## 3 Results
### Spatiotemporal Coverage  
In general, a high earth orbit can offer extensive ground coverage and a relatively short revisit period in the meantime. For example, a Moon-based platform can implement continuous observations of the Earth on a hemispherical scale with a period of approximately one day due to the Earth's rotation and the moon's revolution. As for optical Earth remote sensing, in the case of observing the Earth's facing-Moon disk, most areas of the Earth's hemisphere are visible both at Moon-based platforms and halo orbits, between which a slight difference may exist in the viewing angular characteristics of the Earth disk. However, when it comes to broadside-looking synthetic aperture radar (SAR), the incident angle and azimuth angle need to be limited for data quality. There are relatively many historical studies focused on the spatiotemporal coverage of Moon-based Earth observation [34; 35]. In this section, the effects of the limitations of elevation angle and azimuth angle on the global spatiotemporal coverage of Moon-based SAR as well as the halo orbits are analyzed for comparison. Most of the present spaceborne SAR systems are configured with an incident angle between 20\({}^{\circ}\) and 60\({}^{\circ}\), e.g., RADARSAT-1, RADARSAT-2, TerraSAR-X, and TanDEM-X [37]. This is a little bit harsh for Moon-based SAR. Nevertheless, there are still several satellites configured with a wider range of incident angle, such as ALOS-2, whose range of incident angle is designed between 8\({}^{\circ}\) and 70\({}^{\circ}\). As adopted in previous studies focused on Moon-based SAR [35; 38], for the quality of imaging data, here the incident angle is limited between 15\({}^{\circ}\) and 70\({}^{\circ}\) as an assumption. Moreover, the velocity of halo motion is relatively small compared to the Moon; thus, according to the inverse synthetic aperture imaging mode of Moon-based SAR, the reference azimuth angle, which is the main factor that affects the squint angle, is limited between 30\({}^{\circ}\) and 150\({}^{\circ}\) or between 210\({}^{\circ}\) and 330\({}^{\circ}\). Therefore, with the limitation of incident angle and azimuth angle, the spatiotemporal coverage of the Moon-based platform and halo orbits is calculated. Figure 7 presents the accumulated coverage time of Moon-based platforms (a,b) and halo orbits with a z-direction amplitude of 20,000 km (c,d) during one year from March 2024 to March 2025 (a,c) and from March 2033 to March 2034 (b,d). Results demonstrate the differences induced by the varying declination of lunar orbit in different years and by the halo motion. On the one hand, the comparison of (a) and (b), where the declination of the moon almost reaches the maximum 28.6\({}^{\circ}\) and the minimum 18.3\({}^{\circ}\), reveals the different annual patterns of ground coverage: when the declination of the lunar orbit reaches the maximum, the area with low latitudes from 15\({}^{\circ}\)S to 15\({}^{\circ}\)N are covered most intensively, the mid-latitude zone from 30\({}^{\circ}\) to 50\({}^{\circ}\) is also covered for a considerable time, and the polar area can be observed in at least 755 h during one year. However, when the declination of the lunar orbit reaches the minimum, the ground coverage in a year behaves with a complete pattern, and the coverage time of low-latitude area decreases greatly from 1743 h to 900 h, especially in the equatorial region. Reduction happens in the near-polar region as well, where the north and south poles can be hardly covered during the year. Instead, the mid-latitude area can be covered almost every day with a significantly increased coverage time during the whole year, and the range of latitudes is also extended to 20\({}^{\circ}\) and 60\({}^{\circ}\).