# Evaluation of Orbital Drift Effect on Proba-V Surface Reflectances Time Series
## 5 Discussion  
Evaluating drifts in temporal series of satellite-derived surface reflectances is a complex task, since sensor-related effects, such as calibration drifts or variation in orbit parameters and sun-viewing conditions, are highly interrelated with the natural variability of surface and atmospheric conditions. The two-fold approach chosen in this paper, based on numerical simulations and analysis of real observations, provides the means to reliably characterize the component of the drift induced by the drifting orbit.  
Figure 17: The figure shows the NRMSE of the estimated drifts for the different angular conditions: blue bars are the azimuthal symmetry (difference in the drifts estimated for backward and forward scattering), orange bars are near nadir observations (VZA \(<\) 10\({}^{\circ}\)) and green bars correspond to all observations. The NRMSE is computed for each per land cover class and spectral bands, including NDVI. The red horizontal line is the threshold value used to identify a statistically significant impact of the orbital drift in the TOC archive.  
The simulation study represents an idealized case, where the temporal variability of the synthetic surface reflectances over the mission lifetime is modulated solely by the SZA seasonal changes and the LTDN decay. The results show that the orbital drift induces a positive spurious bias in the westward-looking camera and a negative one in the nadir and eastward-looking cameras. The reason for such azimuthal asymmetry is because the three cameras sense different regions of the BRDF angular distribution (see Figure 6), with the westward camera sensing in the proximity of the retro-reflectance peak and the other cameras in the dips of the opposite azimuthal regions. These findings were used as benchmark in the analysis of real observations.  
The investigation of the orbital effect on real data is based on the assumption that the BRDF-normalized temporal series is free of spurious bias induced by the drifting LTDN. The statistical difference between BRDF-normalized and TOC reflectances therefore provides a quantitative measure of the orbit effect. The statistical analysis of real data is aggregated by land cover classes to understand the impact for different surface types. The statistical population of the classes for the considered ensemble of sites is not equally distributed, and therefore fewer robust statistical metrics were derived for the grassland and, in particular, for sparse vegetation sites. For these classes, large statistical variability remains in the estimated drifts, and so the orbital effect cannot be reliably identified.  
Despite the expected variability in surface anisotropy and atmospheric conditions, the outcomes of the simulation were consistently verified on real data, notably, the azimuthal asymmetry in the drifts, with a distinct positive bias in the backscattering direction. Yet, no clear indication of an orbit effect was found when the whole time series is considered in the analysis. This is due to compensation effects, since the spurious biases in the two scattering directions compensate each other to a large extent. Likewise, for the NDVI, the similar temporal patterns in Red and NIR bands largely cancel out in the NDVI ratio, resulting in no significant orbit-induced effect. The limited sensitivity of NDVI, as compared to EVI, to the sun-viewing azimuthal configuration was confirmed in previous results on MODIS data [10; 12; 38], and the limited impact of the orbital drift on temporal consistency of AVHRR NDVI series was verified with theoretical studies [6].