Understanding ADS-B and Emergency Squawks

Automatic Dependent Surveillance–Broadcast (ADS-B) is a surveillance technology in which an aircraft determines its own position using satellite navigation (GNSS) and broadcasts it automatically, typically twice per second. Unlike primary radar, which detects reflected radio energy, or secondary surveillance radar (SSR), which interrogates the transponder from the ground, ADS-B is "dependent" on the aircraft's own navigation data. Each transmission includes the aircraft's 24-bit ICAO address, position, altitude and velocity.

The Role of Transponder Codes (Squawks)

A squawk is the four-digit Mode A code set on the aircraft's transponder. Each digit ranges from 0 to 7 (octal), giving 4,096 possible codes. ATC assigns discrete codes so that each surveillance target can be correlated with its flight plan, and the code is decoded from the transponder's Mode S identity replies and, on equipped aircraft, from ADS-B emergency/priority status messages. Three codes are reserved under ICAO procedures to communicate critical situations on board.

• 7700: General Emergency
  Indicates an emergency of any kind, such as an engine failure, loss of pressurization or a medical emergency on board. Selecting 7700 generates an alert on ATC displays so the flight can be given priority handling. If ATC has already assigned a discrete code, the crew may be instructed to keep it instead.
• 7600: Lost Communications
  Indicates a failure of two-way radio communication with ATC. Under ICAO procedures, the crew maintains the last assigned speed and level for a prescribed period before continuing in accordance with the flight plan, while ATC keeps other traffic separated from the expected route and altitude.
• 7500: Hijacking
  Indicates unlawful interference, such as a hijacking. 7500 data on this website is intentionally delayed by 2 hours for security purposes.

Coverage and Limitations

Detection depends on ground receiver coverage, so flights over oceans and remote regions may not be captured until they come within range. Emergency codes can also be selected briefly by mistake during a code change, and a crew handling an emergency may keep its assigned code at ATC's request. Squawk events shown here should therefore be read as transponder status reports, not as confirmed accounts of what happened on board.

This website uses ADS-B data to detect and visualize flights transmitting these codes in real time.