## 5 Overall Strategy and Readiness Level  
### Overall Land Cal/Val Strategy  
In order to tackle the recommendations identified in the previous chapters (see Tables 2-4) with an integrated approach, ESA aims to implement a generic Cal/Val framework to be replicated and eventually fine-tuned to the specific needs of the geophysical variable under validation. The building blocks of this generic framework are the following (see Figure 2):  
* Metrology--The application of metrological best practices is the central pillar underpinning the overall ESA Land Cal/Val strategy, since metrology provides the generic guidelines and community-agreed standards to achieve full traceability of the measurements and detailed characterisation of the uncertainty budget. The metrological practices shall be used as underlying principles for developing all other elements of the Cal/Val strategy. * RTM and Inter-comparison--The use of RTM simulations allows advancing our understanding of the uncertainty budget associated to the validation, since instrument-related error sources can be fully characterised and decoupled from modelling and retrieval errors. Inter-comparison exercises are key to understand the discrepancies between the various algorithms used within the community, to address the current limitations and, in the longer-term, to converge to a community-agreed solution. RTM simulations are often essential to support algorithm inter-comparison, notably for building reference datasets, as in the case of ACLX [118]. * FRM and supersites--In order to properly assess the accuracy of satellite EO products, we need to have an initial set of _\"golden\"_ sites that represent our best knowledge of the geophysical parameter(s) under validation. These sites are super-characterised and the in situ measurements are carried out using community-endorsed best practices. Both the FRM and CEOS-LPV supersites share these basic underlying principles, although they are addressed with different, yet complementary viewpoints. While the CEOS-LPV supersites concept stems from the need to assess inter-variables' physical consistency, the FRM concept builds upon the fundamental requirement of adopting metrological practices in EO, ensuring traceability of the measurements and detailed uncertainty budget characterisation. * Protocols--The availability of standard protocols is an essential element in establishing a common approach to the validation problem. When protocols are endorsed at CEOS-LPV level and widely accepted within the community, Cal/Val data from different networks and campaigns can be reliably merged, enhancing spatio-temporal sampling. * Networks--Once the protocols and requirements for setting up a Cal/Val site are consolidated, they shall be replicated in a network of sites to enhance global coverage. The global network shall have sustained funding and ensure continuous operations, as well as possibly providing data in quasi-real time to support services, such as for GBOV [53], and guarantee that data are generated in a consistent manner across the sites with common approach for calibration and measurements and with harmonised data content, format definition and quality information. * Ad-hoc campaigns--While networks are the foremost step for moving towards an operational Cal/Val system, the scientific and technological solutions will continue in parallel to evolve, for example, UAV-based and low-cost automated sensors. It is therefore critical to continue supporting ad-hoc Cal/Val campaigns for in-depth validation and for keeping pace with the evolutions in the science and technology domains. The outcomes of such campaigns are advanced protocols and devices, which can be injected into the overall Cal/Val cycle, quality certified following metrological practices and inter-comparison exercises. As these solutions will enhance their maturity level, they could be potentially considered for complementing and expanding the overall Cal/Val solution.