The earth ellipsoid surface is taken as the reflection reference surface to initially position the specular reflection point, and then the geoid undulation is introduced to correct the reflection reference surface to geoid. Besides the positioning error caused by the static-state elevation error, that caused by the normal-radial difference is corrected without approximation at last. The positioning of the specular reflection point is improved by 28.66 m.  
Based on the static-state elevation error correction, the sea surface dynamic topography with a global amplitude of \(\pm\)2 m is the reflection reference surface time-varying elevation error that must be further eliminated to achieve cm-level accuracy sea surface altimetry. The contribution of ocean tides to the dynamic topography of the sea surface is 80% [23]; it is the most important source of time-varying elevation positioning error of specular reflection points. The standard deviation of ocean tides changes in open seas is 10-60 cm, up to several meters in marginal seas and offshore. The standard deviation contributed by ocean tides in sea level changes is 5-30 cm [24], which is a non-negligible specular reflection point positioning error for cm-level accuracy GNSS-R sea surface altimetry. Therefore, applying ocean tides to construct the ocean tidal correction positioning method is the key to further refining the specular reflection point positioning, and then to increase the GNSS-R altimetry accuracy. As the geometric positioning of the GNSS-R specular reflection point is independent of the instrument, it can be corrected based on the global tidal model. In the 1980s, the Subwiderski ocean tidal model was used as the most important global tidal model for the correction of ocean tides in various precision geodesy observations [25]. In 1992, with the success of TOPEX/Poseidon (T/P) satellite mission, satellite altimetry made it possible to quickly acquire global high-accuracy sea surface heights. In recent years, based on the development of satellite altimetry technology and data assimilation technology, a series of high-accuracy, high-resolution ocean tidal models such as Finite Element Solution (FES) [26], Technical University of Denmark (DTU) model [27], TOPEX/Poseidon global inverse solution (TPXO) [28; 29; 30], Empirical Ocean Tide (EOT) model [31], Goddard/Grenoble Ocean Tide (GOT) empirical model [32], etc. have appeared. Based on these models, tides can be calculated at any position and time in the global ocean. Comparison of the statistical results of the ocean tide correction by different tidal models and the altimeter error correction shows that the accuracy difference between the main tidal models is below 1 cm, while the error is relatively large in the offshore shallow waters [23]. The ocean tidal model and data are mainly applied to correct and calibrate the sea surface height data obtained by the satellite altimeters [33; 34; 35; 36; 37; 38; 39; 40]; it has not been applied in the correction of the GNSS-R specular point positioning, yet. In addition to the important characteristics of time-varying, ocean tides also have different spatial variation characteristics in the offshore and deep seas. In the offshore, the tide difference is larger than that in the deep sea, the tidal wave propagation is more complicated, and complex additional tidal constituents are formed. These differences will have different effects on the accuracy of the tidal elevation correction in the offshore and deep sea. The offshore is the key area for sea surface altimetry with the requirement of accurate altimetry support. Therefore, it is necessary to study the influence of offshore tidal height variation on the accuracy of tidal elevation correction positioning; this is discussed in Section 3. In summary, the existing GNSS-R specular reflection point positioning methods do not use the instantaneous sea surface with real-time variation as the reflection reference surface, resulting in a non-negligible positioning error for the cm-level accuracy sea surface altimetry.