A new space instrumental concept for the measurement of CO2 concentration in the atmosphere

Measuring the concentration of greenhouse gases from space is a topical challenge. They are measured via a precise analysis of the signature of chemical gaseous species (CO2, CH4, CO, etc.) in the spectrum of the Earth's atmosphere. Two types of spectrometer are commonly used. The first is based on the interference between two radiation waves. The Infrared Atmospheric Sounding Interferometer (IASI) aboard the METOP satellite is a good example of a fully-operational instrument of this kind. The second is based on the use of dispersive optical components. These instruments must have high radiometric and spectral resolutions in narrow spectral bands to be able to discriminate absorption lines from various atmospheric chemical species and to quantify their concentration. This is the case, for example, of the instrument aboard NASA's Orbiting Carbon Observatory (OCO). Our analysis led us to define a new instrument concept, based on a dispersive grating spectrometer, offering similar performance in a more compact and therefore less expensive instrument. After describing this instrument, which uses a specific grating component, a preliminary assessment of performances will be presented, including the theoretical calculations and formulae. A mock-up version of this specific grating demonstrated the feasibility of this concept and its capabilities. This preliminary design is encouraging and shows that such a spectrometer may be compatible with a microsatellite bus. Some prospects for improvement are also considered.