## 4 Conclusions  
In this paper, we propose the concept of using a Moon-based transmitter and a high-orbit satellite-based receiver to form a bistatic SAR for Earth observation, called MS-BiSAR, and analyze its geometric characteristics and observation coverage to meet the demand for temporally consistent and spatially continuous Earth observation data on a global scale in geoscience research. A geometric model of the MS-BiSAR's Earth observation was established using JPL ephemerides, Earth orientation parameters (EOP), and SDP4 orbit propagators with reference system transformations, and its geometric characteristics were analyzed. The motion of the Moon has both long-term (18.6 years) and short-term (27.3 days) periods. During these periodic motions, the nadir point of the Moon-based observatory moves across different latitudes, which affects the geometric characteristics of Earth observation. The changes in the nadir points of different satellite orbits vary, resulting in differences in the coverage capability of different satellites. For the GSO orbit, its nadir point always moves along a fixed longitude and latitude range on the Earth's surface, and by increasing the orbit inclination, its latitude range can be expanded. For the HEO orbit, the subsatellite points are mainly distributed in the mid-to-high latitudes of one hemisphere throughout one orbital period, with a very short duration of time for other areas. Based on the geometric model, the accumulated observable times of three different platform configurations within a year were calculated and analyzed. The results show that MS-BiSAR with GSO satellite can provide good coverage over a third of the Earth's sphere, and its coverage can be increased at middle and high latitudes with higher orbital inclination. For the configuration of the HEO satellite platform and Moon-based platform, \begin{table}
\begin{tabular}{l c c c c c c c c c c c c c c c c c c} \hline \hline \multicolumn{1}{c}{**Location**} & \multicolumn{1}{c}{**Acousal Accomulated Observable Time (Observed)**} & \multicolumn{1}{c}{**CV**} \\ \cline{2-13} \multicolumn{1}{c}{} & 2004 & 2005 & 2006 & 2007 & 2008 & 2009 & 2010 & 2011 & 2012 & 2013 & 2014 & 2015 & 2016 & 2017 & 2018 & 2019 & 2020 & 2017 & 2022 \\ \hline \(10^{8}\)-\(15^{\circ}\) & 344.5 & 206.3 & 230.7 & 304.2 & 240.7 & 237.7 & 253.8 & 191.5 & 184.8 & 132.8 & 96.0 & 98.5 & 79.3 & 92.0 & 146.0 & 156.8 & 196.3 & 2018.6 & 275.5 & 48.3/5s \\ \(25^{\circ}\) & 793.5 & 603.7 & 612.5 & 67.0 & 543.7 & 560.8 & 591.5 & 505.5 & 567.0 & 552.3 & 691.2 & 521.8 & 499.3 & 448.0 & 582.2 & 502.3 & 554.5 & 643.3 & 12.0/5s \\ \(40^{\circ}\) & 1112.7 & 1048.5 & 510.22 & 1003.5 & 906.3 & 973.7 & 1005.5 & 998.8 & 1077.0 & 1071.0 & 1012.3 & 1067.7 & 1082.2 & 998.5 & 1012.7 & 968.3 & 1008.3 & 1003.3 & 1003.0 & 4.6/5s \\ \(30^{\circ}\) & 1175.1 & 1723.0 & 1068.7 & 1825.0 & 1066.2 & 1396.0 & 1606.3 & 1716.5 & 1607.5 & 1773.7 & 1772.3 & 1738.8 & 1734.5 & 1702.7 & 1758.2 & 1698.3 & 1701.3 & 1726.7 & 1660.0 & 5.0/5s \\ \(30^{\circ}\) & 1197.5 & 1294.8 & 2306.8 & 2304.2 & 2362.7 & 2226.8 & 2204.7 & 2320.5 & 2203.5 & 2317.7 & 2302.7 & 2304.7 & 2306.5 & 2303.7 & 2301.0 & 2285.3 & 2303.8 & 2278.3 & 2269.8 & 2252.7 & 1.3/5s \\ \(30^{\circ}\) & 2774.5 & 2748.5 & 2772.7 & 2773.7 & 2773.2 & 2778.7 & 2770.5 & 2807.5 & 2805.5 & 2803.5 & 2804.8 & 2807.5 & 2805.7 & 2805.7 & 2805.3 & 2805.3 & 2801.8 & 2806.2 & 2798.3 & 2778.0 & 0.9/5s \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Annual accumulated observable time of MS-BiSAR (GEO configuration) for different locations in one lunar precession period. better coverage of polar regions can be achieved. However, none of these configurations can achieve complete coverage of the Earth. To achieve global observation, a receiving constellation consisting of multiple high-orbital satellites is needed. Finally, this paper took the IGSO satellite receiver as an example and calculated the accumulated observable time of MS-BiSAR for different latitude ground areas during one nodal regression cycle.