## 2 Materials and Methods  
### Simulation Datasets  
Assuming that the surface is Lambertian, the top-of-atmosphere (TOA) radiance over a vegetation target can be expressed as the following radiative transfer equation [22; 23]:  
\[\mathrm{L_{TOA}=L_{0}+\frac{L_{TOC}\cdot\rho_{s}\cdot T_{\uparrow}}{1-S\cdot\rho_ {s}}+\frac{SIF\cdot T_{\uparrow}}{1-S\cdot\rho_{s}}} \tag{1}\]  
where \(\mathrm{L_{0}}\) is the atmospheric path radiance, \(\mathrm{L_{TOC}}\) is the radiance reaching the surface, \(\rho_{s}\) is the surface reflectance, \(S\) is the atmospheric spherical albedo, \(T_{\uparrow}\) is the upward atmospheric transmittance, and SIF is the emitted fluorescence signal at the top-of-canopy (TOC), which should be omitted when simulating radiance over the non-vegetated surface. The Soil Canopy Observation Photosynthesis and Energy (SCOPE) [24] model was employed to simulate the emitted fluorescence signals and reflectance of vegetation under 126 different canopy structures and leaf biochemic characteristics. Ten different reflectance spectra over snow and soil surface were derived from the spectral library of the Environment for Visualizing Images (ENVI) [25]. Three constant reflectance spectra were set in the Moderate-resolution atmospheric TRANsmission (MODTRAN) [26; 27] model to calculate the atmospheric radiation transfer parameters (\(\mathrm{L_{0}}\), \(\mathrm{L_{TOC}}\), and S) under 1280 different atmospheric and observation conditions. The input parameters of the two models are shown in Table 2. Figure 1 shows the simulated canopy SIF spectra of different canopy and leaf conditions in the range of 640-800 nm. The fluorescence spectra have a bimodal distribution, with a more obvious peak at around 740 nm. The spectra of the atmospheric radiation transfer parameters (\(\mathrm{L_{0}}\), \(\mathrm{L_{TOC}}\), S, and \(\mathrm{T_{\uparrow}}\)) simulated by MODTRAN have a spectral resolution of 0.005 nm. The canopy SIF and the reflectance spectra over vegetated and non-vegetated surfaces were then resampled to the same interval. Then, the resampled SIF and reflectance spectra were used to determine the TOA radiance spectra according to Equation (1). Finally, 161,280 simulated radiance spectra over the vegetated surface were generated as the test dataset, and 12,800 simulated radiance spectra over the non-vegetated surface were generated as the training dataset. \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameters of MODTRAN5** & **Value** \\ \hline Atmospheric temperature profile & middle latitude summer/winter \\ Total column water vapor (g cm\({}^{-2}\)) & 0.5, 1.5, 2.5, 4 \\ View zenith angle (degree) & 0, 16 \\ Final altitude (km) & 0.01, 0.05, 1, 2 \\ Aerosol optical thickness at 550 nm (km) & 0.05, 0.12, 0.2, 0.3, 0.4 \\ Solar zenith angle (degree) & 15, 30, 45, 70 \\ \hline
**Parameters of SCOPE** & **Value** \\ \hline Leaf area index (LAI) & 0.5, 1, 2, 3, 4, 5, 7 \\ Fluorescence quantum efficiency (fqe) & 0.01, 0.02, 0.04 \\ Chlorophyll content (Cab) (\(\mu\)g cm\({}^{-2}\)) & 20, 30, 40, 50, 60, 80 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Look–up table (LUT) for the Moderate-resolution atmospheric TRANsmission (MODTRAN) and the Soil Canopy Observation Photosynthesis and Energy (SCOPE) models. Simulated radiance spectra with a high spectral resolution of 0.005 nm were then convolved and resampled according to the specifications of SIFIS (a spectral resolution of 0.3 nm and a sampling interval of 0.1 nm). Figure 2 shows a set of TECIS-like simulated spectra, including the canopy radiance spectra (Rad_Veg), solar irradiance spectra (Solar_Irr), upward atmospheric transmittance spectra (T_u), and SIF signal at the top-of-canopy and atmosphere. As can be seen in Figure 2, the absorption lines are obvious under a spectral resolution of 0.3 nm, and the radiance spectra are affected by the absorption of solar atmosphere and Earth's atmosphere. Since SIF is a weak signal, its retrieval is very sensitive to noise [13; 28; 29].