## 4 Conclusions  
Sentinel-3 SYN VGT products were designed to provide continuity to the SPOT VGT standard products, but since SPOT5 was decommissioned before the launch of the first Sentinel-3 satellite, PROBA-V acted as a gap filler, and thus, consistency with PROBA-V is as (if not more) relevant. In principle, a continuous data series of standard TOA reflectance, daily and 10-day composite TOC reflectance and NDVI products should be available to users; for applications related to long-term, large-scale vegetation monitoring; climate change studies; agricultural monitoring; etc. However, some flaws were identified in the Sentinel-3 SYN VGT processing baselines that were only gradually resolved in the course of recent years. These changes have progressively increased the quality of Sentinel-3 SYN VGT products, as is evidenced through the spatio-temporal intercomparison of the combined time series of SPOT VGT, PROBA-V and Sentinel-3 SYN VGT 10-day NDVI composite products with an external reference from LSA-SAF, and the intercomparison of Sentinel-3 SYN V10 products with the LTS of VGT-C3 resp. PV-C2 for three distinct periods with different levels of product quality.  
For the past 12 months regarding SYN V10 products, mean absolute bias values compared to the PV-C2 climatology of ~1% (for BLUE and RED surface reflectance), ~2% (for NIR and SWIR) and 0.01 (for NDVI) are reached. Consistency with VGT-C3 is slightly higher for BLUE, similar for SWIR, and slightly lower for RED and NIR and the NDVI. Overall, a high dispersion of product retrievals around their expected values, i.e., a high standard deviation of the bias, is observed because of the many factors that influence the intercomparison.  
The subsequent Sentinel-3 SYN VGT processing baseline updates have thus resulted in better-quality products. Nevertheless, there are a few possible improvements that could still lead to better consistency with the SPOT VGT and PROBA-V product archives: (i) an enhancement in the atmospheric correction scheme with more reliable AOT estimation; (ii) improvements in cloud and snow screening and the inclusion of cloud shadow masking; (iii) a combination of S3A and S3B observations in single composite products to increase daily global coverage and to reduce the effect of residual cloud contamination; and (iv) an upgrade of the spatial resolution of SYN VGT products to 300 m (1\({}^{\circ}\)/336) to align with the higher resolution PROBA-V global data products archive.  
Finally, from users' perspective, it is essential to reprocess the entire Sentinel-3 SYN VGT archive using the latest and most advanced processing baseline. The current archive is inconsistent, preventing users from accessing a uniform data set of standard SPOT VGT, PROBA-V and Sentinel-3 SYN VGT products. A consistent data record, spanning over 25 years from 1998 onwards, would provide valuable input for a wide range of applications.  
Conceptualization, C.T. and E.S.; Data Curation, C.T. and C.H.; Formal Analysis, C.T.; Methodology, C.T. and E.S.; Resources, C.T., E.S. and C.H.; Software, C.T. and E.S.; Writing--Original Draft, C.T.; Writing--Review and Editing, C.T., E.S. and C.H. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the European Union and the European Space Agency, contract number 4000136252/21/I-BG, and Federaal Wetenschapsbeleid (Belspo), contract number CB/67/12.  
The views expressed herein can in no way be taken to reflect the official opinions of the European Space Agency or the European Union.  
**Data Availability Statement:** The original data presented in this study are openly available on the Copernicus Data Space Ecosystem at [[https://dataspace.copernicus.eu/](https://dataspace.copernicus.eu/)]([https://dataspace.copernicus.eu/](https://dataspace.copernicus.eu/)) and on the Terrascope platform at [[https://terrascope.be/en](https://terrascope.be/en)]([https://terrascope.be/en](https://terrascope.be/en)).  
**Conflicts of Interest:** The authors declare no conflicts of interest.