# Evaluation of Orbital Drift Effect on Proba-V Surface Reflectances Time Series
## 4 Results
### Analysis of Estimated Drifts in TOC Products  
#### 4.3.1 Analysis of Temporal Series over Four Representative Sites  
The analysis of temporal series over four sample BELMANIP-2 sites is presented in Figures 7-10. This initial set of sites allows evaluating the orbital drift impact over four representative land cover classes, forest, cropland, shrubland and bare soils, in a range of latitudes [\(-40^{\circ}\), \(40^{\circ}\)], where larger reflectances variations were predicted in the simulation study. Results are reported only for the NIR band for the sake of conciseness; similar temporal patterns are also observed for the other bands and NDVI (see Supplementary Materials, Section S.3). Figure 8: The figure shows TOC reflectance in NIR band for a cropland site (Belmanip-232) located in Barrax, Spain (lat/lon = \(39.06^{\circ}\) / \(-2.07^{\circ}\)). The plots present the temporal series of TOC (red) and BRDF-normalized (blue) reflectances for three angular conditions: (**a**): RAA \(<90^{\circ}\), (**b**) RAA \(>90^{\circ}\), (**c**): “All”; the latter corresponds to retaining all observations in the fit. The fitted linear curves are also shown for TOC (purple) and BRDF-normalized (green) data with drift values reported in the legend. In the plot (**c**), the two regression lines lies above each other. Figure 7: The figure shows TOC NIR reflectances over Belmanip-38 site in the Amazon rainforest (lat/lon = \(-9.75^{\circ}\) / \(-60.33^{\circ}\)). The plots present the temporal series of TOC (red) and BRDF-normalized (blue) reflectances for three angular conditions: (**a**): RAA \(<90^{\circ}\), (**b**) RAA \(>90^{\circ}\), (**c**): “All”; the latter corresponds to retaining all observations in the fit. The fitted linear curves are also shown for TOC (purple) and BRDF-normalized (green) data with drift values reported in the legend. The results for a rainforest site in a tropical region (lat/lon = \(-9.75^{\circ}/-60.33^{\circ}\)) are shown in Figure 7. The temporal series of TOC reflectances in the three angular conditions show periodic fluctuations, driven by the SZA seasonal variations, with super-imposed clear long-term drifts. More specifically, in the backscattering direction (RAA < 90\({}^{\circ}\)), there is an increase in TOC NIR reflectances, which is more pronounced during the summer periods, with an estimated positive drift of 0.0039 reflectance/year. Contrarily, in the opposite, forward scattering direction (RAA > 90\({}^{\circ}\)), we observe a decrease in TOC NIR reflectances, with an estimated drift of \(-0.0025\) reflectances/year. This initial finding confirms the results of the previous studies. Figure 10: The figure shows TOC reflectance in NIR band for a bright desert site (Belmanip-207) located in Egypt (lat/lon = \(27.87^{\circ}/28.87^{\circ}\)). The plots present the temporal series of TOC (red) and BRDF-normalized (blue) reflectances for three angular conditions: (**a**): RAA < 90\({}^{\circ}\), (**b**) RAA > 90\({}^{\circ}\), (**c**): “All”; the latter corresponds to retaining all observations in the fit. The fitted linear curves are also shown for TOC (purple) and BRDF-normalized (green) data with drift values reported in the legend. Figure 9: The figure shows TOC reflectance in NIR band for a shrubland site (Belmanip-57) located in the north of Mexico (lat/lon = \(27.57^{\circ}/-103.61^{\circ}\)). The plots present the temporal series of TOC (red) and BRDF-normalized (blue) reflectances for three angular conditions: (**a**): RAA < 90\({}^{\circ}\), (**b**) RAA > 90\({}^{\circ}\), (**c**): “All”; the latter corresponds to retaining all observations in the fit. The fitted linear curves are also shown for TOC (purple) and BRDF-normalized (green) data with drift values reported in the legend. of the simulation study for the same land cover class. When the temporal series of \"all\" observations is considered (Figure 7c), the opposite trends in the forward and backward scattering directions largely cancel out, with remaining small residual drift (\(-0.00048\) reflectance/year).