## 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
Simultaneously, using Equation (9) to compute the ship-to-shore distance based on the VDES ship station location yields a value of 16.1096 nautical miles. Since this ship-to-shore distance is less than the VDES signal coverage range, it indicates that the information broadcast by the base station can be received by the ship station. Considering the received power, the calculated propagation distance of the VDES signal, using Equation (8), is 3.02 nautical miles. At this point, the ship-to-shore distance exceeds the tolerance range of 97.3% of the propagation distance, indicating that the received VDES message was not transmitted by the base station. This suggests the presence of spoofing source interference in the VDES data link at that moment, thereby validating the functionality of the Ship-to-Shore Distance vs. Propagation Distance Matching Analysis module. ## 4 Verification and Analysis of Experimental Results

The VDES data link integrity monitoring software system is a human-computer interaction system based on the C/S model, designed to effectively monitor the presence of radio interference or spoofing sources within the VDES data link. In this section, various interference scenarios are simulated to test the functionality and monitoring effectiveness of the software system. ### Testing of Radio Interference Vulnerability and Threats

In this section, the effectiveness of the radio interference detection method based on message characteristics is validated through tests that assess vulnerabilities to radio interference within the VDES data link. These tests include the verification of VDES message format compliance, legality of VDES message parameters, completeness of VDES message content, and consistency between transmitted and received messages. This validation demonstrates the method's capability to effectively detect radio interference vulnerabilities in the VDES data link. 1. Compliance Testing of VDES Message Formats

We assuming the message received by the VDES base station is \"!AIVEM,1,1,0,A,46:tkW 1 vLohh\(<\)'dD=LF?M3U00D0G,0*6F\". Upon parsing this message using the VDES raw message database, the test results are illustrated in Figure 20.7

As shown in Figure 34, querying the VDES raw message database confirmed that the format of the message is inconsistent with the format stored in the database, preventing the message from being parsed correctly. Consequently, the verification of VDES message format compliance failed, indicating the presence of radio interference within the VDES data link at that time. Figure 20.8 Functional verification results of the ship-to-shore distance and propagation distance matching analysis module. 2. Verification Testing of VDES Message Parameter Legitimacy

We assuming the message received by the VDES base station is \"!AIVEM,1,1,0,A,403t? ]1, vN2 hkR?dKu<F?1 AG02D0O,0?62\". Following a query to the VDES raw message database for parsing this specific message and extracting parameter information from each field of the message, the parameters are detailed in Figure 20.9 A comparison is then made between the parameters listed in Table 1 and the parameters of legitimate messages previously sent by this base station in the AIS/VDES message distributed database. The test outcomes are presented in Figure 20:0 \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameters** & **Explain** \\ \hline MMSI Code & 004132808 \\ \hline Longitude & \(121^{\circ}33.32862^{\prime}\) E \\ \hline Latitude & \(38^{\circ}52.09008^{\prime}\) N \\ \hline Repeat & 0 \\ \hline Position accuracy & 1 \\ \hline Types of electronic positioning devices & 7 \\ \hline Transmission control of long-range & 0 \\ broadcast messages & 0 \\ \hline Spare & 0 \\ \hline Sync State & 0 \\ \hline RAIM flag & 1 \\ \hline Time slot timeout & 5 \\ \hline Received Stations & 31 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Parameter table for VDES message parameter legitimacy verification testing. Figure 34: Results and figures of VDES message format compliance testing.