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Author: Admin | 2025-04-28
On the auxiliary ground professional experiments. An alternate remote sensing approach for CH4 emissions detection is band ratio analysis, such as that used for fire detection [28], which has low computational demands and does not depend on atmospheric and surface parameterizations. Due to the wavelength dependence of atmospheric gas absorption coefficients, negative anomalies in the band ratio images can be indicative of increased gas absorption at the numerator wavelength or decreased gas absorption for the denominator. In this study, the band ratio namely RCH4, has the form: RCH4 = Rabsorbing/Rwindow (2) Here, Rabsorbing is the radiance data at the CH4 absorptions band, and Rwindow is the radiance data at the CH4 window band.It is necessary to note that the band ratio method uses the SASI radiance data that include the radiance of ground objects and atmospheric path radiance (Formula (3)) [29]. Radiance data were not processed by atmospheric correction, also reserved the radiation contribution of atmospheric CH4. The radiance curves show obvious absorption valley characteristics near the absorption bands of CH4 and H2O etc., because the scattering effect of atmospheric aerosols and molecules makes the spectrum curve show strong radiation values in the short wavelength. L = ρ E τ π + L P L G = ρ E π (3) where L is total radiance measured by the sensor; LP is atmospheric path radiance; LG is radiance of ground object; ρ is reflectance of the ground object; τ is atmospheric transmittance; E is irradiance of ground object.CH4 atmospheric window is the spectral bands with transmittance close to 1, such as the transmittance spectra in 2100–2150 nm as shown in Figure 2b. CH4 atmospheric window band with less absorption influence of CO2 and H2O should be selected for band ratio calculation. It is critical to obtain CH4 absorption band (without interference of other information) from the absorptions window between 2200 nm and 2400 nm, as shown in Figure 2b, using the spectral analysis method. This study determines the absorption characteristic band of CH4 by the singularity detection processed with a wavelet transform [30,31], which is performed through four steps (Figure 3): (1)Processing the spectrum with envelope removal can effectively highlight the absorption, reflection and emission characteristics of the spectral curve, and can normalize them to a consistent spectral background, which is conducive to the comparison of characteristic values with other spectral curves [32]. First is the envelope calculation of the
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