## 2 Polarization State and Observation Analysis
### Wavelength Selection  
The sunlight reflected by the Earth not only has polarization characteristics but also has wavelength selectivity. The optical properties (such as scattering, absorption, reflection) and microphysical properties (such as particle size, shape, concentration) of the light radiated by different observation targets are all functions of the wavelength. For example, at certain wavelengths, the scattering and absorption of light by aerosol particles will show very different characteristics, and the scattering characteristics of cloud droplets will also change with the wavelength [12; 16]. Through the comprehensive utilization of polarization information in different spectral bands, the inversion accuracy of different  
Figure 2: A schematic diagram of the Earth–Moon system observed from the Earth–Moon L1 point. target characteristic parameters in Earth observation can be effectively improved. This is because the polarization information of different bands can reflect the characteristics of the target from multiple dimensions, and can obtain more comprehensive and accurate information, so as to reduce errors in the inversion of parameters such as aerosol optical thickness and the effective radius of cloud droplets. Considering the practical application requirements of the Earth's surface information, atmospheric aerosols and cloud optical and microphysical properties, we refer in depth to the band selection of other orbital polarization detectors [16; 17], such as the detailed data shown in Table 1. After rigorous screening of a large amount of existing data, a total of seven bands were selected. On this basis, we innovatively propose two new bands for measurement. Among them, the band near 532 nm has unique optical properties and shows strong penetration to light. In the field of vegetation monitoring, this band has extremely important application value. It can be used to estimate key indicators such as vegetation coverage and leaf area index. Vegetation coverage reflects the degree of vegetation coverage on the Earth's surface. For the global carbon cycle, the accurate estimation of its coverage is helpful to accurately evaluate carbon absorption and then provide indispensable data support for global carbon cycle and ecological balance research. The 600 nm band shows a significant advantage in the detection of soil characteristics. Through the measurement and analysis of the polarization information of this band, a variety of key characteristic information of the soil can be accurately obtained, which has important guiding significance for agricultural production planning. In summary, the whole instrument is finally determined to have a total of nine bands. The specific selection of each band and its corresponding detailed uses are clearly listed in Table 2. The comprehensive application of these bands will provide a comprehensive and accurate database for ground polarization measurements, and promote the research and application of related fields. \begin{table}
\begin{tabular}{c c c} \hline
**Satellite/Sensor** & **Country** & **Band Center (nm)** \\ \hline DPC & China & 490; 670; 865 \\ APS & U.S.A. & 412; 443; 555; 672; 865; 910; 1370; 1610; 2200 \\ POSP & China & 380; 410; 443; 490; 670; 865 \\
3 MI & Eumetsat & 410; 443; 490; 555; 670; 865; 1370; 1650; 2130 \\ MAI & China & 560; 670; 865 \\ CAPI & Japan & 670; 1640 \\ HARP2 & NASA & 440; 550; 670; 870 \\ POLDER-1,2 & Japan & 443; 670; 865 \\ POLDER-3 & France & 490; 670; 865 \\ \hline \end{tabular}
\end{table} 
Table 1: Polarization band selection of on-orbit instrument.