Geometric distortions of radar include foreshortening, layover and shadowing.
Foreshortening and layover distortions are related to terrain. These errors are unlikely to occur in flat terrain. In rugged terrain higher elevation is detected by the sensor first which makes it seem closer than it actually is. Any terrain feature with a slope inclined toward the radar will seem compressed (foreshortened) especially in comparison to slopes inclined away. This distortion increases as slope and feature size increase. Similarly for layover, the beam at the top of the feature returns before the beam at the base. This affect is decreased as the sensor moves away from the feature. Image layover is an extreme case of foreshortening and cannot be corrected in post-processing. Shadowing is a result of both of these distortions where the area behind the imaged spot is black due to a lack of pulse interaction. The angle of the imaging can greatly affect these shadows (think of the sun at noon vs dusk, at a lower sun angle shadows elongate).
Flat surfaces also have distortion. This is due to a changing slant range (near to far). This is similar to off-nadir imaging. Features further from the sensor can appear distorted or more elongate.
These distortions can be decreased by increasing the distance between the sensor and the ground. Hence satellite radar imagery is less distorted than aircraft images.
Jensen, John R. (2006) Remote Sensing of the Environment: An Earth Resource Perspective, New Jersey, Prentice Hall.