## 5 Overall Strategy and Readiness Level
### Readiness Level  
The application of the above-mentioned generic Cal/Val approach to ESA operational land products at different processing stages is here discussed and the readiness level for each element of the generic framework is assessed. The readiness levels, ranked with a score (from 0 to 5) are additionally reported in Tables 5-7 and illustrated in Figure 3. The objective of this review is to identify gaps in terms of infrastructure and methods. By addressing these gaps and increasing the readiness level of the various Cal/Val elements, we will enhance our capability to properly assess the accuracy of the relevant satellite EO products and verify whether the mission has effectively met the science objectives defined in the MRD. This is ultimately the main goal of the ESA Cal/Val strategy. Overall, we see that we already reached a good level of readiness for Level 1 products, owing to the efforts spent in the past decades in developing and consolidating the protocols for calibration and intercalibration, as well as the availability of a ground-based network of SI-traceable measurements, such as RadCalNet [107]. New technological solutions are also being successfully tested, which can further complement the current Cal/Val facilities (see, in particular, the SPecular Array for Radiometric Calibration (SPARC) system [183]). Conversely to Level 1 TOA products, significant gaps remain in the ability to assess radiometric quality at BOA level. As a matter of fact, our current capability in validating SR products is the weakest point in terms of readiness level over the whole processing  
Figure 2: The figure presents the overall ESA generic framework for land Cal/Val, which is replicated and fine-tuned for the considered geophysical variable under validation. chain (see Figure 3). This is a major concern for reaching interoperability of current and future optical land imaging sensors and should be addressed with the highest priority in the coming years. The major gaps in this respect are the lack of a ground-based network of surface reflectance measurements and of community agreed protocols for SR validation. Significant discrepancies also remain in terms of RTM accuracies in the simulation of radiative interactions within the coupled surface-atmosphere system. Finally, crucial knowledge and data gaps were identified for cloud mask validation, which is an essential preprocessing step for the generation of SR products. ESA is currently tackling these gaps with high priority in the frame of the ACLX-CMIX [118], RAMI4 ATM [125] and HYPERNETS [136] initiatives. However, much effort will be required in coordination with the other Space Agencies and the CEOS-LPV to fulfil these gaps with a sustained long-term solution. When we consider Level 2 bio-geophysical products, some important needs were identified by the science community [3]. The CEOS-LPV sub-group was essential in addressing several of these gaps, notably with the release of new protocols, the definition of the super-sites concept and the work towards standardised validation frameworks. The ESA FRM4 VEG project [152] further contributed to developing the advanced practices and upscaling procedures for SR, fAPAR and chlorophyll content validation. The GBOV initiative [53] additionally addressed the need of interoperability across networks and started filling the gaps in geographical coverage, particularly for vegetation parameters. Despite these on-going efforts, several challenges remain, owing to the disparity of the used practices, the lack of community-agreed protocols for some variables, the scarcity of Cal/Val data over some geographical areas, and the difficulty in discovering and accessing Cal/Val data. ESA in collaboration with the other Space Agencies and in coordination with CEOS-LPV sub-group, will contribute to filling these gaps with the aim of enhancing preparedness of infrastructure and methods for sustaining Cal/Val needs of current and future ESA/Copernicus land-focused optical missions.