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.