## 1 Introduction  
Low Earth Orbit (LEO) remote sensing missions present a main limitation regarding their revisit time of several days or weeks. They cannot provide continuous monitoring over the same area of the planet. The introduction of the Geosynchronous Synthetic Aperture Radar (GEOSAR) aims to mitigate this limitation. The main strength of GEOSAR is the ability to monitor continental regions with a revisit time of less than 24 h owing to the nature of the Geosynchronous Orbit (GEO). Radar systems are sensitive to the backscatter of Earth's surface, as well as the refractive index of the atmosphere. Therefore, GEOSAR is able to provide continuous monitoring of Earth's surface and atmosphere. Potential applications that require continuous monitoring are, for instance: flooding monitoring and forecasting, glacier motion and snow cover monitoring, earthquakes, volcano and landslide forecasting, and subsidence and infrastructure deformation monitoring. Several geosynchronous SAR remote sensing missions have been considered in the past or are being studied presently. The first GEOSAR proposal considered different orbital configurations including a Near-Zero Inclination (NZI) scheme [1]. The principle of operation of GEOSAR is based on the daily orbits described by the satellite over a continent. NZI GEO satellites present an inherent limited motion with respect to Earth in the order of hundreds of kilometers. This allows the spacecraft to receive the echoes from the surface in different satellite orbital positions and to coherently add them in order to form synthetic apertures of a few hours. This is sufficient to form images with a resolution of dozens of meters [2]. Otherwise, GEOSAR missions would not be feasible since they would require forming synthetic apertures artificially by means of a fuel-spending station-keeping process. As a consequence, this would reduce the lifetime of the mission. Morerecently, two NZI GEOSAR proposals were submitted to the Earth Explorer (EE) program of the European Space Agency (ESA), including EE-9 and EE-10 [3; 4]. The latter (G-CLASS, renamed Hydroterra) was selected as one of the three candidate projects. The proposal consisted of a satellite using a C-band antenna with a diameter of 7 m. The elliptical relative motion of the spacecraft with respect to Earth, located over central Africa, allows the radar to monitor the water cycle over Europe and Africa, as qualitatively represented in Figure 1.  
The single-platform GEOSAR concept considered for the Hydroterra mission can be fractionated by means of a swarm of N SAR platforms, operating in a multistatic SAR configuration in order to enhance its performance. In this way, the synthetic aperture required to obtain high-resolution images would be obtained in a fraction of the time according to the number of platforms. The coherent combinations of the echoes obtained from different tracks would improve the Signal-to-Noise Ratio (SNR) by a factor of N\({}^{2}\), leading to metric resolution, a 20-40 min minimum observation time, and multi-polarimetric and interferometric imaging [5; 6]. The NZI GEOSAR scheme implies restricted geographical coverage. Therefore, a new GEOSAR scheme with a 50\({}^{\circ}\) inclination was proposed in order to provide imaging over the entire American continent [7; 8]. The larger coverage is achieved by means of a nadir-point trajectory in a large \"figure 8\" shape, which implies shorter integration times with respect to the NZI proposal. In this case, the transmitter power and antenna diameter requirements were 15 kW and 30 m, respectively. Studies from China propose a GEOSAR mission with an inclination of more than 50\({}^{\circ}\) with a large \"figure 8\" shape of the nadir-point trajectory, taking into account the large latitude and longitude ranges of the country [9; 10]. Such a high-inclination scheme could provide multiple observation angles for many areas, allowing the formation of three-dimensional (3D) deformation maps.