The brightness and darkness in the SAR images are due to the structure surface of the target object on the ground. The roughness of the surface decides the brightness of the SAR images. When the antenna transmits the high-frequency radar waves and they hit the ground, only a portion of the signals reflects back to the antenna. The antenna has the capability of both transmitting and receiving radar signals.
When the adequate portion of the radar signals reflects back to the antenna then those parts are shown brightly in the SAR image. When a very small portion of the radar signals is reflected back to the antenna then those areas are projected dark in the SAR images. There are certain factors over which the returning radar signals depend upon like electrical properties, roughness, the geometrical positioning of the surface, and the polarization direction of the returning radar wave. Figure 1. shows the bright and dark areas in the SAR image.
Figure 2. Surface orientation (a) Smooth surface (b) Rough surface
Figure 2. shows the surface orientation of the target ground over which the radar signals hit. The smooth and rough surfaces are shown in the above figure. When radar signal hits the smooth surface of the terrain as shown in Figure 2.(a) the angle of incidence of the radar signal is almost equal to the angle of reflection of the reflected radar signal. This means that there is either no radar return or a very little energy returns back to the radar antenna. In this case, the projection is shown dark in the SAR image. This type of case is mainly seen in the calm water body like a river which has a specular reflection.
When radar signal hits the rougher surface which has a random and irregular surface orientation, then the radar energy is reflected back in many directions as shown in Figure 2.(b). In this case, a small portion of the radar energy is reflected back and received by the SAR antenna which produces a bright signature in the SAR image. This type of case is mainly seen in SAR images of rocky mountains and trees.
The orientation of the local surface of the target region relative to radar wave travel path is responsible for the variation of brightness in the SAR images. Other various surface orientations responsible for variation in brightness are shown in Figure 3. When radar signal hit the surfaces that are perpendicular to the travel path of the signal then this scenario generates the strongest return as they are directly facing the sensor shown in Figure 3(a).
The slopes that face away from the SAR sensor produce the weak returns of the target information. Similarly, the slopes of the hills are steeper than the depression angle then they are not illuminated by the radar signals and are completely shadowed and in this case, least or no information of the returned back to the antenna which also appears darker in the SAR image. As the depression angle becomes smaller causing more gentle back slopes to become shadowed.
The visual appearance of the buildings in the populated area is bright and clear in the SAR image because of the corner reflector shape as shown in Figure 3.(b) and (c). In Figure 3(b), corner reflector is shown that produces the strongest return of the radar signals. It can be noticed that the radar signals when hit the surface it completely reflects back to the SAR sensor regardless of the depression angle. The objects with metal surfaces like bridges and towers of power line appear brighter in the SAR images because the value of the dielectric constant is high. The dry natural materials like rammed earth and earth sheltering have low dielectric constants, but the occurrence of humidity in the soil, snow, or vegetation raises this value and their radar reflectivity too. SAR image processing assesses humidity content of surface materials.
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Note: If you are taking help from the above work and also incorporating the above content in your research work, Kindly make sure you are citing the below papers.
- https://www.sciencedirect.com/science/article/pii/S2215098617314003
- https://www.sciencedirect.com/science/article/pii/S1319157817301106
- https://www.sciencedirect.com/science/article/pii/S1319157817303865
- https://link.springer.com/article/10.1007/s11831-021-09548-z
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February 05, 2022
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