## 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
At this moment, with a wind speed of 10 m/s, a sea surface temperature of 15 degC, a tidal level of 0.3 m, and relative humidity of 50%, the received signal power at the VDES ship station is \(-\)70 dBm. Consequently, the VDES signal propagation distance, calculated using Equation (8), is 35 nautical miles. Since the ship-to-shore distance is less than the VDES signal propagation distance, the VDES ship station can successfully receive messages from the VDES base station, indicating the absence of a spoofing source in the VDES data link. Therefore, at this point, the VDES data link can be considered credible. The simulation environments in Figure 18 employs the C# language within Visual Studio 2019 in a Windows 11 operating system environment. As shown in Figure 18, 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. At this moment, with a wind speed of 15 m/s, a sea surface temperature of 16 degC, a tidal level of 0.1 m, and relative humidity of 67%, the received signal power at the VDES ship station is \(-\)70 dBm. Consequently, the VDES signal propagation distance, calculated using Equation (8), is 15 nautical miles. Since the ship-to-shore distance exceeds the VDES signal propagation distance, the VDES ship station is unable to receive the message sent by the VDES base station. This establishes that the VDES message received by the VDES ship station originates from a spoofing source, indicating the presence of a spoofing threat in the VDES data link. Therefore, at this point, the VDES data link is not considered credible. In this section, the range and position of the spoofing source were determined using a localization algorithm that analyzes the reception of deceptive messages by one, two, and three monitoring stations. For the case of a single monitoring station, variations in the coverage of VDES signals under different sea conditions were examined to establish the potential location range of the spoofing source. In the scenario involving two monitoring stations, the positions of each station and the time differences in signal arrival were utilized, along with the coverage of VDES signals, to track the position trajectory of the spoofing source. When three monitoring stations were employed, a cross-location method was used to accurately pinpoint the position of the spoofing source. 1. [label=(0)]
2. Localization of Spoofing Sources by Single Monitoring Stations

When only one monitoring station receives a deceptive message, the spoofing source is uniformly distributed within a range defined by the propagation distance of the VDES signal. Taking into account the influence of hydro-meteorological factors--such as sea surface wind speed, tidal level variations, and evaporation duct height--on the signal propagation distance from the VDES base station, the positions of the spoofing source were determined under different sea conditions for received powers of \(-83\) dBm, \(-77\) dBm, \(-69\) dBm, and \(-60\) dBm. The results, indicating the possible locations of the spoofing source, are illustrated in Figure 19.

Figure 19: Results of spoofing source localization by a single monitoring station. Figure 19a shows the equal probability distribution of the spoofing source in a circle with varying propagation distances as the radius, under different wind speed conditions. It also depicts the variation of the spoofing source location across different VDES signal propagation distances when the wind speeds are 1 m/s, 5 m/s, 10 m/s, and 15 m/s, respectively. When the wind speed is 1 m/s and the received signal power is \(-\)83 dBm, the VDES signal propagation distance is 40 km; thus, the spoofing source is located within a circle centered on the VDES monitoring station, with a radius of 40 km, reflecting an equal probability distribution.