Transponder

A transponder (short for transmitter-responder) is an onboard electronic device that receives interrogation signals from radar stations and automatically replies with coded information. It allows ATC to identify, track, and separate aircraft on radar.

• Enhances radar visibility by adding identity and altitude to the radar return
• Sends a squawk code, sometimes combined with altitude (Mode C) or additional data (Mode S)
• Plays a critical role in collision avoidance systems (e.g., TCAS)

• Mode A: Basic identification (used in all aircraft with transponder)
• Mode C: Enables altitude readout for radar separation
• Mode S: Required in most of Europe for all controlled flights and many Class G operations

• “Squawk 4712” → Set the code 4712
• “Squawk Mode Charlie” → Ensure altitude transmission is active
• “Squawk Standby” → Stop transmitting temporarily
• “Squawk Ident” → Press IDENT button for radar highlighting

• Set the assigned squawk code before departure clearance
• Switch to 7000 when flying VFR (unless otherwise instructed)
• Use Ident only when instructed
• Switch to Standby after landing if instructed or when clear of the runway (aerodrome policy-dependent)

• Incorrect squawk settings can cause radar confusion, ATC errors, or trigger unnecessary alerts
• Mode C errors (wrong altitude transmission) may lead to loss of separation
• Some aircraft automatically switch transponder modes via weight-on-wheels or airspeed detection

• TCAS (Traffic Collision Avoidance System) relies on transponder returns from nearby aircraft
• ADS-B systems build on Mode S functionality to broadcast full positional data (used by tools like FlightRadar24)

• Transponders must be function-tested regularly (typically during annual inspection)
• Use is mandatory in many controlled and transponder-mandatory zones (TMZ)
• In some airspace (e.g., RMZ), transponder use is not mandatory but highly recommended