## 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
\(L_{b\lambda h}(d,\Delta h)\) indicates the transmission loss affected by tidal water level changes. \(\Delta h\) stands for the tidal water level change in meters. \(L_{b\lambda_{e}}(d,T_{a},T_{s},U,RH)\) represents the transmission loss influenced by evaporation duct height. \(T_{a}\) refers to air temperature in Kelvin. \(T_{s}\) denotes sea surface temperature in Kelvin. \(U\) signifies sea surface wind speed in meters per second. \(RH\) represents relative humidity, and \(d\) is the propagation distance. To obtain an accurate expression for the propagation loss model, actual measurements of VHF signal reception power were utilized for data fitting using the least squares method. These data were obtained from real experiments conducted by the author's team in the Dalian coastal area, with detailed experimental results available in reference [43]. The propagation loss model for VHF signals in complex sea surface environments, after fitting, is as follows:

Figure 6.7

According to the conclusion in reference [36], the goodness of fit for Equation (8) is 0.9675, indicating a deviation of only 3.3% between the complex sea surface VHF signal transmission loss calculated by this equation and the actual data. This meets the criteria for a good fit, making it suitable for calculating the propagation distance of VDES signals. As VDES stations regularly report their vessel positions via telegrams, and the location of the VDES base station is known, the calculation of ship-to-shore distance can be accomplished using the Haversine formula. This formula is used to determine the distance between two given latitude and longitude points on Earth, typically measured in nautical miles, kilometers, or miles. This revision improves clarity and coherence while maintainingthe original content. Assuming the specific coordinates of the ship and base station are (\(lat1\), \(lon1\)) and (\(lat2\), \(lon2\)), respectively, the expression is as follows:

Figure 6.8

In the equation, R represents the radius of the Earth. #### 2.2.2 Spoofing Sources Localization Method

In this paper, the approach for locating the spoofing sources existing in the VDES data link is to employ the VDES communication signals, and the localization method based on this is the land-based positioning method. When a VDES ship station receives messages from three VDES base stations, the positions of each base station can be considered the centers of circles, with the coverage range of each signal serving as the radius. These circles intersect pairwise. Line MN denotes the common chord formed by the signal coverage circles of Base Stations A and B, and Line EF denotes the common chord formed by the signal coverage circles of Base Stations A and C. By connecting the intersection points of any two circles, three common chords can be derived from the intersections of the three pairs of circles, all converging at a single point, which represents the location of the VDES ship station, as shown in Figure 6.9

Subsequently, the distances between the VDES ship station and each VDES base station are calculated. Using a model for VHF signal propagation loss and received power under complex sea surface conditions, the propagation distances of the signals from each VDES base station are determined. The analysis then checks whether the ship-to-shore distance falls within the tolerance range of the propagation distance from each base station. If all distances are within this range, there is no indication of a spoofing source; however, if any propagation distance exceeds this range, it suggests the presence of a spoofing source, implying that one of the base station messages is being broadcast by an unauthorized source, as shown in Figure 5:0

Figure 5:1 Positioning the vessel by using VDES shore station signals.