The incidence angle is the angle between the incident radar beam on a surface and the normal to a reference surface. Generally, it is distinguished between the local incidence angle and the incidence angle to the ellipsoid. The local incidence angle considers the normal to the surface at target location, i.e. it considers the local topography. The incidence angle to the ellipsoid corresponds to the angle between the incident radar beam and the normal to the local ellipsoid, regardless of the local slope and terrain.
For a flat surface and neglecting the Earth’s curvature, the incidence angle corresponds to the angle between the incident radar beam and the vertical, and it equals the look angle of the sensor, which characterizes the angle between the nadir view and the radar beam. Considering a flat surface, the incidence angle varies continuously within a SAR scene: it increases from near to far range. Depending on the considered sensor and acquisition modes, variations of the incidence angle up to 20° can be observed between near and far range.
The incidence angle has an influence on the radar backscatter intensity. Considering a surface with diffuse reflection, increasing incidence angles lead to decreasing backscatter intensities. This effect is less pronounced for rough than for smooth surfaces. A change in incidence angle may also induce a change in the occurring backscattering mechanisms or geometric distortions of the image. For example, for high incidence angles, terrain distortion due to the side-looking geometry is reduced. Due to the high dependency of the radar backscatter from the incidence angle, the choice of the optimal configuration should happen depending on the application. For example, whereas low incidence angles are more sensitive to biomass in forestry applications, higher incidence angle are preferred for distinguishing different forest types due to their structural characteristics.