## 3. Results and Discussion
### Intercomparison with VGT-C3 and PV-C2 LTS for P1, P2 and P3  
In order to evaluate the evolution of the consistency between Sentinel-3 SYN V10 and the SPOT VGT and PROBA-V data archives, Figures 3 and 4 show the results of the statistical consistency analysis between VGT-C3 LTS resp. PV-C2 LTS and S3A SYN V10 for P1, P2 and P3 and for the NDVI and four spectral bands. The results for S3B are very similar and are not displayed. The large scatter in the scatter density plots overall (leading to high values for _P_) is partly related to the fact that two different periods are compared: the 5-year period on which the respective LTS are based versus the 12-month periods in the more recent SYN V10 product archive. Land cover transitions, e.g., vegetation loss, urban expansion, reforestation or agricultural expansion, will have taken place on the surface in a minority of the pixels sampled. Also, natural fluctuations in vegetation development occur, plus human-induced variations, such as, e.g., crop rotation. In addition, and more importantly, the satellite observations are influenced by angular and atmospheric effects, although these effects are attenuated to some extent through the maximum NDVI value composite algorithm [35] that is used to generate the 10-day composite products. Both SPOT VGT and PROBA-V have a very wide swath coverage, with swath widths above 2200 km, whereas both OLCI and SLSTR have more narrow swath widths (around 1400 km). The OLCI swath is not centered at nadir but tilted 12.6\({}^{\circ}\) westward to minimize the impact of sun glint contamination [2]. In addition, whereas Sentinel-3 flies in a sun-synorous orbit with descending node equatorial crossing at 10 h mean local time, both SPOT VGT and PROBA-V experienced orbital drift (see above). Different sampling in both viewing and illumination angles contributes to unsystematic differences observed in the comparison. Secondly, incomplete cloud and cloud shadow masking can influence the intercomparison, especially in areas with persistent cloud cover where the probability of not having clear sky observations in the 10-day compositing period is higher. In this respect, it should noted that currently, no cloud shadow masking is carried out in Sentinel-3 SYN VGT products. Finally, differences in atmospheric correction lead to unsystematic differences in TOC reflectances and the NDVI. One aspect is the estimation of the aerosol optical thickness (AOT). For VGT-C3 atmospheric correction, the AOT is estimated from the BLUE band and the NDVI through an optimization process [36]. PV-C2 uses an external AOT data set, namely the MERRA-2 (Modern-Era Retrospective analysis for Research and Applications, version 2), as an input for atmospheric correction [37]. In Sentinel-3 SYN Level-2 processing, the AOT is retrieved using collocated OLCI and SLSTR TOA radiances based on methods developed for the predecessors of both sensors [38], and previous studies have shown that this AOT tends to be overestimated [39]. Apart from the large scatter, discussed above, overall high linear correlations are found between the VGT-C3 resp. PV-C2 LTS and the SYN V10 products, with R\({}^{2}\) values around or well above 0.7. The largest discrepancies are found for the NDVI in P1, with a GMR slope well below 1, and for the SWIR band in P1 and P2, with GMR intercepts well below 0. Figure 5 shows the gradual improvements in _APU_ statistics from P1 to P2 and to P3. The largest improvement for the NDVI, leading to drops in \(A\) from 0.1 to 0.05 (in comparison to SPOT VGT) and 0.04 (in comparison to PROBA-V), is related to the correction in the definition of the product in June 2021: in P1, the product was incorrectly based on TOA reflectances. Finally, corrections and adaptations in the spectral band mapping and calibration, applied in July 2023 (P3), resulted in \(A\) around 0.02 and 0.01 in comparison to SPOT VGT and PROBA-V, respectively. This effect is obtained by small improvements in the consistency for the RED band.