## 3. Results and Discussion
### Intercomparison with VGT-C3 and PV-C2 LTS for P1, P2 and P3
The largest effects of the changes applied in July 2023 are visible in the SWIR band, where \(A\) drops from around 0.09 in P1 and P2 to values around \(-0.02\) in P3. Figure 3: Scatter density plots and GM regression between VGT-C3 LTS (X) and S3A SYN V10 (Y) for P1 (**left**), P2 (**middle**) and P3 (**right**). From top to bottom: NDVI, BLUE, RED, NIR and SWIR. The overview of the _APU_ statistics in Figure 5 also shows that there is very little difference between the results based on Sentinel-3A and Sentinel-3B SYN V10 products. The application of the OLCI-A calibration adjustments (implemented between P2 and P3) seem to have only minor impacts. Values for \(P\) remain roughly constant over the three periods because the reasons for high scatter in the comparison (see above) have not been influenced by the subsequent processing baseline updates. Figure 4: Scatter density plots and GM regression between PV-C2 LTS (X) and S3A SYN V10 (Y) 10-day composite surface reflectance for P1 (**left**), P2 (**middle**) and P3 (**right**). From top to bottom: NDVI, BLUE, RED, NIR and SWIR. The Current Consistency Level between Sentinel-3 SYN V10 Products and the SPOT VGT and PROBA-V Archives  
Finally, Table 2 provides the _APU_ metrics that are based on the latest 12 months of available Sentinel-3 SYN V10 data (P3), as these provide a measure of the current consistency level between SYN V10 and the SPOT VGT resp. PROBA-V archives. Overall, the consistency is higher with the PV-C2 LTS, with absolute \(A\) values around 0.01 (i.e., \(\pm\)1% surface reflectance) for BLUE and RED, around \(-\)0.02 (\(-\)2%) for NIR and SWIR, and around 0.01 for the NDVI. Consistency with VGT-C3 is slightly higher for BLUE (\(A\) below 1%), similar for SWIR, and lower for RED and NIR (\(-\)2% and \(-\)4%, respectively) and the NDVI (0.02). \begin{table}
\begin{tabular}{l c c c c c c} \hline \hline
**Intercomparison** & **Metric** & **BLUE** & **RED** & **NIR** & **SWIR** & **NDVI** \\ \hline VGT-C3 LTS vs. & A & \(-\)0.003 & \(-\)0.024 & \(-\)0.035 & \(-\)0.022 & 0.022 \\  & P & 0.023 & 0.037 & 0.057 & 0.059 & 0.083 \\ S3A SYN V10 & U & 0.023 & 0.044 & 0.067 & 0.063 & 0.086 \\ \hline VGT-C3 LTS vs. & A & \(-\)0.002 & \(-\)0.024 & \(-\)0.037 & \(-\)0.020 & 0.018 \\ S3B SYN V10 & P & 0.023 & 0.037 & 0.057 & 0.057 & 0.083 \\ S3B SYN V10 & U & 0.023 & 0.045 & 0.068 & 0.060 & 0.085 \\ \hline PV-C2 LTS vs. & A & 0.012 & \(-\)0.014 & \(-\)0.019 & \(-\)0.025 & 0.013 \\ S3A SYN V10 & U & 0.025 & 0.036 & 0.057 & 0.056 & 0.08 \\ \hline PV-C2 LTS vs. & U & 0.028 & 0.039 & 0.06 & 0.061 & 0.081 \\ \hline PV-C2 LTS vs. & A & 0.012 & \(-\)0.014 & \(-\)0.021 & \(-\)0.023 & 0.009 \\ S3B SYN V10 & P & 0.026 & 0.036 & 0.056 & 0.055 & 0.080 \\ U & 0.029 & 0.039 & 0.06 & 0.059 & 0.080 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: _APU_ metrics for the intercomparison between the VGT-C3 and PV-C2 LTS and Sentinel-3 SYN V10 products for P3 (August 2023–July 2024). The statistics give a measure for the current consistency levels between Sentinel-3 SYN V10 and the SPOT VGT resp. PROBA-V product archives. Figure 5: The evolution of _APU_ metrics of the intercomparison of VGT-C3 LTS (**left**) and PV-C2 LST (**right**) and Sentinel-3A (solid lines) and Sentinel-3 (dashed lines) SYN V10 products from P1 to P3. \(P\) (and consequently \(U\)) values are relatively high, indicating a high dispersion of product retrievals around their expected values. This is caused by many factors: the inter-comparison between different periods (recent products vs. product archive); differences in temporal, spatial and angular sampling; the performance of the spectral resampling algorithm; atmospheric effects and the performance of atmospheric correction; and the performance of pixel classification algorithms.