TRANSPONDER

The transponder is a transceiver that receives "interrogations" from air traffic control radar systems and replies with a digital code. This secondary radar reply permits the radar system to detect the aircraft more reliably and at greater distances than are possible with primary radar. This system of secondary radars and transponders is known collectively as the air traffic control radar beacon system, or ATCRBS.

A basic "mode A" transponder responds with a 4-digit code with each digit ranging from 0 to 7. This is called a 4,096 code transponder. This pilot sets the code according to the type and status of the flight or as directed by air traffic control.

A "mode C" transponder also replies with the pressure altitude of the aircraft encoded to the nearest 100 feet (30 m). Modern "mode S" transponders can respond with a longer digital identifier that is unique for each aircraft (thus allowing each aircraft to be uniquely identified even when there is no voice communication between the aircraft and air traffic control) and can receive digital traffic information from air traffic control radar systems and display them for the pilot.

An IFF transponder, "Identification friend or foe", is used in military aircraft and has additional modes of operation beyond those used in civil air traffic control.