/*
* Copyright (C) 2014 Jared Boone, ShareBrained Technology, Inc.
* Copyright (C) 2016 Furrtek
*
* This file is part of PortaPack.
*
* This program 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 2, or (at your option)
* any later version.
*
* This program 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; see the file COPYING. If not, write to
* the Free Software Foundation, Inc., 51 Franklin Street,
* Boston, MA 02110-1301, USA.
*/
#include "adsb.hpp"
#include "sine_table.hpp"
#include "utility.hpp"
#include <math.h>
namespace adsb {
void make_frame_adsb(ADSBFrame& frame, const uint32_t ICAO_address) {
frame.clear();
frame.push_byte((DF_ADSB << 3) | 5); // DF=17 and CA
frame.push_byte(ICAO_address >> 16);
frame.push_byte(ICAO_address >> 8);
frame.push_byte(ICAO_address & 0xFF);
}
// Civil aircraft ADS-B message type starts with Dowlink Format (DF=17) and frame is 112 bits long.
// All known DF's >=16 are long (112 bits). All known DF's <=15 are short (56 bits).(In this case 112 bits)
// Msg structure consists of five main parts :|DF=17 (5 bits)|CA (3 bits)|ICAO (24 bits)|ME (56 bits)|CRC (24 bits)
// Aircraft identification and category message structure, the ME (56 bits) = TC,5 bits | CA,3 bits | C1,6 bits | C2,6 bits | C3,6 | C4,6 | C5,6 | C6,6 | C7,6 | C8,6
// TC : (1..4) : Aircraft identification Type Code . // TC : 9 to 18: Airbone postion // TC : 19 Airbone velocity .
// In this encode_frame_identification function we are using DF = 17 (112 bits long) and TC=4)
void encode_frame_id(ADSBFrame& frame, const uint32_t ICAO_address, const std::string& callsign) {
std::string callsign_formatted(8, '_');
uint64_t callsign_coded = 0;
uint32_t c, s;
char ch;
make_frame_adsb(frame, ICAO_address); // Header DF=17 Downlink Format = ADS-B message (frame 112 bits)
frame.push_byte(TC_IDENT << 3); // 5 top bits ME = TC = we use fix 4 , # Type aircraft Identification Category = TC_IDENT = 4,
// Translate and encode callsign
for (c = 0; c < 8; c++) {
ch = callsign[c];
for (s = 0; s < 64; s++)
if (ch == icao_id_lut[s]) break;
if (s == 64) {
ch = ' '; // Invalid character
s = 32;
}
callsign_coded <<= 6;
callsign_coded |= s;
//callsign[c] = ch;
}
// Insert callsign in frame
for (c = 0; c < 6; c++)
frame.push_byte((callsign_coded >> ((5 - c) * 8)) & 0xFF);
frame.make_CRC();
}
std::string decode_frame_id(ADSBFrame& frame) {
std::string callsign = "";
uint8_t * raw_data = frame.get_raw_data();
uint64_t callsign_coded = 0;
uint32_t c;
// Frame bytes to long
for (c = 5; c < 11; c++) {
callsign_coded <<= 8;
callsign_coded |= raw_data[c];
}
// Long to 6-bit characters
for (c = 0; c < 8; c++) {
callsign.append(1, icao_id_lut[(callsign_coded >> 42) & 0x3F]);
callsign_coded <<= 6;
}
return callsign;
}
/*void generate_frame_emergency(ADSBFrame& frame, const uint32_t ICAO_address, const uint8_t code) {
make_frame_mode_s(frame, ICAO_address);
frame.push_byte((28 << 3) + 1); // TC = 28 (Emergency), subtype = 1 (Emergency)
frame.push_byte(code << 5);
frame.make_CRC();
}*/
// Mode S services. (Mode Select Beacon System). There are two types of Mode S interrogations: The short (56 bits) . and the long (112 bits )
// All known DF's >=16 are long frame msg (112 bits). All known DF's <=15 are short frame msgs (56 bits).(In this case 112 bits)
// Identity squawk replies can be DF=5 (Surveillance Identity reply)(56 bits) / DF 21 (Comm-B with Identity reply) (112 bits)
// DF 21: Comm-B with identity reply structure = |DF=21(5 bits)|FS (3 bits)|DR (5 bits)|UM (6 bits) |Identity squawk code (13 bits) |MB (56 bits) |CRC (24 bits) (total 112 bits)
// Comm-B messages count for a large portion of the Mode S selective interrogation responses.(means, only transmitted information upon selective request)
// Comm-B messages protocol supports many different types of msg's (up to 255).The three more popular ones are the following ones:
// (a) Mode S ELementary Surveillance (ELS) / (b) Mode S EnHanced Surveillance (EHS) / (c) Meteorological information
// Comm-B Data Selector (BDS) is an 8-bit code that determines which information to be included in the MB fields
void encode_frame_squawk(ADSBFrame& frame, const uint16_t squawk) {
uint16_t squawk_coded;
uint8_t UM_field=0b111101, FS=0b010, DR=0b00001 ;
// To be sent those fields, (56 bits). We should store byte by byte into the frame , and It will be transmitted byte to byte same FIFO order.
