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dump1090/comm_b.c
Oliver Jowett 39aeb7a48c Remove comm-b "consistency" bonuses, look at inconsistent data only. 
Rescale penalties so badly inconsistent messages will tend to get
the same penalty in all formats, so the ambiguity check kicks in.

With this final change the HEADING_SPEED and TRACK_TURN message rates
are almost identical, which is what we'd expect to see if the
interrogator is asking for both at about the same rate.

Before:

    567 AMBIGUOUS
   7445 GICB_CAPS
  19173 EMPTY_RESPONSE
  75252 UNKNOWN
  76469 AIRCRAFT_IDENT
 127603 ACAS_RA
 212654 HEADING_SPEED
 223068 DATALINK_CAPS
 229807 TRACK_TURN
 269267 VERTICAL_INTENT
    466 "suspicious"

After:

   7445 GICB_CAPS
  17432 AMBIGUOUS
  19173 EMPTY_RESPONSE
  75252 UNKNOWN
  76469 AIRCRAFT_IDENT
 127603 ACAS_RA
 212718 HEADING_SPEED
 212878 TRACK_TURN
 223068 DATALINK_CAPS
 269267 VERTICAL_INTENT
    328 "suspicious"
2019-05-02 18:19:27 +08:00

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// Part of dump1090, a Mode S message decoder for RTLSDR devices.
//
// comm_b.c: Comm-B message decoding
//
// Copyright (c) 2017 FlightAware, LLC
// Copyright (c) 2017 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/>.
#include "dump1090.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[9];
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;
}
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