# Evaluation of Orbital Drift Effect on Proba-V Surface Reflectances Time Series
## 6 Conclusions  
The impact of the orbital drift on Proba-V surface directional reflectances (TOC products) and NDVI temporal series at 1 km resolution was evaluated and quantitatively assessed within this study. Analysis was conducted on both synthetic and real observations, showing consistent results.  
The largest impact of the orbital drift is detected when observations from backward and forward scattering conditions are treated separately. This effect is larger for the NIR and SWIR bands and for densely vegetated sites. Specifically, a clear, positive spurious trend is induced in the backward direction, corresponding to acquisitions from the westward looking camera, while a negative one is detected in the opposite forward scattering direction, which corresponds to the eastward looking camera acquisitions. A noticeable impact of the orbital drift is also observed for near-nadir acquisitions, with a spurious negative bias particularly for the NIR and SWIR bands and for forest, cropland and shrubland sites.  
When observations from all angular conditions are merged, the opposite drifts in the two scattering conditions largely cancel out, with no remaining statistically significant impact of the orbital drift.  
In conclusion, as long as data from all angular conditions are considered, there is no statistical evidence of an orbit-induced effect in the consistency of Proba-V surface reflectances and NDVI time series at 1 km resolution. This dataset is therefore suitable for further exploitation for inter-annual vegetation studies.  
**Supplementary Materials:** The following are available at [[https://www.mdpi.com/article/10.3](https://www.mdpi.com/article/10.3)]([https://www.mdpi.com/article/10.3](https://www.mdpi.com/article/10.3)) 390/rs13122250/s1 in the Supplementary Material document: S.1: SZA Predicted and Observed Evolutions; S.2: BRDF predicted temporal evolution for all spectral bands; S.3: Temporal series over the four sites for all spectral bands; S.4: Statistical analysis of NDVI Azimuthal Asymmetry.  
**Funding:** This research was carried out as part of the European Space Agency (ESA) Quality Assurance for Earth Observation (IDEAS-QA4 EO) framework contract.  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** The Proba-V data presented in this study are openly available within the Proba-V Mission Exploitation Platform (MEP), which can be accessed here (last accessed on 8 June 2021): [[https://proba-v-mep.esa.int/](https://proba-v-mep.esa.int/)]([https://proba-v-mep.esa.int/](https://proba-v-mep.esa.int/)). All analysis was made within this platform using the embedded Python 3.5 scientific libraries. The MODIS Albedo daily products V6 (MCD43A1.006) was accessed within the Google Earth Engine (GEE) platform (last accessed on 8 June 2021): [[https://code.earthegine.google.com/](https://code.earthegine.google.com/)]([https://code.earthegine.google.com/](https://code.earthegine.google.com/)).  
**Acknowledgments:** The author greatly acknowledges the anonymous reviewers for the insightful and valuable comments that were instrumental in improving the quality of the manuscript. The author wishes to thank [PERSON] (Serco) and the VITO MEP team who supported the deployment of the Python code in the MEP cluster. The Proba-V data used within this study are provided by the European Space Agency (ESA) and the Belgian Science Policy Office (BELSPO), they are generated and distributed by VITO.  
**Conflicts of Interest:** The author declares no conflict of interest.