# VENuS-Derived NDVI and REIP at Different View Azimuth Angles
## 3 Results and Discussion  
To explore the view azimuth angle's effect on the spectral reflectance, the change in the VIs (Equation (4)) was examined in three selected polygons (Figure 3a). The NDVI pixel subtraction (\(\Delta\)NDVI) of the two tiles (S01-W08) showed the distinct difference as a function of the land use (Figure 3b,c). For example, on April 18, the southern half-circle of the chickpea crop field (i.e., CircleField-S, yellow-colored polygon in Figure 3a) was characterized by negative values of \(\Delta\)NDVI, while the post-sowing bare soil of the northern half-circle of the corn crop field (i.e., CircleField-N, green-colored polygon in Figure 3a) was characterized by positive \(\Delta\)NDVI values. Similarly, most northern corn fields showed negative values, while the southern dry chickpea field showed values distributed around zero on June 27. On September 11, bare soil was found in both cultivated fields with \(\Delta\)NDVI values distributed similarly at around zero difference. On all the selected dates, the relatively homogeneous half-circle fields were characterized by narrow distributions (yellow and green-colored bars in Figure 3c). In contrast, the inhomogeneous urban area (i.e., Urban) was characterized by a wider distribution of \(\Delta\)NDVI (brown-colored bars in Figure 3c). In accordance with its relatively low seasonal variability, the urban area was characterized by similar distributions for all selected dates with no negative-positive dominance. Examination of the \(\Delta\)REIP values (Figure 3d,e) for the exact dates and polygons also showed distinct differences. Areas with dominant vegetation, such as the chickpea field's southern half-circle (yellow-colored polygon on April 18) or the corn field's northern half-circle (green-colored polygon on June 27), showed nearly zero differences (Figure 3e). On the other hand, bare soil and urban areas consistently showed wider distributions and larger differences. The \(\Delta\)Albedo values (Figure 3f,g) showed a mainly positive change as expected based on the solar and view angles (Table 2). The CVA magnitude for the NDVI-albedo combination showed that pixels with a CV magnitude (Figure 4a) lower than the threshold (1-STD) were considered as no-change pixels and colored black. The same threshold was also used for the REIP-albedo combination (Figure 4d). The urban area and the bare soil in the circle fields showed relatively large magnitude changes, while areas such as field crops or dry matter showed no change. Therefore, the NDVI values, as obtained by VEN\(\mu\)S, of vegetative areas were less affected by the view azimuth angle than those of soil areas. Nevertheless, dry standing vegetative biomass (i.e., CircleField-S on June 27) areas or bare soil fields a relatively long time after harvest (i.e., CircleField-S on September 11) had a magnitude below the threshold (i.e., showed no change). Figure 4a shows the CV magnitudes as a function of land use, similar to the direction in Figure 4b. In addition, the polar plots in Figure 4c illustrate both the magnitude and the direction changes for each polygon of interest on the selected dates. Following the positive changes in albedo for the two half-circle crop fields (Figure 3f,g), their direction continued to be on the upper part (i.e., positive \(\Delta\)Albedo). In contrast, the urban area was characterized by negative and positive changes in albedo, with some patterns connecting these changes of \(\Delta\)Albedo with changes in \(\Delta\)NDVI (Figure 4c). The similarities between NDVI-albedo and REIP-albedo (Figure 4) included the dominant positive \(\Delta\)Albedo in the crop field areas vs. positive or negative \(\Delta\)Albedo in the urban area and the distinct narrow distribution of \(\Delta\)REIP in the vegetative crop field areas vs. the wider distribution of \(\Delta\)REIP in the bare soil, dry matter, and urban areas.