## 3 Fiducial Reference Measurements (FRMs)
### Achieving and Evidencing FRM Status  
The FRM label is now becoming used internationally, including within the recently published Global Climate Observing System (GCOS) implementation plan [25], and many data providers are seeking to claim that they are FRM compliant. Similarly, satellite operators are requesting that validation data come from an FRM-compliant source. However, whilst the FRM principles are clear, they still necessarily leave room for interpretation, and there is no real authority to stop the label being used in an unjustified or misleading manner even by those using it with the best intentions. CEOS WGCV has thus recognised the need to establish a means to enable Cal/Val data providers to robustly evidence that they are FRM compliant or, at least, show their progress towards being fully FRM compliant to allow their users (satellite operators and data users) to assess and weight the utility of the Cal/Val data in validating the performance of a particular satellite. To this end, they have added the prefix CEOS-FRM, allowing some ability to control the usage of the label. Recognising that the number of potential CEOS-FRM data providers may become very large, it is necessary that any 'endorsement' process needs to be time efficient and low cost but also that it maintains integrity. It also needs to be flexible to address different levels of maturity in the methodologies and products that are being validated as well as technology domains. It is critical that the characteristics and performance of any CEOS-FRM can be unequivocally considered consistent and trustworthy (within their self-declared metrics) and readily interpretable so that any information derived from it can be reliably utilized by a user. This is not to say that FRM for any particular measurement of a measurand or application must all meet the same level of performance or indeed necessarily fully meet all the CEOS-FRM criteria (at least whilst developing a capability) but that any potential user can readily identify and assess suitability for their application. Ensuring that the CEOS-FRM classification has meaning requires not only a set of common criteria but also obliges some form of independent assessment against them, or at least the potential for such assessment. This could be achieved through the creation of a formal process implemented \begin{table}
\begin{tabular}{c c c} \hline \hline
**FRM Project** & **Parameter** & **Indicative ESA Satellite** \\  & & **Mission Sensor** \\ \hline FRM4 SM & Soil moisture & SMOS/CIMR \\ FRM4 STS & SST, LST & Sentinel-3 SLSTR/CIMR \\ FRM4 SOC & Ocean colour & Sentinel-3 OLCI \\ FRM4 ALT & Altimetry & Sentinel-3 topography, \\ St3 TART & Water surface height, land and & Sentinel-6 \\  & sea ice thickness & (CRISTAL) \\ FRM4 RADAR & Cloud products & EarthCARE \\  & Tropspheric \& stratospheric \\  & NO\({}_{2}\) vertical profiles, total O\({}_{3}\) & \\  & column, tropospheric HCHO & Sentinel-5P/Sentinel-5/ \\ FRM4 DOAS & profile & Sentinel-4 \\  & Under development: aerosol & Sentinel-2 \\  & product; advanced urban & \\  & tropospheric NO\({}_{2}\) product & \\  & CO, CH\({}_{4}\) & \\  & Characterisation and & \\ FRM4 GHG & comparison of small portable & Sentinel-5P, Sentinel-5, CO2M \\  & FTIR instrument products & \\  & with established TCCON; CO\({}_{2}\) & \\ FRM4 Veg & Surface reflectance, FAPAR & Sentinel-2, Sentinel-3 OLCI \\ FRM4 AQ (Pandonia FRM) & O\({}_{3}\), NO\({}_{2}\), SO\({}_{2}\), H\({}_{2}\)O, CH\({}_{4}\), & Sentinel-5P/Sentinel-5/ \\  & CO, HCHO and aerosols & Sentinel-4 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: ESA FRM projects, the parameters they consider and the primary satellite sensor considered. The FRMs are considered to apply to other similar sensors in this class. though a body such as ISO, where independent assessment/audit against a set of criteria is undertaken by an approved body funded by the entity seeking accreditation.