# 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 BRDF-normalized reflectances shows, as expected, a drastic reduction in the day-to-day and seasonal fluctuations, owing to the removal of directional effects in the temporal series, while retaining the annual phenological cycle over this site. More importantly, after BRDF normalization, the estimated drifts appear consistent for the three angular configurations, showing the same sign and comparable magnitudes. The temporal series over a cropland site, Barrax in Spain (lat/lon = \(39.06^{\circ}/-2.07^{\circ}\)), is presented in Figure 8. The evolution of TOC NIR reflectances is more complex over this agricultural site, showing sharp increases during the growing seasons and a slow decrease moving towards the wintertime. Despite the more complex intra-annual changes, the analysis of estimated drifts shows similar patterns as observed for the rainforest site. Notably, in the backscattering direction we observe an increase in TOC NIR reflectances throughout the mission, mostly driven by the maxima at the end of the growing seasons, while a decrease is observed in the forward scattering direction. The linear fit indicates a spurious positive drift (\(0.0055\) reflectance/year) for RAA \(<90^{\circ}\), while a negative one is observed for RAA \(>90^{\circ}\) (\(-0.00037\)). When observations for all angular conditions are merged, the drift observed in the two scattering directions is strongly reduced, owing to compensation effects (\(0.0013\) reflectance/year). The BRDF normalization procedure allows reducing the day-to-day and seasonal variations, while maintaining the phenological cycle at the site. Furthermore, the estimated drifts in the different angular conditions show consistent values in terms of both sign and magnitude, as already pointed out over the forest site. For this agricultural site, the adopted BRDF procedure proves robust in resolving the short-term variations in directional reflectances and the azimuthal asymmetry in the drifts, despite the limitations of the adopted assumptions, in particular the use of a 20-day gathering period. The temporal series of NIR reflectances over a dry shrubland site, located in the north of Mexico (lat/lon = \(27.57^{\circ}/-103.61^{\circ}\)) (see Figure 9) confirms the previously observed asymmetry in the estimated drift for backward and forward scattering conditions. The spurious positive drift in the backscattering is particularly evident over this site, with maxima of NIR reflectances increasing by up to 15% in mid-2020 as compared to the values at the beginning of the mission. In the opposite direction, the decrease is also evident. When all observations are merged, the opposite drifts in the two angular conditions--0.0042 and \(-0.0018\) reflectance/year, respectively--largely cancel out, with a resulting drift of \(0.000049\) reflectance/year. The BRDF normalization procedure dramatically reduces the day-to-day and seasonal changes and resolves the azimuthal asymmetry of the estimated drifts, since the drifts in the three angular conditions have now the same sign and comparable values. The BRDF normalization is particularly efficient over this temporally stable dry shrubland site, since the fluctuations in NIR reflectances induced by SZA seasonal and inter-annual changes are largely smoothed out. Yet, some localized anomalies are observed in the BRDF-normalized time series, such as a minimum during winter 2015 and a maximum during summer 2017. These localized anomalies do not impact the drift estimation, as confirmed by the plotted fitted line, following the slowly varying temporal changes of reflectances along the mission lifetime. Finally, an example of temporal series for a bare soil class is shown in Figure 10, corresponding to a bright desert site in Egypt (lat/lon = \(27.87^{\circ}/28.87^{\circ}\)). This site was chosen to investigate the effect of the drift for a spatially and temporally invariant site. The evolution of TOC NIR reflectances clearly shows, as expected, the absence of any seasonal or phenological changes.