Redundant Transmitting System in Aircraft (RTSA)

In air navigation, a flight transponder (transmitter-responder) is a device that emits an identifying signal in response to an interrogating received signal. Transponders are used as Identification of Friend or Foe (IFF) systems in military aviation, and also in Air Traffic Control (ATC) Secondary Surveillance Radar (beacon radar) systems for general aviation and commercial aviation.

Primary radar is limited by terrain and weather, and also detects unwanted objects such as automobiles, hills, and trees. It also cannot estimate the attitude of an aircraft. Secondary Surveillance Radar (SSR) overcomes these limitations, but it de pends on a transponder in the aircraft to respond to interrogations from the ground station in order to make the plane more visible. Depending on the type of interrogation, the transponder sends back a transponder code (or “squawk code”) or altitude information to maintain separation between planes. Mode S (Mode Select) is designed to help avoid over-interrogation of the transponder (having many radars in busy areas), and to allow automatic collision avoidance.