A transponder is a device that receives a signal and automatically transmits a different signal in response. They are commonly used in various applications, such as satellite communication, aviation, and vehicle tracking. For example, in aviation, transponders help air traffic control identify and track aircraft by sending back information like altitude and identification. In satellite systems, transponders receive signals from the ground, amplify them, and then send them back to Earth. This process allows for efficient communication over long distances. Transponders are essential for technologies like GPS and satellite television, ensuring reliable connectivity and accurate location services.