Layover 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 steep slopes oriented towards the sensor. These slopes appear in the radar image as if being flipped over. Due to the side looking geometry and the mapping of the SAR image based on range and time measurement, the summit of a mountain is closer to the sensor that the foot of that same mountain, on the side facing the sensor. The signal from the top comes back to the sensor before the signal from the foot and is therefore mapped in nearer range than the foot of the mountain. Making an analogy to sound waves, an echo from the top of the mountain will arrive sooner at the sensor than an echo from the bottom of the mountain. Due to this “leaning over” effect, the sensor facing slope signal usually overlaps with ground signal, and a “ghost” effect appears as both signals overlap. The resulting layover area is usually very bright in the SAR image, as it superimposes backscatter signals from the slope of the mountains and the ground before it. When considering SAR images of urban areas, even up to three signals may overlap in the layover area: ground, building façade and (part of the) roof area.
Layover occurs for slopes whose inclination is larger than the look angle of the radar antenna. Due to the variation of the look angle in the SAR image, layover occurs more often in near range than in far range. Layover is therefore greater for small incidence angles. It represents the extreme case of foreshortening, when the slope inclination becomes higher than the look angle.