## 2 VDES Data Link Credibility Monitoring Approach
### Radio Interference Detection Based on Message Characteristics
#### 2.1.1 VDES Message Integrity Verification Method Based on the MD5 Algorithm
Therefore, the distance between point E and point O is equal to half the length of the common chord, expressed as follows:

Figure 6:0

The coordinates of the intersection points E and F of the two monitoring stations can be derived from Equation (19) and the slope \(k_{EF}=\frac{y_{E}-y_{O}}{x_{E}-x_{O}}\) of the common chord EF as follows:

\[\left\{ \begin{array}{c}x_{E}=x_{O}+\sqrt{\frac{r_{ef}^{2}}{1+k_{EF}^{2}}}\\ y_{E}=y_{O}+k_{EF}(x_{E}-x_{O})\end{array} \right.,\;\left\{ \begin{array}{c}x_ {F}=x_{O}-\sqrt{\frac{r_{ef}^{2}}{1+k_{EF}^{2}}}\\ y_{F}=y_{O}+k_{EF}(x_{F}-x_{O})\end{array} \right. \tag{20}\]

This yields the expression for the common chord EF between monitoring stations A and B as follows:

\[y=-\frac{1}{k_{AB}}x+y_{E}+\frac{1}{k_{AB}}x_{E} \tag{21}\]

By utilizing the method for determining the common chord EF between monitoring stations A and B, the expression for the common chord MN between monitoring stations B and C can be derived as follows:

\[-\frac{x_{B}-x^{\prime}}{y_{B}-y^{\prime}}\times x_{C}+\frac{y_{B}+y^{\prime} }{2}+\frac{x_{B}-x^{\prime}}{y_{B}-y^{\prime}}\times\frac{x_{B}+x^{\prime}}{2 }=y_{C} \tag{22}\]

In the equation, \(x\prime\) represents the abscissa of any point on the common chord MN between monitoring stations B and C, \(y\prime\) denotes the ordinate of any point on MN, \(x_{B}\) represents the abscissa of monitoring station B, \(y_{B}\) signifies the ordinate of B, \(x_{C}\) stands for the abscissa of monitoring station C, and \(y_{C}\) denotes the ordinate of C.

With Equations (21) and (22), the position \((x_{S},y_{S})\) of the spoofing source S can be determined as follows:

\[ \begin{split} x_{S}&=\frac{-a+\sqrt{a^{2}-4\times \left[1-\left(\frac{x_{A}-y_{B}}{x_{A}-y_{B}}\right)^{2}\right]\times\left[ \frac{y_{B}\left(y_{A}-y_{B}\right)\left(\frac{x_{A}^{2}-y_{B}^{2}}{x_{A}-y_{B }}\right)^{2}-\left(\frac{y_{A}^{2}-y_{B}^{2}+x_{A}^{2}-y_{B}^{2}}{4\left(x_{A }-y_{B}\right)^{2}}\right)^{2}-2x_{C}x_{B}-x_{B}^{2}+y_{A}y_{B}\right]}}{2\left[ 1-\left(\frac{x_{A}-x_{B}}{x_{A}-y_{B}}\right)^{2}\right]}},\\ y_{S}&=\frac{-b+\sqrt{b^{2}-4\left[\left(\frac{y_{A}-y_{B }}{x_{A}-y_{B}}\right)^{2}-1\right]\left[-3x_{B}x_{C}-x_{A}x_{C}-x_{B}^{2}-y_{ B}^{2}-\frac{4x_{C}\left(x_{A}-x_{B}\right)\left(\frac{y_{A}^{2}-y_{B}^{2}}{x_{A}-y_{B }^{2}}\right)+y_{A}^{2}-\frac{y_{B}^{2}+x_{A}^{2}-y_{B}^{2}}{4\left(x_{A}-x_{B }\right)^{2}}\right]}}}{2\left[\left(\frac{y_{A}-y_{B}}{x_{A}-y_{B}}\right)^{2 }-1\right]}\\ &\text{where }a=\frac{\left[-\left(y_{A}-y_{B}\right)^{2}-x_{A}^{2}+x_{B }^{2}\right]\left(x_{A}-x_{B}\right)}{\left(y_{A}-y_{B}\right)^{2}}+2x_{C},\,b= \frac{\left[\left(2x_{C}-x_{A}-x_{B}\right)\left(x_{A}-x_{B}\right)-y_{A}^{2} +y_{B}^{2}\right]\left(y_{A}-y_{B}\right)}{\left(x_{A}-x_{B}\right)^{2}}+2y_{B}.\end{split}  \tag{23}\]

#### 2.2.3 Experimental Testing and Results Analysis

To assess the detection capability of the spoofing source identification and localization method based on distance matching analysis against deceptive threats in the VDES data link, various experimental scenarios were conducted in this section to validate and analyze the proposed design method. 1. Spoofing Source Detection Experiment 1. No Spoofing Source DetectedAt a certain moment, the VDES ship station received a message from a VDES base station, which read: \"IABTDB,1,1,0,A,403 sooQvN!\(<\)?W\(<\)Cfj0 FG9 hG00D0c,0\"$C\". Upon decoding, the location information of the VDES base station was identified as (39 deg4.8' N, 117 deg4.8' E), as illustrated in Figure 17, according to the experimental results. The simulation environment in Figure 17 employs the C# language within Visual Studio 2019 in a Windows 11 operating system environment. As shown in Figure 17, when the VDES ship station is located at (38 deg54.594' N, 117 deg34.146' E), the ship-to-shore distance calculated using Equation (9) is 25.0319 nautical miles.