The TRUTHS cryogenic solar absolute radiometer (CSAR)

TRUTHS (Traceable Radiometry Underpinning Terrestrial – and Helio- Studies) is primarily an operational climate space mission of European Space Agency (ESA) [1], to be launched in 2030, aiming to enhance by an order of magnitude our ability to estimate the Earth radiation budget through direct measurements and to establish a “metrology laboratory in space” to create a fiducial (International System of units – SI traceable) reference data, setting the ‘gold standard’ reference for climate measurements to cross-calibrate other sensors and improve the quality of their data. TRUTHS main objective is to establish a reference baseline measurement (benchmark) of the state of the planet, against which future observations can be compared to allow climate model improvements and forecast testing and to provide observational evidence of climate change including mitigation strategies in the shortest time possible. AIRBUS UK, as prime contractor, is responsible to implement the TRUTHS satellite that carries a payload made essentially by two main instruments: the Cryogenic Solar Absolute Radiometer (CSAR) and the Hyperspectral Imaging Spectrometer (HIS), as well as a novel On-Board Calibration System (OBCS) measuring both the incoming and the Earth reflected solar radiation. The Payload will be used to evaluate the Earth’s energy-in to energy-out ratio. CSAR instrument is conceived to measure the Total Sun Irradiance (TSI) as a standalone product. It will also measure, along with the HIS, the optical power coming from the On-Board Calibration System (OBCS) to anchor the HIS to the SI, an uncertainty of less than 0.3 % (k = 2). Thales Alenia Space in Switzerland leads a Swiss consortium responsible to develop and to test CSAR. The goal of providing SI-traceable means of measuring the TSI with an uncertainty of less than 0.02 % (k=2), as well as calibrating its sister instrument - the Hyperspectral Imaging Spectrometer (HIS) - poses engineering challenges across all disciplines. We present the challenges to design such an instrument. A particular focus will be made on the optical, thermo-mechanical and electrical design. The specificities linked to the specific cryogenic environment will also be presented along with the associated verification equipment. The ways towards building a blackbody as part of the CSAR, analyzing the CSAR radiometric performances, the peculiarities of the opto-mechanical and thermo-mechanical design for the specific cryogenic environment, the related verification in the representative thermal environment will be presented. Last, the design and breadboarding of the acquisition electronics, specific to all highly accurate for electrical-substitution radiometers in space will be detailed.