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comm_b.c
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comm_b.c
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// Part of readsb, a Mode-S/ADSB/TIS message decoder.
//
// comm_b.c: Comm-B message decoding
//
// Copyright (c) 2020 Michael Wolf <michael@mictronics.de>
//
// This code is based on a detached fork of dump1090-fa.
//
// Copyright (c) 2017 FlightAware, LLC
// Copyright (c) 2017 Oliver Jowett <oliver@mutability.co.uk>
//
// This file is free software: you can redistribute it and/or modify
// it under the terms of the GNU General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// 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/>.
#include "readsb.h"
#include "ais_charset.h"
typedef int (*CommBDecoderFn)(struct modesMessage *, bool);
static int decodeEmptyResponse(struct modesMessage *mm, bool store);
static int decodeBDS10(struct modesMessage *mm, bool store);
static int decodeBDS17(struct modesMessage *mm, bool store);
static int decodeBDS20(struct modesMessage *mm, bool store);
static int decodeBDS30(struct modesMessage *mm, bool store);
static int decodeBDS40(struct modesMessage *mm, bool store);
static int decodeBDS50(struct modesMessage *mm, bool store);
static int decodeBDS60(struct modesMessage *mm, bool store);
static CommBDecoderFn comm_b_decoders[] = {
&decodeEmptyResponse,
&decodeBDS10,
&decodeBDS20,
&decodeBDS30,
&decodeBDS17,
&decodeBDS40,
&decodeBDS50,
&decodeBDS60
};
void decodeCommB(struct modesMessage *mm) {
mm->commb_format = COMMB_UNKNOWN;
// If DR or UM are set, this message is _probably_ noise
// as nothing really seems to use the multisite broadcast stuff?
// Also skip anything that had errors corrected
if (mm->DR != 0 || mm->UM != 0 || mm->correctedbits > 0) {
return;
}
// This is a bit hairy as we don't know what the requested register was
int bestScore = 0;
CommBDecoderFn bestDecoder = NULL;
int ambiguous = 0;
for (unsigned i = 0; i < (sizeof (comm_b_decoders) / sizeof (comm_b_decoders[0])); ++i) {
int score = comm_b_decoders[i](mm, false);
if (score > bestScore) {
bestScore = score;
bestDecoder = comm_b_decoders[i];
ambiguous = 0;
} else if (score == bestScore) {
ambiguous = 1;
}
}
if (bestDecoder) {
if (ambiguous) {
mm->commb_format = COMMB_AMBIGUOUS;
} else {
// decode it
bestDecoder(mm, true);
}
}
}
static int decodeEmptyResponse(struct modesMessage *mm, bool store) {
for (unsigned i = 0; i < 7; ++i) {
if (mm->MB[i] != 0) {
return 0;
}
}
if (store) {
mm->commb_format = COMMB_EMPTY_RESPONSE;
}
return 56;
}
// BDS1,0 Datalink capabilities
static int decodeBDS10(struct modesMessage *mm, bool store) {
unsigned char *msg = mm->MB;
// BDS identifier
if (msg[0] != 0x10) {
return 0;
}
// Reserved bits
if (getbits(msg, 10, 14) != 0) {
return 0;
}
// Looks plausible.
