Secondary surveillance radar
From Biocrawler, the free encyclopedia.
A Secondary Surveillance Radar (SSR) is a radar device installed in air traffic control facilities to allow the precise identification of aircraft. An SSR transmits an interrogation pulse; aircraft contain a transponder that listens for the pulse and sends a responding pulse. This is similar to conventional radar, but has the advantage that only aircraft will return pulses (i.e. birds, clouds, and terrain will not appear on the radar). Transponders also encode a 4-digit transponder code that can be used to identify an aircraft. More advanced transponders can also encode the aircraft's pressure altitude and callsign.
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