SAR Series Part3: SAR Image Distortions and Interpretation

This part covers geometric and radiometric distortions, surface parameters affecting radar backscatter and interpretation of SAR image.

Geometric distortions

Due to the side-looking geometry of SAR systems, terrain features such as slopes lead to geometric distortions in the acquired SAR data. Most relevant distortions are foreshortening, layover and shadow.

Figure 3.1: Main geometric distortions in SAR image (Credit: [1])

Foreshortening: Slopes facing the sensor appear foreshortened such that a mountain is compressed and appears as if leaning towards the sensor. The front side of the slope is displayed as a bright, narrow band. The slope between points A and B have been foreshortened to an image area A’B’. An example is shown in Figure 3.1.

Figure 3.2: Foreshortening effects seen in before terrain correction (left) image; (right) image obtained after applying terrain correction (Credit: [2])

Layover: An extreme case of foreshortening is Layover. This occurs in areas of steep slopes and steep incidence angles. In this situation, returns from the top of the mountain are received at the same time as returns from the base. Because of this, area in the front of the slope is projected onto the back side in the slant range image [3]. This leads to information loss from the front slope.