## 3 Fiducial Reference Measurements (FRMs)
### FRMs in Practice  
One of the first projects that could be considered an FRM (although the terminology did not exist at the time) was dedicated to Sea Surface Temperature (SST) validation; it was performed in Miami, hosted by Miami University, more than 20 years ago. The objective of the campaign was to calibrate and compare infrared radiometers used in the validation of the different sea and land surface temperature products derived from satellites. The objective included an assessment of the relative performance of each instrument as well as ensuring that surface measurements used in satellite product validation were traceable to SI units at the highest possible confidence level. This inter-comparison exercise ensured a coherence between the validation measurements used in various international Cal/Valprograms and provided increased confidence in the results used for validation of satellite-derived surface temperatures. This first inter-comparison of infrared radiometers was held at Rosenstiel School of Marine, Atmospheric, and Earth Science (RSMAS) in Miami during March 1998. This comparison involved several high-quality radiometers and some off-the-shelf devices. The National Institute of Standards and Technology (NIST) provided their standard blackbody target [14] for calibration of each radiometer. The first exercise was followed by a second in 2001 prior to the launch of Aqua Modis and Envisat, and the third one took place in May 2009, under the auspices of the CEOS WGCV Infrared and Visible Optical Sensors Subgroup (IVOS), prior to the launch of Suomi-NPP. This third comparison followed the newly established QA4 EO guidelines, with protocols developed to ensure a robust traceability to SI (via NIST and National Physical Laboratory--NPL) and establish the degree of equivalence between participants. The results were then published [15; 16]. Following the creation of the FRM label and criteria, an enhanced version of the previous 'Miami series' became FRM4 STS--Fiducial Reference Measurements for Surface Temperature Satellites [17]. Started in 2014, the aim was to establish and maintain SI traceability of global FRMs for satellite-derived surface temperature product validation (water and land) and to help develop a case for their long-term sustainability. It required:  
* Comparisons to ensure consistency between measurement teams with predefined protocols;
* Accessible common descriptions and evaluation of uncertainties;
* Robust links to SI;
* Experiments to evaluate sources of bias/uncertainty under different operational conditions;
* Guidance for prospective validation teams on efforts needed to establish and maintain their measurement instrumentation in the laboratory and on campaign;
* Traceability of their instruments and also on their use in the laboratory and field. The CEOS plenary endorsed the project in 2014 with the objective to facilitate international harmonisation. With the success of FRM4 STS [17] as an example, the European Space Agency developed a series of projects, following the same logic, aimed at improving the validation and qualifying the validation measurements as fiducial in various domains (see e.g. [18] in the context of optical remote sensing applied for the generation of marine climate-quality products). Table 1 lists (not exhaustive) some ESA projects with the relevant parameters and satellite missions, and in most cases, their websites provide access to the results and documentation. In addition to these FRM labelled projects, there are initiatives such as CEOS WGCV RadCalNet [19], and also Hypernets [20], that can both justify the FRM label not only for an individual site but also for a network of sites. In both of these examples, each individual site contributing to the network must adhere to FRM criteria to provide data to a central processing hub. However, in addition, the hub serving as the interface to the community, delivering commonly processed data, is itself also of FRM quality, with documented procedures, uncertainties and comparisons to underpin the evidence.