Radiometric calibration of GOSAT TANSO-FTS SWIR bands: comparison of vicarious to on-orbit results

The Thermal And Near-infrared Sensor for carbon Observation - Fourier Transform Spectrometer (TANSO-FTS) aboard the Greenhouse gases Observing SATellite (GOSAT) has been providing global, space-based measurements of solar reflected radiances since early 2009. Several operational or semi-operational algorithms exist to invert the measured radiances, producing column-averaged carbon dioxide (CO₂) dry air mole fraction (XCO₂). The resulting XCO₂ are used as inputs to flux inversion models to determine sources and sinks of CO₂. An accurate radiometric calibration of the TANSO-FTS short wave infrared (SWIR) channels is required in order to yield results with high accuracy. In this work we summarize the latest estimation of ground-based vicarious calibration coefficients (VCC) from four separate field campaigns conducted at the Railroad Valley playa in June of 2009-2012. We then provide a comparison of the time-dependent VCC with the results from the radiometric calibration performed using on-orbit solar observations. While both approaches indicate some radiometric degradation in the SWIR bands, with the strongest decay in the Oxygen-A band, the magnitude of the changes disagree.