"A radar Beacon"

A radar beacon transponder provides positive identification and location of an aircraft on the radar screens of ATC. Two types of radar are used by air traffic control. Plan position indicator (PPI) displays a blip or target representing the aircraft. Secondary surveillance radar (SSR) use to verify the aircraft’s position and to add the third dimension of altitude to its location. Transponder control must place in the ALT or Mode C position to gain altitude clarification. Inspect and test transponders every 24 calendar months. Altitude encoders convert the aircraft’s pressure altitude into a code sent by the transponder to ATC. Transmission of certain codes reserved for emergencies or military activity must be avoided. The procedure to select a code during ground operation is to do so with the transponder in the OFF or STANDBY mode to avoid inadvertent transmission. Mode S is sometimes referred to as mode select.