// DF 5 bits 5 DF=21 (5 top bits) Downlink Format
// FS 3 bits 0b000, FS (3 bottom bits) (Flight status ) = 000 : no alert, no SPI, aircraft is airborne
// DR 5 bits 0b00001 DR (Downlink request) (5 top bits) = 00000 : no downlink request (In surveillance replies, only values 0, 1, 4, and 5 are used.)
// UM 6 bits 0b000010 UM (Utility message)= 000000, Utility message (UM): 6 bits, contains transponder communication status information.(IIS + IDS)
// Identity_code 13 bits squawk id_code in special interleaved format.
// MB 56 bits
// CRC partity 24 bits parity checksum , cyclic redundancy chek.
frame.clear();
frame.push_byte( ( DF_EHS_SQUAWK << 3 ) | FS ); // DF=21 (5bits) + FS (3bits, 010 : alert, NO SPI, aircraft is airborne)
frame.push_byte(( DR <<3 ) | ( UM_field>>3) ); // DR (5bits, 00001 : downlink request + 3 top bits of UM , let's use 0b111000
// 12 11 10 9 8 7 6 5 4 3 2 1 0 (Original notes) bit weight position----------------------
// 32 31 30 29 28 27 26 25 24 23 22 21 20 (it was wrong , now corrected) bit order inside frame msg
// D4 B4 D2 B2 D1 B1 __ A4 C4 A2 C2 A1 C1 standard spec order of the 13 bits, to be sent , each octal digit = 3 bits , (example A=7 binary A4 A2 A0 = 111
// ABCD = code (octal, 0000~7777)
// FEDCBA9876543210
// xAAAxBBBxCCCxDDD 4 x 3 bits (each octal digit)
// x421x421x421x421 binary weight of each binary position, example AAA = 7 = 111 -------------------------
// Additional , expanded notes -------------------------------
// Identity squawk code ABCD = code (octal, 0000~7777) , input concatenated squawk : 4 octal digits ,A4 A2 A1-B4 B2 B1-C4 C2 C1-D4 D2 D1.
// 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 bit position of the frame msg, (Squawk id is bit 20-32, from C1..D4).
// UM4 UM2 UM1 C1 A1 C2 A2 C4 A4 X B1 D1 B2 D2 B4 D4 3 lower bit UM4,UM2,UM1 of the UM (6bits), and we should re-order the 13 bits ABCD changing 12 bit poistion based on std.
// 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 Two bytes , bit position to be send.
squawk_coded = ( (( UM_field & (0b111)) <<13) | ((squawk << 9) & 0x1000) ) | // C1 It also leaves in the top 3 lower bottom bitd part of UM field.
((squawk << 2) & 0x0800) | // A1
((squawk << 6) & 0x0400) | // C2
((squawk >> 1) & 0x0200) | // A2
((squawk << 3) & 0x0100) | // C4
((squawk >> 4) & 0x0080) | // A4
((squawk >> 1) & 0x0020) | // B1
((squawk << 4) & 0x0010) | // D1
((squawk >> 4) & 0x0008) | // B2
((squawk << 1) & 0x0004) | // D2
((squawk >> 7) & 0x0002) | // B4
((squawk >> 2) & 0x0001); // D4
frame.push_byte(squawk_coded>>8); // UM4 UM2 UM1 C1 A1 C2 A2 C4 that is the correct order, confirmed with dump1090
frame.push_byte(squawk_coded); // A4 X(1) B1 D1 B2 D2 B4 D4 that is the correct order, confirmed with dupm1090
// DF 21 messages , has 56 bits more after 13 bits of squawk, we should add MB (56 bits)
// In this example, we are adding fixed MB = Track and turn report (BDS 5,0) decoding MB example = "F9363D3BBF9CE9" (56 bits)
// # -9.7, roll angle (deg)
// # 140.273, track angle (deg)
// # -0.406, track angle rate (deg/s)
// # 476, ground speed (kt)
// # 466, TAS (kt)
frame.push_byte(0xF9);frame.push_byte(0x36);frame.push_byte(0x3D);frame.push_byte(0x3B); // If we deltele those two lines, to send this fixed MB (56 bits),
frame.push_byte(0xBF);frame.push_byte(0x9C);frame.push_byte(0xE9); // current fw is padding with 56 x 0's to complete 112 bits msg.
