## 4 Discussion  
Different from traditional artificial satellites, a Moon-based platform is equipped with Earth's natural satellite. Our study mainly focuses on the characteristics of the angular combination of Moon-based Earth observations. The observational angles are mainly affected by the positions of the observed point, solar, and lunar nadirs. Thus, we directly derived the observational angles from the analytical orbits of the Moon and Sun. Based on the observational angular model, an ACNI was proposed to characterize the angular performance. Furthermore, we analyzed the effect of the observed point's position and time sampling interval on the angular combination, which is beneficial for selecting observed points with optimal observation angle combinations and an appropriate time sampling interval. When the region of interest is at mid-low latitudes, a smaller sampling interval should be selected to avoid loss of the angular combination coverage. In contrast, if the region is at high latitude, a larger time sampling interval can minimize redundant observations while ensuring minimal angular combination coverage losses.  
The angular sampling characteristic of an observation platform depends on its observation geometry. As shown in Table 2, the GEO satellite has continuous solar angles and constant viewing angles. On the contrary, the L1 satellite has continuous viewing angles and a constant relative azimuth angle. The Moon, as the sole natural satellite, always faces the Earth on the same side because of the tidal lock. As the Earth rotates, a Moon-based  
Figure 10: Angular sampling of **(a)** LEO satellite, **(b)** GEO satellite, **(c)** L1 satellite, **(d)** Moon-based platform over a lunar month. (The polar axis represents the relative azimuth angle, polar diameter represents the viewing zenith angle, and color represents the solar zenith angle).  
sensor can obtain varied and continuous observational angles on a hemispheric-scale view (Figures 4-6). Additionally, the Moon's orbital inclination and altitude are variable. Therefore, a Moon-based sensor can obtain abundant angular combination sampling. In this sense, a Moon-based platform has continuous viewing and solar observational angles, which can complement angular sampling observations from existing satellites.  
As can be inferred from the results, the time sampling interval has a different effect on the angular combination for different observed points. From Figure 8, the difference maps between different time sampling intervals show regular variations. It is found that the time sampling interval significantly affects the angular combination characteristics for the observed points in the mid-low latitude zone. The angular combination of observed points in the high latitude zone is less sensitive to time sampling intervals. Therefore, the Earth's surface can be divided into two major zones, as shown in Figure 11. Zone I is in the mid-low latitude zone, and its latitude varies from 30\({}^{\circ}\)S to 30\({}^{\circ}\)N. For the observed point in Zone I, the angular combination is sensitive, however, to the variety of the time sampling interval. The Zone II consists of two parts, which are between 30\({}^{\circ}\)N and 90\({}^{\circ}\)N, or 30\({}^{\circ}\)S and 90\({}^{\circ}\)S. The observed points in these latitude zones are not sensitive to the variations of the time sampling interval.