Transponder

Transponder is an electronic device aboard an aircraft that receives radar interrogation signals and automatically transmits a coded response containing the aircraft identification (squawk code) and, in Mode C/S, pressure altitude.

• Mode A: Transmits 4-digit identification code
• Mode C: Transmits code plus pressure altitude
• Mode S: Selective interrogation with enhanced data (aircraft type, call sign, selected altitude). Required for TCAS and ADS-B

Transponders are required in most controlled airspace worldwide. The coded responses appear on ATC radar screens as data blocks associated with each aircraft target.