frame.make_CRC();
}
float cpr_mod(float a, float b) {
return a - (b * floor(a / b));
}
int cpr_NL_precise(float lat) {
return (int) floor(2 * PI / acos(1 - ((1 - cos(PI / (2 * NZ))) / pow(cos(PI * lat / 180), 2))));
}
int cpr_NL_approx(float lat) {
if (lat < 0)
lat = -lat; // Symmetry
for (size_t c = 0; c < 58; c++) {
if (lat < adsb_lat_lut[c])
return 59 - c;
}
return 1;
}
int cpr_NL(float lat) {
// TODO prove that the approximate function is good
// enough for the precision we need. Uncomment if
// that is true. No performance penalty was noticed
// from testing, but if you find it might be an issue,
// switch to cpr_NL_approx() instead:
//return cpr_NL_approx(lat);
return cpr_NL_precise(lat);
}
int cpr_N(float lat, int is_odd) {
int nl = cpr_NL(lat) - is_odd;
if (nl < 1)
nl = 1;
return nl;
}
float cpr_Dlon(float lat, int is_odd) {
return 360.0 / cpr_N(lat, is_odd);
}
// An ADS-B frame Civil aircraft message type starts with Dowlink Format (DF=17) and frame is 112 bits long.
// All known DF's >=16 are long (112 bits). All known DF's <=15 are short (56 bits). (In this case 112 bits)
// Msg structure consists of five main parts :|DF=17 (5 bits)|CA (3 bits)|ICAO (24 bits)|ME (56 bits)|CRC (24 bits)
// Airborne position msg struct, the ME (56 bits) = |TC,5 bits| SS, 2 bits | SAF, 1 | ALT, 12 | T, 1 | F, 1 | LAT-CPR, 17 | LON-CPR, 17
// TC : (1..4) : Aircraft identification Type Code. // TC : 9 to 18: Airbone postion and altitude // TC : 19 Airbone velocity .
// Airborne position message is used to broadcast the position and altitude of the aircraft. It has the Type Code 9–18 and 20–22. (here , we use TC=11)
void encode_frame_pos(ADSBFrame& frame, const uint32_t ICAO_address, const int32_t altitude,
const float latitude, const float longitude, const uint32_t time_parity) {
uint32_t altitude_coded;
uint32_t lat, lon;
float delta_lat, yz, rlat, delta_lon, xz;
make_frame_adsb(frame, ICAO_address); // Header DF=17 (long frame 112 bits)
frame.push_byte(TC_AIRBORNE_POS << 3); // Bits 2~1: Surveillance Status, bit 0: NICsb
altitude_coded = (altitude + 1000) / 25; // 25ft precision, insert Q-bit (1)
altitude_coded = ((altitude_coded & 0x7F0) << 1) | 0x10 | (altitude_coded & 0x0F);
frame.push_byte(altitude_coded >> 4); // Top-most altitude bits
// CPR encoding
// Info from: http://antena.fe.uni-lj.si/literatura/Razno/Avionika/modes/CPRencoding.pdf
delta_lat = 360.0 / ((4.0 * NZ) - time_parity); // NZ = 15
yz = floor(CPR_MAX_VALUE * (cpr_mod(latitude, delta_lat) / delta_lat) + 0.5);
rlat = delta_lat * ((yz / CPR_MAX_VALUE) + floor(latitude / delta_lat));
if ((cpr_NL(rlat) - time_parity) > 0)
delta_lon = 360.0 / cpr_N(rlat, time_parity);
else
delta_lon = 360.0;
xz = floor(CPR_MAX_VALUE * (cpr_mod(longitude, delta_lon) / delta_lon) + 0.5);
lat = cpr_mod(yz, CPR_MAX_VALUE);
lon = cpr_mod(xz, CPR_MAX_VALUE);
frame.push_byte((altitude_coded << 4) | ((uint32_t)time_parity << 2) | (lat >> 15)); // T = 0
frame.push_byte(lat >> 7);
frame.push_byte((lat << 1) | (lon >> 16));
frame.push_byte(lon >> 8);
frame.push_byte(lon);
frame.make_CRC();
}
// Decoding method from dump1090
adsb_pos decode_frame_pos(ADSBFrame& frame_even, ADSBFrame& frame_odd) {
uint8_t * raw_data;
uint32_t latcprE, latcprO, loncprE, loncprO;
float latE, latO, m, Dlon, cpr_lon_odd, cpr_lon_even, cpr_lat_odd, cpr_lat_even;
int ni;
adsb_pos position { false, 0, 0, 0 };
uint32_t time_even = frame_even.get_rx_timestamp();
uint32_t time_odd = frame_odd.get_rx_timestamp();
uint8_t * frame_data_even = frame_even.get_raw_data();
uint8_t * frame_data_odd = frame_odd.get_raw_data();
// Return most recent altitude
if (time_even > time_odd)
raw_data = frame_data_even;
