On-orbit radiometric calibration of SWIR bands of TANSO-FTS onboard GOSAT

Abstract. The Greenhouse gases Observing SATellite (GOSAT) was launched on 23 January 2009 to monitor global distributions of carbon dioxide and methane. The Thermal And Near-infrared Sensor for carbon Observation-Fourier Transform Spectrometer (TANSO-FTS) onboard GOSAT measures the short-wavelength infrared (SWIR) spectra. Radiometric accuracy directly influences the accuracy of the retrieved greenhouse gas concentrations. From a 2.5-yr retrieval analysis of GOSAT data, we found that the minimum of the mean-squared value of the residuals (the difference between observed and fitted spectra) and the radiance adjustment factor (one of the ancillary parameters to be retrieved with the gas concentrations for adjusting the radiance level between the bands) changed with time, possibly due to inaccurate degradation correction. In this study, the radiometric degradation of TANSO-FTS was evaluated from the on-orbit solar calibration data and modeled as a function of time and wavenumber for each spectral band. The radiometric degradation of TANSO-FTS Band 1 (centered at 0.76 μm) after the launch was found to be about 4 to 6%, varying with wavenumber, whereas the other two bands (Band 2: 1.6 μm and Band 3: 2.0 μm) showed about 1% degradation and small wavenumber dependency. When we applied the new degradation model in the retrieval analysis, the above-mentioned issues disappeared.