## 2 Experimental Data
Therefore, this study recalculated the altimetry accuracy of PARIS based on the error budget table proposed by [PERSON], taking into account the impact of the orbital altitude of FY-3E/F, 10 km along-track resolution, and geophysical error correction. The results showed that at the orbit altitude  
Figure 2: Daily specular point trajectory map for FY-3E and FY-3F. of FY-3E/F, the altimetry accuracy for IOD \(\sigma_{IOD}\) and Operational scenarios \(\sigma_{Operational}\) are 41.52 cm and 16.17 cm, respectively, with a 10 km along-track resolution. Figure 3 illustrates the process of simulating spaceborne GNSS-R altimetry data, while the detailed calculation process can be found in our previous work [60]. Prior to simulating sea surface height (SSH) data for spaceborne GNSS-R interferometric altimetry, sparse sampling was applied to the specular point trajectory data of the FY-3E and FY-3F satellites. This approach was adopted to address the 5.6 km ground spacing between adjacent along-track specular points of FY-3E/F satellites, which is below the minimum achievable along-track resolution (10 km) for spaceborne GNSS-R interferometric altimetry. The sparse sampling ensures the independence between adjacent sampling points along the track, thereby eliminating data redundancy. As shown in Figure 4, the minimum sampling distance was set at 10 km. Figure 4: Diagram of reflection area construction for spaceborne GNSS-R interferometric altimetry. Figure 3: Flowchart of data simulation process for spaceborne GNSS-R altimetry. To simulate SSH in spaceborne GNSS-R interferometric altimetry, the DTU21 MSS model from the Technical University of Denmark (DTU) was selected as the input mean sea surface (MSS) model. With a grid resolution of 1 arc-minute, the DTU21 MSS model is significantly finer than the 10 km spatial resolution achievable by spaceborne GNSS-R interferometric altimetry, except in polar regions. To accurately represent the impact of the spatial resolution of spaceborne GNSS-R interferometry on the SSH observations, the average SSH within the spatial resolution of spaceborne GNSS-R interferometry was employed as the background field. As shown in Figure 4, a 10 km \(\times\) 10 km specular reflection area was constructed with each spaceborne GNSS-R specular point as the center, and the reflection area was resampled into points with a 2 km interval. The SSH value at each resampling point within the reflection area was calculated by interpolating with the DTU21 MSS model, and the average SSH of all resampling points within the reflection area was calculated as the average MSS within the specular reflection area \(MSS_{Sp\_area}\). At a single point, the altimetry accuracy can also be seen as the difference error between spaceborne GNSS-R SSH interferometric altimetry results and the true SSH. Furthermore, in practical scenarios, the altimetry errors of adjacent points along the track are generally correlated. However, from a global and long-term observation perspective, the distribution of all altimetry errors follows a normal distribution. Therefore, in the evaluation of the global monthly marine gravity field recovered by spaceborne GNSS-R interferometric altimetry, altimetry errors can be assigned to all points based on the central limit theorem. This is achieved by generating a random number from a normal distribution with a mean of 0 and a standard deviation of altimetry accuracy (41.52 cm for the IOD scenario and 16.17 cm for the Operational scenario). Lastly, the final simulated results of spaceborne GNSS-R interferometric altimetry results \(SSH_{GNSS-R}\) will be determined by adding altimetry errors \(\epsilon_{altimetry}\) to the \(MSS_{Sp\_area}\).