 0, 0, 0, 0, 0 };
if (!memcmp(all_zeros, uncorrected, sizeof(all_zeros)))
return SR_ALL_ZEROS;
// try to produce a corrected DF11/17/18, including correcting the DF bits
unsigned char corrected[14];
uint32_t short_syndrome, long_syndrome;
int corrections = correctMessage(uncorrected, corrected, &short_syndrome, &long_syndrome);
unsigned df = getbits(corrected, 1, 5); // Downlink Format
switch (df) {
case 0: // short air-air surveillance
case 4: // surveillance, altitude reply
case 5: // surveillance, altitude reply
{
if (short_syndrome == UNCHECKED_SYNDROME)
short_syndrome = modesChecksum(corrected, MODES_SHORT_MSG_BITS);
bool recent = icaoFilterTest(short_syndrome);
return recent ? SR_UNRELIABLE_KNOWN : SR_UNRELIABLE_UNKNOWN;
}
case 16: // long air-air surveillance
case 20: // Comm-B, altitude reply
case 21: // Comm-B, identity reply
{
if (long_syndrome == UNCHECKED_SYNDROME)
long_syndrome = modesChecksum(corrected, MODES_LONG_MSG_BITS);
bool recent = icaoFilterTest(long_syndrome);
return recent ? SR_UNRELIABLE_KNOWN : SR_UNRELIABLE_UNKNOWN;
}
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)
{
if (!Modes.enable_df24)
return SR_UNCORRECTABLE;
if (long_syndrome == UNCHECKED_SYNDROME)
long_syndrome = modesChecksum(corrected, MODES_LONG_MSG_BITS);
bool recent = icaoFilterTest(long_syndrome);
return recent ? SR_UNRELIABLE_KNOWN : SR_UNRELIABLE_UNKNOWN;
}
case 11:
{
// DF11 All-call reply
uint32_t addr = getbits(corrected, 9, 32);
if (short_syndrome == UNCHECKED_SYNDROME)
short_syndrome = modesChecksum(corrected, MODES_SHORT_MSG_BITS);
uint32_t iid = short_syndrome & 0x7F;
bool recent = icaoFilterTest(addr);
switch (corrections) {
case 0:
if (iid == 0)
return recent ? SR_DF11_ACQ_KNOWN : SR_DF11_ACQ_UNKNOWN;
else
return recent ? SR_DF11_IID_KNOWN : SR_DF11_IID_UNKNOWN;
case 1:
if (iid == 0)
return recent ? SR_DF11_ACQ_1ERROR_KNOWN : SR_DF11_ACQ_1ERROR_UNKNOWN;
else
return recent ? SR_DF11_IID_1ERROR_KNOWN : SR_DF11_IID_1ERROR_UNKNOWN;
default:
return SR_UNCORRECTABLE;
}
}
case 17: // Extended squitter
{
uint32_t addr = getbits(corrected, 9, 32);
bool recent = icaoFilterTest(addr);
switch (corrections) {
case 0:
return recent ? SR_DF17_KNOWN : SR_DF17_UNKNOWN;
case 1:
return recent ? SR_DF17_1ERROR_KNOWN : SR_DF17_1ERROR_UNKNOWN;
case 2:
return recent ? SR_DF17_2ERROR_KNOWN : SR_DF17_2ERROR_UNKNOWN;
default:
return SR_UNCORRECTABLE;
}
}
case 18: // Extended squitter/non-transponder
{
uint32_t addr = getbits(corrected, 9, 32);
bool recent = icaoFilterTest(addr | ICAO_FILTER_ADSB_NT); // only look for previous DF18 activity
switch (corrections) {
case 0:
return recent ? SR_DF18_KNOWN : SR_DF18_UNKNOWN;
case 1:
return recent ? SR_DF18_1ERROR_KNOWN : SR_DF18_1ERROR_UNKNOWN;
case 2:
return recent ? SR_DF18_2ERROR_KNOWN : SR_DF18_2ERROR_UNKNOWN;
default:
return SR_UNCORRECTABLE;
}
}
default:
// unknown message type
return SR_UNKNOWN_DF;
}
}
static const char *score_to_string(score_rank score)
{
switch (score) {
case SR_NOT_SET: return "NOT_SET";
case SR_UNKNOWN_THRESHOLD: return "UNKNOWN_THRESHOLD";
case SR_ACCEPT_THRESHOLD: return "ACCEPT_THRESHOLD";
case SR_ALL_ZEROS: return "ALL_ZEROS";
case SR_UNKNOWN_DF: return "UNKNOWN_DF";
case SR_UNCORRECTABLE: return "UNCORRECTABLE";
case SR_UNRELIABLE_UNKNOWN: return "UNRELIABLE_UNKNOWN";
case SR_UNRELIABLE_KNOWN: return "UNRELIABLE_KNOWN";
case SR_DF11_IID_1ERROR_UNKNOWN: return "DF11_IID_1ERROR_UNKNOWN";
case SR_DF11_ACQ_1ERROR_UNKNOWN: return "DF11_ACQ_1ERROR_UNKNOWN";
case SR_DF11_IID_UNKNOWN: return "DF11_IID_UNKNOWN";