if (store) {
mm->commb_format = COMMB_DATALINK_CAPS;
}
return 56;
}
// BDS1,7 Common usage GICB capability report
static int decodeBDS17(struct modesMessage *mm, bool store) {
unsigned char *msg = mm->MB;
// reserved bits
if (getbits(msg, 25, 56) != 0) {
return 0;
}
int score = 0;
if (getbit(msg, 7)) {
score += 1; // 2,0 aircraft identification
} else {
// BDS2,0 is on almost everything
score -= 2;
}
// unlikely bits
if (getbit(msg, 10)) { // 4,1 next waypoint identifier
score -= 2;
}
if (getbit(msg, 11)) { // 4,2 next waypoint position
score -= 2;
}
if (getbit(msg, 12)) { // 4,3 next waypoint information
score -= 2;
}
if (getbit(msg, 13)) { // 4,4 meterological routine report
score -= 2;
}
if (getbit(msg, 14)) { // 4,4 meterological hazard report
score -= 2;
}
if (getbit(msg, 20)) { // 5,4 waypoint 1
score -= 2;
}
if (getbit(msg, 21)) { // 5,5 waypoint 2
score -= 2;
}
if (getbit(msg, 22)) { // 5,6 waypoint 3
score -= 2;
}
if (getbit(msg, 1) && getbit(msg, 2) && getbit(msg, 3) && getbit(msg, 4) && getbit(msg, 5)) {
// looks like ES capable
score += 5;
if (getbit(msg, 6)) {
// ES EDI
score += 1;
}
} else if (!getbit(msg, 1) && !getbit(msg, 2) && !getbit(msg, 3) && !getbit(msg, 4) && !getbit(msg, 5) && !getbit(msg, 6)) {
// not ES capable
score += 1;
} else {
// partial ES support, unlikely
score -= 12;
}
if (getbit(msg, 16) && getbit(msg, 24)) {
// track/turn, heading/speed
score += 2;
if (getbit(msg, 9)) {
// vertical intent
score += 1;
}
} else if (!getbit(msg, 16) && !getbit(msg, 24) && !getbit(msg, 9)) {
// neither
score += 1;
} else {
// unlikely
score -= 6;
}
if (store) {
mm->commb_format = COMMB_GICB_CAPS;
}
return score;
}
// BDS2,0 Aircraft identification
static int decodeBDS20(struct modesMessage *mm, bool store) {
char callsign[sizeof (mm->callsign)];
unsigned char *msg = mm->MB;
// BDS identifier
if (msg[0] != 0x20) {
return 0;
}
callsign[0] = ais_charset[getbits(msg, 9, 14)];
callsign[1] = ais_charset[getbits(msg, 15, 20)];
callsign[2] = ais_charset[getbits(msg, 21, 26)];
callsign[3] = ais_charset[getbits(msg, 27, 32)];
callsign[4] = ais_charset[getbits(msg, 33, 38)];
callsign[5] = ais_charset[getbits(msg, 39, 44)];
callsign[6] = ais_charset[getbits(msg, 45, 50)];
callsign[7] = ais_charset[getbits(msg, 51, 56)];
callsign[8] = 0;
// score based on number of valid characters
int score = 8;
int valid = 1;
for (unsigned i = 0; i < 8; ++i) {
if ((callsign[i] >= 'A' && callsign[i] <= 'Z') || (callsign[i] >= '0' && callsign[i] <= '9') || callsign[i] == ' ') {
score += 6;
} else if (callsign[i] == '@') {
// Padding (sometimes we get @@@@@@@@, i.e. BDS2,0 with all zeros - we do want to accept this as a BDS2,0 but not actually use the callsign)
valid = 0;
} else {
// Invalid
return 0;
}
}
if (store) {
mm->commb_format = COMMB_AIRCRAFT_IDENT;
if (valid) {
memcpy(mm->callsign, callsign, sizeof (mm->callsign));
mm->callsign_valid = 1;
}
}
return score;
}
// BDS3,0 ACAS RA
static int decodeBDS30(struct modesMessage *mm, bool store) {
unsigned char *msg = mm->MB;
// BDS identifier
if (msg[0] != 0x30) {
return 0;
}
if (store) {
mm->commb_format = COMMB_ACAS_RA;
}
// just accept it.
