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19. Shadow


A slope away from the radar illumination with an angle that is steeper than the sensor depression angle provokes radar shadows.
It should be also noted that the radar shadows of two objects of the same height are longer in the far range than in the near range.
Shadow regions appear as dark (zero signal) with any changes due solely to system noise, sidelobes, and other effects normally of small importance.

Let us consider the mountain as sketched in the animation. Points A, B, C and D are defining different parts of the target when vertically projected on the ground (the orthogonal projection, as it is done in conventional cartography). However, the segment between B and C is not giving any contribution in the slant range direction which is the SAR projection ( B'' - C'' ), due to the geometry of the mountain.

Note also the distance AB becoming orthogonal projected A' B'. In the slant-range projection this is A'' B'', much shorter, due to the foreshortening effect.
Cote D'Azur

Cote D'Azur
This multitemporal (13-19-25/09/1991) SAR image has been acquired over the Cote D'Azur area (France). The Gran Canon du Verdon visible in the central part of the image has a very steep gorge that descends swiftly to the valley bottom, causing Radar shadow. This is shown by the dark zones in the central part of the image.

A map of the area may be useful to localise the feature.