A222-0009
Improvement of the Greenhouse Gas Retrieval Algorithm for the GOSAT Series Satellites

Wednesday, 16 December 2020
Poster
Nobuhiro Kikuchi1, Akihiko Kuze2, Fumie Kataoka3, Kei Shiomi2, Makiko Hashimoto2 and Hiroshi Suto2, (1)Japan Aerospace Exploration Agency, Kanagawa, Japan, (2)Japan Aerospace Exploration Agency, Tsukuba, Japan, (3)Remote Sensing Technology Center of Japan, Tsukuba, Japan
Abstract:
There are three unique features that distinguish TANSO-FTS onboard GOSAT (and also FTS-2 onboard GOSAT-2) from other satellite sensors for the greenhouse gas observation from space.

First, the SWIR and TIR spectra are measured simultaneously at the exactly same footprint. SWIR has information on the total column amount of CO2 and CH4, while TIR is sensitive to these gases mainly in the upper troposphere. With these two spectral windows, GOSAT and GOSAT-2 are capable of retrieving CO2 and CH4 concentrations in the lower troposphere, which helps estimates of the surface fluxes.

Second, both TANSO-FTS and FTS-2 measures two linear polarizations of the reflected sunlight, which can be used to correct undesirable effects of aerosols on the accurate retrieval of the greenhouse gas concentrations.

Thirdly, the flexible pointing mechanism enables GOSAT and GOSAT-2 to observe targeted points in addition to regular grid points, which meets a growing demand for monitoring greenhouse emission from local point sources such as megacities.

For the successful retrieval of the greenhouse gas concentrations from space, satellite sensors need to be characterized with sufficient accuracy. We have evaluated on-orbit performance of TANSO-FTS and FTS-2 by utilizing solar calibration observations and sun-glint observations. For FTS-2, we found some discrepancies between pre-launch and on-orbit evaluation of the sensor characteristics. The spectral resolution of FTS-2 in orbit seems to be degraded than the pre-launch evaluation. In particular, the instrument line shape (ILS) function in Band 1 (O2 A band) varies with time. We modeled time-dependence of the Band 1 ILS by analyzing the Fraunhofer lines measured by the solar calibration observations. The response of the FTS-2 sensor to the polarization state of the incident light is somewhat mysterious, for which we have made a correction scheme by analyzing sun-glint observations.

With this sensor characterization, GOSAT and GOSAT-2 measurements are now being processed to retrieved CO2 and CH4 concentrations using both SWIR and TIR spectral windows. We retrieved vertical profiles of CO2 and CH4 with five layers in total, two of five in the troposphere. The data will be released from the JAXA/EORC website.

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