Mode

Mode, in the context of an aircraft transponder, refers to the type of information transmitted by the transponder in response to an SSR interrogation. Different modes transmit different types of information and respond to different interrogation frequencies.

Transponder Modes

  • Mode A: Transmits the 4-digit squawk code — identifies the aircraft for ATC
  • Mode C: Transmits the 4-digit code plus pressure altitude (from the altimeter encoder) — allows ATC to see altitude readout
  • Mode S: Advanced mode; transmits the aircraft’s unique 24-bit ICAO address, squawk code, altitude, and supports two-way datalink with ATC — enables enhanced surveillance

Required Mode

In most Canadian controlled airspace above 12,500 ft and in designated airspace, Mode C (or Mode S with altitude reporting) is required. Mode A alone is insufficient in these areas. When operating in Class C airspace or in the ADIZ, Mode C must be selected and operating.

Related Terms

See also: Transponder, Transponder Codes, Automatic Altitude Reporting.