Spectral Parameters and Signal-to-noise Ratio Requirement for CO₂ Hyper Spectral Remote Sensor

With the stable increase of carbon dioxide (CO₂) concentrations, space based measurement of CO₂ concentration in lower atmosphere by reflected sunlight in near infrared band has become a hot research topic. Recently, instruments sensitive to total CO₂ column data in near-surface have become available through the SCIAMACHY instrument on ENVISAT and TANSO-FTS on GOSAT. The developing hyper spectral CO₂ detector in China carried by TANSAT is going to be launched in 2015. Hyper spectral CO₂ detector is designed to provide global measurements of CO₂ in lower troposphere, employing high resolution spectra of reflected sunlight taken simultaneously in near-infrared CO₂ (1.61 μm and 2.06 μm) and O₂ (0.76 μm) bands.Associated with climate change and observation requirements of carbon sources and sinks, the feasibility of making CO₂ column concentration measurements with high-resolution and high-precision is studied by high resolution atmosphere radiation transfer model. In consideration of the application requirements, effects of key specifications of the hyper spectral CO₂ detector such as spectral resolution, sampling ratio and sign-to-noise ratio (SNR) on CO₂ detection are analyzed.Typical characteristics of hyper spectral CO₂ detector on TANSAT are grating spectrometer and array-based detector. To achieve the column averaged atmospheric CO₂ dry air mole fraction (X_CO2) precision requirements of 1×10^-6-4×10^-6, hyper spectral CO₂ detector should provide high resolution at first to resolve CO₂ absorption lines from continuous spectra of reflected sunlight. Compared to a variety of simulated spectral resolutions, the spectral resolution of hyper spectral CO₂ detector on TANSAT can resolve CO₂ spectral features and maintain the moderate radiance sensitivity. Since small size array detector-based instruments may suffer from undersampling of the spectra, influences of spectral undersampling to CO₂ absorption spectra are studied, indicating that sampling ratio should exceed 2 pixels/FWHM to ensure the accuracy of CO₂ spectrum.SNR is one of the most important parameters of hyper spectral CO₂ detectors to ensure the reliability. SNR requirements of CO₂ detector to different detection precisions are explored based on the radiance sensitivity factors. Results show that it is difficult to achieve SNR to detect 1×10^-6-4×10^-6 CO₂ concentration change in the boundary layer by solar shortwave infrared passive remote sensing, limited by the instrument development condition and level at present. However, the instrument SNR to detect 1% change in the CO₂ column concentration is attainable. These results are not only conductive to universal applications and guides on developing grating spectrometer, but also helpful to better understand the complexity of CO₂ retrieval.