else
raw_data = frame_data_odd;
// Q-bit must be present
if (raw_data[5] & 1)
position.altitude = ((((raw_data[5] & 0xFE) << 3) | ((raw_data[6] & 0xF0) >> 4)) * 25) - 1000;
// Position
latcprE = ((frame_data_even[6] & 3) << 15) | (frame_data_even[7] << 7) | (frame_data_even[8] >> 1);
loncprE = ((frame_data_even[8] & 1) << 16) | (frame_data_even[9] << 8) | frame_data_even[10];
latcprO = ((frame_data_odd[6] & 3) << 15) | (frame_data_odd[7] << 7) | (frame_data_odd[8] >> 1);
loncprO = ((frame_data_odd[8] & 1) << 16) | (frame_data_odd[9] << 8) | frame_data_odd[10];
// Calculate the coefficients
cpr_lon_even = loncprE / CPR_MAX_VALUE;
cpr_lon_odd = loncprO / CPR_MAX_VALUE;
cpr_lat_odd = latcprO / CPR_MAX_VALUE;
cpr_lat_even = latcprE / CPR_MAX_VALUE;
// Compute latitude index
float j = floor(((59.0 * cpr_lat_even) - (60.0 * cpr_lat_odd)) + 0.5);
latE = (360.0 / 60.0) * (cpr_mod(j, 60) + cpr_lat_even);
latO = (360.0 / 59.0) * (cpr_mod(j, 59) + cpr_lat_odd);
if (latE >= 270) latE -= 360;
if (latO >= 270) latO -= 360;
// Both frames must be in the same latitude zone
if (cpr_NL(latE) != cpr_NL(latO))
return position;
// Compute longitude
if (time_even > time_odd) {
// Use even frame2
ni = cpr_N(latE, 0);
Dlon = 360.0 / ni;
m = floor((cpr_lon_even * (cpr_NL(latE) - 1)) - (cpr_lon_odd * cpr_NL(latE)) + 0.5);
position.longitude = Dlon * (cpr_mod(m, ni) + cpr_lon_even);
position.latitude = latE;
} else {
// Use odd frame
ni = cpr_N(latO, 1);
Dlon = 360.0 / ni;
m = floor((cpr_lon_even * (cpr_NL(latO) - 1)) - (cpr_lon_odd * cpr_NL(latO)) + 0.5);
position.longitude = Dlon * (cpr_mod(m, ni) + cpr_lon_odd);
position.latitude = latO;
}
if (position.longitude >= 180) position.longitude -= 360;
position.valid = true;
return position;
}
// An ADS-B frame is 112 bits long. Civil aircraft ADS-B message starts with the Downlink Format ,DF=17.
// Msg structure consists of five main parts :|DF=17 (5 bits)|CA (3 bits)|ICAO (24 bits)|ME (56 bits)|CRC (24 bits)
// Airborne velocities are all transmitted with Type Code 19 ( TC=19 ) inside ME (56 bits)
// [units] : speed is in knots, vertical rate climb / descend is in ft/min
void encode_frame_velo(ADSBFrame& frame, const uint32_t ICAO_address, const uint32_t speed,
const float angle, const int32_t v_rate) {
int32_t velo_ew, velo_ns;
uint32_t velo_ew_abs, velo_ns_abs, v_rate_coded_abs;
// To get NS and EW speeds from speed and bearing, a polar to cartesian conversion is enough
velo_ew = static_cast<int32_t>(sin_f32(DEG_TO_RAD(angle) ) * speed); // East direction, is the projection from West -> East is directly sin(angle=Compas Bearing) , (90º is the max +1, EAST) max velo_EW
velo_ns = static_cast<int32_t>(sin_f32( (pi/2 - DEG_TO_RAD(angle) ) ) * speed); // North direction,is the projection of North = cos(angle=Compas Bearing), cos(angle)= sen(90-angle) (0º is the max +1 NORTH) max velo_NS
v_rate_coded_abs = (abs(v_rate) / 64) + 1; //encoding vertical rate source. (Decoding, VR ft/min = (Decimal v_rate_value - 1)* 64)
velo_ew_abs = abs(velo_ew) + 1; // encoding Velo speed EW , when sign Direction is 0 (+): West->East, (-) 1: East->West
velo_ns_abs = abs(velo_ns) + 1; // encoding Velo speed NS , when sign Direction is 0 (+): South->North , (-) 1: North->South
make_frame_adsb(frame, ICAO_address); // Header DF=17 (long frame 112 bits)
// Airborne velocities are all transmitted with Type Code 19 ( TC=19, using 5 bits ,TC=19 [Binary: 10011]), the following 3 bits are Subt-type Code ,SC= 1,2,3,4
// SC Subtypes code 1 and 2 are used to report ground speeds of aircraft. (SC 3,4 to used to report true airspeed. SC 2,4 are for supersonic aircraft (not used in commercial airline).
