Calibration, Level 1 Processing, and Radiometric Validation for TANSO-FTS TIR on GOSAT

The Greenhouse gases Observing SATellite (GOSAT) carries the Thermal and Near-Infrared Sensor for carbon Observations Fourier Transform Spectrometer (TANSO-FTS). TANSO-FTS covers a wide spectral range from the shortwave infrared to the thermal infrared (TIR). This paper describes the updated calibration algorithm of the TANSO-FTS Level 1B TIR spectra and the radiometric validation of the new V210.210 products by comparison with the previous version of V201.202. The revised nonlinearity correction for V210.210 product creates a decade-long, well-calibrated radiance set while minimizing the effect of two major anomalies: rotation stop of the one of the solar paddles in 2014 and a cryocooler shutdown in 2015, which caused abrupt changes in the thermal environment of the TANSO-FTS sensor. To check the improved nonlinearity correction and onboard calibration in TANSO-FTS V210.210 processing, we validated the entire spectral range by comparing with aircraft-based Scanning High-resolution Interferometer Sounder coincident in time with the GOSAT overpass. Also selected CO₂ and CH₄ channels are validated with the Atmospheric infrared sounder and the window channel with the in situ SST Quality Monitor data at temporally coincident and spatially collocated points. We have confirmed that the new V210.210 products exhibit no significant time trend in the window channel and a reduced spectral bias in the CO₂ and CH₄ channels. There remains some spectral bias, especially in the CO₂ ν ₂ channel and CH₄ channel, which are attributed to the uncertainty of orbital and seasonal variations in the average direct current level of TANSO-FTS without photon input.