## 3 Underlying Terminology and Concepts
### Validation Stage and Maturity Matrix  
As the set of validation reference data increases in spatial and temporal representativeness, and the documented traceability and uncertainty estimate are provided following FRM best practices, the confidence in the relevant EO satellite products increases, but how can we best define this confidence level for EO data? This question was tackled under several projects in recent years [59; 60; 61; 62; 63], and various approaches were proposed on how to express the maturity and usability of a given EO dataset. Among the various attempts, the following are worth mentioning and will be considered for elaborating the ESA Cal/Val strategy for land:  
* CEOS LPV validation stage [59]--One of the most appropriate and suitable definitions of confidence level for terrestrial ECVs is the validation stage concept, defined and put forward by the CEOS-LPV sub-group. The increase in the stage, from 0 to 4, is driven by the use of more representative sets of reference data and by the adoption of rigorous approaches for the uncertainty estimate. The validation stage should be confirmed and endorsed by the scientific community through peer-reviewed publication, and the validation result should be periodically reviewed and maintained as long as new versions of satellite EO data become available. * CCI Maturity Matrix [60]--The Maturity Matrix (MM) concept, tailored in the frame of ESA Climate Change Initiative (CCI) [61], is a systematic approach for assessing the usability of any CDR. The CCI MM offers a standard and efficient way to quantify for a given climate dataset six attributes: software readiness, data format, documentation, uncertainty characterisation, accessibility and usability. The maturity level is rated via a score (from 1 to 6), which is assigned on each aspect of the data record. The CCI MM provides an overview on the status of climate datasets, highlighting areas for further improvement and allowing to compare the fitness for purpose of different products. * GAIA CLIM Maturity Matrix [62]--The GAIA-CLIM MM was specifically designed for assessing the level of maturity of ground-based atmospheric composition networks. The assessment is based on the following aspects: metadata, documentation, uncertainty characterisation, public access, feedback and update, usage, sustainability  
Figure 1: The figure shows a schematic representation of FRM concept. As long as the community-endorsed best practices and metrological guidelines are adopted, e.g., through uncertainty budget analysis and regular inter-comparisons, the confidence in the reference Cal/Val data is enhanced with a resulting increased confidence in the satellite EO products being validated. The traceability to SI is eventually attained when comparisons are made and regularly maintained against internationally agreed upon standards provided by National Metrological Institutes (NMI). and software. The MM can be used both to aid user decision making as to which measurement series are suitable for particular use cases and support networks' owners in improving the maturity of their measurements, facilitating interoperability across existing ground-based measurement systems. * EDAP Product Quality Evaluation Matrix [63]--The Earthnet Data Assessment Pilot (EDAP) matrix, which is an evolution of the QA4 ECV matrix [40], aims at providing to the users a readily accessible overview of the relevant EO satellite data products quality and fitness for purpose. The quality assessment addresses specifically the following main EO products characteristics: format and accessibility, products and algorithm documentation and quality flags, uncertainty characterisation and validation results. For each feature of the identified product, a ranking is provided with colour coding, providing easy and quick information on the overall quality of the considered EO data.