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A simulation model of a methane spectrointerferometer based on GHGSat-D was established.
background
Methane (CH₄) is the world's second-largest greenhouse gas, with an atmospheric lifetime of only about 10 years. Targeted emission reductions can rapidly mitigate short-term global warming, and 60% to 90% of methane is anthropogenic.Point source emissionsConcentrating on a few large emission sources, high-precision point source identification and observation are crucial for methane emission reduction. This paper uses GHGSat-D, the world's first methane point source detection microsatellite launched in 2016, as a reference, and focuses on its core wide-angle fixed cavity.Fabry-PérotFor the imaging spectrometer payload, a numerical simulation model was built that covers the complete signal link of solar radiation, atmospheric transmission and detector output.
FP interferometerprinciple
The FP etalon consists of two optical plates spaced d apart and with an inner surface reflectivity R. The transmittance of light at the incident angle θ and wavelength λ is determined by...Airy functiondescribe:
Where n is the refractive index of the intercavity medium, and F is the fineness coefficient related to reflectivity. GHGSat-D employs a fixed cavity length design, relying on variations in the incident angle to achieve spectral tuning. For the m-th order interference, the wavelength corresponding to the transmission peak satisfies the interference maxima condition:
Simulation model end-to-end
The numerical simulation model built in this paper covers the complete signal flow of satellite observation, reproducing the entire link transmission process from solar radiation incident to detector output. The overall model consists of...Atmospheric radiative transferThe system consists of three main modules connected in series: the interferometer optical system, the detector response, and so on. The architecture is shown in Figure 1.
Figure 1.Simulation model full-link architecture diagram
Atmospheric radiation transfer module
This module primarily calculates the atmospheric top spectral radiance reaching the satellite entrance pupil after atmospheric absorption and attenuation. The model assumes the Earth's surface is a Lambertian reflector and calculates the radiation reference for surface reflection by combining the solar zenith angle (θsza), Earth-Sun distance (RE-S), and surface spectral reflectance (a). For typical shortwave infrared methane observation scenarios, the model neglects the effects of atmospheric scattering and surface thermal emission to simplify the calculation; the calculation formula is as follows:
Atmospheric absorption coefficients were obtained from the HITRAN spectral database and integrated layer by layer using a stratified atmospheric model. The atmospheric temperature, humidity, and pressure profiles were obtained using a standard atmospheric model, ultimately yielding the atmospheric top spectral radiance for the target band.
Optical systems and signal generation
This module simulates the propagation and modulation of light through an optical system, mapping the atmospheric top radiance to the detector image plane. The entire optical system consists of a telephoto lens group and an imaging lens, as shown in Figure 2. Correspondingly on the detector plane, the pixel coordinates (i,j) and the polar angle and azimuth angle of the incident angle satisfy the following mapping relationship:
Where (i₀, j₀) is the center pixel of the optical axis, ψ is the polar angle of the incident angle, and φ is the azimuth angle. In a single frame of a two-dimensional image, the signal exhibits a regular change with the off-axis radius: the image center corresponds to the longest transmission wavelength FP, and the transmission peak continuously shifts towards shorter wavelengths as the radius increases; at the image edges, the shortest wavelength transmission peak shifts out of the bandpass range, resulting in spectral overlap between adjacent interference orders.
Figure 2.Schematic diagram of the GHGSat-D optical system
(Image source: The GHGSat-D imaging spectrometer.)
(i) OSF, (ii) Fabry-Perot interferometer and (iii) detector.
Simulation output interferogram features
This module can directly output the two-dimensional interferometric pattern of the detector plane, that is, the original signal shape of a single frame observation.
Geometrically, the interference pattern is distributed in concentric rings centered on the optical axis: light rays on the same radius have the same incident angle and correspond to the same transmission peak wavelength. In terms of intensity distribution, the interference rings are formed by the FP interference effect, characterized by narrow transmission peaks and high contrast. Simultaneously, due to atmospheric methane absorption modulation, the brightness of the rings at positions corresponding to strong absorption lines shows a significant attenuation; the brightness fluctuations of the rings are essentially a projection of the atmospheric absorption spectrum into the spatial dimension.
Summarize
In summary, this paper uses the GHGSat-D satellite payload as a model to construct a complete full-link simulation framework for a methane FP imaging spectrometer. This simulation framework can provide reliable numerical support for the subsequent optimization of FP-type spectral payload indicators and the development of interferogram demodulation algorithms.
References
[1] Jervis D, et al. The GHGSat-D imaging spectrometer. *Atmos. Meas. Tech.*, 2021.
[2] Sloan J J, et al. Fabry–Perot interferometer based satellite detection of atmospheric trace gases. U.S. Patent 9,228,897, 2016.
[3] Rodgers C D. *Inverse Methods for Atmospheric Sounding*. World Scientific, 2000.
[4] Varon D J, et al. Quantifying methane point sources from fine-scale satellite observations. *Atmos. Meas. Tech.*, 2018.
[5] Jacob D J, et al. Satellite observations of atmospheric methane. *Atmos. Chem. Phys.*, 2016.