**6. Conclusions**

A spectral reflectance time-series reconstruction (RTSR) method has been proposed to reconstruct optical remote-sensing time-series data that are hampered by missing or noisy observations due to varying atmospheric conditions. This method differs from existing approaches in that it adjusts smoothed values of surface reflectance values to approach those of trustworthy observations, using a vegetation index as a proxy for reliability. The results show that the RTSR method is effective in retaining trustworthy observations in the original time series, with RMSE values in the order of 0.01 to 0.03 in terms of surface reflectance. The method was evaluated on 100 sites in Europe, with a focus on vegetation. The Savitzky–Golay filter was employed herein as the smoothing algorithm, but other filters can also be used. The potential of this method is substantial now that optical remote-sensing data have become publicly available at a finer spatial and temporal resolution. We hope that this study will encourage further research in this area, and that the RTSR method will be applied to create analysis-ready data for other applications beyond vegetation monitoring.

**Author Contributions:** Conceptualization, P.K., M.C. and R.d.; methodology, P.K.; software, P.K.; evaluation, P.K. and M.C.; writing—original draft preparation, P.K.; review and editing, all authors; All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** This study was reviewed by the Joint Research Centre editorial review board (JERB).

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Source code of main image-processing library used for this work (pyjeo) is freely available online https://github.com/ec-jrc/jeolib-pyjeo (accessed on 5 December 2022). Data and results are available on https://jeodpp.jrc.ec.europa.eu/ftp/jrc-opendata/DRLL/RTSR/ (accessed on 19 April 2023).

**Conflicts of Interest:** The authors declare no conflict of interest.
