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CarbonSat is one of the two candidate missions for the 8th cycle of European Space Agency (ESA) Earth Explorers, currently undergoing feasibility studies with two industrial consortia. The mission aims at quantifying the spatial distribution of carbon dioxide (CO2) and methane (CH4) with high precision (3.0 ppm for CO2 and 12.0 ppb for CH4) and accuracy (0.5 ppm for CO2 and 5 ppb for CH4) at a high spatial resolution (2km x 3km) and with global coverage above 40° latitude every 12 days. It consists of three pushbroom spectrometers measuring the Earth reflectance in each of the following bands: NIR (747nm- 773nm @0.1nm resolution), SWIR-1 (1590-1675nm @0.3nm) and SWIR-2 (1925-2095nm @0.55nm). Although most requirements for the CarbonSat phase A are defined over spatially homogeneous scenes, it is known from previous missions and studies that the observation of real, spatially heterogeneous scenes create specific measurement errors. One obvious mechanism is a distortion of the instrument spectral response function (ISRF) induced by a non-uniform slit illumination in the along-track (ALT) direction. This error has been analysed for several missions (OMI, Sentinel-4, Sentinel-5). The combination of spectrometer smile with across-track (ACT) scene non-uniformities induces similar errors. In this paper, we report about the analysis efforts carried out during CarbonSat preliminary phases to evaluate and mitigate these effects. In a first section, we introduce common concepts and notations for heterogeneous scenes analysis. An exhaustive list of known error mechanisms is presented. In section 2 we discuss the effect of inhomogeneous slit illumination, and describe hardware mitigation with a slit homogeniser. The combination of spectrometer smile and ACT heterogeneities is studied in section 3.

The CarbonSat candidate mission: radiometric and spectral performances over spatially heterogeneous scenes