# Evaluation of Orbital Drift Effect on Proba-V Surface Reflectances Time Series
## 4 Results
### Analysis of Estimated Drifts in TOC Products
#### 4.3.3 Statistical Analysis of Azimuthal Asymmetry  
The statistical distribution of the azimuthal asymmetry is here analyzed over the BELMANIP-2 sites. The sites above 50\({}^{\circ}\) are not included in the analysis to limit the impact of undetected clouds. The statistics are aggregated per land cover classes: Crop, Tree, Shrubland, Grassland, Sparse, Bare. The statistical population within each class after the latitudinal filtering is: cropland (34), forest (96), shrubland (47), grassland (34), sparse vegetation (22), bare soils (65). The aggregation works as follows. For each land cover class, the estimated drifts over all sites belonging to the class are considered as part of the statistical population; the median and the inter-quartile range (IQR) of this ensemble of data is hence computed. These statistical metrics may be impacted by the number population within each class as well as the natural variability of surface and atmospheric conditions over the considered sites. In particular, for grassland and sparse vegetation classes, the lower number of sites could potentially lead to less robust statistics as compared to the other classes. The statistical analysis for TOC and BRDF-normalized reflectances is presented in Figures 12 and 13. The results of Figure 12 provide a comprehensive verification at a global scale of the azimuthal asymmetry observed over the four sample sites. In particular, estimated drifts in TOC products are higher and generally positive in the backward scattering direction, and lower and mostly negative in the opposite direction. This general behavior is particularly evident for NIR and SWIR bands, as predicted in the simulation study. The Blue and Red bands (Figure 12, panels (a) and (b)) show comparable patterns, although the drift difference (\(drift_{diff}=drift_{backward}-drift_{forward}\)) is significantly lower than the infrared bands; this is due to the lower dynamic range of surface reflectance in the visible range. In the infrared bands, the asymmetry is typically higher over cropland, forest and shrubland sites. Larger variability in the estimated drift is observed for sparse vegetation and grassland sites, which can be related to the less representative statistical population within these classes. The drift difference is lower, but still clearly observed, over bare soils for all spectral bands, particularly in the infrared bands. The statistical analysis for BRDF-normalized data (Figure 13) clearly demonstrates that the azimuthal asymmetry in the TOC drifts is dramatically reduced after BRDF normalization, as already observed over the four sample sites. Specifically, the statistical distribution of the drift difference (\(drift_{diff}\)) is much narrower than the original TOC data and the median is centered around the zero value for all spectral bands and land cover  
Figure 12: The figure shows boxplots of estimated drifts in TOC products aggregated per land cover class for the backward (blue) and forward (red) scattering directions, and their differences (green). Results are presented for bands: (**a**) Blue, (**b**) Red, (**c**) NIR, (**d**) SWIR. Aggregation is performed per land cover class: Crop (CR), Tree (TR), Shrubland (SH), Grassland (GR), Sparse vegetation (SP), Bare soils (BA). The height of the box extends from the upper to the lower quartile, \(Q_{3}\) and \(Q_{1}\), corresponding to the 75 th and 25 th percentiles, respectively. The median is plotted inside the box and the bars extend to 1.5 \(\times\) IQR from the lower and upper quartiles, where IQR = \(Q_{3}-Q_{1}\). Figure 13: The figure shows boxplots of estimated drifts in BRDF-normalised products aggregated per land cover class for the backward (blue) and forward (red) scattering directions, and their differences (green). Results are presented for bands: (**a**) Blue, (**b**) Red, (**c**) NIR, (**d**) SWIR. Aggregation is performed per land cover class: Crop (CR), Tree (TR), Shrubland (SH), Grassland (GR), Sparse vegetation (SP), Bare soils (BA).