The visual indication on a situation display of unwanted signals.
Source: ICAO Doc 4444 PANS-ATM
The electromagnetic signals processed by the radar receiver (especially for PSR) consist of three main components:
- useful signal (echoes from aircraft)
- clutter, which comprises:
- echoes from targets other than aircraft (e.g. birds, clouds, terrain, buildings, etc.)
- signals received by lobes other than the main one (e.g. side lobes).
- multiple reflections, e.g. instead of radar-aircraft-radar, the path of the signal can be radar-aircraft-terrain-radar
- noise, which can be:
- external, e.g. random ambient electromagnetic signals on the frequency, used by the radar.
- internal (i.e. caused by processes in the receiver itself). It exists in all electronic equipment.
If the combined signal (useful+noise+clutter) is shown on the situation display directly, two problems occur:
- targets (i.e. aircraft) can remain undetected due to being indistinguishable. An example of this is when a (small) aircraft flies low and there is a mountain behind it.
- false alarms, e.g.:
- other objects being recognized as aircraft
- duplicated aircraft, i.e. two signals from the same target, one of which is with wrong parameters (distance and/or azimuth)
The main objective of radar data processing is to separate the useful signals (i.e. the aircraft) from noise and clutter. While both phenomena have the same effect on the equipment (reduce the probability of finding a real targer and increase the chance of false alarms), they are created differently, and, therefore, are mitigated using different techniques.
Internal noise is reduced by using high quality equipment and external noise is mitigated by increasing the transmitter's power. This leads to stronger reflected signals, and therefore, better detection, since external noise is not affected. It should be noted, that power cannot be increased indefinitely.
Increasing transmitter power will have no (or even detrimental) effect on clutter suppression, as the undesired signals will also become stronger. Therefore, more spohisticated methods are used to mitigate clutter, e.g.:
- antenna designs that provide better directivity, i.e. stronger main lobe and smaller side lobes
- radar processing using a moving target detector (MTD). The MTD uses the Doppler effect to filter out stationary targets. This method is very good at suppressing clutter from terrain and buildings (stationary objects). However, it relies on the radial relative speed (i.e. how fast the aircraft is closing on or going away from the radar). Therefore, sometimes an aircraft echo can be suppressed depending on the direction of the flight and relative position to the radar antenna. Slow moving targets (e.g. helicopters in the hover) may be filtered out as well.
- use of transponders almost eliminates clutter. While there are other issues with similar effects (e.g. garbling and FRUIT), their impact on the controller's job is not as significant and they can be mitigated to a great extent. This is one of the reasons why in most parts of the world, transponder equippage and operation is mandatory in controlled airspace.
It should be noted that, depending on the purpose of the equipment, the clutter described above may be considered as useful signal, e.g.: