The Earth Climate Observatory space mission concept for innovative continuity in the monitoring of the Earth Reflected Solar Radiation

Monitoring the Earth Energy Imbalance (EEI) is crucial for predicting climate change and assessing global progress under the Paris Climate Agreement. Currently, the most accurate EEI estimates are derived from in situ observations, with a dominant contribution from the time derivative of the Ocean Heat Content (OHC). These observations, however, require long time periods—typically a decade or more—to yield meaningful insights. In contrast, direct space-based EEI observations have the potential to provide measurements at the annual mean timescale.To fully understand EEI, it is essential to spectrally separate the Total Outgoing Radiation (TOR) into the two components of the Earth Radiation Budget (ERB): Reflected Solar Radiation (RSR) and Outgoing Longwave Radiation (OLR). This separation is critical for understanding radiative forcing (e.g., aerosol radiative forcing) and climate feedback mechanisms (e.g., ice-albedo feedback), as well as for validating climate models.The state-of-the-art observation of the RSR is provided by the CERES scanning 3-channel broadband radiometer on the Sun-synchronuous afternoon orbit satellites Aqua, Suomi NPP and NOAA 20.The Earth Climate Observatory (ECO) mission concept was recently selected by the European Space Agency as one of the 4 candidate Earth Explorer 12 missions, that will be further studied in Phase 0 until mid 2026. The ECO mission proposes an innovative continuity for RSR measurements by replacing the scanning broadband radiometer by a multispectral wide field of view cameras. The wide-field-of-view design enables full angular coverage, significantly reducing the dominant angular conversion error. To leverage this capability, an advanced Deep Learning-based angular conversion method is proposed.The multispectral bands of the camera are designed to reconstruct the broadband RSR within the state of the art accuracy. Furthermore, the spatial resolution of the cameras will be sufficient to discriminate cloudy from clear-sky scenes. For the calibration of the cameras we propose an on-board shutter for the dark current determination, vicarious calibration for the gain determination, and cross-calibration with the sun-earth radiometer for the final broadband calibration directly tied to the incoming solar radiation.This mission concept addresses critical challenges in EEI monitoring and represents a significant advancement in Earth Radiation Budget observations. The ECO mission holds the potential to deepen our understanding of climate processes, improve climate models, and provide timely, actionable insights for monitoring climate change.