Transponders

A transponder is an electronic device that automatically responds to an incoming signal with a transmitted response. The term is a contraction of transmitter-responder.

In radar and air traffic control, transponders on aircraft reply to ground radar interrogations with digital information such as identification codes and altitude. Modes A and C provide an identity and altitude, while Mode S enables selective addressing and higher data exchange. The use of radar transponders with secondary surveillance radar improves identification, tracking, and overall airspace efficiency.

In satellite communications, a satellite transponder receives an uplink signal, shifts its frequency to a downlink band, amplifies it, and retransmits it back to Earth. Each transponder covers a defined bandwidth and carries multiple channels. Satellites may host many transponders to support a large number of simultaneous connections.

Transponders can be passive or active. Passive transponders have no internal power and derive energy from the interrogating signal, as in many RFID tags and certain identification systems. Active transponders carry their own power source and can initiate responses, offering longer range and more complex data both on terrestrial and space links.

Other uses include telemetry and data links for ground and aerospace systems, maritime identification, and various forms of contactless identification and access control. The transponder concept underpins many modern communication and identification networks by providing automatic, remote, and often bidirectional signaling.