## 5 Discussion  
In this paper, we introduce the MEPI based on the Lagrange L1 point of the Earth-Moon system, which can measure the multi-spectral polarization of the entire Earth and has the ability to produce images. The system provides critical multi-spectral polarization data for various applications, such as Earth cloud and aerosol properties, global water cycle monitoring, and extrasolar planetary exploration. The MEPI uses a CCD detector, the optical system transmittance is 0.63, the integration time is 1 s, and F is 7.6. We calculated the signal-to-noise ratio of the system when measuring the Earth. The calculation results show that the SNR of the system is better than 400; as shown in Figure 11, it has good observation performance. As a stable and easy-to-observe natural celestial body, the Moon is an ideal polarization calibration source [5; 6; 7]. In the field of polarization calibration, the Moon shows significant advantages: (1) Stability: the polarization characteristics of the Moon's reflected sunlight fluctuate very little in a long time scale, which provides a very stable reference for polarization calibration, ensures the reliability of the reference signal during the calibration process, and effectively reduces the measurement error caused by the instability of the calibration source; (2) Predictability: The parameters such as the trajectory and phase change of the Moon can be accurately predicted, so that researchers can reasonably arrange the time and conditions of calibration observation, which greatly improves the efficiency and success rate of calibration observation. (3) Wide-band response: The spectral coverage of the lunar reflected light is wide, which can be highly adapted to the needs of multi-channel observation systems. The lunar reflected light provides a rich and comprehensive data source, which helps to comprehensively evaluate the polarization response characteristics of the system in different spectral ranges [21]. Using the Moon as the calibration source is the key to verify the reliability and accuracy of the polarization performance of the system. In the on-orbit calibration, the polarization performance of the instrument is evaluated from the dimensions of polarization degree, polarization angle and channel consistency. According to the orbital parameters and phase changes of the Moon, the phase angles (such as 0\({}^{\circ}\), 30\({}^{\circ}\), 60\({}^{\circ}\), 90\({}^{\circ}\)) are selected as the observation window to cover the range of polarization characteristics [22], and the random error is reduced by repeated observations at each observation point. The difference between the lunar polarization degree and polarization angle measured by the system and the theoretical value is compared, and the measurement accuracy and polarization  
Figure 11: SNR in Earth observation. direction judgment accuracy are accurately evaluated. By comparing the consistency of the observation results of the three channels in the same area of the Moon, the channel performance balance is judged to help the instrument performance optimization. At the same time, because the reflection characteristics of the lunar surface are stable and the radiation characteristics can be accurately calculated or measured, the satellite observes the sunlight reflected by the Moon and compares it with the known radiation model or reference data to determine the radiation response characteristics of the sensor and realize the calibration of the observation data. This calibration method provides a stable calibration target, realizes multiple calibrations to reduce errors and improve accuracy, and meets the requirements of multi-spectral calibration. There are many difficulties in arranging the observation system at the L1 point of the Earth-Moon. The position of this point is unstable, and a correction model must be constructed to continuously correct the position [23; 24]. The radiation intensity of the universe is far greater than that of the radiation of the Earth.