# Evaluation of Orbital Drift Effect on Proba-V Surface Reflectances Time Series
## 4 Results
### Analysis of Estimated Drifts in TOC Products
#### 4.3.2 Geographical Distribution of Azimuthal Asymmetry  
In order to verify the results obtained over the four sample sites, the analysis is here extended at a global scale over the full set of 420 BELMANIP-2 sites. The first step consists of a qualitative assessment of the geographical distribution of the estimated drifts. The rationale is to investigate potential latitudinal or regional patterns.  
The geographical distribution of the azimuthal asymmetry in the estimated drifts for backward and forward scattering directions and for TOC and BRDF-normalized NIR reflectances is presented in Figure 11 over all BELMANIP-2 sites. The azimuthal asymmetry is presented in the maps in terms of difference between the drifts estimated in the backward and forward scattering directions (\(drift_{diff}=drift_{backward}-drift_{forward}\)), respectively. The sign of this difference is visualized in the map with colors (green for positive and red for negative), while the size of the points is proportional to the magnitude of the absolute difference.  
Several considerations can be inferred from the maps. Firstly, when considering the TOC data, the azimuthal asymmetry in the estimated drifts, observed over the four sample sites, is now confirmed at global scale; particularly, the drift difference is positive for most of the analyzed sites. The magnitude of the difference is generally larger over vegetated sites, notably over tropical forests in South America, Africa and Asia, as well as over cropland, shrubland and grassland sites in North America and Asia. There is no evidence of a latitudinal or regional dependence of the \(drift_{diff}\), although larger occurrence of negative values is observed for high latitude in the Northern Hemisphere. When comparing the  
Figure 11: The two maps show the geographical distribution of the differences in the estimated drift (\(drift_{diff}=drift_{backward}-drift_{forward}\)) of NIR reflectances over BELMANIP-2 sites for the two temporal series of (**a**) TOC data and (**b**) BRDF-normalized data. The size of the points is proportional to the magnitude of the differences; the color indicates the sign, i.e., green is positive, red is negative.  
spatial distribution of the \(drift_{diff}\) for TOC and BRDF-normalized reflectances, we can clearly see that the BRDF normalization allows for smoothing out the asymmetry in the two angular conditions. The magnitude of the drift differences for BRDF-normalised data is in fact strongly reduced for most of the sites, except a single outlier in the Southern Hemisphere, meaning that the drifts estimated in the two scattering conditions are generally consistent after the BRDF normalization. On the other hand, this smoothing is not globally uniform, since large differences, both positive and negative, remain at high latitude in the Northern Hemisphere. This behavior seems to point to a flaw in the BRDF-procedure over this latitude region, which can be reliably explained by residual impact of undetected clouds. It is in fact known that the cloud detection algorithm, used to generate the input Proba-V TOC products Collection 1, suffers a systematic issue for latitude above 50\({}^{\circ}\) North ([PERSON], 1976), resulting in high amounts of undetected clouds. This issue can clearly impact the normalization procedure, notably for high latitude regions and during wintertime, when the combination of persistent cloudy conditions and the presence of remaining undetected clouds can lead to scarce and unreliable input data, with resulting noisy BRDF retrieved parameters ([PERSON], 1976).