Correction of Spectral Distortion on Oxygen a-Band Due to Non-Linear Phase Delay Onboard Tanso-FTS/Gosat

Abstract:

To observe the global column concentration of carbon dioxide (CO₂) and methane (CH₄) from space, the Greenhouse gases Observing SATellite (GOSAT) was launched on January 23, 2009, and has started the operational observation. Thermal and Near Infrared Sensor for Carbon Observation – Fourier Transform Spectrometer (TANSO-FTS) has been continuously measuring CO₂ and CH₄ distributions globally, and the retrieved column CO₂ and CH₄ data have been distributed to the public. To make a successful retrieval of X_CO2 and X_CH4, the spectral quality of Oxygen A-band is the most importance. Over five years in-orbit operation of TANSO-FTS, the spectral distortion related with input radiance on Oxygen A-band have been observed and reduced the retrieval accuracy and precision of X_CO2 and X_CH4. It suggests that the Oxygen A-band signal chain has non-linear response against input radiance. To characterize the non-linear response of signal chain against input signal levels, the test procedure is newly developed coupled with the modulated laser light, simultaneous signal acquisition system and on-ground TANSO-FTS, which called engineering model. The results present clearly that the analogue signal chain of Oxygen A-band excites the non-linear response both of amplitude and phase delay against input signal levels. Also, the non-linear interferogram drives both of the artificial spectra on the out-band region and the spectral distortion linked with absorption spectral lines. To improve the spectral quality of Oxygen A-band, these artificial and distorted spectra have to correct with properly. The newly correction algorithm for level-1 processing was developed and the corrected spectra were retrieved and validated by applying RemoTeC algorithm. Comparing with the previous version of level-1 products, the agreement between observation and theoretical calculation is well improved and the biases of biases of X_CO2 and X_CH4 against ground validation site are reduced.