## 4 Results and Discussion  
### Assessment of Monthly Marine Gravity Field Recovered a Single GNSS-R Satellite  
To explore the feasibility of using spaceborne GNSS-R interferometric altimetry for monthly marine gravity anomaly inversion, this study first simulated GNSS-R SSH interferometric altimetry data based on one-month trajectories from the FY-3E satellite and then inverted the monthly global marine gravity anomaly within latitudes between \(\pm 80^{\circ}\). Based on our previous research, spaceborne GNSS-R has the advantages of rapid coverage and revisits, which can compensate for the poor accuracy of single altimetry and obtain high-precision global marine vertical deflections. As the accuracy of vertical deflections directly affects the accuracy of gravity anomaly inversion, it can be expected that altimetry accuracy and revisits are also important factors affecting the accuracy of marine gravity anomaly inversion using spaceborne GNSS-R interferometric altimetry. Altimetry accuracy depends on instrument performance and error correction accuracy, which can be demonstrated through simulations using PARIS IOD and Operational scenarios. When the time period is fixed to one month, the size of the grid used is the key factor affecting the number of revisits. Therefore, the accuracy of monthly gravity anomaly simulations was evaluated for grid sizes of \(10^{\prime}\), \(15^{\prime}\), \(20^{\prime}\), \(25^{\prime}\), and \(30^{\prime}\) under both PARIS IOD and Operational scenarios. The results are shown in Table 1, which reflects the ability of spaceborne GNSS-R interferometric altimetry to invert monthly marine gravity anomaly. Figure 6: Global gravity anomaly from EGM2008 with \(5^{\prime}\) grid resolution. According to Table 1, at a grid resolution of 10\({}^{\prime}\), the inversion accuracy is poor in both the IOD and Operational scenarios, at 28.37 mGal and 13.22 mGal, respectively. The superior accuracy of the Operational scenario highlights its advantage over the IOD scenario, primarily due to the insufficient coverage rate (72.62%) and low average revisits (2.34 times) of a single spaceborne GNSS-R satellite at this resolution. However, in the Operational scenario, the inversion accuracy reaches within 10 mGal at a grid resolution of 15\({}^{\prime}\) or finer, and within 5 mGal at a grid resolution of 30\({}^{\prime}\), specifically 4.93 mGal. In the IOD scenario, the accuracy reaches within 10 mGal at a grid resolution of 20\({}^{\prime}\) or finer. In conclusion, the accuracy of monthly gravity field inversion is significantly influenced by the accuracy of GNSS-R interferometric altimetry under the observation of a single satellite. Furthermore, [PERSON] et al. used CryoSat-2 conventional radar altimeter data to successfully invert the monthly gravity anomaly, achieving an accuracy of 5.063 mGal with a 60\({}^{\prime}\) search radius [26]. Comparing this to the Operational scenario's inversion accuracy of 4.93 mGal at a grid resolution of 30\({}^{\prime}\), and the IOD scenario's accuracy of 5.7 mGal, it can be seen that the inverted gravity field from spaceborne GNSS-R interferometric altimetry can achieve comparable accuracy at a finer spatial resolution. Figure 7 shows the 30\({}^{\prime}\) monthly marine gravity anomaly inverted using simulated interferometric altimetry data from a single GNSS-R satellite in the Operational scenario. Despite some gaps in the 30\({}^{\prime}\) marine gravity field, most marine areas are covered, as shown in Figure 7. According to Table 1, the coverage rate at this resolution reaches 91.91%, with an average revisit rate of 21.14. Additionally, it can be observed that there are significantly fewer detection gaps in the Indian Ocean and Western Pacific regions compared to other areas. This is due to the presence of IGSO satellites from BeiDou Navigation Satellite System (BDS-2 and BDS-3) in these regions, resulting in a higher spatial coverage range for the BDS signals received by FY-3E in these areas [65].