## 1 Introduction
A medium inclination (16\({}^{\circ}\)) GEOSAR scheme with a small \"figure 8\" nadir-point trajectory has also been studied owing to its longer coverage time over China [11]. Signals of Opportunity-Reflectometry (SoOp-R) is an emerging technology that can be used to measure the Snow Water Equivalent (SWE). The system is based on passive receivers and uses existing, strong, satellite transmissions, available across most radio bands, penetrating the Earth's atmosphere (P to Ka band) in a bistatic radar configuration. P-band communication satellites in GEO orbits, such as the Navy's Mobile Users Objective System (MUOS), provide higher illumination power density than Global Navigation Satellite Systems (GNSSs) and better surface penetration for snow and soil moisture sensing, thus allowing small P-band SoOp satellite receivers operating at LEO to capture the reflection  
Figure 1: Hydroterra Near-Zero Inclination (NZI) scheme. Operating from geosynchronous orbit (GEO), imaging can be provided over most of Europe and Africa. The size of the ellipse has been exaggerated with respect to a real NZI GEO satellite orbit for clarity. with a high SNR. The use of P-band SoOp from MUOS has also been receiving increasing attention for soil moisture sensing and other land hydrology elements [12; 13; 14]. The previously described GEOSAR missions are currently under study. Regardless of the configuration, correct focusing of GEOSAR images depends on the compensation of the echo phase histories, which requires determining the orbit tracks with precisions in the order of 10 m or better [15]; this precision can be relaxed by one order of magnitude using SAR autofocusing techniques based on entropy minimization [15]. Therefore, GEOSAR missions present two main challenges. On the one hand, they need to compensate for the impact of distances greater than 36,000 km on the received echo power. Therefore, they require the use of high-gain antennas, significant average transmitted power, and long integration times between minutes and hours depending on the desired resolution [2; 8; 9; 16; 17]. On the other hand, they require unprecedented orbit determination precision [18]. The satellite navigation techniques used for LEOSAR missions present limitations when applied to higher orbits beyond Medium Earth Orbit (MEO). Since the GEO trajectories lie above the GNSS constellations, some studies propose to use the sidelobes GNSS signals from the other side of the Earth in order to determine the orbit of GEO satellites with precision [19]. The coverage time from a single GNSS satellite from the other side of the Earth is limited. Therefore, only dual- or multi-GNSS could provide continuous navigation solutions with a receiver threshold of 15 dB-Hz [20]. Other studies from China have improved the orbit determination precision of the BeiDou satellites in GEO orbits by combining GNSS navigation with ground tracking [21]. Hence, the advent of emerging geosynchronous remote sensing missions relies on the development of novel, precise orbit determination techniques, both spaceborne and ground-based. The technique presented in this paper aims to complement other orbit determination techniques, such as the ones based on GNSS, in order to enhance the tracking precision of future remote sensing missions from GEO.