Radar is an essential tool for air traffic controllers in safely routing aircraft through our busy skies. Ground-based radar systems are enhanced by transponders installed in aircraft. A transponder is a receiver-transmitter that responds to a pulse from the Air Traffic Control Radar Beacon System with its own high-energy return pulse. In addition to strengthening the return signal received by the radar system, the transponder can encode further information for display on the air traffic controller's radar screen. This can include identifying information for the particular aircraft, the aircraft's altitude, and critical status information. A transponder that encodes altitude is required equipment to enter many crowded areas of controlled airspace.
A typical transponder can be set to any of 4,096 identifying codes. Setting the transponder to broadcast a code of 1200 indicates that the flight is being conducted under visual flight rules. The air traffic controller may direct the pilot of any flight to transmit other codes to aid in identification.
Certain transponder codes are reserved for special purposes. Codes 0000, 4000, and 7777 are exclusively reserved for military use. The pilot can set the transponder to broadcast code 7600, indicating to the air traffic controller that the aircraft's communications radios have failed. Code 7700 indicates an emergency, and code 7500 indicates a hijacking.
Signals broadcasted from transponders are also read and used by Traffic Collision Avoidance Systems (TCAS) that can be installed in aircraft.
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