Current Status and Trends of Spaceborne Methane Monitoring

There are three main methods for atmospheric CH4 detection: ground-based, airborne, and satellite-based. Ground-based detection was the earliest to develop and has now formed an observation system represented by the Total Carbon Column Observation Network (TCCON) and the National Disturbance and Change in Atmospheric Composition Detection Network (NDACC). Airborne detection has also seen the development of a series of detectors, such as the Airborne Laser Infrared Absorption Spectrometer (ALIAS) and the second-generation ALIAS (ALIAS-II), the Airborne Visible/Infrared Imaging Spectrometer (AVIRIS-Classic), and the next-generation AVIRIS (AVIRIS-NG). While ground-based and airborne observations offer high precision, their spatial coverage is limited, restricting their application to localized observations. Satellite observation, unaffected by numerous natural conditions, enables continuous, stable, and high-precision observations globally, and provides verification and support for bottom-up emission inventories through a top-down approach.

methane monitoring satellite sensor

Methods for detecting atmospheric CH4 using satellite remote sensing technology can be divided into passive and active remote sensing. Passive remote sensing mainly relies on the 1.65 μm and 2.3 μm short-wave infrared (SWIR) bands and the approximately 8 μm thermal infrared (TIR) ​​band. Accordingly, passive CH4 detection sensors are also divided into thermal infrared sensors and short-wave infrared sensors. Short-wave infrared sensors acquire information by measuring atmospheric backscattering, while thermal infrared sensors measure atmospheric thermal radiation. Active remote sensing, exemplified by the CH4 remote sensing lidar mission (MERLIN), involves sensors that autonomously emit radiation, which is then received after interacting with the atmosphere, thus enabling detection. In the short-wave infrared region, CH4 molecules exhibit many overtones and combination bands, especially near 1.66 μm and 2.3 μm. Using these two bands, CH4 absorption characteristics can be effectively characterized, thereby obtaining CH4 concentration information. Moreover, short-wave infrared is more sensitive to near-ground CH4 concentration changes, compensating for the limitations of thermal infrared. Therefore, most of the spaceborne sensors developed later used shortwave infrared satellites for CH4 detection.

△ Typical methane monitoring satellites at home and abroad

Regional-scale exploration satellites

1. SCIMACHE

The Scanning Imaging Absorption Spectroscopic Atmospheric Mapper (SCIAMACHY), carried by ESA's large environmental monitoring satellite ENVISAT, was the world's first spaceborne detector to use the short-wave infrared absorption band as its detection wavelength. Launched in March 2002, SCIAMACHY features nadir, occultation, and liminal observation modes, enabling effective measurement of the chemical composition of Earth's atmosphere. It is a radiation detector covering the ultraviolet to short-wave infrared spectrum, with eight channels and a wavelength range of 0.24–2.38 micrometers. Channel 6 (1.63–1.67 micrometers) and Channel 8 (2.26–2.38 micrometers) can both detect CH4, with spectral resolutions of 1.48 nm and 0.26 nm, respectively.

2. Japan's GOSAT satellite

The first-generation GOSAT satellite was launched into a sun-synchronous orbit in January 2009, carrying the Thermal Infrared and Near-Infrared Carbon Observatory (TANSO) to achieve global monitoring of greenhouse gases. TANSO consists of a Fourier Transform Spectrometer (FTS) and a Cloud and Aerosol Imager (CAI). The former is used for greenhouse gas detection, while the latter is used to simultaneously collect cloud and aerosol information for coordinated inversion. TANSO-FTS includes four bands: 0.758–0.775, 1.56–1.72, 1.92–2.08, and 5.56–14.3, generally detecting and inverting CH4 around 1.6 micrometers, with a spectral resolution of 0.2 cm⁻¹. The GOSAT-2 satellite, launched in October 2018, directly succeeded the GOSAT satellite and also carries the second-generation Fourier Transform Spectrometer (TANSO-FTS-2). It adds SIF and CO detection channels, improving the signal-to-noise ratio and resulting in higher accuracy for greenhouse gas detection. The sensor has a spectral resolution of less than 0.2 cm⁻¹, covering the ranges of 0.75–0.77 μm, 1.56–1.69 μm, 1.92–2.38 μm, and 5.6–14.30 μm. The detection window for CH₄ is ​​around 1.67 μm, and the target accuracy above a 500 km land grid and a 2000 km ocean grid is 5 × 10⁻⁹. GOSAT and GOSAT-2 demonstrate stable performance and high accuracy, making significant contributions to global greenhouse gas monitoring.