case SR_DF11_ACQ_UNKNOWN: return "DF11_ACQ_UNKNOWN";
case SR_DF11_IID_1ERROR_KNOWN: return "DF11_IID_1ERROR_KNOWN";
case SR_DF11_ACQ_1ERROR_KNOWN: return "DF11_ACQ_1ERROR_KNOWN";
case SR_DF11_IID_KNOWN: return "DF11_IID_KNOWN";
case SR_DF11_ACQ_KNOWN: return "DF11_ACQ_KNOWN";
case SR_DF17_2ERROR_UNKNOWN: return "DF17_2ERROR_UNKNOWN";
case SR_DF17_2ERROR_KNOWN: return "DF17_2ERROR_KNOWN";
case SR_DF17_1ERROR_UNKNOWN: return "DF17_1ERROR_UNKNOWN";
case SR_DF17_1ERROR_KNOWN: return "DF17_1ERROR_KNOWN";
case SR_DF17_UNKNOWN: return "DF17_UNKNOWN";
case SR_DF17_KNOWN: return "DF17_KNOWN";
case SR_DF18_2ERROR_UNKNOWN: return "DF18_2ERROR_UNKNOWN";
case SR_DF18_2ERROR_KNOWN: return "DF18_2ERROR_KNOWN";
case SR_DF18_1ERROR_UNKNOWN: return "DF18_1ERROR_UNKNOWN";
case SR_DF18_1ERROR_KNOWN: return "DF18_1ERROR_KNOWN";
case SR_DF18_UNKNOWN: return "DF18_UNKNOWN";
case SR_DF18_KNOWN: return "DF18_KNOWN";
}
return "<bad value>";
}
static void decodeExtendedSquitter(struct modesMessage *mm);
//
//=========================================================================
//
// Decode a raw Mode S message demodulated as a stream of bytes by detectModeS(),
// and split it into fields populating a modesMessage structure.
//
// return 0 if all OK
// <0 if it's a bad message
//
int decodeModesMessage(struct modesMessage *mm, const unsigned char *in)
{
// score the message if needed (it might be coming off the network)
if (mm->score == SR_NOT_SET)
mm->score = scoreModesMessage(in);
if (mm->score < SR_UNKNOWN_THRESHOLD)
return -1;
if (mm->score < SR_ACCEPT_THRESHOLD)
return -2;
// Preserve the original uncorrected copy for later forwarding
memcpy(mm->verbatim, in, MODES_LONG_MSG_BYTES);
// Apply corrections to our local copy
uint32_t short_syndrome, long_syndrome;
int corrections = correctMessage(in, mm->msg, &short_syndrome, &long_syndrome);
const unsigned char *msg = mm->msg;
// Get the message type ASAP as other operations depend on this
mm->msgtype = getbits(msg, 1, 5); // Downlink Format
mm->msgbits = modesMessageLenByType(mm->msgtype);
if (mm->msgtype & 16) {
if (long_syndrome == UNCHECKED_SYNDROME)
long_syndrome = modesChecksum(mm->msg, MODES_LONG_MSG_BITS);
mm->crc = long_syndrome;
} else {
if (short_syndrome == UNCHECKED_SYNDROME)
short_syndrome = modesChecksum(mm->msg, MODES_SHORT_MSG_BITS);
mm->crc = short_syndrome;
}
mm->correctedbits = corrections > 0 ? corrections : 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, identity reply
case 16: // long air-air surveillance
// These message types use Address/Parity
// so we can't check the CRC and must infer the transmitter's address
mm->source = SOURCE_MODE_S;
mm->addr = mm->crc;
mm->reliable = 0;
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;
mm->source = SOURCE_MODE_S_CHECKED;
mm->reliable = (mm->IID == 0 && mm->correctedbits == 0);
break;
case 17: // Extended squitter
case 18: { // Extended squitter/non-transponder
// These message types use Parity/Interrogator, but are specified to set II=0
mm->source = SOURCE_ADSB; // TIS-B decoding will override this if needed
mm->reliable = (mm->correctedbits == 0);
break;
}
case 20: // Comm-B, altitude reply
case 21: // Comm-B, identity reply
// These message types either use Address/Parity
// 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.
mm->source = SOURCE_MODE_S;
mm->addr = mm->crc;
mm->reliable = 0;
break;
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 messages use Address/Parity,
// and also use some of the DF bits to carry data. Remap them all to a single
// DF for simplicity.
