Ultra-precise measurement of CO 2 from space

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The experimental data on CO₂ and O₂ detection in atmosphere using Fabry-Perot technique are presented. The atmosphere's irradiance measurements are an important tool for the remote sensing study. We show results from lab, ground and flight testing of a new instrument called FPICC (Fabry-Perot Interferometer for Column CO₂) which is intended for a very precise measurements of atmospheric carbon dioxide and oxygen. The optical setup consists of three channels. The first channel is built to measure the carbon dioxide. This channel operates using the reflected sunlight off the ground and solid Fabry-Perot etalon to restrict the measurement to light in CO₂ absorption bands. The free spectral range of the etalon is calculated to be equal to the almost regular spacing between the CO₂ spectral bands located near 1,571 μm, R band, where CO₂ absorption is significant. The precise alignment of the transmission peaks of the Fabry-Perot etalon to the CO₂ absorption lines is achieved through altering the refractive index of the material (fused silica) using its temperature dependence. The second and third channels foucs on the O₂ A band (759 - 771 nm) composed of about 300 absorption lines, which vary in strength and width according to pressure and temperature. We performed measurements using solid Fabry-Perot etalons with different FSR and two different pre-filters. The first pre-filter selects a spectral range around 762 nm which is between the P and R branches, where the absorption coefficient is insensitive to temperature, but is sensitive to pressure changes and therefore to the variations in the O₂ column. The second pre-filter is selecting several absorption bands between 765 and 770 nm, which are more sensitive to temperature changes. The experimental data presented show excellent agreement with our theoretical expectations. They are recorded at different gas pressures, temperatures and different weather conditions. Some of the major advantages of the optical setup are its compactness, high sensitivity, high signal-to-noise ratio, and stability.