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.

Both layover and foreshortening decrease with increasing look angle; however, larger look angle will result in more image shadow!

Shadow: This occurs in situations where angle of the back slope is steep enough such that the sensor cannot illuminate it at all — areas on that slope are not imaged! Shadow is not dependent on the time of data acquisition or the Sun angle in the sky and it is always away from the sensor flight line.

Note that topography related image distortions cannot be entirely removed from the image. Terrain correction is a critical step in the pre-processing of a SAR image; while foreshortened areas can be corrected using a Digital Elevation Model (DEM), areas impacted by layover or shadow do not have any return data to correct.

Radiometric Distortions

SAR images, in addition to geometric distortions, are characterized by grainy appearance that resembles “salt and pepper” noise…