# Evaluation of Orbital Drift Effect on Proba-V Surface Reflectances Time Series
## 4 Results
### Analysis of Simulated Changes in Synthetic Surface Reflectances  
The simulated surface reflectances changes over the considered rainforest site are presented in Figure 5 for Proba-V equivalent NIR band as a function of date and latitude. The results for the other bands are provided in the Supplementary Materials, Section S.2. Three plots are presented in Figure 5, corresponding to the viewing directions at the center of each Proba-V camera (left, nadir and right), corresponding to VZA = \(-34^{\circ},\,0^{\circ}\) and \(34^{\circ}\), respectively. The simulation was run over the principal plane to identify the maximum potential impact of the directional effects. Although Proba-V cross-track geometry does not sense the principal plane exactly, its measurement geometries are symmetrically distributed in the forward and backward scattering directions (see Figure 1). Specifically, the left camera senses in the westward direction (away from the sun), while the right camera points to the eastward direction (towards the sun). early morning overpasses (see Figure 2). Reflectance changes remain within a range of [\(-\)0.01, 0.01] until mid-2017, when larger deviations start to appear for all cameras. This is the time when LTDN reaches 10:30 a.m. and the rate of decay starts to grow. Overall, the reflectances absolute changes are stronger in the latitude range [\(-\)40\({}^{\circ}\), 40\({}^{\circ}\)] during hemispherical summertime conditions, in correspondence to the larger SZA variations (see Figure 4). The impact of the orbital drift is not homogeneous along the across-track swath, but it strongly depends on the considered camera, with distinct and opposite temporal patterns in the left camera as compared to the nadir and right cameras. In the left camera and over tropical and sub-tropical regions, the simulated reflectances are predicted to increase during summertime by up to 0.08 in reflectance units at the end of the simulation period. Conversely, in the nadir and left camera, the reflectances are predicted to drift progressively towards lower values. Similar temporal patterns are observed in the SWIR and visible bands, although in the Blue and Red bands the magnitude of the changes is one order of magnitude lower than infrared bands, owing to the lower surface reflectance at these wavelengths. The observed dependence on the camera is related to the changes in the sun viewing azimuthal geometry induced by the orbital decay. This dependence is better explained in the polar plots in Figure 6, showing the BRDF angular distribution for different SZA with superimposed VZA of the three cameras. As the SZA increases, the BRDF angular distribution becomes strongly asymmetric, with enhancement of the retro-reflectance peak (hot-spot) in the backscattering direction, while reflectances decrease in the opposite, forward scattering direction. The different cameras are sensing different portions of the BRDF, meaning that their response to the orbital drift is different. In the left camera, there is a prevalence of observations lying close to the hot-spot; this yields to an increase in reflectances at increases in SZA. The opposite occurs in the nadir and right cameras, causing the reflectances to progressively decrease when SZA increases. Figure 6: The figure shows Proba–V NIR band BRDF for (**a**) SZA = 10\({}^{\circ}\), (**b**) SZA = 20\({}^{\circ}\), (**c**) SZA = 30\({}^{\circ}\), (**d**) SZA = 40\({}^{\circ}\). Distance from the origin is the zenith angle, while azimuth angles span the polar plot plane. The VZA positions of the left (L), nadir (N) and right (R) cameras are shown in each plot with black circles. In the used convention, the SAA is 180\({}^{\circ}\), the VAA spans 0–360\({}^{\circ}\), i.e., the region with relative azimuth angle (RAA = SAA–VAA) \(<\) 90\({}^{\circ}\) corresponds to the backscattering, while RAA \(>\) 90\({}^{\circ}\) to the forward scattering directions.