Transponder

Definition

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.

Function

  • 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)

Main Modes

ModeNameFunction
OFFPower offNo transmission
STBYStandbyPowered but not transmitting
ON (Mode A)IdentityTransmits 4-digit squawk code only
ALT (Mode C)AltitudeTransmits squawk + pressure altitude
Mode SSelectiveSends squawk, altitude, and extra aircraft info (e.g., callsign, heading)

Usage Scenarios

  • 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

Phraseology Examples

  • “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

Transponder Use in Practice

  • 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)

Important Considerations

  • 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

Special Uses

  • 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)

Maintenance and Legality

  • 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