## 5 Conclusions  
This study utilized simulation to invert global monthly marine gravity anomalies using sea surface specular point trajectories from the FY-3E and FY-3F satellites over one month. The altimetry performance of PARIS IOD and Operational satellite GNSS-R interferometric altimetry missions, proposed by [PERSON], served as examples. The potential contribution of spaceborne GNSS-R interferometric altimetry to global monthly time-variable gravity field inversion was assessed based on coverage, revisit frequency, and inversion accuracy. The main conclusions are as follows:  
1. Spaceborne GNSS-R interferometric altimetry technology allows for high-resolution and precise inversion of monthly marine gravity anomaly, enhancing the spatial resolution of current time-variable gravity field products. In the single GNSS-R satellite Operational scenario, the accuracy of monthly marine gravity anomaly inversion on a \(30^{\prime}\) grid can reach 4.93 mGal. 2. Considering the low cost of constellation formation features of spaceborne GNSS-R technology, a dual-satellite constellation simulation experiment was conducted. The dual-satellite constellation improves the spatial resolution of time-variable gravity fields that can be inverted with the same level of accuracy. For example, in an Operational dual-satellite constellation scenario with a 5 mGal precision requirement for gravity field inversion, a dual-satellite constellation can invert a global monthly marine gravity anomaly on a \(20^{\prime}\) grid, achieving an accuracy of 4.82 mGal. 3. Increasing the number of revisits can compensate for the precision limitations of spaceborne GNSS-R interferometric altimetry, thereby enhancing the accuracy of the monthly marine gravity anomaly. It is worth noting that our study utilized only two GNSS-R satellites, each with fewer than four effective channels. Future improvements could involve increasing the number of satellites in the constellation or observation channels to enhance monthly coverage and average revisits. 4. When inverting the global monthly marine gravity anomaly at a resolution finer than \(10^{\prime}\), the precision of the inversion is significantly impacted by the accuracy of spaceborne GNSS-R interferometric altimetry. As the grid resolution improves, the need for high spatial resolution observations grows approximately proportional to the square of the grid resolution enhancement. Beyond a certain threshold, relying solely on increasing the number of satellites for network observations will also make it difficult to achieve sufficient revisits within a month. Therefore, enhancing the accuracy of spaceborne GNSS-R interferometric altimetry becomes particularly important at this stage. In recent years, spaceborne GNSS-R technology has rapidly advanced and is expected to provide higher-quality sea surface altimetry data in the future. With advantages such as multi-track observations, low-cost constellation formation, and rapid global coverage and revisiting capabilities, it presents a promising tool for high-resolution monthly marine gravity anomaly inversion. This will complement the time-variable gravity field data from gravity satellites, advancing research and applications in Earth's time-variable gravity fields. Conceptualization, L.D., Z.Z. and J.X. ; methodology, L.D. ; software, L.D. ; validation, L.D. ; investigation, L.D. ; resources, W.B., Y.S. ([PERSON]), Q.D., X.W. and D.W.; data curation, L.D. and C.Y. ; writing--L.D. and Y.L. ; writing--review and editing, W.B., J.X., Z.Z., F.H.,C.Y., Y.L. and Y.S. [PERSON]); funding acquisition, W.B., J.X., F.H., Y.S. ([PERSON]) and Q.D. All authors have read and agreed to the published version of the manuscript.