A benchmark for climate data
The Traceable Radiometry Underpinning Terrestrial- and Helio-Studies mission, or TRUTHS for short, would have laid any such doubts to rest. The satellite, which ESA was building under the umbrella of its Earth Watch programme and on behalf of the UK, would have provided traceable International System of Units (SI) measurements of incoming solar radiation and of radiation reflected from Earth back out into space, with which to calibrate data from other satellites. In effect, TRUTHS would have been a ‘standards laboratory in space’, setting the ‘gold standard’ for climate measurements.
However, at the end of 2025, the UK withdrew its funding, leading to the suspension of the mission. At the time development was halted, the mission was progressing as planned, having successfully demonstrated its performance and technological readiness. Its scientific value had also been clearly validated by strong engagement and endorsement from data user communities at multiple events.
Access the video
There is little doubt that human activity is having a disastrous effect on Earth’s natural processes – and it seems we are on the brink of catastrophe unless the climate crises is addressed. When it comes to underpinning decision-making, satellite data records are critical – but confidence in these data is of utmost importance. TRUTHS would have provided the benchmark reference for radiation measurements to improve our understanding of climate change and help increase the precision of climate models. This would have helped to monitor the effectiveness of international policy to fight the climate crisis.
Essentially, the amount of incoming solar energy compared to the amount that bounces back to space controls our climate. An accurate knowledge of these energy exchanges is fundamental to understanding and monitoring change. The TRUTHS satellite was designed to carry two main instruments: the Cryogenic Solar Absolute Radiometer and the Hyperspectral Imaging Spectrometer as well as a novel onboard calibration system.
Together, these instruments would have made continuous measurements of both incoming solar radiation and reflected radiation. These two observations would have been used to evaluate the energy-in to energy-out ratio. The spatial and spectral resolution of the mission would have allowed greater understanding of the causes of change in energy balance and helped assess progress stemming from climate action. The unprecedented quality of these measurements would have offered the standard reference against which observations from other satellites can be corrected.
NASA is developing the CLARREO Pathfinder instrument, which will fly on the International Space Station, and an overarching CLARREO satellite concept – both similar to TRUTHS.
TRUTHS was being developed by ESA and led by the UK Space Agency with participation from Switzerland, Greece, Romania and Czech Republic, with Airbus UK as the prime contractor. However, at the end of 2025, the UK withdrew its funding, leading to the suspension of the mission.