return 56;
}
// BDS4,0 Selected vertical intention
static int decodeBDS40(struct modesMessage *mm, bool store) {
unsigned char *msg = mm->MB;
unsigned mcp_valid = getbit(msg, 1);
unsigned mcp_raw = getbits(msg, 2, 13);
unsigned fms_valid = getbit(msg, 14);
unsigned fms_raw = getbits(msg, 15, 26);
unsigned baro_valid = getbit(msg, 27);
unsigned baro_raw = getbits(msg, 28, 39);
unsigned reserved_1 = getbits(msg, 40, 47);
unsigned mode_valid = getbit(msg, 48);
unsigned mode_raw = getbits(msg, 49, 51);
unsigned reserved_2 = getbits(msg, 52, 53);
unsigned source_valid = getbit(msg, 54);
unsigned source_raw = getbits(msg, 55, 56);
if (!mcp_valid && !fms_valid && !baro_valid && !mode_valid && !source_valid) {
return 0;
}
int score = 0;
unsigned mcp_alt = 0;
if (mcp_valid && mcp_raw != 0) {
mcp_alt = mcp_raw * 16;
if (mcp_alt >= 1000 && mcp_alt <= 50000) {
score += 13;
} else {
// unlikely altitude
return 0;
}
} else if (!mcp_valid && mcp_raw == 0) {
score += 1;
} else {
return 0;
}
unsigned fms_alt = 0;
if (fms_valid && fms_raw != 0) {
fms_alt = fms_raw * 16;
if (fms_alt >= 1000 && fms_alt <= 50000) {
score += 13;
} else {
// unlikely altitude
return 0;
}
} else if (!fms_valid && fms_raw == 0) {
score += 1;
} else {
return 0;
}
float baro_setting = 0;
if (baro_valid && baro_raw != 0) {
baro_setting = 800 + baro_raw * 0.1;
if (baro_setting >= 900 && baro_setting <= 1100) {
score += 13;
} else {
// unlikely pressure setting
return 0;
}
} else if (!baro_valid && baro_raw == 0) {
score += 1;
} else {
return 0;
}
if (reserved_1 != 0) {
return 0;
}
if (mode_valid) {
score += 4;
} else if (!mode_valid && mode_raw == 0) {
score += 1;
} else {
return 0;
}
if (reserved_2 != 0) {
return 0;
}
if (source_valid) {
score += 3;
} else if (!source_valid && source_raw == 0) {
score += 1;
} else {
return 0;
}
// small penalty for inconsistent data
if (mcp_valid && fms_valid && mcp_alt != fms_alt) {
score -= 4;
}
if (mcp_valid) {
unsigned remainder = mcp_alt % 500;
if (!(remainder < 16 || remainder > 484)) {
// mcp altitude is not a multiple of 500
score -= 4;
}
}
if (fms_valid) {
unsigned remainder = fms_alt % 500;
if (!(remainder < 16 || remainder > 484)) {
// fms altitude is not a multiple of 500
score -= 4;
}
}
if (store) {
mm->commb_format = COMMB_VERTICAL_INTENT;
if (mcp_valid) {
mm->nav.mcp_altitude_valid = 1;
mm->nav.mcp_altitude = mcp_alt;
}
if (fms_valid) {
mm->nav.fms_altitude_valid = 1;
mm->nav.fms_altitude = fms_alt;
}
if (baro_valid) {
mm->nav.qnh_valid = 1;
mm->nav.qnh = baro_setting;
}
if (mode_valid) {
mm->nav.modes_valid = 1;
mm->nav.modes =
((mode_raw & 4) ? NAV_MODE_VNAV : 0) |
((mode_raw & 2) ? NAV_MODE_ALT_HOLD : 0) |
((mode_raw & 1) ? NAV_MODE_APPROACH : 0);
}
if (source_valid) {
switch (source_raw) {
case 0:
mm->nav.altitude_source = NAV_ALT_UNKNOWN;
break;
case 1:
mm->nav.altitude_source = NAV_ALT_AIRCRAFT;
break;
case 2:
mm->nav.altitude_source = NAV_ALT_MCP;
break;
case 3:
