## 5 Conclusions  
The positioning error of the specular reflection point is the main error source reducing the accuracy of GNSS-R sea surface altimetry, and reflection reference surface correction is the key to improving the positioning accuracy. Integrating a variety of ocean dynamic parameters to establish a global all-sea-state reflection reference surface model is not only an important task to improve the accuracy of GNSS-R sea surface altimetry, but also a precondition for exerting GNSS-R's high-spatial-resolution observation advantage and realizing its application value. As an important factor determining the sea surface elevation, the earth's gravity field plays a significant role in constructing the all-sea-state reflection reference surface model. In this study, based on the correction of the GNSS-R reflection reference surface to geoid, a gravity field normal projection reflection reference surface combination correction method is proposed to improve the positioning accuracy of the specular reflection point. Firstly, the position of the specular reflection point was preliminarily calculated with the WGS-84 ellipsoid as the reflection reference surface using the receiver and the transmitter positions contained in the dataset of SGR-ReSI carried by the TDS-1 satellite. Secondly, the elevation correction calculated based on the earth's gravity field model EGM2008 was introduced in the positioning iteration to correct the reflection reference surface to geoid, which is much closer to the sea surface than the WGS-84 ellipsoid surface. This step reduces the specular reflection point positioning error caused by the elevation difference between the reflection reference surface and the sea surface. Finally, based on the GFRSCM, the normal projection reflection reference surface correction method corrected the normal-radial difference, and reduced the influence of the approximate substitution on the calculation accuracy by directly solving the spatial geometry related to the normal reflection reference plane. Applying the GF-NPRRSCCM proposed in this study, the positioning accuracy of the GPS-R specular reflection point with the WGS-84 ellipsoid as the reflection reference surface was improved by 28.66 m. Specifically, the GFRSCM improved the positioning accuracy by 25.15 m, based on which the NPRRSCM further improved the positioning accuracy by 13.05 m towards the normal. The elevation difference between the reflection reference surface and the sea surface was the main error source of the specular reflection point positioning relative to the normal-radial difference. Based on the quantitative evaluation and comparison of the reflection geometry accuracy, the GFRSCM was more accurate than the TDS-1 positioning, which was improved by more than 40 km by the GFRSCM. The amplification of the positioning error related to the satellite orbit height was much better controlled by the GFRSCM. The reflection geometric relationship error of TDS-1 data increases with the the angle of incidence increases. When the angle of incidence exceeds 20\({}^{\circ}\), the specular reflection point position data should be used with caution for accuracy concerns, while the GFRSCM does not have this problem. The TDS-1 SGR-ReSI black-body calibration on the noise reference results in error of its own specular reflection point positioning, but no obvious impact of this calibration is observed in the geometric relationship determined by the GFRSCM positioning result. [PERSON] and [PERSON] conceived the experiments and wrote the paper; [PERSON] designed and performed the experiments; [PERSON] provided theory support for the index requirement of GNSS-R sea surface altimetry; [PERSON] provided suggestions for the experiments and the analysis of the results.