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SSR Transponder Sun 16.Dec 18
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Primary Surveillance Radar (PSR)
The Primary Surveillance Radar operates on the principle of sending a narrow beam of radio energy, which is reflected from the aircraft. Noting the time elapsed between the radar pulse transmission and its received echo allowes to measure the distance to the aircraft. This reflected signal is detected and processed to display a blip on the screen, indicating the position of the target.

The PSR system works well in low traffic areas. However, as the air traffic increases in a given area, the Radar display becomes cluttered and specific targets may be difficult to distinguish. Further, the PSR has the disadvantage that the operator has no way of knowing the altitude of the aircraft unless the pilot reports it. These problems have been addressed with the introduction of the Secondary Surveillance Radar (SSR).

Air Traffic Controller Radar Screen

Air Traffic Controller Radar Screen

Secondary Surveillance Radar (SSR)
The Secondary Surveillance Radar operates on the coded reply sent from the airborne radio beacon Transponder in response to an interrogation sent from the ground station. Usually, on a radar station the PSR and SSR antennas are co-located and scan synchronized, and both Radars are used in conjunction to develop the total air traffic situation display.

The SSR was developed from the military Identification-Friend-or-Foe (IFF) system, in which an airborne radio beacon Transponder responds to ground radar interrogations with coded replies. These replies, displayed as short lines on the display, allow the controllers to identify the various targets by having each one send back a different code.

The desired code can be manually selected by the pilot on the Transponder control head in, or automatically set by an encoding altimeter or altitude digitizer for reporting the Aircraft's altitude in Mode "C" operation. Since the reply signal from the airborne Transponder is stronger than the reflected PSR signal, it will reinforce the "blip" on the display to provide positive aircraft identification.

SSR Transponder Modes
Mode A interrogations are sent to request the specified aircraft identification code. Mode C is used to request altitude reporting together with identification. The coded reply signal is composed of a series of pulses. In Mode A operation, the number of pulses generated in a reply signal is determined by setting the four octal (0 to 7) digit code switches on the Transponder to the assigned identification code. This allowes for 4'096 different identification codes. Certain transponder codes are reserved for special applications to activate an alert on the controller's console:
    7500 indicates that a hijack is in process
  • 7600 reports a communication radio failure
  • 7700 indicates an emergency condition
Replies to Mode C interrogations generate pulses that correspond to the aircraft's altitude. The received altitude information is then displayed directly on the controller's console. Altitude information is not selected by the code switches of the transponder, but is obtained directly from an encoding altimeter.

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