## 2 Materials and Methods  
### Data  
#### 2.1.1 Sentinel-3 SYN VGT V10  
Sentinel-3 SYN VGT products and processing steps are described in [1]. In summary, the SYN VGT processing module consists of (i) the Level-1 module dedicated to the co-registration of OLCI and SLSTR acquisitions and the production of internal SYN Level-1 products, (ii) the SYN Level-2 module dedicated to aerosol retrieval and atmospheric correction and (iii) the SYN VGT module that performs spectral mapping to simulate the SPOT VGT or PROBA-V spectral bands (see Table 1) and projection and compositing to generate simulated VGP, VG1 and V10 products [18]. In this study, we focus on V10 products, i.e., 10-day maximum NDVI value composites of surface reflectance measurements and the NDVI at 1 km spatial resolution.  
As highlighted above, important updates, improvements and bug fixes were implemented into the SYN VGT processing lines since the first release of SYN VGT products in 2018 (exact dates and processing baseline documents are available at [[https://sentiwiki.copernicus.eu/web/synergy-processing](https://sentiwiki.copernicus.eu/web/synergy-processing)]([https://sentiwiki.copernicus.eu/web/synergy-processing](https://sentiwiki.copernicus.eu/web/synergy-processing)), accessed on 25 August 2024). The most impactful changes are listed below and schematized in Figure 1:  
* September 2019: The correction of a 0.5-pixel displacement in latitude and longitude directions in product gridding. Measurements are provided on the same regular latitude-longitude grid as previous SPOT VGT and 1 km PROBA-V products with an equatorial sampling distance of approximately 1 km (1\({}^{\circ}\)/112).
* June 2020: The alignment of the temporal compositing scheme of 1-day and 10-day synthesis products with previous SPOT VGT and PROBA-V products. Per month, three V10 products are provided based on observations in days 1-10, days 11-20 and day 21 to the end of the month, respectively.
* June 2021: Correction in the definitions of VG1 and V10 NDVI to be based on surface reflectance in the RED and NIR bands. Until May 2021, NDVI products were erroneously based on top-of-atmosphere (TOA) reflectances.
* August/September 2022: Improved handling of saturated values.
* July 2023: The last important update includes two aspects, namely (i) correction in the spectral band mapping procedure and (ii) S3A/OLCI and SLSTR calibration adjustments. The first aspect includes a change to the use of PROBA-V spectral response functions (instead of SPOT4/VGT1) in the spectral band mapping procedure. Although differences between the spectral responses between SPOT VGT and PROBA-V are rather small [3], this change has a slight impact on the retrievals in the RED and SWIR bands. A much larger impact is induced by the implementation of important bug fixes, including the correct exclusion of atmospheric absorption bands and the correct handling of wavelength units. The second aspect includes the application of a 2% calibration bias correction on S3A OLCI, as evidenced through the tandem phase study [19], and the application of the Sentinel-3 SLSTR vicarious calibration adjustments [20].  
For the statistical evaluation of the consistency between SYN VGT products and the SPOT VGT and PROBA-V product archives, 3 periods of 12 months were selected in between these processing baseline updates as follows(Figure 1):  
* Period 1 (P1): June 2020-May 2021;
* Period 2 (P2): August 2021-July 2022;
* Period 3 (P3): August 2023-July 2024.  
Sentinel-3 SYN VGT products are made available through the Copernicus Data Space Ecosystem ([[https://dataspace.copernicus.eu/](https://dataspace.copernicus.eu/)]([https://dataspace.copernicus.eu/](https://dataspace.copernicus.eu/)), accessed on 25 August 2024).