Primary radar is used to detect aircraft range and azimuth, this system utilizes ground based equipment and is not dependent upon special components on board the aircraft. How ever, because of its many disadvantages which limit its effectiveness in air traffic control, the air traffic control radar beacon system (ATCRBS) was developed to overcome the problems of primary radar. This system known as secondary radar requires the use of three additional components to the basic radar equipment. Interrogator - mounted on the radar antenna, this ground based transmitter receiver scans in synchronism with the primary radar and transmits discrete radio signals which repetitively request all transponders, on the mode being used, to reply. Decoder - attached to the radarscope console. This equipment enables the controller to assign discrete transponder codes to each aircraft under his control. Transponder -located in the aircraft. This airborne transmitter-receiver automatically receives the signals from the interrogator and selectively replies with a specific pulse group (code) only to those interrogators being received on the mode to which it is set. The advantages of secondary radar over primary radar include reinforcement of radar targets and rapid target identification. It also provides a unique display of selected codes as well as displaying altitude readout. Currently four radar systems have been put into use. Two provide azimuth and range data only and are called surveillance systems, they are airway surveillance radar and aerodrome surveillance radar. The other two provide altitude information, as well as azimuth and range, they are the automated radar terminal systems and precision approach radar. Airway surveillance radar is a long range radar system designed primarily to provide a display of aircraft locations over large areas for ATC controllers to separate en route aircraft. Aerodrome surveillance radar provides short range coverage in the vicinity of an airport and to serve as an expedition means of handling terminal area traffic through observation of precise aircraft locations on a radarscope. Approach controllers mainly use ASR as an instrument approach aid.3. doesn＇t go with the other three.