Foreshortening is a geometric distortion occurring in the SAR image due the side-looking geometry of imaging radar sensors. It occurs principally in SAR images of mountainous areas, on slopes oriented towards the sensor. These slopes appear in the radar image as if being compressed. Due to the side looking geometry and the mapping of the SAR image based on range and time measurement, the distance in the SAR image between two points situated on a slope facing the sensor appears smaller than it is in the reality and than the same distance between two points situated in flat area. This results in a compression of the radiometric information of the slope. The resulting foreshortening area is brighter in the SAR image than its surroundings, as it compresses in a few pixels the backscatter information of the whole slope.
Foreshortening occurs for slopes whose inclination is smaller than the look angle of the radar antenna. Due to the variation of the look angle in the SAR image, the foreshortening is more pronounced in near range than in far range. Foreshortening is therefore greater for small incidence angles. The extreme case of foreshortening happens when the slope inclination is equal to the look angle: in this case, the whole slope is mapped in one pixel of the SAR image, which results in a very bright line. When the slope inclination becomes higher than the look angle, layover occurs.