## Materials and Methods
### Methodologies  
#### 2.2.1 The Gravity Field Reflection Reference Surface Correction Method (GFRRSCM)  
The coordinate system of this study is the WGS-84. The [PERSON] Wu method is used to position the specular reflection point with the ellipsoid surface as the reflection reference surface, in each iteration correction, the corresponding geoid undulation of the specular reflection point position is calculated and substituted, and the specular reflection point is finally positioned onto geoid. The evaluation and comparison of the positioning accuracy of specular reflection point is based on the Fresnel reflection law [2, 27, 28, 29, 33, 34]. When the positions of the transmitter, the receiver and the reflection reference surface are determined, the angle of incidence, the angle of emergence and the normal of the reflection are determined by the position of the specular reflection point. According to the Fresnel reflection law, the criteria for judging whether the position of the specular reflection point is accurate are as follows: (1) the angle of emergence is equal to the angle of incidence; (2) the normal of the specular reflection point is perpendicular to the reflection surface. Because of the limited accuracy of the computation, the criteria above cannot be fully satisfied, the following standard is used to estimate the positioning accuracy. The smaller the difference between the angle of incidence and the angle of emergence as well as the difference between the normal of the specular reflection point and the vertical direction of the reflection surface, the more accurate the reflection geometric relationship, so that the higher the positioning accuracy of the specular reflection point. Steps of positioning the specular reflection point are as follows [28]:  
1. Position of \(M\). In Figure 2, \(O\) is the center of the earth, and the position vectors of the receiver, the transmitter, and the specular reflection point are respectively \(\overrightarrow{\textbf{R}}\), \(\overrightarrow{\textbf{T}}\), and \(\overrightarrow{\textbf{S}}\). \(H_{\text{R}}\) and \(H_{\text{T}}\) are the geoid undulations of the nadirs of the transmitter and the receiver. \(M\) is the intersection of the \(OS\) extension line and the \(TR\) connection line, \(R^{\prime}\) is the mirror point of \(R\) to \(OM\), and \(M^{\prime}\) is the mirror point of \(M\) to \(RR^{\prime}\), then:  
\[\overrightarrow{\textbf{M}}=\overrightarrow{\textbf{R}}+H_{\text{R}}/(H_{ \text{R}}+H_{\text{T}})(\overrightarrow{\textbf{T}}-\overrightarrow{\textbf{R }}), \tag{1}\]  
2. Calculation and correction of the initial position of the specular reflection point. The latitude and the longitude of \(S\) are the same as those of \(M\), thus the geoid undulation \(H_{\text{S}}\) of \(S\) can be calculated using EGM2008 model, and then \(H_{\text{S}}\) is substituted in the conversion of \(S\) from the geodetic coordinates system into the space coordinates system, see Equations (2) and (3). Thus, the reflection reference surface is corrected from the ellipsoid towards geoid. \[ \begin{array}{l}X_{\text{S}}=(N_{\text{S}}+H_{\text{S}})\text{cos}(B_{\text {S}})\text{cos}(L_{\text{S}})=N_{\text{S}}\text{cos}(B_{\text{S}})\text{cos}(L _{\text{S}})+\sigma_{\text{x}}\\ Y_{\text{S}}=(N_{\text{S}}+H_{\text{S}})\text{cos}(B_{\text{S}})\text{sin}(L_{ \text{S}})=N_{\text{S}}\text{cos}(B_{\text{S}})\text{sin}(L_{\text{S}})+ \sigma_{\text{y}}\\ Z_{\text{S}}=(N_{\text{S}}\;(1\;-\text{e}^{2})+H_{\text{S}})\text{sin}(B_{ \text{S}})=N_{\text{S}}(1-\text{e}^{2})\;\text{sin}(B_{\text{S}})+\sigma_{ \text{z}},\end{array}