3. ESA Sentinel-5P satellite

Sentinel-5P is a global atmospheric pollutant monitoring satellite launched by ESA on October 13, 2017. It carries a technologically advanced, high-spatial-resolution atmospheric monitoring spectrometer, the Tropospheric Monitor (TROPOMI). This instrument has an imaging swath of 2600 km and a resolution of 7 km × 5.5 km, enabling daily global coverage and significantly improving the satellite's spatiotemporal coverage capabilities. TROPOMI is a pushbroom spectrometer that conducts measurements in three main spectral regions: the ultraviolet-visible spectral region (UVVIS) of 270–495 nm, the near-infrared spectral region (NIR) of 675–775 nm, and the short-wave infrared spectral region (SWIR) of 2305–2385 nm. TROPOMI uses spectral information near 2.3 micrometers for CH4 inversion, achieving a spectral resolution of 0.25 nm and an inversion accuracy of 0.8%.

4. GMI payload of Gaofen-5 satellite

△ Schematic diagram of Gaofen-5 payload

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Launched in May 2018, the GF-5 satellite is the satellite with the most payloads and the highest spectral resolution among my country's major high-resolution scientific and technological projects. It is also the world's first full-spectrum hyperspectral satellite to achieve comprehensive observation of the atmosphere and land. Its Greenhouse Gas Monitor (GMI) employs spatial heterodyne spectroscopy, featuring arbitrary band selection and hyperspectral resolution, making it the world's first spaceborne greenhouse gas remote sensing device based on this technology. The instrument has four bands for O2, CO2, and CH4, covering the spectral range of 0.759–0.769 nm and 1.568–2.058 μm. The CH4 detection channel is located at 1.642–1.658 μm, with a spectral resolution of 0.27 cm⁻¹, aiming to acquire global CH4 column concentration data.

5. FY-3D satellite

△ FY3D GAS Greenhouse Gas Monitor

In January 2019, my country launched its new-generation polar-orbiting meteorological satellite, FY-3D, which carries a near-infrared hyperspectral greenhouse gas monitor (GAS). GAS is China's first spaceborne interferometric greenhouse gas sensor. Its detection method is grating-based spectral dispersion, with two near-infrared bands and two short-wave infrared bands, corresponding to wavelengths of 0.75–0.77 μm, 1.56–1.72 μm, 1.92–2.08 μm, and 2.20–2.38 μm, respectively. Bands 2 and 4 can both be used for CH4 detection, with center wavelengths of 1.6 μm and 2.3 μm, respectively, and a spectral resolution of 0.27 cm⁻¹.

Point source detection satellite

In addition to the aforementioned global or large-scale observation satellites, point source detection has also become a research hotspot. In recent years, point source detection satellites and sensors based on hyperspectral imaging have experienced explosive growth.

1. GHGsat satellite

△ GHGSAT WAF-P monitor

The GHGSat constellation is a satellite system developed by GHGSat Inc. of Canada for high spatial resolution global greenhouse gas detection, primarily targeting the detection and quantification of point sources. The GHGSat constellation currently consists of six satellites: GHGSat-D (Claire), launched in June 2016; GHGSat-C1 (Iris), launched in September 2020; GHGSat-C2 (Hugo), launched in January 2021; and GHGSat-C3 (Luca), C4 (Penny), and C5 (Diako), launched in 2022. Each satellite is equipped with a wide-field Fabry-Perot (WAF-P) imaging spectrometer, covering the 1630–1675 nm short-wave infrared band, for CH4 detection. The GHGSat series satellites boast a spatial resolution of up to 25 m and an XCH4 inversion accuracy of 18 × 10⁻⁹, which is of great significance for high-precision estimation of point source emissions.

