There are two principal types of radar system used for ATC, namely primary surveillance radar (PSR) and secondary surveillance radar (SSR). A PSR transmits radar pulses and detects the reflected signals from the aircraft concerned. It provides information on the range and bearing of aircraft in the form of a target blip on the radar screen. An SSR provides information on the position, altitude and identity of aircraft in the form of target label on the radar screen. It does so by transmitting interrogation signals to trigger the transponder on board an aircraft, which in turn transmits special coded signals back to the SSR to provide the concerned information. Thus, effective operation of an SSR is subject to the availability and proper functioning of the aircraft’s transponders. Not all aircraft have transponders. Some military aircraft, for instance, do not. A PSR, on the other hand, does not rely on any transponder. To provide an adequate radar picture with a cont...