## 2 Data and Methods
### Satellite Orbit  
The coverage of MS-BiSAR for Earth observation is mainly determined by the positions of both the transmitting and receiving platforms. Therefore, to meet the requirements for observing large-scale geoscience phenomena, a relatively high orbit altitude is necessary for the receiving satellite.  
Figure 3 shows the observation geometry of the MS-BiSAR in the Earth-Centered Inertial Coordinate System (ECI) and the antenna coordinate system (ACS). It can be seen from the figure that different orbit parameters can have an impact on the satellite orbit, thereby affecting the Earth observation characteristics of MS-BiSAR.  
Figure 2: The main work content and steps of this paper.  
This paper primarily considers two types of satellite orbits: geosynchronous orbit and highly elliptical orbit (HEO). The former provides continuous coverage of one-third of the Earth's sphere, while the latter provides better coverage of the Earth's high latitudes.  
Geosynchronous orbit refers to the orbit of an anterograde satellite whose orbital period is equal to the rotation period of the Earth. Its orbital semi-major axis is approximately 42,000 km. Based on orbital inclination, GSO can be divided into geostationary orbit (GEO) and inclined geosynchronous orbit (IGSO). Satellites in GEO have an inclination of zero degrees and are stationary above the equator, providing continuous observation of a fixed coverage area. IGSO has a non-zero inclination and its nadir tracks across the northern and southern hemispheres, with the intersection point being on the equator. Satellites in such orbits provide better coverage at middle and high latitudes than GEO satellites. HEO is an elliptical orbit with a low perigee and an extremely high apogee. With a perigee of less than 1000 km and an apogee higher than 36,000 km, HEO satellites spend a long time passing through apogee and a short time passing through perigee, enabling them to cover the ground area below the apogee for more than 12 h. By adjusting orbit parameters appropriately, HEO satellites are suitable for long-term polar region observations. This paper utilizes Molniya orbits to conduct research on MS-BiSAR with an HEO configuration.  
To simulate satellite orbits, an orbit propagator is required. This paper utilizes Simplified Deep-Space Perturbations-4 (SDP4) orbit propagators [22], which is a solver that calculates the position and velocity of an object whose motion is predominantly influenced by gravity from celestial bodies. By using SDP4 orbit propagators, the position and velocity of satellites with variant orbital parameters in ITRS at a given moment can be determined. Since the satellites discussed in this paper follow quasi-periodic orbits, accurately predicting their long-term orbits poses significant challenges. Therefore, an effective method has been employed in this study to reduce the errors of the long-term predictor by using SDP4 orbit propagators to predict the position and velocity of the satellite within a single orbital period, repeating this process to obtain the motion state of the satellite throughout the entire study period.  
Figure 3: Observation geometry of the MS-BiSAR in the ECI and ACS, where \(u\) is the argument of latitude and \(\theta\) and \(\Omega\) are the orbit inclination and right ascension node, respectively (not to scale for the sake of clarity).