Earth observation (EO) satellites are indispensable for the global monitoring of Essential Climate Variables (ECVs). The exploitation of EO data products, however, is hampered by a lack of complete and traceable information about their error budget. Obtaining such information is far from trivial, because the error traceability chain is easily broken when launching satellites into space and applying models to retrieve ECVs from their radiance measurements. In fact, error characteristics of ECV products are most commonly assessed using in situ reference measurements whose error budget has been specified either in an untraceable manner or not at all. Moreover, reference sites are too sparsely and unevenly distributed to properly represent the heterogeneity of the Earth surface, or even heterogeneity within satellite footprints. To address this issue, many initiatives have been launched to establish so-called Fiducial Reference Measurements (FRMs). FRMs are defined as fully error-characterised and traceable ground measurements, which follow community-agreed guidelines that are based upon metrological principles as outlined by the Quality Assurance framework for Earth Observation (QA4EO). The ESA project “Fiducial Reference Measurements for Soil Moisture” (FRM4SM) is one such initiative aiming to define standards for reliable and fully-traceable in situ measurements of SM. It builds upon the International Soil Moisture Network (ISMN; https://ismn.earth/), which is currently the most extensive public SM reference data base holding in situ measurements from networks all around the world, the Quality Assurance for Soil Moisture (QA4SM; https://qa4sm.eu/) platform, which is a free online SM validation tool, and good practice guidelines for satellite SM validation that have been recently developed by the SM community and are endorsed by the Committee of Earth Observation Satellites. Specifically, the goals of the FRM4SM project are to: • develop Quality Indicators (QIs) that allow to fully describe the error characteristics of in situ SM measurements; • define protocols to obtain a fully traceable error budget of in situ SM measurements based on metrological principles; • calculate QIs for all ISMN stations and apply the developed protocols to assess the degree to which these stations can be considered FRMs; and • demonstrate how identified “FRM super-sites” can be used to validate satellite SM products within the QA4SM platform. In this talk, we will report on the findings and progress made within the FRM4SM project thus far, and on the most pressing issues currently preventing full end-to-end error traceability for EO data sets.