## 3 Results
### Distribution of Viewing Angles  
The elevation angle and azimuth angle are very important angular parameters in remote sensing observation of the Earth, both for optical and radar remote sensing. The elevation angle is the angle between the line-of-sight direction and the local horizontal plane, which is the residual angle of the zenith angle. As defined above, the azimuth angle is the angle between the projection of the line-of-sight direction on the local horizontal plane and the reference direction, which is set as east for the convenience of visualization. In this section, a series of experiments was carried out to analyze the distributions of viewing elevation angle and azimuth angle during the period from 20 March 2024 to 20 March 2025 at four locations with different attitudes on the prime meridian. As contrasted with the Moon-based platform (a), the viewing angular characteristics of two representative halo orbits, of which the parameters of orbital amplitude are set at 10,000 km (b) and 20,000 km (c), are shown in Figure 6. The pre-test shows that the differences in angular distribution between different longitudes are small, and it is symmetrical in the northern and southern hemispheres. Thus, with the sampling interval set at an hour, the distribution of the combination of elevation angle and azimuth angle at four representative locations with different latitudes, namely 0\({}^{\circ}\)N, 28\({}^{\circ}\)N, 66\({}^{\circ}\)N, and 90\({}^{\circ}\)N, are presented in the polar coordinate system, where the polar radius and polar angle corresponds to elevation angle and azimuth angle, respectively. In order to maximize the observation angle combination of Moon-based Earth observation in the polar region, the one-year period selected in this experiment is when the lunar orbital inclination is close to the maximum value of 28.5\({}^{\circ}\). On the one hand, the results demonstrated that at low latitudes, there is little difference in the combination of angular distribution between Moon-based platforms and halo orbits throughout the year, where for both of them, the azimuth angle during one year covers the whole range from 0\({}^{\circ}\) to 360\({}^{\circ}\) and the elevation angle during one year also covers the whole range from 0\({}^{\circ}\) to 90\({}^{\circ}\). While at critical latitude near 28\({}^{\circ}\)N, the viewing azimuth angle of the Moon-based platform is limited from 180\({}^{\circ}\) to 360\({}^{\circ}\). Moreover, for areas of interest exceeding 28\({}^{\circ}\)N, the data observed from the whole north side is missing. In the case of a halo orbit, especially of greater z-directional amplitude, the critical latitude could be extended to over 42\({}^{\circ}\)N, depending on the designed orbit. Thus, generally, the range of elevation angle and azimuth angle can both be extended by halo orbits. On the other hand, the period of the halo orbit around L1 of the Earth-Moon system is shorter than half of the Moon's orbital period (e.g., the period for the chosen halo orbit with a 20,000 km amplitude is approximately 12 days), as a result of which the revisiting period of the nadir point can be reduced compared to the Moon-based platform. Generally, the Moon's halo orbits around L1 are expected to enhance the ability of Moon-based Earth observation to some extent with the complementation of an extended viewing angle and a shortened period of nadir point revisiting. For example, one of the scientific aims of Moon-based Earth observation is to understand the Earth's radiation  
Figure 5: Variation of FOV from 20 March 2024 to 20 March 2025. (**a**) halo orbit with an amplitude of around 10,000 km; (**b**) halo orbit with an amplitude of around 20,000 km; (**c**) mean FOV varying with z-directional amplitude. budget and its interaction with global climate change, which is focused on the changes in outgoing radiation at the top of the atmosphere (TOA).