Analyzing the reliability of in situ soil moisture measurements for satellite product validation: What makes fiducial reference measurements fiducial?

Abstract

To this day, in situ soil moisture data is viewed as ground truth by the satellite soil moisture (SSM) community. In general, little is still commonly known regarding the traceability of ground measurement uncertainty and their overall in uncertainty budget, which can impact satellite SSM product quality assessments.

Within ESA’s “Fiducial Reference Measurement for Soil Moisture (FRM4SM, May 2021 - May 2023)” project, objectives are set towards building fully characterized and traceable (i.e., fiducial) in situ measurements following community-agreed guidelines from the GEOS/CEOS Quality Assurance for Soil Moisture (QA4EO) framework. These so called “fiducial reference data” (FRM) should have associated Quality Indicators (QI) attached to evaluate their fitness for purpose building upon agreed reference standards (SI if possible). Moreover, such data should be easily and openly accessible, validation case studies should demonstrate their utility and reliability, and protocols and procedures should be established for the usage of such FRM datasets to make scientific studies intercomparable and reproducible.

As part of the FRM4SM project, the following questions were addressed using the International Soil Moisture Network (ISMN) as a ground reference database and the Soil Moisture and Ocean Salinity (SMOS) mission as an example satellite product:

(1) What makes “fiducial reference data” fiducial?

(2) Is the creation of a globally-representative FRM subset already feasible for SSM?

(3) What are the current limitations of in situ observations that limit fiduciality?

(4) What is needed to create a full traceability chain from in situ point measurements to the satellite footprint scale?

In this presentation, we will discuss these questions in detail and report on related findings of the FRM4SM project.