page_title: Air traffic control radar beacon system
section_title: Airborne Transponder Equipment
hierarchical_section_title: Air traffic control radar beacon system / Parts of the system / Airborne Transponder Equipment
caption_attribution_description: English: A transponder
context_page_description: The air traffic control radar beacon system is a system used in air traffic control to enhance surveillance radar monitoring and separation of air traffic. It consists of a rotating ground antenna and transponders in aircraft. The ground antenna sweeps a narrow vertical beam of microwaves around the airspace. When the beam strikes an aircraft, the transponder transmits a return signal back giving information such as altitude and the Squawk Code, a four digit code assigned to each aircraft that enters a region. Information about this aircraft is then entered into the system and subsequently added to the controller's screen to display this information when queried. This information can include flight number designation and altitude of the aircraft. ATCRBS assists air traffic control surveillance radars by acquiring information about the aircraft being monitored, and providing this information to the radar controllers. The controllers can use the information to identify radar returns from aircraft and to distinguish those returns from ground clutter.
context_section_description: The equipment installed in the aircraft is considerably simpler, consisting of the transponder itself, usually mounted in the instrument panel or avionics rack, and a small L band UHF antenna, mounted on the bottom of the fuselage. Many commercial aircraft also have an antenna on the top of the fuselage, and either or both antennas can be selected by the flight crew.
  Typical installations also include an altitude encoder, which is a small device connected to both the transponder and the aircraft's static system. It provides the aircraft's pressure altitude to the transponder, so that it may relay the information to the ATC facility. The encoder uses 11 wires to pass altitude information to the transponder in the form of a Gillham Code, a modified binary Gray code.
  The transponder has a small required set of controls and is simple to operate. It has a method to enter the four-digit transponder code, also known as a beacon code or squawk code, and a control to transmit an ident, which is done at the controller's request (see SPI pulse below). Transponders typically have 4 operating modes: Off, Standby, On (Mode-A), and Alt (Mode-C). On and Alt mode differ only in that the On mode inhibits transmitting any altitude information. Standby mode allows the unit to remain powered and warmed up but inhibits any replies, since the radar is used for searching the aircraft and exact location of aircraft.
caption: A light aircraft transponder
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