Secondary Surveillance Radar
Secondary Surveillance Radar (SSR) is referred to as "secondary", to distinguish it from the "primary radar" that works by passively bouncing a radio signal off the skin of the aircraft. Primary radar determines range and bearing to a target with reasonably high fidelity, but it cannot determine target elevation (altitude) reliably except for at close range. SSR uses an active transponder (beacon) to transmit a response whenever it senses the beam of the primary radar hitting the aircraft. This response most often includes the aircraft's pressure altitude and a 4-digit octal identifier.
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