Transponder

A transponder is a coupled radio or radar receiver-transmitter that will generate a reply signal, upon proper interrogation. Transponders aboard airplanes are the basis of modern air traffic control (ATC); it is incorrect to assume that ATC tracks aircraft by radar alone.

In maritime safety, the new automatic identification system will provide similar collision avoidance and traffic management for vessels.

Military identification-friend-or-foe systems are based on transponders, which, when sent an appropriately verifiable signal, respond with a "don't kill me" response that avoids fratricide. Such systems, in combat, have to be designed to respond only to securely authenticated interrogators, so they do not give away position information to an enemy.