transponder

A transponder is an electronic device that receives an input signal and automatically emits a corresponding response. The term combines transmitter and responder, and transponders are used in systems that need automatic identification, signaling, or relaying of information without human intervention. They may perform frequency translation, amplification, demodulation, and encoding as required by the specific application.

In aviation, aircraft transponders reply to radar interrogations with coded information, enabling air traffic control to identify and track airplanes. They typically operate in several modes: Mode A returns an identity code; Mode C adds altitude information; Mode S provides selective addressing and more data. Modern systems also support ADS-B, where aircraft periodically broadcast position and other data without a radar prompt.

In satellite communications, a transponder is the onboard relay that receives a signal on an uplink frequency, downconverts it, and re-transmits it on a downlink frequency, usually with fixed gain. Transponders can be bent-pipe (transparent) or regenerative. Frequencies commonly used include C-, Ku-, and Ka-band, with varying polarization and bandwidth.

In RFID and related systems, a transponder (tag) responds to a reader’s field with stored data; passive tags derive power from the reader, while active tags have an internal power source. In optical networks, transponders convert or translate wavelengths and formats to support multiplexed channels in WDM systems.

Across domains, transponders serve as automated, remote, and efficient relays of information, enabling identification, tracking, and long-distance communication.