TABLE OF CONTENTS A transponder is a beacon that replies with a unique identification code after being interrogated on a specific frequency. There are RF transponders on military and civil aircraft and on military ships. The interrogation signal is usually sent from a transmitter that could be on the ground, in an aircraft, or on shipboard. The transponder's response is called a reply. It replies only when interrogated with the proper signal for security reasons or to conserve power.
Radio-frequency transponders are important equipment in the commercial air traffic control (ATC) system. A transponder on a commercial aircraft permits ground-based ATC personnel to monitor the position of the aircraft by interrogating its transponder when the aircraft has entered a designated airspace, typically near an airport. The ATC radar transmits an interrogation code, and the transponder on the aircraft replies in its own unique code that gives its airline, flight number, and altitude. This reply is converted to an alphanumeric message that appears on the ATC radar screen in the correct relative position so that the range and bearing can be determined. These transponder replies provide a more distinctive return than could be obtained by radar echoes from the aircraft, and they also permit the ATC controllers to rapidly distinguish between aircraft in the immediate airspace, as shown on a crowded ATC radar screen.
Military and commercial communications satellites are equipped with electronic circuitry, also called transponders, that receive RF transmissions from...