mm->nav.altitude_source = NAV_ALT_FMS;
break;
default:
mm->nav.altitude_source = NAV_ALT_INVALID;
break;
}
} else {
mm->nav.altitude_source = NAV_ALT_INVALID;
}
}
return score;
}
// BDS5,0 Track and turn report
static int decodeBDS50(struct modesMessage *mm, bool store) {
unsigned char *msg = mm->MB;
unsigned roll_valid = getbit(msg, 1);
unsigned roll_sign = getbit(msg, 2);
unsigned roll_raw = getbits(msg, 3, 11);
unsigned track_valid = getbit(msg, 12);
unsigned track_sign = getbit(msg, 13);
unsigned track_raw = getbits(msg, 14, 23);
unsigned gs_valid = getbit(msg, 24);
unsigned gs_raw = getbits(msg, 25, 34);
unsigned track_rate_valid = getbit(msg, 35);
unsigned track_rate_sign = getbit(msg, 36);
unsigned track_rate_raw = getbits(msg, 37, 45);
unsigned tas_valid = getbit(msg, 46);
unsigned tas_raw = getbits(msg, 47, 56);
if (!roll_valid || !track_valid || !gs_valid || !tas_valid) {
return 0;
}
int score = 0;
float roll = 0;
if (roll_valid) {
roll = roll_raw * 45.0 / 256.0;
if (roll_sign) {
roll -= 90.0;
}
if (roll >= -40 && roll < 40) {
score += 11;
} else {
return 0;
}
} else if (!roll_valid && roll_raw == 0 && !roll_sign) {
score += 1;
} else {
return 0;
}
float track = 0;
if (track_valid) {
score += 12;
track = track_raw * 90.0 / 512.0;
if (track_sign) {
track += 180.0;
}
} else if (!track_valid && track_raw == 0 && !track_sign) {
score += 1;
} else {
return 0;
}
unsigned gs = 0;
if (gs_valid && gs_raw != 0) {
gs = gs_raw * 2;
if (gs >= 50 && gs <= 700) {
score += 11;
} else {
return 0;
}
} else if (!gs_valid && gs_raw == 0) {
score += 1;
} else {
return 0;
}
float track_rate = 0;
if (track_rate_valid) {
track_rate = track_rate_raw * 8.0 / 256.0;
if (track_rate_sign) {
track_rate -= 16;
}
if (track_rate >= -10.0 && track_rate <= 10.0) {
score += 11;
} else {
return 0;
}
} else if (!track_rate_valid && track_rate_raw == 0 && !track_rate_sign) {
score += 1;
} else {
return 0;
}
unsigned tas = 0;
if (tas_valid && tas_raw != 0) {
tas = tas_raw * 2;
if (tas >= 50 && tas <= 700) {
score += 11;
} else {
return 0;
}
} else if (!tas_valid && tas_raw == 0) {
score += 1;
} else {
return 0;
}
// small penalty for inconsistent data
if (gs_valid && tas_valid) {
int delta = abs((int) gs_valid - (int) tas_valid);
if (delta > 150) {
score -= 6;
}
}
// compute the theoretical turn rate and compare to track angle rate
if (roll_valid && tas_valid && tas > 0 && track_rate_valid) {
double turn_rate = 68625 * tan(roll * M_PI / 180.0) / (tas * 20 * M_PI);
double delta = fabs(turn_rate - track_rate);
if (delta > 2.0) {
score -= 6;
}
}
if (store) {
mm->commb_format = COMMB_TRACK_TURN;
if (roll_valid) {
mm->roll_valid = 1;
mm->roll = roll;
}
if (track_valid) {
mm->heading_valid = 1;
mm->heading = track;
mm->heading_type = HEADING_GROUND_TRACK;
}
if (gs_valid) {
mm->gs_valid = 1;
mm->gs.v0 = mm->gs.v2 = mm->gs.selected = gs;
}
if (track_rate_valid) {
mm->track_rate_valid = 1;
mm->track_rate = track_rate;
}
if (tas_valid) {