2. AHSI payload of Gaofen-5 satellite

△ Schematic diagram of Gaofen-5 payload

The Visible-Shortwave Infrared Hyperspectral Camera (AHSI) aboard my country's GF-5 satellite is a typical example of point source detection. It was the first spaceborne hyperspectral sensor to utilize convex grating spectrophotometry and an improved three-concentric mirror (Offner) configuration. It has 330 bands, covering a spectral range of 0.4–2.5 μm, with spectral resolutions of 5 nm for VNIR and 10 nm for SWIR. The AHSI has a swath width of 60 km and a spatial resolution as high as 30 m. Using spectral channels from 2.11 to 2.45 μm, it can detect the concentration of CH4 from point sources with an accuracy of 63 × 10⁻⁹.

3. PRISMA satellite

PRISMA is a medium-resolution hyperspectral imaging mission developed by the Italian Space Agency (ASI) in 2008, a successor to the terminated Hyperspectral Earth Observation Satellite (HypSEO) mission. Launched on March 22, 2019, the PRISMA satellite, carrying a hyperspectral camera (HYC), covers a spectral range of 400–2505 nm with a spectral resolution better than 10 nm and a spatial resolution of up to 30 m, providing hyperspectral observations in the visible, near-infrared, and short-wave infrared bands. Similar to AHSI on the Gaofen-5 satellite, HYC utilizes a window of 2110–2450 nm to detect CH4 concentration with an accuracy of 149 × 10⁻⁹.

4. ENMAP satellite

The Environmental Mapping and Analysis (EnMAP) program is a German-developed satellite mission designed to acquire hyperspectral imagery data of the Earth's atmosphere and surface. The mission launched its namesake satellite on April 1, 2022.

It is equipped with a hyperspectral imager (HSI). This instrument is a pushbroom imager capable of observing the spectral range of 430–2450 nm. EnMAP has 228 spectral bands set in the VNIR and SWIR channels, with spectral resolutions of 6.5 nm and 10 nm, respectively. The spatial resolution is 30 m, and the maximum swath width is 30 km. Among them, 2200–2400 nm can be used as the detection window for CH4, with a detection accuracy of 3%–7%.

5. EMIT payload

△ The EMIT payload is carried on the International Space Station

Based on NASA's Earth Surface Mineral Dust Source Survey (EMIT) mission, the EMIT payload was launched on July 14, 2022, by SpaceX's 25th Commercial Resupply Services Mission (CRS-25) and installed on the International Space Station. EMIT is a hyperspectral imaging spectrometer covering the spectral range of 380–2500 nm, with a spectral sampling interval of 7.4 nm and a spatial resolution of 60 m. Due to its CH4 characteristic channel of 2200–2400 nm, EMIT can also be used for global CH4 point source detection.

6. MethaneSAT satellite

The MethaneSAT mission, jointly developed by the United States and New Zealand, deploys an Earth observation satellite, MethaneSAT, scheduled for launch in 2023. Its primary objective is to observe the Earth's resources, particularly those accounting for 80% of global oil and gas production.

The above-mentioned regions will be subject to systematic monitoring of CH4 emissions. The MethaneSAT satellite has a swath width of approximately 200 km and a high spatial resolution of 100 m × 400 m, enabling it to obtain quantitative observations of CH4 emissions. MethaneSAT carries a high-performance spectrometer CH4 sensing system covering the spectral range of 1605–1683 nm, with a spectral sampling rate of 0.1 nm and a spectral resolution of 0.3 nm, and a CH4 detection threshold of 2 × 10⁻⁹.

Methane satellite sensor monitoring trends

In the future, CH4 detection satellite sensors will continue to advance towards the goals of high spatiotemporal resolution, high precision, high accuracy, and integrated continuous observation. Meanwhile, since a single satellite is still insufficient to meet the demands of high-precision global CH4 detection, it is necessary to accelerate the development of satellite networks. Observations using a multi-satellite network can achieve higher revisit rates, sensitivity, and accuracy, providing rapid global coverage and further enhancing the remote sensing capabilities of atmospheric CH4 satellites.

To address the need for satellite monitoring of carbon emissions from key point sources, advanced methods such as the Gaussian plume model are employed, integrating high spatiotemporal resolution wind field data with fine-resolution CH4 greenhouse gas satellite products to establish a comprehensive point source emission monitoring and simulation system. This system enables dynamic simulation of greenhouse gas diffusion, achieving high-precision estimation of point source emissions using satellite-observed atmospheric greenhouse gas data, and establishing operational capability.