## 3 Results
### Fusion and Transformation of Multispectral and Radar Data  
LS8 and PALSAR image fusion (Figure 5), using the Gram Schmid algorithm, along with multispectral and radar image transformation, using the MNF approach, have significantly boosted  
Figure 4: Radar images of the structure with different wavelengths and polarizations. A close-up look at the structure in (**a**) Sentinel-1 C-HV, (**b**) Sentinel-1 C-VV, (**c**) PALSAR L-HV, and (**d**) PALSAR L-HH polarizations. Bright areas in the radar images indicate rocky surfaces and coarse-grained materials, whereas dark areas are fine-grained materials, such as Aeolian and fluvial deposits. Note the superiority of the PALSAR L-bands (**c.d**) in revealing the complete circularity of the structure when compared to Sentinel-1 C-bands. This is due to the longer wavelength of the L-band which is capable of penetrating sand deeper than the shorter wavelength of the C-band. (**e**) A close-up look of the Sentinel-1 C-band. Note the superiority of the shorter wavelength in revealing the radial pattern drainage network and traces of concentric faults on the inner wall of the northern flank of the structure.  
the visual interpretability of the structure's morphology. Fusion and transformation of LS8 and radar data enabled the combining of information of spectral properties (from multispectral data) and surface roughness (from microwave data) to enhance the visibility of subtle surface and near-surface features of the Rimaal structure. The fused image reveals clearly the circularity of the structures and the radial drainage patterns buried beneath the Aeolian deposits. The fused image highlights extensive systems of NE-SW trending elongated sand dunes that run across the Rimaal structure and obscure much of the detail of the inner ring and the outer rim. The dunes' orientation indicates a strong influence of the easterly trade wind in the area.  
Figure 5 displays an RGB color composite image of a stacked Sentinel-1 C-VV and PALSAR L-HH and L-HV channels, which clearly reveals the morphology of a large part of the Rimaal structure. The structure is a 10.5 km-diameter near-circular bowl-shaped depression. Impact craters larger than 10 km in diameter are most probably created by a meteorite with diameter larger than 1 km [28]. Assuming a spherical projectile, the volume of the impactor of the Rimaal structure is estimated to be about 606 km\({}^{3}\). It comprises a small, irregularly-shaped central peak surrounded by a near-circular 1 to 1.5 km wide ring feature of a strong radar backscatter. This is followed by a slightly flat radar dark annular trough, of a fairly uniform width of \(\sim\)2.5 km surrounded by a radar bright outer rim. The inner wall of the rim exhibits traces of concentric faults. PALSAR surface roughness profile indicates a much coarser textured material of the crater interior than the surroundings. Here, the inner ring of the structure is made up of a significantly coarse textured material (e.g., bedrocks and boulders) than that of the annual trough of fine textured grains (e.g., fine Aeolian and fluvial deposits) (Figure 6). Typically, in complex impact craters, the central uplift structures are often more resistant to weathering than the surrounding crater fill of impact breccias and, thus, could be the only relic of deeply eroded impact craters [4].  
Figure 5: Fusion and transformation of multispectral and radar data of the Rimaal structure. (**a**) Landsat-8 true color (bands 4, 3, 2 as R, G, B); (**b**) fusion of LS8 with Palsar HH polarization images using Gram Schmid algorithm; (**c**) MNF image transformation of a stacked LS8 multispectral and PALSAR microwave image. The figure shows that combining of spectral and roughness data dramatically enhances the visibility of the subtle surface and near-surface features of the structure. (**d**) Sentinel-1 and PALSAR stacked image (C-VV, L-HH, and L-HV as RGB) clearly reveals large parts of the morphology of the Rimaal structure.