mm->msgtype = 24;
mm->source = SOURCE_MODE_S;
mm->addr = mm->crc;
mm->reliable = 0;
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_baro = decodeAC13Field(mm->AC, &mm->altitude_baro_unit);
if (mm->altitude_baro != INVALID_ALTITUDE)
mm->altitude_baro_valid = 1;
}
}
// 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, see Figure 2-2 ADS-B Message BaselineFormat Structure)
if (mm->msgtype == 18) {
mm->CF = getbits(msg, 6, 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->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) {
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->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 == 11 && mm->IID == 0))) {
// DF17 ADS-B or DF11 acquisition squitter. Mark as known Mode-S source
icaoFilterAdd(mm->addr);
}
if (!mm->correctedbits && mm->msgtype == 18) {
// Mark as known ADS-B (NT) source
icaoFilterAdd(mm->addr | ICAO_FILTER_ADSB_NT);
}
// MLAT overrides all other sources
if (mm->remote && mm->timestampMsg == MAGIC_MLAT_TIMESTAMP)
mm->source = SOURCE_MLAT;
// all done
return 0;
}
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)];
mm->callsign[8] = 0;
mm->callsign_valid = 1;
// actually valid?
for (unsigned 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] != ' ') {
// Bad callsign, ignore it
mm->callsign_valid = 0;
break;
}
}
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;
// 1-5: ME type
// 6-8: ME subtype
mm->mesub = getbits(me, 6, 8);
if (mm->mesub < 1 || mm->mesub > 4)
return;
// 9: IMF or Intent Change
if (check_imf && getbit(me, 9))
setIMF(mm);
// 10: reserved
// 11-13: NACv (NUCr in v0, maps directly to NACv in v2)
mm->accuracy.nac_v_valid = 1;
mm->accuracy.nac_v = getbits(me, 11, 13);
// 14-35: speed/velocity depending on subtype
switch (mm->mesub) {
case 1: case 2:
{
// 14: E/W direction
// 15-24: E/W speed
// 25: N/S direction
// 26-35: N/S speed
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->gs.v0 = mm->gs.v2 = mm->gs.selected = sqrtf((ns_vel * ns_vel) + (ew_vel * ew_vel) + 0.5);
mm->gs_valid = 1;
if (mm->gs.selected > 0) {
float ground_track = atan2(ew_vel, ns_vel) * 180.0 / M_PI;
// We don't want negative values but a 0-360 scale
if (ground_track < 0)
ground_track += 360;
mm->heading = ground_track;
mm->heading_type = HEADING_GROUND_TRACK;
mm->heading_valid = 1;
}
}
break;
}
case 3: case 4:
{
// 14: heading status
// 15-24: heading
if (getbit(me, 14)) {
mm->heading_valid = 1;
mm->heading = getbits(me, 15, 24) * 360.0 / 1024.0;
mm->heading_type = HEADING_MAGNETIC_OR_TRUE;
}
// 25: airspeed type
// 26-35: airspeed
unsigned airspeed = getbits(me, 26, 35);
if (airspeed) {
unsigned speed = (airspeed - 1) * (mm->mesub == 4 ? 4 : 1);
if (getbit(me, 25)) {
mm->tas_valid = 1;
mm->tas = speed;
} else {
mm->ias_valid = 1;
mm->ias = speed;
}
}
break;
}
}
// 36: vert rate source
// 37: vert rate sign
// 38-46: vert rate magnitude
unsigned vert_rate = getbits(me, 38, 46);
unsigned vert_rate_is_baro = getbit(me, 36);
if (vert_rate) {
int rate = (vert_rate - 1) * (getbit(me, 37) ? -64 : 64);
if (vert_rate_is_baro) {
mm->baro_rate = rate;
mm->baro_rate_valid = 1;
} else {
mm->geom_rate = rate;
mm->geom_rate_valid = 1;
}
}
// 47-48: reserved
// 49: baro/geom delta sign
// 50-56: baro/geom delta magnitude
unsigned raw_delta = getbits(me, 50, 56);
if (raw_delta) {
mm->geom_delta_valid = 1;
mm->geom_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;
mm->airground = AG_GROUND; // definitely.
mm->cpr_valid = 1;
mm->cpr_type = CPR_SURFACE;
// 6-12: Movement
unsigned movement = getbits(me, 6, 12);
if (movement > 0 && movement < 125) {
mm->gs_valid = 1;
mm->gs.selected = mm->gs.v0 = decodeMovementFieldV0(movement); // assumed v0 until told otherwise
mm->gs.v2 = decodeMovementFieldV2(movement);
}
// 13: Heading/track status
// 14-20: Heading/track
if (getbit(me, 13)) {
mm->heading_valid = 1;
mm->heading = getbits(me, 14, 20) * 360.0 / 128.0;
mm->heading_type = HEADING_TRACK_OR_HEADING;
}
// 21: IMF or T flag
if (check_imf && getbit(me, 21))
setIMF(mm);
// 22: F flag (odd/even)
mm->cpr_odd = getbit(me, 22);
// 23-39: CPR encoded latitude
mm->cpr_lat = getbits(me, 23, 39);
// 40-56: CPR encoded longitude
mm->cpr_lon = getbits(me, 40, 56);