The Primary Surveillance Radar (PSR) output data uses a polar coordinate system, it provides range and bearing of the targets found in respect of the antenna position.
- The range is determined by the time difference of the emitted and received pulse.
- The rotation speed of the antenna is usually between 5 and 12 rpm.
- PSR is the only surveillance sensor used in civil aviation that does not require any on-board equipment to locate aircraft.
- Limitations – No level data available, difficult correlation, relies on reflected signals.
Secondary Surveillance Radar (SSR) relies on targets equipped with a radar transponder, that reply to each interrogation signal by transmitting encoded data such as an identity code, the aircraft’s altitude and further information depending on its chosen mode.
- Developed during World War II for Identification Friend or Foe (IFF).
- SSR ground station transmits interrogation pulses on 1030 MHz.
- Aircraft transponder ‘listens’ for the SSR interrogation signal and transmits a reply on 1090 MHz that provides aircraft info.
- SSR system is used in air traffic control (ATC).
Primary and Secondary Surveillance Radar
- The initial use of radar was to locate aircraft and display their range and bearing on a monitor (ether ground based or in another aircraft).
- This type of radar is termed Primary radar: Energy is radiated via a rotating radar antenna to illuminate a ‘target’; this target could be an aircraft, the ground or a cloud. Some of this energy is reflected back from the target and is collected in the same antenna. The strength of the returned energy is measured and used to determine the range of the target. A rotating antenna provides the directional information such that the target can be displayed on a screen.
- Primary radar has its disadvantages, one of which is that the amount of energy being transmitted is very large compared with the amount of energy reflected from the target.
- An alternative method is secondary radar that transmits a specific low energy signal (the interrogation) to a known target. This signal is analysed and a new (or secondary) reply signal, i.e. not a reflected signal, is sent back to the origin.
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