mm->tas_valid = 1;
mm->tas = tas;
}
}
return score;
}
// BDS6,0 Heading and speed report
static int decodeBDS60(struct modesMessage *mm, bool store) {
unsigned char *msg = mm->MB;
unsigned heading_valid = getbit(msg, 1);
unsigned heading_sign = getbit(msg, 2);
unsigned heading_raw = getbits(msg, 3, 12);
unsigned ias_valid = getbit(msg, 13);
unsigned ias_raw = getbits(msg, 14, 23);
unsigned mach_valid = getbit(msg, 24);
unsigned mach_raw = getbits(msg, 25, 34);
unsigned baro_rate_valid = getbit(msg, 35);
unsigned baro_rate_sign = getbit(msg, 36);
unsigned baro_rate_raw = getbits(msg, 37, 45);
unsigned inertial_rate_valid = getbit(msg, 46);
unsigned inertial_rate_sign = getbit(msg, 47);
unsigned inertial_rate_raw = getbits(msg, 48, 56);
if (!heading_valid || !ias_valid || !mach_valid || (!baro_rate_valid && !inertial_rate_valid)) {
return 0;
}
int score = 0;
float heading = 0;
if (heading_valid) {
heading = heading_raw * 90.0 / 512.0;
if (heading_sign) {
heading += 180.0;
}
score += 12;
} else if (!heading_valid && heading_raw == 0 && !heading_sign) {
score += 1;
} else {
return 0;
}
unsigned ias = 0;
if (ias_valid && ias_raw != 0) {
ias = ias_raw;
if (ias >= 50 && ias <= 700) {
score += 11;
} else {
return 0;
}
} else if (!ias_valid && ias_raw == 0) {
score += 1;
} else {
return 0;
}
float mach = 0;
if (mach_valid && mach_raw != 0) {
mach = mach_raw * 2.048 / 512;
if (mach >= 0.1 && mach <= 0.9) {
score += 11;
} else {
return 0;
}
} else if (!mach_valid && mach_raw == 0) {
score += 1;
} else {
return 0;
}
int baro_rate = 0;
if (baro_rate_valid) {
baro_rate = baro_rate_raw * 32;
if (baro_rate_sign) {
baro_rate -= 16384;
}
if (baro_rate >= -6000 && baro_rate <= 6000) {
score += 11;
} else {
return 0;
}
} else if (!baro_rate_valid && baro_rate_raw == 0) {
score += 1;
} else {
return 0;
}
int inertial_rate = 0;
if (inertial_rate_valid) {
inertial_rate = inertial_rate_raw * 32;
if (inertial_rate_sign) {
inertial_rate -= 16384;
}
if (inertial_rate >= -6000 && inertial_rate <= 6000) {
score += 11;
} else {
return 0;
}
} else if (!inertial_rate_valid && inertial_rate_raw == 0) {
score += 1;
} else {
return 0;
}
// small penalty for inconsistent data
// Should check IAS vs Mach at given altitude, but the maths is a little involved
if (baro_rate_valid && inertial_rate_valid) {
int delta = abs(baro_rate - inertial_rate);
if (delta > 2000) {
score -= 12;
}
}
if (store) {
mm->commb_format = COMMB_HEADING_SPEED;
if (heading_valid) {
mm->heading_valid = 1;
mm->heading = heading;
mm->heading_type = HEADING_MAGNETIC;
}
if (ias_valid) {
mm->ias_valid = 1;
mm->ias = ias;
}
if (mach_valid) {
mm->mach_valid = 1;
mm->mach = mach;
}
if (baro_rate_valid) {
mm->baro_rate_valid = 1;
mm->baro_rate = baro_rate;
}
if (inertial_rate_valid) {
// INS-derived data is treated as a "geometric rate" / "geometric altitude"
// elsewhere, so do the same here.
mm->geom_rate_valid = 1;
mm->geom_rate = inertial_rate;
}
}
return score;
}