frame.push_byte((TC_AIRBORNE_VELO << 3) | 1); // 1st byte , top 5 bits Type Code TC=19, and lower 3 bits (38-40 bits), SC=001 Subtype Code SC: 1 (subsonic) ,
// Message A, (ME bits from 14-35) , 22 bits = Sign ew(1 bit) + V_ew (10 bits) + Sign_ns (1 bit) + V_ns (10 bits)
// Vertical rate source bit VrSrc (ME bit 36) indicates source of the altitude measurements. GNSS altitude(0) / , barometric altitude(1).
// Vertical rate source direction,(ME bit 37) movement can be read from Svr bit , with 0 and 1 referring to climb and descent, respectively (ft/min)
// The encoded vertical rate value VR can be computed using message (ME bits 38 to 46). If the 9-bit block contains all zeros, the vertical rate information is not available.
// + Sign VrSrc (vert rate src) (1 bit)+ VrSrc (9 bits).
frame.push_byte(((velo_ew < 0 ? 1 : 0) << 2) | (velo_ew_abs >> 8));
frame.push_byte(velo_ew_abs);
frame.push_byte(((velo_ns < 0 ? 1 : 0) << 7) | (velo_ns_abs >> 3));
frame.push_byte((velo_ns_abs << 5) | ((v_rate < 0 ? 1 : 0) << 3) | (v_rate_coded_abs >> 6)); // VrSrc = 0
frame.push_byte(v_rate_coded_abs << 2);
frame.push_byte(0);
frame.make_CRC();
}
// Decoding method from dump1090
adsb_vel decode_frame_velo(ADSBFrame& frame){
adsb_vel velo {false, 0, 0, 0};
uint8_t * frame_data = frame.get_raw_data();
uint8_t velo_type = frame.get_msg_sub();
if(velo_type >= 1 && velo_type <= 4){ //vertical rate is always present
velo.v_rate = (((frame_data[8] & 0x07 ) << 6) | ((frame_data[9] >> 2) - 1)) * 64;
if((frame_data[8] & 0x8) >> 3) velo.v_rate *= -1; //check v_rate sign
}
if(velo_type == 1 || velo_type == 2){ //Ground Speed
int32_t raw_ew = ((frame_data[5] & 0x03) << 8) | frame_data[6];
int32_t velo_ew = raw_ew - 1; //velocities are all offset by one (this is part of the spec)
int32_t raw_ns = ((frame_data[7] & 0x7f) << 3) | (frame_data[8] >> 5);
int32_t velo_ns = raw_ns - 1;
if (velo_type == 2){ // supersonic indicator so multiply by 4
velo_ew = velo_ew << 2;
velo_ns = velo_ns << 2;
}
if(frame_data[5]&0x04) velo_ew *= -1; //check ew direction sign
if(frame_data[7]&0x80) velo_ns *= -1; //check ns direction sign
velo.speed = fast_int_magnitude(velo_ns,velo_ew);
if(velo.speed){
//calculate heading in degrees from ew/ns velocities
int16_t heading_temp = (int16_t)(int_atan2(velo_ew,velo_ns)); // Nearest degree
// We don't want negative values but a 0-360 scale.
if (heading_temp < 0) heading_temp += 360.0;
velo.heading = (uint16_t)heading_temp;
}
}else if(velo_type == 3 || velo_type == 4){ //Airspeed
velo.valid = frame_data[5] & (1<<2);
velo.heading = ((((frame_data[5] & 0x03)<<8) | frame_data[6]) * 45) << 7;
}
return velo;
}
} /* namespace adsb */