This is a C library for decoding Mode S messages from aviation aircrafts. It supports both standard Mode S Acquisition Squitter messages (56 bits) and Mode S Extended Squitter messages (112 bits) that also carry ADS-B information.

This project is a refactoring of the popular dump1090 project by Salvatore Sanfilippo. It modularizes the code into separate functions and removes all non-essentials, so that only the decoding logic is left.

#include "mode-s.h"
#include <stdio.h>

void on_msg(mode_s_t *self, struct mode_s_msg *mm) {
  printf("Got message from flight %s at altitude %d\n", mm->flight, mm->altitude);
}

int main(int argc, char **argv) {
  mode_s_t state;
  uint32_t data_len = 262620;
  unsigned char data[data_len];
  uint16_t mag[data_len / 2];

  // initialize the decoder state
  mode_s_init(&state);

  // get some raw IQ data somehow
  get_samples(&data);

  // compute the magnitude of the signal
  mode_s_compute_magnitude_vector(&data, &mag, data_len);

  // detect Mode S messages in the signal and call on_msg with each message
  mode_s_detect(&state, &mag, data_len/2, on_msg);
}

Check out tests/test.c for a complete example.

The provided callback to mode_s_detect will be called with a mode_s_msg struct. This struct contain the following fields:

• unsigned char msg[14] - Binary message
• int msgbits - Number of bits in message
• int msgtype - Downlink format #
• int crcok - True if CRC was valid
• uint32_t crc - Message CRC
• int errorbit - Bit corrected. -1 if no bit corrected
• int aa1 - ICAO Address byte 1
• int aa2 - ICAO Address byte 2
• int aa3 - ICAO Address byte 3
• int phase_corrected - True if phase correction was applied

• int ca - Responder capabilities

• int metype - Extended squitter message type
• int mesub - Extended squitter message subtype
• int heading_is_valid
• int heading
• int aircraft_type
• int fflag - 1 = Odd, 0 = Even CPR message
• int tflag - UTC synchronized?
• int raw_latitude - Non decoded latitude
• int raw_longitude - Non decoded longitude
• char flight[9] - 8 chars flight number
• int ew_dir - 0 = East, 1 = West
• int ew_velocity - E/W velocity
• int ns_dir - 0 = North, 1 = South
• int ns_velocity - N/S velocity.
• int vert_rate_source - Vertical rate source
• int vert_rate_sign - Vertical rate sign
• int vert_rate - Vertical rate
• int velocity - Computed from EW and NS velocity

• int fs - Flight status for DF4, 5, 20, 21
• int dr - Request extraction of downlink request
• int um - Request extraction of downlink request
• int identity - 13 bits identity (Squawk)

• int altitude
• int unit

To test the library, simply run:

make && make test

Note that the first time you run make test, a large (ca. 50MB) test fixture will be downloaded to tests/fixtures. You can delete this folder at any time if you wish.

BSD-2-Clause