Improvement of the basic performance of the sensors with which the Greenhouse Gases Observing Satellite (GOSAT)−2 is equipped improves the accuracy of greenhouse gas measurement, and using these improved measurement data, more precise analysis of the source/sink strength of the gases through inversion analysis is expected. With the addition of new spectral channels, carbon monoxide and light-absorbing aerosols such as black carbon can be observed. Based on these improvements of the observation performance, the science-plan of the project, which is a guideline for research using GOSAT-2 data, has been issued. It describes: 1) Progress on carbon cycle research based on long-term observation data on greenhouse gases; 2) the integration of various approaches to estimating the source/sink strength of greenhouse gases; 3) the advancement of terrestrial ecosystem models; 4) the identification of hot spots and the monitoring of the vegetation response to short-term events; and 5) the importance of aerosol monitoring.

Greenhouse Gases Observing Satellite (GOSAT)-2, which is the successor of GOSAT, was launched on 29^th October, 2018. Thermal And Near infrared Sensor for carbon Observation (TANSO)- Fourier Transform Spectrometer (FTS)-2 on board the GOSAT-2 has two thermal infrared (TIR) bands (Bands 4 and 5) with which to observe the vertical profiles of atmospheric temperature and greenhouse gases such as carbon dioxide (CO₂) and methane (CH₄). We adopted a non-linear maximum a posteriori (MAP) method with linear mapping to retrieve the temperature and gas concentrations. The current TANSO-FTS-2 retrieval system developed before the GOSAT-2 launch utilizes the CO₂ absorption bands at around 9, 10, and 15μm (690-750cm^－1, 790-795cm^－1, 930-990cm^－1, and 1040-1090cm^－1) to retrieve CO₂, water vapor, ozone, and temperature, and utilizes the CH₄ and N₂O absorption bands at around 7 and 8μm (1140-1370cm^－1) to retrieve CH₄, N₂O, water vapor, ozone, and temperature. The signal-to-noise ratio (SNR) values of the TIR bands of the TANSO-FTS-2 are expected to be significantly larger than those of the TANSO-FTS, which gives the TANSO-FTS-2 higher sensitivity to gas concentrations. According to a preliminary retrieval simulation, the degree of freedom (DF) of the CO₂ observations of the TIR bands of the TANSO-FTS-2 is 2.44 in low latitudes, while the DF value of the TANSO-FTS TIR CO₂ observations is 2.06 under the same atmospheric conditions. The DF values of the TANSO-FTS-2 TIR CH₄ and N₂O, observations are 3.07 and 1.84, respectively, which are much higher than those of the TANSO-FTS TIR observations (0.50 and 0.14).

GOSAT product validation activities and future issues are described in this article. An overview of the GOSAT-2 validation plan is also given. The present main ideas of acquiring validation data for the GOSAT-2 are the same as those for the GOSAT. The TANSO-FTS-2 SWIR L2 products of the column-averaged dry-air mole fractions of CO₂, CH₄, CO, H₂O were obtained as follows. Ground-based high-resolution Fourier transform spectrometer data of the TCCON (Total Carbon Column Observing Network) and NDACC IRWG (Network for the Detection of Atmospheric Composition Change-InfraRed Working Group) sites will be used, and some sites will be co-located with lidar and sky-radiometer systems for extensive validation data acquisition. In-situ measurements and/or sampling of CO₂, CH₄, and CO onboard commercial and charter aircraft by CONTRAIL (Comprehensive Observation Network for TRace gases by AIrLiner) and NOAA (National Oceanic and Atmospheric Administration) will also be employed. The National Institute for Environmental Studies (NIES) validation team, organizations responsibly participating in the GOSAT series project, and research teams selected through Research Announcement will conduct the validation of the products.

The Greenhouse gases Observing SATellite-2 (GOSAT-2) was launched on 29 October, 2018. GOSAT-2 is equipped with two earth-observing instruments: the Thermal And Near-infrared Sensor for carbon Observation Fourier Transform Spectrometer 2 (TANSO-FTS-2), and the Cloud and Aerosol Imager 2 (TANSO-CAI-2). Cloud contamination within the instantaneous field-of-view (IFOV) of the FTS-2 leads to incorrect estimates of major greenhouse gases concentration. To solve this problem, the FTS-2 data suspected to have cloud contamination must be identified by a new cloud discrimination algorithm using a support vector machine, the Cloud and Aerosol Unbiased Decision Intellectual Algorithm-3 (CLAUDIA3), used with the CAI-2. The cloud discrimination results will be publicly available as the CAI-2 L2 cloud discrimination products. In this paper, we deal with the CLAUDIA3 and the application of the CLAUDIA3 to CAI-2 data. Moreover, we show several cloud discrimination results of CLAUDIA3 applied to CAI data.

Observational data acquired by satellites are at present a valuable tool for exploring the global carbon cycle. A key element of satellite observations is their high-frequency and global coverage, which can compensate for gaps in surface observation networks. The Greenhouse gases Observing SATellite (GOSAT) and its successor GOSAT-2 are designed to measure atmospheric carbon dioxide (CO₂) and methane (CH₄) so as to estimate their sources and sinks on subcontinental scales. This commentary gives an overview of the frameworks for estimating global surface CO₂ and CH₄ fluxes using GOSAT and GOSAT-2 observations.

We describe past, ongoing, and planned space-based instruments with the capability to observe the column abundance of greenhouse gases such as carbon dioxide (CO₂) and methane (CH₄). The key features of the satellite and spectral instruments are concisely summarized.

The GOSAT observation is overviewed in terms of CO₂ and CH₄ variations in the atmosphere and their global budgets, and the results obtained are discussed in comparison with those from other studies. Expectations for the GOSAT-2 project are also described.