Secondary surveillance radar
Secondary surveillance radar (SSR) is a radar system used in air traffic control (ATC), that unlike primary radar 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 to each interrogation signal by transmitting encoded data such as an identity code, the aircraft's altitude and further information depending on its chosen mode. SSR is based on the military identification friend or foe (IFF) technology originally developed during World War II; therefore, the two systems are still compatible. Monopulse secondary surveillance radar (MSSR), Mode S, TCAS and ADS-B are similar modern methods of secondary surveillance.
SSR antenna of Deutsche Flugsicherung at Neubrandenburg, in Mecklenburg/Western Pomerania
Transponder in a private aircraft squawking 2000
Secondary surveillance radar antenna (flat rectangle, top) mounted on an ASR-9 primary airport surveillance radar antenna (curved rectangle, bottom).
Independent secondary surveillance radar (ISSR), designation YMT, north of Chibougamau, Quebec, Canada
Air traffic control (ATC) is a service provided by ground-based air traffic controllers (people) who direct aircraft on the ground and through a given section of controlled airspace, and can provide advisory services to aircraft in non-controlled airspace. The primary purpose of ATC worldwide is to prevent collisions, organise and expedite the flow of traffic in the air, and provide information and other support for pilots.
The air traffic control tower of Mumbai International Airport in India.
São Paulo–Guarulhos International Airport's control tower.
Control tower at Birmingham Airport, England.
Small control tower at Räyskälä Airfield in Loppi, Finland.