## 4 CEOS FRM Self-Assessment  
The CEOS Working Group Cal/Val (WGCV) recently developed a tool to enable Cal/Val data providers to evaluate to what extent they are CEOS-FRM compliant and, if not, to report on their intended progress towards reaching full compliance. CEOS-Fiducial Reference Measurements are defined as \"_independent, fully characterised, and traceable measurements, tailored specifically to address the calibration/validation needs of satellite borne instruments making measurements of a particular measurand, that follow the guidelines outlined by the GEO/CEOS Quality Assurance framework for Earth Observation (QA4 EO)_\" [65]. In practice, the evaluation framework takes a pragmatic approach relying on a self-assessment of the  
Figure 19: Overview plot of the S5p tropospheric NO\({}_{2}\) v2.4 vs. MAX-DOAS comparisons results based on the NIDFORVAL (N4V), FRM4 DOAS (FRM) and automated validation server (AVS) treatments for a subset of sites (see text). Panels (**a–c**) present scatter plots for each case including statistics information, and panels (**d,e**) summarise the final comparison spread in absolute and relative cases as bar plots. status of the FRM against a set of criteria that are subject to peer review through a board of experts led by CEOS WGCV. The degree of compliance with those criteria is based on a gradation scaling rather than a simple fail/pass approach. The outcome of the evaluation is presented in the Maturity Matrix model [65], which provides a simple visual assessment of the state of the CEOS-FRM for all given criteria, making visible where it is mature and where evolution and effort are needed. Table 11 shows the Maturity Matrix for the FRM4 DOAS system obtained by self-assessment following the guidelines proposed by the CEOS WGCV. Several categories are graded as Ideal or Excellent, including the Operator Expertise, Automation Level, Availability and Data Format. Most of the other categories are graded as Good, except for \"Utilisation/feedback\" and \"Independent verification\", which are currently graded as Basic. The low level of assessment for these two criteria is largely due to the current demonstrational status of the service, which limits its usage and verification. We anticipate significant progress on these criteria from 2025, when FRM4 DOAS will enter an operational phase, and a greater number of users and applications will be served by the system. Concerning other criteria currently assessed as Good, more efforts will be needed to progressively meet higher class criteria and ultimately reach class A. To this aim, special attention will be brought to the following actions:  
* Improve access to necessary ancillary data and complementary measurements. For MAX-DOAS measurements, the ideal configuration is to collocate instruments with sun photometers providing ancillary data on aerosol optical properties. * Further develop the documentation on instruments, especially for non-commercial ones and make it available on the service website. Likewise, we will work on establishing systematic calibration and maintenance reports for all instruments. * In the current state of the service, the set of instruments served by FRM4 DOAS already covers a wide range of latitudes as well as different levels of pollution. When working \begin{table}
\begin{tabular}{c c c c c c} \hline \hline Descriptor & Operator expertise & Automation level & Data completeness & Uncertainty & Guidelines adherence \\ \hline Range of instruments & Instrument documentation & Measured sampling & Availability and usability & Traceability & Utilisation/ feedback \\ \hline Complementary observations & Evidence of traceable calibration & ATBD on processing/software & Data format & Comparison/ calibration of FRM & Metrology verification \\ \hline Location/availability of FRM assessment & QA/maintenance & Guidance on transformation to satellite sensor & Ancillary data & Adequacy for class of instrument/measurand & Independent verification \\ \hline \hline \end{tabular}
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