## 3 Fiducial Reference Measurements (FRMs)
### What Is an FRM? A Fiducial Reference Measurement (FRM) is:  
_a suite of independent, fully characterised, and traceable (to a community agreed reference, ideally SI) measurements of a satellite relevant measurand, tailored specifically to address the calibration/validation needs of a class of satellite borne sensor and that follow the guidelines outlined by the GEO/CEOS Quality Assurance framework for Earth Observation (QA4 EO)_[3]. In many cases, FRMs may be a subset of 'in situ' measurements, originally designed to make observations in their own right at a specific geo-location, both individually and/or as part of a network, but they could also include measurements from a campaign, from airborne or seaborne platforms, or even from another satellite, providing it meets the FRM criteria. The newly emerging SITSats (SI Traceable Satellites) [10] are a good example of a potential satellite-based FRM. These FRMs provide the maximum Return On Investment (ROI) for a satellite mission by delivering, to satellite operators and their users, the required confidence in data products, in the form of independent validation results and satellite measurement uncertainty estimation, over the entire end-to-end duration of a satellite mission. The defining mandatory characteristics for an FRM are:  
1. **Traceability--**FRM measurements should have documented evidence of their traceability (including assessment of any bias and its associated uncertainty) to a community agreed reference ideally tied to the International System of units, SI, (e.g. via a comparison 'round robin' or other with peers and/or a metrology institute together with regular pre- and post-deployment calibration of instruments). This should be carried out using SI-traceable'metrology' standards and/or community recognised best practices, for both instrumentation and observations;
2. **Independence of satellite-under-test--**FRM measurements are independent from the satellite (under test) geophysical retrieval process;
3. **Uncertainty budget--**A comprehensive uncertainty budget for all FRM instruments and for the derived (processed) measurements (including any transformation of the measurand to match the measurand of the satellite product) is available and maintained;
4. **Documented protocols--**FRM measurement protocols, procedures and community-wide quality management practices (measurement, processing, archive, documents, etc.) are defined, published and adhered to by FRM instrument deployments;
5. **Accessibility--**FRM datasets, including metadata and reports documenting processing, are accessible to other researchers, allowing independent verification of processing systems;
6. **Representativeness--**FRM datasets are required to determine the on-orbit uncertainty characteristics of satellite geophysical measurements via independent validation activities. The degree of representativeness of the FRM to that of the satellite observation as well as the satellite to FRM comparison process are thus required to be documented and the uncertainty assessed. Note for any individual satellite sensor, the exact sampling and elements of the comparison process may differ, even within a generic sensor class, but the documentation and evidence to support the uncertainty analysis must be presented in a manner that can be readily interpreted by a user. The data and associated uncertainty information should be available in a timely manner and in a form that is readily utilizable by a satellite operator. 7. **Adequacy of Uncertainty --**The uncertainty of the FRM, including the comparison process, must be commensurate with the requirements of the class of satellite sensor they are specified to support. 8. **Utility--**FRM datasets are designed to apply to a class of satellite missions. They are not mission-specific. For example, in the spectral domain, it is expected that differentspectral bands of different sensors can be addressed and not only tailored exclusively to the exact bands of a single sensor such as Sentinel 2.