Assessment of the Potential of Spaceborne GNSS-R Interferometric Altimetry for Monthly Marine Gravity Anomaly  
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7 School of Management, China University of Mining and Technology (Beijing), Beijing 100083, China;  
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transport and exchange within the Earth's system [1]. With approximately 71% of the Earth's surface covered by oceans, the recovery of marine gravity field is essential for building a comprehensive Earth gravity field model. The time-variable marine gravity field is crucial for studying changes in the Earth's system and engineering applications, such as the water cycle, climate change, movement of materials within the Earth, and energy development [2]. Additionally, the higher the resolution of the marine time-variable gravity field, the more information it provides about dynamic processes in the Earth's system. Therefore, obtaining high-resolution, time-varying information about the marine gravity field is essential for both research and applications in Earth sciences. In 1969, [PERSON] proposed the GRACE mission concept, employing inter-satellite ranging measurements between two low Earth co-orbiting satellites to observe the Earth's time-variable gravity field [3]. At present, GRACE/GRACE-FO serve as the primary tools for detecting the global gravity variation, having continuously provided the monthly Earth's gravity field data for over two decades [4]. These missions have significantly advanced the monitoring and understanding of global environmental changes, encompassing variations in ground-water storage [5], deformation caused by earthquakes [6], sea level rise [7], deep ocean currents [8], and ocean heat content [9]. However, the orbital configuration of GRACE/GRACE-FO satellites (~500 km satellite altitude, ~220 km inter-satellite distance) fundamentally limits the spatial resolution of their derived time-variable gravity fields to hundreds of kilometers [10]. Furthermore, the accuracy of the gravity field degrades significantly with increasing spherical harmonic degree, as high-degree coefficients dominated by noise [11]. To mitigate the impact of strong longitudinal stripes and random noises, spatial filtering is required, further reducing the spatial resolution of GRACE/GRACE-FO time-variable gravity data [12]. Currently, the monthly gravity field products from GFZ GRACE Release 06 (RL06) solutions are truncated at spherical harmonic degrees of 60 and 96 [13], corresponding to spatial resolutions of approximately 330 km and 220 km (half-wavelength) [14], respectively. Consequently, the coarse spatial resolution of the gravity field products provided by GRACE/GRACE-FO satellites hinders further research and applications of time-variable gravity fields [15,16]. Marine gravity anomalies from satellite altimetry have greatly improved the accuracy and spatial resolution of static marine gravity anomaly results [17, 18, 19, 20], allowing for a better understanding of deep-sea seabed topography and structure [21, 22, 23]. However, due to the satellite's limited view confined to the nadir and the high cost of constructing large-scale constellations, traditional radar altimeters struggle to achieve rapid global coverage and revisiting capabilities [24]. As a result, radar altimeter data are mainly used for inverting static marine gravity anomaly rather than for time-variable marine gravity fields, due to their insufficient observational efficiency. In 2021, [PERSON] et al. first utilized the yearly marine time-variable gravity field from radar altimetry, revealing the undersea volcano magma mass motions of the Izu-Bonin arcs near Japan. However, it was also pointed out that the low time resolution of altimetry-derived marine time-variable gravity fields makes it challenging to study rapidly evolving volcanic systems [25]. In 2023, [PERSON] et al.