## 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
Upon detecting a spoofing source while monitoring the VDES data link, the VDES ship station, as known information, is used to locate the spoofing source based on the vessel position reported by the VDES shipborne terminal and the two base station positions received by the ship station within the tolerance range. By utilizing the positions of each monitoring station as circle centers with the coverage range of each station's signal as the radius, similar pairwise circle intersections occur. Line EF denotes the common chord formed by the signal coverage circles of Base Stations A and B. Connecting the intersection points of any two circles yields three common chords that converge at a single point, indicating the location of the spoofing source. Therefore, this method accurately identifies the position of the spoofing source, as shown in Figure 5:2

Figure 5:3 The spoofing source broadcasting the spoofing VDES message. Figure 5:4 Positioning the spoofing source. The formula for estimating the location of a spoofing source using a land-based positioning method is as follows:

Figure 5:5

In the equation, \(r_{i}\) represents the propagation distance of the \(i\)-th VDES signal monitoring station, where \(i=1\ldots N\), with \(N\) denoting the number of VDES signal monitoring stations, and \(s_{i}\) indicating the position and timing information of the \(i\)-th monitoring station. When \(N=1\), indicating that only one monitoring station receives the message from the spoofing source. The VDES signal propagation distance is calculated using a model for VHF signal propagation loss and received power under complex sea surface conditions. The ship-to-shore distance is then analyzed to determine if it falls within the tolerance range of this propagation distance. If it does, no spoofing source is present in the VDES data link; however, if it exceeds this range, the message from that VDES base station is deemed to originate from a spoofing source, which is uniformly distributed within a circle defined by the VDES signal propagation distance as the radius. The single monitoring station positioning scenario is shown in Figure 5:6

When \(N=2\), meaning two monitoring stations receive the message from the spoofing source. The propagation distances of the signals from each base station are calculated using the same model for VHF signal propagation loss and received power under complex sea surface conditions. The ship-to-shore distance is analyzed to determine if it falls within the tolerance range of the propagation distance from each base station. If all distances are within this range, no spoofing source exists; if any distance exceeds this range, it indicates that a message from one base station originates from a spoofing source. Upon detecting a spoofing source in the VDES data link, the VDES ship station is used as known information to locate the spoofing source. This is based on the vessel position information reported by the VDES shipborne terminal and the position of one VDES base station within the tolerance range. Considering the positions of the VDES base station and ship station, along with the time difference in message arrival, the spoofing source can be localized uniformly along the circumference of a circle with the VDES base station's position as the center. The two monitoring stations positioning scenario is shown in Figure 5:7

Figure 5:8 The single monitoring station positioning scenario.