## 1 Introduction
The Wagner method is based on the premise of the earth standard sphere model, and the geodetic coordinates of the specular reflection point on the spherical surface are obtained by the polar spherical triangle relationship [29; 34]. The Gleason method estimates the position of the specular reflection point on the spherical surface based on the vector collinear method [35]. The existing geometric specular reflection point positioning methods take the reference ellipsoid or standard sphere as the reflection reference surface, but do not consider the positioning errors caused by the elevation difference between the sea surface and the reflection reference surface (Figure 1), and this error cannot be ignored. For the approximation of the earth surface, if the WGS-84 ellipsoid is selected, the elevation difference between the ellipsoid surface and the instantaneous sea level is tens of meters, while if global earth gravity model geoid is applied, the accuracy at the sea surface can reach the decimeter level [1]. A number of altimetry experiments by shore-based and air-based GNSS-R equipments on calm lake surface and in specific areas of the sea have been able to obtain measurements with high accuracy comparable to those by satellite altimeters [8; 9; 10; 11; 12]. However, the global ocean surface topography and roughness, which are complex and variable, are determined by various ocean dynamic parameters such as ocean gravity field, tidal field, circulation field, and sea surface wind field. The reflection reference surface error would introduce uncertainties to the spaceborne GNSS-R global sea surface altimetry. [PERSON] carried out the first spaceborne GPS-R observation of SSH with about 8 m Root Mean Squared Error. The TDS-1 metadata's specular reflection point position, which is used in [PERSON]'s study and taken as reference in this study, takes the WGS-84 ellipsoid as reflection reference surface, resulting in positioning error which is propagated into SSH. Additionally, the GPS-R instruments onboard TDS-1 are not optimized for SSH estimation [19]. Therefore, integrating a variety of ocean dynamic parameters to establish a global all-sea-state reflection reference surface model is not only a basic work to improve the accuracy of sea surface altimetry and other applications of GNSS-R,but also a precondition for exerting its high spatial resolution observation advantage and realizing its application value. Different from previous studies, this study proposes the gravity field--normal projection reflection reference surface combination correction method (GF-NPRRSCCM). The purpose is establishing the geoid reflection reference surface model to improve the positioning accuracy of the specular reflection point based on one of the important parameters determining the sea surface elevation--the earth's gravity field. Firstly, the WGS-84 ellipsoid is used as the reflection reference surface to preliminarily position the specular reflection point. Then the elevation correction based on the earth's gravity field model is introduced in the iteration, so that the reflection reference surface is corrected to geoid. The specular reflection point positioning error caused by the elevation difference between the reflection reference surface and the sea surface is reduced. Secondly, based on the elevation correction, the reflection reference surface is corrected from the radial to the normal of the specular reflection point, reducing the positioning error introduced by the normal-radial difference. Additionally, the influence of the approximate substitution on the positioning accuracy is reduced by directly solving the spatial geometric relationship between the normal projection on the plane and the reflection path. Thirdly, based on the combined application of the above two correction methods, the positioning error caused by the difference between the reflection reference surface and the sea surface is reduced, and the positioning accuracy is improved.