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Abstract

It is essential to monitor the global annual mean Earth’s Energy Imbalance (EEI) at the top of the atmosphere, as this excesss energy is accumulated in the Earth climate system, causing global warming. We propose the Earth Climate Observatory (ECO) as a new space mission concept specifically targeted to the monitoring of the global annual mean EEI. In order to reach high accuracy – with a target of 1 W/m² – and stability – with a target of 0.1 W/m²dec – we are sacrificing resolution. The core instrument is an advanced wide field of view radiometer; this instrument is capable of measuring both the incoming solar radiation, and the outgoing terrestrial radiation – both of the order of 340 W/m² - with the same instrument, and thus to make a significant measurement of the small difference – of the order of 1 W/m² (0.3%) - between these two nearly equal terms. For the sampling of the diurnal cycle of Earth’s outgoing radiation, we propose a combination of 3 orbits: 1) an orbit with precession period of 6 months, and an inclination close to 82°, providing a statistical sampling of the full diurnal cycle for midlatitude and equatorial regions every 3 months 2) an orbit with precession period of 12 months and inclination of 90°, providing a statistical sampling of the full diurnal cycle for polar regions every 6 months, 3) a noon-midnight sun synchronous orbit, providing a stable sampling of the diurnal cycle at the monthly mean level.