## 1 Introduction  
The Multispectral Earth Polarization Observation System is an indispensable instrument for analyzing the polarization information of the Earth's reflected sunlight. The livability of the Earth is the result of the exquisite synergy of complex systems such as geological structure, atmospheric circulation, marine ecology, and biological evolution. Earth polarization observation, with its keen capture of the microscopic characteristics of each layer of the Earth, breaks the limitations of traditional spectral observation. With its high sensitivity and accuracy, it has become the core pillar technology for understanding global climate change, evaluating ecosystems, and exploring livable planets [1, 2, 3]. The Multispectral Earth Polarization Observation System is located at the Earth-Moon system's Lagrange point L1, which is about 327,000 km away from the Earth. At this position, the Earth's rotation characteristics can be used to realize the stable observation of the whole Earth, and polarization observation is carried out on a daily time scale. The complete Earth image information can be obtained at the same time at the L1 point, which effectively guarantees the time synchronization of data in different regions [4]. The Earth-Moon L1 point is a special position with great scientific research value. At this point, the polarized spectral data generated by the Moon's reflection of sunlight can be effectively obtained. The analysis of polarization information can accurately identify typical terrain features such as craters and impact debris on the lunar surface. Secondly, due to its high stability of reflectivity and emissivity, the Moon has become an ideal radiation source for on-orbit calibration and attenuation inspection. In the practice of polarimeter calibration, the polarization state of the Moon's reflected light is used as a reference, and through accurate comparison and calculation, the measurement deviation of the polarimeter can be effectively corrected and the measurement accuracy can be significantly improved. Therefore, the Moon can be regarded as an extended linearly polarized plane source for the relative calibration of polarimeters [5; 6; 7]. From the system point of view, located at the L1 point of the Earth and the Moon, by integrating the polarization characteristics data of the Earth's atmosphere and the polarization spectrum information of the Moon, a more perfect and accurate Earth-Moon system model can be constructed, which provides a solid data support and theoretical basis for the interdisciplinary research of astronomy, earth science, space physics and other disciplines. So far, polarization remote sensing technology has been used to detect and image the Earth, such as DPC of China, HARP2 of NASA, POLDER of France and so on [8; 9; 10]. However, the current instrument is too small; it cannot fully obtain the desired multi-spectral information, polarization information, and images of the Earth [9]. In the polarization detection technology scheme, the American HARP2 instrument and the French POLDER instrument have adopted two completely different technical routes. The former uses the amplitude-division method combined with a narrow-band filter to measure, and it uses a prism to separate the light of different polarization states in space and guide the three detectors, thereby simultaneously capturing images of the three polarization states. The POLDER instrument in France is mainly based on a wide-field telescope optical system, and the polarization measurement is carried out by using rotating polarizer technology combined with a single detector [10]. These two measurement techniques have inevitable problems. Polarization measurement by prism beam splitting depends on a multi-detector array. Each detector unit has inherent differences in sensitivity, noise and other properties, which makes it difficult to be fully calibrated. This affects the accuracy and consistency of polarization measurements and limits its application in high-precision tasks.