Squawk

Squawk refers to the four-digit octal code (0000–7777) set on an aircraft's transponder, which allows air traffic control to identify and track the aircraft on radar. The term originates from the IFF (Identification Friend or Foe) system developed during World War II, codenamed "Parrot."

Special squawk codes with universal meaning:

7700: Emergency — activates special alerting on ATC radar displays
7600: Communication failure (radio failure/NORDO)
7500: Hijacking — triggers silent security alert to ATC and authorities
1200: VFR flight (North America)
2000: Default code when entering a region without specific assignment (ICAO)

Transponder modes:

Mode A: Transmits the squawk code only (identification)
Mode C: Transmits code plus pressure altitude
Mode S: Selective interrogation with additional data (aircraft ID, altitude, intent). Basis for ADS-B.