Secondary surveillance radar

Secondary surveillance radar (SSR) is a radar system used in air traffic control (ATC) that, unlike Primary Surveillance Radar (PSR) systems that measure the bearing and distance of targets using the detected reflections of radio signals, relies on targets equipped with a radar transponder, that reply the Mode that an interrogation pulse code corresponds to, by transmitting a pulse telegram containing, e.g. the identity code as 4 digit octal number in Mode A, the aircraft's altitude from the barometric pressure sensor of an aircraft in Mode C and a unique 24-bit address and further information in other Modes.

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