One of the most important reasons for performance degradation of air-surveillance radars (but also of other radio systems) are reflections with the resulting multi-path propagation causing interference. Those reflections may occur at large structures like buildings or wind turbines. The figure on the left shows the calculated reflections of the signal of a secondary surveillance radar (1030 MHz) at a tower building a few kilometres west of the radar. Additionally the beam on the left side of the figure is caused by the shadowing.  

A further example is the simulation of the interference of an instrument landing system (ILS) caused by aircraft moving on nearby taxiways or parked on the apron. The aim of this study was to determine which areas have to be kept free during the critical part of the approach.

When siting radars (or wireless radio systems in general) it is important to prevent possible shadowing caused by mountains. While lower frequencies (like FM-radio) propagate into the optical shadow area behind the obstacle due to diffraction, this mechanism is negligible at microwave frequencies and above. Therefore the antenna site has to be selected carefully with the help of the so-called coverage diagrams. This picture shows the coverage of the Inn valley around Innsbruck airport provided by the local multilateration system 5000 ft above sealevel.