## 5 Results
### Ground-Based Direct Validation  
To investigate the accuracy of C3S PROBA-V SA v1.0 and MCD43A3 C6 satellite albedo products, scatter plots versus field measurements (GBOV RM) were produced for the 2014-2018 period over 20 homogeneous sites (see Section 3) with different vegetation types. Figure 11 and Table 14 show the scatter-plots, and relevant statistics from the direct validation exercise, whilst Table 15 summarizes the compliance of both satellite products with user requirements (predefined in Section 4.1). The relevant statistics per biome type are presented in Tables 16 and 17 for PROBA-V SA v1.0 and MCD43A3 C6, and the scatter-plots per biome type can be found in Appendix A. The temporal trajectories of both PROBA-V SA v1.0 and MCD43A3 C6 compared to the daily GBOV RMs are presented in Appendix B.  
C3S PROBA-V SA v1.0 shows an overall accuracy (median error) of 18.2%, with a tendency to overestimate ground values (positive bias of 11.5%). This positive bias was mainly observed for low albedo values (up to 0.2, forest sites) showing an offset of 0.07 and a slope of 0.7 in the MAR relationship. MCD43A3 C6 provides a lower median error of 11.2% with a negative mean bias of \(-\)5.9%. C3S PROBA-V SA v1.0 products provide similar results in terms of uncertainty (RMSD of 22.4%) than MCD43A3 C6 (RMSD of 24.8%). \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & \multicolumn{6}{c}{**Inter-Annual Precision (Median Absolute Deviation)**} \\ \cline{2-7}  & **AL-DH-VI** & **AL-DH-NI** & **AL-DH-BB** & **AL-BH-VI** & **AL-BH-NI** & **AL-BH-BB** \\ \hline
**PROBA-V** & \multirow{2}{*}{0.004 (1.2\%)} & \multirow{2}{*}{0.006 (1.1\%)} & \multirow{2}{*}{0.004 (0.9\%)} & \multirow{2}{*}{0.004 (1.3\%)} & \multirow{2}{*}{0.008 (1.3\%)} & \multirow{2}{*}{0.006 (1.2\%)} \\
**SA v1.0** & & & & & & \\ \hline
**SPOT/VGT** & \multirow{2}{*}{0.008 (2.6\%)} & \multirow{2}{*}{0.012 (2\%)} & \multirow{2}{*}{0.009 (2\%)} & \multirow{2}{*}{0.011 (3.7\%)} & \multirow{2}{*}{0.02 (3.3\%)} & \multirow{2}{*}{0.014 (3\%)} \\
**SA v1.0** & & & & & & \\ \hline
**MCD43A3 C6** & \multirow{2}{*}{0.003 (0.9\%)} & \multirow{2}{*}{0.005 (0.8\%)} & \multirow{2}{*}{0.004 (0.8\%)} & \multirow{2}{*}{0.003 (1.2\%)} & \multirow{2}{*}{0.006 (1.1\%)} & \multirow{2}{*}{0.005 (1.1\%)} \\ \hline \hline \end{tabular}
\end{table} 
Table 13: The median absolute deviation between two consecutive years of the PROBA-V SA v1.0, SPOT/VGT SA v1.0 and MCD43A3 C6 products. Computation over desert calibration sites. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & \multicolumn{6}{c}{**Median of 3-Point Difference (i.e., Median \& Values)**} \\ \cline{2-7}  & **AL-DH-VI** & **AL-DH-NI** & **AL-DH-BB** & **AL-BH-VI** & **AL-BH-NI** & **AL-BH-BB** \\ \hline
**PROBA-V SA v1.0** & 0.0026 & 0.0052 & 0.0035 & 0.0039 & 0.0074 & 0.0050 \\
**SPOT/VGT SA v1.0** & 0.0042 & 0.0047 & 0.0041 & 0.0057 & 0.0063 & 0.0055 \\
**MCD43A3 C6** & 0.0012 & 0.0022 & 0.0016 & 0.0018 & 0.0032 & 0.0023 \\ \hline \hline \end{tabular}
\end{table} 
Table 12: Intra-annual precision indicator (Median of the 3-point difference) of the C3S PROBA-V SA v1.0, C3S SPOT/VGT SA v1.0 and MCD43A3 C6 products. Computation over LANDVAL sites during the overlap period (December 2013–May 2014 period). Note that 15.5% of the 1715 PROBA-V SA v1.0 samples achieved the optimal predefined level (i.e., GCOS requirements) and 28.6% of the target level (i.e., C3S KPI), as shown in Table 15. Slightly improved results were found for MCD43A3 C6 (23.7% and 45.5% of the optimal and target levels, respectively). Per biome type, PROBA-V SA v1.0 provides a large positive bias for forest (22.4%) and desert (10.4%) sites compared to crop (7.4%) and grassland/shrubland (4.2%) sites. MCD43A3 C6 systematically provides a large negative bias for most biome cases (croplands, grassland/shrublands and desert) except for forests, where a low positive bias was found (2.8%).