The QA4EO framework and a metrological approach to FRMs, FDRs and TDPs
by Dr. Emma Woolliams, Prof. Nigel Fox, Dr. Jonathan Mittaz, Mr. Samuel Hunt, Dr. Paul Green, Dr. Bernardo Mota

Abstract

The Group on Earth Observations (GEO) and its space arm, the Committee on Earth Observation Satellites (CEOS) endorsed in 2010 the Quality Assurance Framework for Earth Observation (QA4EO), as part of its strategy to ensure that Earth observation data are “accessible, of identified quality and provenance and interoperable to support the development of tools and delivery of information services.” The term ‘interoperable’ covers several well-defined concepts. It requires common geographical grids and data formats, but it also requires observations to be based on a common measurement scale, referenced to a well-understood reference, ideally the international system of units (the SI). QA4EO realises the following principle regarding Earth Observation data quality: ‘It is critical that data and derived products are easily accessible in an open manner and have an associated indicator of quality traceable to reference standards (preferably SI) so users can assess suitability for their applications i.e. ‘fitness for purpose’.’ QA4EO defines high level processes to achieve these objectives, such as may be achieved through well-documented procedures, participation in comparisons, and uncertainty assessments, applicable to all EO data records. Since 2010 the Earth observation communities, working with metrology institutes have developed methods to apply QA4EO to different types of observations. Some of the most important efforts have been in the introduction of “fiducial reference measurement” (FRM) systems that provide in-situ observations to support calibration and validation of satellite data sets, “fundamental data records” (FDRs) that provide uncertainty-quantified and traceable “level 1” (unprocessed) satellite data, and “thematic data products” (TDPs) that provide higher level satellite data products (I.e. processed records of bio or geophysical quantities) with robust uncertainties and metrological traceability. NPL has recently updated the www.qa4eo.org website to provide detailed guidance on how to perform uncertainty analysis for FRMs, FDRs and TDPs in a metrologically robust manner, based on expertise developed over multiple projects with the observation communities with passive and active satellite sensors and the in situ observations that support them. Here we present this updated material and discuss what is still required.