What Is GOSAT?

GOSAT, the Greenhouse gases Observing SATellite and known in Japan as Ibuki, is an Earth observation spacecraft dedicated to measuring the atmospheric concentrations of carbon dioxide and methane from orbit. Launched on 23 January 2009 aboard an H-IIA rocket from the Tanegashima Space Center, it was the first satellite built specifically for greenhouse gas monitoring, and it established the measurement approach that later missions of this kind have followed. The mission is operated jointly by the Japan Aerospace Exploration Agency, the Ministry of the Environment, and the National Institute for Environmental Studies, a division of labor in which JAXA provides the spacecraft and NIES processes the science data.

Ground-based greenhouse gas monitoring networks are accurate but sparse, with large gaps over oceans, tropical forests, and much of the developing world. GOSAT was conceived to fill those gaps with globally distributed measurements taken by a single, consistently calibrated instrument, so that regional sources and sinks could be estimated where no surface stations exist.

Spacecraft and Orbit

The satellite operates in a sun-synchronous orbit near 666 kilometers altitude with an equator crossing time close to 13:00 local solar time, chosen so that observations are made under reasonably high sun elevation. Its ground track repeats every three days, which gives near-global coverage of clear-sky scenes at that cadence. Rather than sweeping a continuous swath, the instrument points at discrete targets, and an agile pointing mirror allows the observation grid to be adjusted, to be directed at validation sites, or to be steered away from persistent cloud. The design life was five years, and the spacecraft has continued returning data far beyond it.

The TANSO Instruments

The primary payload is TANSO-FTS, a Fourier transform spectrometer covering three shortwave infrared bands near 0.76, 1.6, and 2.0 micrometers along with a thermal infrared band. Sunlight reflected from the surface passes twice through the atmosphere before reaching the instrument, and the depth of the absorption lines in the recorded spectra is inverted to yield column-averaged dry air mole fractions, written XCO2 and XCH4. The oxygen A band near 0.76 micrometers supplies the air mass and light path information the retrieval needs. A second instrument, TANSO-CAI, is a pushbroom imager that screens each footprint for cloud and characterizes aerosol, since undetected cloud is the dominant error source in column retrievals. The Ministry of the Environment describes the resulting observation series as the foundation of Japan's satellite greenhouse gas program.

Data Products and Flux Inversion

Retrieved column concentrations are only the first step. To convert them into estimates of where carbon is being emitted and absorbed, the concentration fields are assimilated into an atmospheric transport model and inverted for surface fluxes, a process that must separate a small anthropogenic signal from a large and variable background. JAXA and NIES have published global monthly methane flux estimates derived from Ibuki observations using this approach, resolving regional sources at a scale that surface networks alone cannot reach. Retrieval accuracy is anchored to ground truth from the Total Carbon Column Observing Network of upward-looking spectrometers.

Successor Missions

GOSAT-2, also called Ibuki-2, launched in October 2018 with an improved spectrometer that adds carbon monoxide to the retrieved species, an intelligent pointing system that selects cloud-free targets on board, and better signal-to-noise performance. Its mission overview documents the instrument changes and early validation. A third mission, GOSAT-GW, launched in June 2025 and extends the series by combining a wide-swath imaging spectrometer with a microwave radiometer for water cycle observation.

Applications

Data from the GOSAT series supports work in a range of fields, including:

• National greenhouse gas inventory verification under international climate agreements
• Carbon cycle science and terrestrial sink estimation
• Methane emission detection from wetlands, agriculture, and fossil fuel infrastructure
• Atmospheric transport model development and data assimilation
• Remote sensing instrument design and retrieval algorithm research
• Solar-induced chlorophyll fluorescence studies of vegetation productivity