Traceable Radiometry Underpinning Terrestrial- and Helio- Studies (TRUTHS) – A ‘gold standard’ reference for an integrated space-based observing system for environment and climate action
by Prof. Nigel Fox, Dr. Thorsten Fehr, Dr. Paul Green, Mr. Sam Hunt, Mr. Andrea Marini, Mr. Kyle Palmer

Abstract

The number, of Earth-viewing optical sensors and range of applications is increasing rapidly, with climate change (its sensitivity, mitigation, and underpinning radiation balance) being the major drivers of this proliferation and criticality. With satellites increasingly operated from not only national/international space agencies but also commercial organisations, the need for a holistic earth observing system with harmonised trustworthy data has never been greater. To achieve this vision of an integrated earth observing system has led to international initiatives, coordinated by bodies, such as the Committee on Earth Observation Satellites (CEOS) and Global Space Inter-Calibration System (GISCS) of WMO, to facilitate interoperability and removal of bias through post-launch Calibration and Validation and linkage to SI units. One challenge from the BIPM/WMO 2010 workshop to address this was the call to ‘establish satellite missions with high-accuracy SI-traceability to serve as in-orbit anchors, harmonising space-based observations and establishing robust benchmarks from which decadal change can be detected in as short a timescale as possible’. The 2020s will see the realisation of this ambitious goal with the launch of missions explicitly designed to provide spectrally resolved SI-traceability evidenced in orbit. In 2023/24, the NASA CLARREO Pathfinder will be installed on-board the International Space Station followed by the free-flying ESA TRUTHS launch in 2029/30, will create this new paradigm heralding the era of SITSats (SI-Traceable Satellites). TRUTHS is a UK-led mission under development by ESA within its EarthWatch program and is based on a concept conceived at the UK National Metrology Institute, NPL, some 20 years ago. The mission not only embeds high-accuracy SI-traceability on-board, but will also make transparent the sources and quantification of uncertainty of the whole processing chain. TRUTHS will make spectrally resolved measurements of incoming solar and Earth/moon reflected radiation from the UV to SWIR with an uncertainty goal of 0.3% (k=2), globally, continuously sampled with a native GSD of 50 m. In addition to establishing benchmark observations of the radiation state of the planet, its SI-traceable uncertainty can be transferred to other sensors through in-orbit reference calibration. In this way creating a ‘metrology laboratory in space’, providing a ‘gold standard’ reference to anchor and improve the calibration and SI-traceability of other sensors. In addition to the technical aspects of TRUTHS, the paper will emphasise the role and value of SITSats in a future global space-based climate observing system, and the necessary complementarity with other elements of the Earth observing system e.g. in-situ Fiducial Reference Measurements (FRMs) used for validation.