Methane-borne Spaceborne Remote Sensing Payloads: FP Interferometry and Applications

In current global climate change research, accurate monitoring of atmospheric greenhouse gas concentrations is a crucial foundation for constructing a carbon emission reduction assessment system. Methane (CH4), as a gas with extremely high warming potential, exhibits significant transient and spatially discrete emission sources, placing higher demands on the spatial resolution, spectral resolution, and detection sensitivity of spaceborne remote sensing payloads. Among numerous technological approaches, remote sensing technology based on the Fabry-Pérot (FP) interferometry principle, with its high light throughput, miniaturized structure, and excellent narrowband spectral selectivity, is gradually becoming an important solution for high-resolution methane remote sensing.

The core physical model of the Fabry-Perot interferometer is multi-beam interference. As shown in Figure 1, when sunlight reflected into a beam composed of two highly reflective parallel plane mirrors...resonant cavityAt this time, the light beam is reflected multiple times within the cavity, and the intensity distribution of the final emitted light follows the well-known...Airy function（Airy Function）。

Figure 1: Schematic diagram of FP interferometer

The relationship between its transmittance T and the incident light wavelength can be expressed as follows:

In this formula, F represents the fineness coefficient (Finesse) of the interferometer, which directly determines the sharpness of the transmission peak; n is the refractive index of the cavity medium; L is the cavity length; θ is the incident angle of the light; and λ is the wavelength. As can be seen from the formula, the FP interferometer is essentially a wavelength selector, allowing only wavelengths that meet specific phase-matching conditions to pass through efficiently, thereby forming a series of periodic narrow transmission peaks in the spectrum, the so-called "interference comb".

Figure 2:FP interferometer transmission curve

The key to applying FP interferometers to methane monitoring lies in the precise coupling of their spectral characteristics with those of methane absorption. Methane molecules exhibit dense absorption lines in the short-wave infrared band (SWIR, especially around 1.6 μm or 2.3 μm). By precisely controlling the cavity length L of the interferometer, the transmission peak frequency of the FP interferometer can be precisely aligned with the characteristic absorption lines of the methane molecules. This design concept is similar to "fingerprint recognition" in optics: when methane is present in the atmosphere, solar radiation attenuates as it passes through the absorption lines, and the FP interferometer precisely captures the signal changes at these specific locations. By comparing the energy difference between the reference channel and the measurement channel, the system can extract methane concentration information with an extremely high signal-to-noise ratio.

Compared to traditional distributed grating spectrometers, FP interferometry offers a significant "Jacquinot Advantage": at the same spectral resolution, its light throughput is far higher than that of slit systems, achieving a narrow-band spectral response while maintaining a high aperture. This results in better sensitivity for the payload when observing weak signals. Furthermore, FP systems can achieve spectral scanning by changing the cavity length or tilt angle, and can also be combined with area array detectors.Staring ImagingIn spaceborne applications, this compact structure significantly reduces payload weight and power consumption, enabling the deployment of satellite constellations with high revisit rates.

In actual spaceborne quantitative inversion processes, the observed radiation intensity is not solely determined by methane concentration. The signal I(λ) received by the detector is a combined convolution of surface reflectivity, atmospheric aerosol scattering, water vapor interference, and the instrument line shape function (ILS). The inversion algorithm requires the use of...Radiative transfer model(RTM) constructs a forward simulation and minimizes the residual between the simulated and observed values ​​through optimization algorithms such as nonlinear least squares.

In the aforementioned cost function, yobs represents the measured spectrum, and F(x,b) represents the simulated spectrum generated by the forward model. By continuously correcting the state vector x (including methane column concentration, surface pressure, etc.), the average atmospheric column mixing ratio XCH4 is finally obtained. This inversion process requires extremely high wavelength calibration accuracy, typically requiring the FP cavity length to achieve picometer (pm) stability in the space environment.

With the development of high-performance optical coatings andActive thermal control technologyWith the maturation of FP interferometers, spaceborne remote sensing payloads have achieved a leap from scientific demonstration to engineering application. They can not only monitor large-scale global methane background fields, but also demonstrate the potential to capture micro-scale events such as industrial point source leaks and fugitive emissions from mining areas. This technological solution provides objective and transparent data support for the implementation of global greenhouse gas emission reduction agreements and is a typical example of how spaceborne remote sensing technology serves climate governance.