Secondary surveillance radar (SSR) is nearly as old as radar itself. It has evolved from a need in the last war for an Identification system between Friend and Foe (IFF)

Primary radar has a scanning aerial beam which transmits a burst of energy, and the reflected signal from an aircraft is detected. In the simplest form of IFF the aircraft carries a receiver/transmitter (transponder), which detects the primary radar beam and transmits a pulse (on the same frequency) a few microseconds later. Thus two ‘blips’ appear on the radar display, one at the shorter range due to reflected energy from the aircraft followed by a stronger reply from the transponder. The system requires all aircraft to carry a transponder which upon receipt of an ‘up link’ signal or message transmits a ‘down link’ signal or message. SSR has evolved (and is still evolving) from this early IFF technique into the present-day system which serves both military IFF needs and the requirements for Air Traffic Control. This paper concentrates on the civil use of SSR where it is the prime sensor for the control of aircraft.