## 4 Conclusions  
In this paper, we define criteria and describe examples of a 'new' category, at least in terms of formalized labels, of Earth observation data expressly designed and characterised to meet the Cal/Val needs of the world's satellite sensors in a coordinated cost-efficient manner. These FRM data sets are typically derived as a subset of a broad-based set of similar data which have been tailored so that they can provide data representative of that measured by a particular class of satellite sensors and that have well-defined and documented estimates of their uncertainty to international references such that they can be unequivocally relied upon for satellite Cal/Val.  
FRMs, and in particular CEOS-FRMs, will provide the means to facilitate confidence and harmonisation not only in the data delivered from national and international space agency missions but also 'new space' commercial providers seeking to evidence the quality of their data. The criteria needed to fully achieve FRM status are detailed, as is the outline of a new endorsement process currently being developed for implementation by CEOS. It is anticipated that many post-launch Cal/Val data providers will seek to achieve full CEOS-FRM status and, through the different classes, allow an evolutionary development with evidence of progress towards CEOS-FRM to avoid barriers to uptake and encourage the development of new FRMs.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & & & & & & \\ \multicolumn{6}{c}{Self-development} \\  & & & & & & & \\ \multicolumn{6}{c}{Assessor} \\  & & & & & & \\ \multicolumn{6}{c}{Projects/} \\  & & & & & & \\ \multicolumn{6}{c}{Simultaneous/} \\  & & & & & \\ \multicolumn{6}{c}{Simultaneous/} \\  & & & & & \\ \multicolumn{6}{c}{Simultaneous/} \\  & & & & & \\ \multicolumn{6}{c}{Simultaneous/} \\  & & & & & \\ \multicolumn{6}{c}{Simultaneous/} \\  & & & & & \\ \multicolumn{6}{c}{Simultaneous/} \\  & & & & \\ \multicolumn{6}{c}{Simultaneous/} \\  & & & & \\ \multicolumn{6}{c}{Simultaneous/} \\  & & & & \\ \multicolumn{6}{c}{Simultaneous/} \\  & & & & \\ \multicolumn{6}{c}{Simultaneous/} \\  & & & & \\ \multicolumn{6}{c}{Simultaneous/} \\  & & & & \\ \multicolumn{6}{c}{Simultaneous/} \\  & & & & \\ \multicolumn{6}{c}{Simultaneous/} \\  & & & & \\ \multicolumn{6}{c}{Simultaneous/} \\  & & & & \\ \multicolumn{6}{c}{Simultaneous/} \\  & & & & \\ \multicolumn{6}{c}{Simultaneous/} \\  & & & & \\ \multicolumn{6}{c}{Simultaneous/} \\  & & & & \\ \multicolumn{6}{c}{Simultaneous/} \\  & & & & \\ \multicolumn{6}{c}{Simultaneous/} \\  & & & & \\ \multicolumn{6}{c}{Simultaneous/} \\  & & & & \\ \multicolumn{6}{c}{Simultaneous/} \\  & & & & \\ \multicolumn{6}{c}{Simultaneous/} \\  & & & & \\ \multicolumn{6}{c}{Simultaneous/} \\  & & & & \\ \multicolumn{6}{c}{Simultaneous/} \\  & & & & \\ \multicolumn{6}{c}{Simultaneous/} \\  & & & & \\ \mul  
**Author Contributions:** Conceptualization, C.D., P.G. and N.F.; methodology, N.F., P.C. and P.G.; writing--original draft preparation, N.F., P.G. and C.D.; writing--review and editing, N.F., P.C. and PG. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research received no external funding.  
**Data Availability Statement:** No new data were created for this article.  
**Acknowledgments:** The FRM principles formed the basis for an on-going series of projects funded by European organisations: EU, ESA, EUMETSAT. We acknowledge the support from these organisations that led to a \"joint vision by ESA and EUMETSAT for Fiducial Reference Measurements\" that was presented at European Commission--ESA--EUMETSAT Trilateral meeting on 5 June 2020. The authors would also like to thank [PERSON] for useful discussion and comments on the drafting of this paper.  
**Conflicts of Interest:** The authors declare no conflict of interest.