ทรานสปอนเดอร์ (Transponder)

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An aircraft transponder (transmitter-responder) is an avionics device that listens for interrogation signals from ground-based secondary surveillance radars (SSR) and automatically replies with a coded response containing the aircraft's assigned squawk code and, on Mode C/S transponders, pressure altitude. This allows air traffic controllers to positively identify and track specific aircraft on radar, rather than relying solely on primary radar returns (which show only a blip with no identity information).

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Ground SSR antennas transmit interrogation pulses at 1030 MHz. The aircraft's transponder receives the interrogation, processes it, and replies at 1090 MHz within microseconds. Three transponder modes are in common use:

• Mode A: Replies with a 4-digit octal squawk code (e.g., 7700 for emergency, 7600 for radio failure, 7500 for hijack)
• Mode C: Adds pressure altitude (from the aircraft's altimeter encoder) to the reply — allows controllers to see altitude on radar
• Mode S (Select): Assigns each aircraft a unique 24-bit ICAO address. Supports selective interrogation (reducing radio frequency congestion) and is the foundation for TCAS and ADS-B

Pilots set squawk codes via a 4-digit selector on the transponder panel. ATC assigns codes at pushback or in flight; discrete codes (0001–7777, excluding emergency codes) uniquely identify each flight within a facility's airspace.

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Military IFF (Identify Friend or Foe) transponders from World War II evolved into civil aviation SSR by the 1950s. Mode S was developed in the 1970s and mandated in controlled airspace in many countries by the 1990s. The Mode S Extended Squitter (1090ES) is the data link used by ADS-B Out. Modern transponders like the Garmin GTX 345 or Collins TDR-94 are integrated with TCAS processors and ADS-B transceivers in a single unit, reducing avionics box count and wiring.

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FAA FAR 91.215 requires Mode C transponders in Class A, B, and C airspace, and above 10,000 ft MSL. EASA mandates Mode S transponders with Elementary Surveillance (ELS) and Enhanced Surveillance (EHS) in most European controlled airspace. Emergency squawk codes (7500/7600/7700) are universally recognized. From January 2020, FAA mandated ADS-B Out (which requires a Mode S transponder) in most controlled US airspace.