Under Clutter refers to targets that the radar detects and displays, but they are undesirable because they, the detection probability reduce the ultimate goals (the term is taken from the English language and called clutter and confusion ). The application of this term varies from radar device to radar device. When an aircraft flies in a dense cloud, the echo signals from that cloud are a weather clutter to the airborne surveillance radar . For the weather radar , however, it is precisely this cloud that is very important, but the echo signal from the aircraft interferes and is therefore referred to as a clutter in weather radar.
The electromagnetic waves from a radar device are reflected on all electrically conductive surfaces, including houses, raised areas and even the damp leaves of trees. These echo signals are undesirable in some radars and are therefore called ground clutters. An older German-language name is called Festzielechos . These fixed target echoes form large-area brightenings on the viewing device , which are concentrated around your own location, as more distant objects on the ground disappear behind the horizon due to the curvature of the earth . This cluster of fixed target echoes in the center of the screen was formerly called the "local rose". Within this local rose, recognizing an aircraft with a small target mark was almost impossible because the fixed target echoes completely brightened this area.
Since these fixed targets are always at a constant distance from the fixed location of the radar device, the echoes from fixed targets are relatively constant in amplitude and phase position and can be filtered out by special assemblies ( Moving Target Indication MTI).
The increasing number of wind power plants makes radar monitoring more difficult. The blades of the wind power plants are partly made of carbon fibers and therefore reflect well. As a result of the rotation, they also have a Doppler frequency and can therefore no longer be suppressed by the MTI. In the picture above, the bottom clutter without Doppler frequency is already filtered out up to a distance of 20 nautical miles (the very light ring is the mark for 25 nautical miles).
The echoes that come from waves at sea are called sea clutters. The wind also gives these sea clutters a small Doppler frequency and is therefore very difficult to suppress. In practice, an STC function is usually used to suppress the disturbances that usually occur in close proximity.
A weather clutter is used when clouds, rain, snow or fog are recorded and displayed as targets by a reconnaissance radar. Since here too the wind causes a movement relative to the radar device, these false echoes sometimes also come through due to the moving target indication .
These interfering signals mostly originate from a so-called volume target (as opposed to a point target), and this interference can therefore be weakened by using circular polarization .
As a military application, additional reflection surfaces are emitted into the atmosphere, which are intended to disrupt a reconnaissance radar. These light reflective objects (thin strips of aluminum foil, metal-coated glass fibers or carbon fibers) stay in the air for a relatively long time due to the thermals, form extensive bright areas on the screen and are called chaff .
In order to achieve a large effective reflection surface, the lengths of these strips or fibers are matched to the wavelength of the radar.
- Richard K. Moore, "Ground Echo", in M. Skolnik, Radar Handbook, 3rd Ed., 2008 New York: McGraw-Hill ( ISBN 978-0-07-148547-0 )
- Lewis B. Wenzel, "Sea Clutter", in M. Skolnik, Radar Handbook, 3rd Ed., 2008 New York: McGraw-Hill ( ISBN 978-0-07-148547-0 )
- Simon Haykin et al .: Uncovering Nonlinear Dynamics - The Case Study of Sea Clutter (PDF file; 597 kB) , Proc. IEEE, 2002.