S1TBX-SE-15 Layover/shadow masks in Range Doppler Terrain Correction


Layover and shadow are two phenomena common in SAR imagery of steep terrain areas. For any backscatter based analysis, it is good to know the location of affected areas. Since the SAR geometry is side looking, some areas on the ground will not be illuminated by the radar signal. These areas are “shadowed” by higher terrain that is closer to the SAR instrument. Since no backscatter will be returned from this area, a dark patch will be seen in the SAR image. The shadow area can be detected during the first pass of the layover computation. When processing from near to far range, i.e. from point A to D, we compute the off-nadir angle θ for each DEM element. If θ increases continuously in an image swath, then there is no shadow. However, if θ decreases then the pixel is in shadow area. With layover, the ordering of surface elements on the radar image is the reverse of the ordering on the ground. When the foreslope of the mountain is very steep, target B in the valley will have a larger slant range than target E on the mountain top, then the foreslope is "reversed" in the slant range image. The layover areas occur where the distance between the sensor and target decreases while the horizontal distance between sub-satellite track and target increases. The computation of the layover area can therefore be done by suitable processing of the DEM surface and the slant range as a decision criterion. An algorithm to detect layover and shadow treats the selected DEM in two passes. For each given row, the first pass runs from near to far range. The slant range distance between the sensor and the DEM elements are computed, starting from the smallest distance at A and continuing to the largest distance at D. As long as the slant range increases continuously, no layover occurs. The layover starts when the slant range decreases. During this first pass, the area between point B and C is recognized as layover. The second pass is done from far to near range. Starting at point D, the slant range distance gets smaller up to point E with slant range distance RSE. From here to point F, the slant range distances increases and therefore all pixels between E and F have to be considered as layover pixels. From F to the near range edge at A the slant range distances decrease again. There are no more layover pixels. Combining the layover areas found during the two passes, we have all pixels between F and C as layover. The functionality for the layover and shadow masks generation is currently available with “SAR Simulation” operator and “SAR Simulation Terrain Correction” graph in SNAP. For this project, the functionality will be added to “Range Doppler Terrain Correction” operator.




Jun Lu


Lisa House

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