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Primary Surveillance Radar (PSR)

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A surveillance radar system which uses reflected radio signals.

Source: ICAO Doc 4444 PANS-ATM


Principle of operation The radar antenna rotates (usually at 5-12 rpm) emits a pulse of radio wave. Upon reaching an aircraft (or other object) the wave is reflected and some of the energy is returned to the antenna.

PSR principle of operation

The PSR output data uses polar coordinate system - it provides range and bearing of the targets found in respect of the antenna position. Note that the range is the slant distance from the antenna and not the horizontal distance.

The range is determined by the time difference of the emitted and received pulse (the speed of propagation is the speed of light) and the bearing is obtained from the antenna azimuth. The rotation speed of the antenna is usually between 5 and 12 rpm.

The antenna radiation patern is a narrow beam when seen from above and, whith some approximation, can be considered as a trapezium if seen from the side.

PSR antenna radiation pattern


PSR is the only surveillance sensor used in civil aviation that does not require any on-board equipment to locate aircraft. Unlike SSR, ADS-B and MLAT it can discover an aircraft experiencing Transponder Failure or an intruder.


  • Cone of silence. Due to the radiation pattern, there is a part of the airspace above the antenna that cannot be surveyed. This effect is mitigated by placing an array of radars in such a way that each radar's cone of silence is covered by another radar.
  • Targets with the same slant range (at different levels) are hard to distinguish (the blips received will overlap). This is mitigated by combining the PSR with an SSR that can recognize the different aircraft by their transponder codes.
  • Difficult correlation. Due to the data received (position only) it is not possible to use automatic correlation. Manual correlation (after proper aircraft identification) is possible, however. The downside is that this requires a lot of controller effort and is therefore not suitable for busy airspaces.
  • 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. This can somewhat be mitigated by processing the data usind an MTD (moving target detector). This feature uses the doppler shift of the received signal to determine whether it came from a stationary or a moving target.
  • No level data available. Civil PSRs do not have the ability to obtain target level. This may be mitigated either by receiving pilot reports or by combining the PSR with other types of sensors. Note that some military radars have this feature (either by using a second antenna or an antenna array). Nevertheless, this information is not to be used for air traffic control purposes as it is geometric and not pressure-derived.
  • Unambiguous range limit. When receiving the signal it is not possible to determine the corresponding emitted pulse. Therefore, a false target can be detected (usually close to the radar) if the reflected signal reaches the antenna after a second pulse is transmitted. This effect is mitigated by adjusting the transmitted energy and the antenna rotation speed.
  • Minimum range limit. The PSR operates on one frequency which means that it cannot emit and receive siganl at the same time. If the target is too close to the radar antenna, the reflected signal may be received before the end of the transmission. If that happens, the target will not be detected. Note that shortening the pulse will also reduce the amount of emitted energy thus limiting the maximum range of the radar. This is mitigated by adjusting the pulse lenght and the antenna rotation speed.

The advantages of the PSR and SSR are often combined by co-locating them.

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