Primary Surveillance Radar (PSR) works on the pulse technique and echo principle which used electromagnetic waves to detect objects and detect their returning echoes. It provides bearing and distance information of detected objects. The radar relies on reflected signals but is not aware if they are received from aircraft or from other objects (e.g. terrain, buildings, clouds). Such reflections are called clutter.
A Secondary Surveillance Radar (SSR) system uses transmitters/receivers (interrogators) and transponders. The radar antenna rotates and transmits a pulse which is received by the onboard equipment (transponder). The transponder sends back a reply containing at least a code (if operating in Mode A) but more often this is combined with level (mode C) or other information, e.g. aircraft identification, selected level, etc. (Mode S). The information received depends on the interrogation mode (A, C or S) and the transponder capability.
- The Secondary Surveillance Radar (SSR) is independent of echoing capabilities or the size of the aircraft and the transponder transmission results in cluter-free responses.
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