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.
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.
Radar shadow 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 away from the sensor. In optical imagery, a shadow area is an area characterized by less sun illumination whose reflection is therefore weaker. In SAR imagery, shadow areas receive no signal. It occurs for example at the backside of mountains or buildings. The areas facing away from the sensor are not illuminated by the SAR sensor, as they are “hidden” from it. Also, ground area situated behind high object with respect to the sensor position are not illuminated and are situated in the radar shadow. They receive no signal information and send no information back to the sensor. Those areas are therefore very dark in SAR images. The size of the shadow area in range direction corresponds to the time delay between the last echo from the top of the mountain and the first echo of the far edge of the shadow region, where the area is not hidden from the sensor anymore.
Radar shadow occurs when the slope inclination of the slope facing away from the sensor is larger than 90° minus the antenna look angle. As for the other geometric effects, the size of a shadow area for the same object depends on its situation in the image. But, unlike as for foreshortening and layover, shadow is more pronounced in far range than in near range, i.e. large incidence angles produce more shadow.
A SAR image may show a return signal in a shadow area: this is principally due to internal sensor noise and does not correspond to any target return signal.