###### Abstract
Fourth, as a permanent platform, the Moon-based observation platform can operate much longer than the spaceborne platform with proper maintenance, which makes it possible to provide long-term consistent calibrated observation data. Fifth, a Moon-based observation station can provide a unique and unparalleled perspective for observing phenomena such as oceanic and solid tides caused by the tidal force of the Moon. Until now, various studies have been conducted on Moon-based Earth observations, and synthetic aperture radar is considered one of the primary sensors for the Moon-based platform [15; 16; 17; 18; 19]. However, the fixed lunar orbits and the viewing angles of Moon-based platforms not only limit the flexibility of interference baselines for Moon-based interferometric synthetic aperture radar (InSAR), but also make it difficult to avoid various electromagnetic interferences from the Earth. In addition, recent studies have suggested that more information can be derived from bistatic scattering than from backscattering [20], but Moon-based monostatic SAR can only receive the backscattered signal of the target's scattering echo. Meanwhile, due to the lack of forward-looking imaging and nadir imaging capabilities, Moon-based monostatic SAR may have limitations in observing some regions of interest, making it difficult to meet the requirement of continuous monitoring over time for a given study area. Moreover, the long distance between the Moon and the Earth brings large propagation losses, requiring the radar system to produce hundreds of kilowatts of transmission power, which brings great challenges to engineering implementations. In accordance with these problems, the MS-BiSAR with a Moon-based transmitter and a high-orbit satellite-based receiver for Earth observation is put forward in this paper. Figure 1 shows a conceptual illustration of MS-BiSAR Earth observation. Compared with Moon-based monostatic SAR, MS-BiSAR can achieve more flexible interferometric baselines to obtain three-dimensional surface information. A flexible position of the receiving satellite can also avoid interference signals from the Earth. At the same time, MS-BiSAR has the capability to receive bistatic scattering echoes, which expands the potential applications of surface parameter inversion. By designing the receiving satellite orbit, BiSAR can adjust the imaging area and achieve forward and subsatellite point imaging that cannot be achieved by monostatic SAR, which is beneficial for the continuous large-scale monitoring of a study area. Finally, the shorter propagation distance of MS-BiSAR can significantly reduce the demand for transmission power, and in some cases even reduce the transmission power by two orders of magnitude, compared to Moon-based monostatic SAR. To better understand the capability of MS-BiSAR for Earth observation, it is necessary to accurately calculate its monitoring capability on the Earth at different times with multiple configurations in order to obtain high-quality homogenized data on a global scale and support the analysis and prediction of complex geoscience phenomena, such as environmental changes and solid Earth motion. Therefore, analyzing the geometric characteristics and observation coverage of Earth observation systems is crucial for the acquisition of global Earth observation data and a better understanding of the Earth system. In this paper, three contributions are made. Firstly, we develop a reference systems transformation approach for MS-BiSAR Earth observation. Secondly, we establish the geometric model of MS SAR. Finally, we determine the conditions under which an area on Earth can be observed by MS-BiSAR and calculate as well as analyze the coverage for three configurations of MS-BiSAR Earth observation.