Secondary surveillance radar

Common card · No. 434035

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 in the same Mode that an interrogation pulse code corresponds to, by transmitting a pulse telegram containing, e.g. the identity code as a 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. SSR is based on the military Identification Friend or Foe (IFF) initially standardized in the U.S. and later adopted by NATO (North Atlantic Treaty Organization); however, the first IFF systems date back to World War II. SSR Mode A, C and S are compatible with IFF Mode 3A, C and S. SSR Mode B and D are no longer specified in ICAO Annex 10. Additional SSR Mode S based systems...

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