Low flux NGP characterisation for microcarb application

CNES (French Space Agency) is developing a microsatellite to monitor and characterize CO2 surface fluxes, that is, the exchanges between sources (natural or anthropogenic) and sinks (atmosphere, ocean, land and vegetation). A better assessment of carbon fluxes is necessary to improve our understanding of the mechanisms governing the exchanges between sources and sinks, their seasonal variability, and their evolution in response to climate change. Values of CO2 concentrations need to be measured with high precision, of the order of 1 ppm (to be compared with the CO2 concentration of 400 ppm) to be able to estimate gradients which amounts to a few ppm. The instrument on board MicroCarb is an infrared passive spectrometer operating in four wavelengths using an echelle grating (dispersive element) to achieve spectral dispersion. The spectral bands cover vissible end Short Wave infrared domain, from 764 μm to 2,075 μm. The selected detector is the NGP (new Generation Panchromatic) manufactured by Sofradir, supplied with a specific AntiReflection coating in order to optimize both sensitivity and stray light. The high accuracy level of the mission requires a high performance detector, operating at low incident flux, and whose imperfections will be very well known, in order to be corrected. The detector non-linearity is the main performance that has to be calibrated in order to allow the overall scientific objectives. CNES has developed a specific test bench in order to assess this performance. This paper describes in detail - The test bench constitution - The test bench calibration - The first detector measurements