Operation
The purpose of SSR is to improve the ability to detect and identify aircraft while it additionally provides automatically the Flight Level (pressure altitude) of a flight. An SSR ground station transmits interrogation pulses on 1030 MHz (continuously in Modes A and C and selectively in Mode S) as its antenna rotates, or is electronically scanned, in space. A transponder on an aircraft that is within line-of-sight range 'listens' for the SSR interrogation signal and sends back a reply on 1090 MHz that provides aircraft information. The reply sent depends on the mode that was interrogated. The aircraft is then displayed as a tagged icon on the controller's radar screen at the measured bearing and range. An aircraft without an operating transponder still may be observed by primary radar, but would be displayed to the controller without the benefit of SSR derived data. It is typically a requirement to have a working transponder in order to fly in controlled air space and many aircraft have a back-up transponder to ensure that this condition is met.
Read more about this topic: Secondary Surveillance Radar
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