## 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
Under conditions of a sea surface wind speed of 10 m/s, a tidal water level variation of 0.2 m, an evaporation duct height of 0.1 m, and received powers of \(-\)61 dBm, \(-\)68 dBm, \(-\)75 dBm, and \(-\)82 dBm, the positioning results of the spoofing source at VDES signal propagation distances of 10 km, 20 km, 30 km, and 40 km calculated using Equation (8) are depicted in Figure 20.

From Figure 20, it is evident that at VDES signal propagation distances of 10 km, 20 km, 30 km, and 40 km, the range of the spoofing source's location exhibits an equal probability distribution along the circumference, with monitoring station A (38 deg58.8' N, 117 deg22.8' E) as the center. (3) Spoofing Source Localization with Three Monitoring Stations

When three or more monitoring stations simultaneously receive signals from the spoofing source, circles are constructed with the positions of each monitoring station as centers and the VDES signal coverage ranges as radii. These circles intersect pairwise, and the common chord of two intersecting circles is used to determine the location of the

Figure 20: Results of spoofing source localization by a two monitoring stations. spoofing source. Assuming monitoring station A is positioned at (38 deg33.2' N, 121 deg8.04' E) and monitoring station B is located at (38 deg43.176' N, 121 deg10.02' E), under conditions of a sea surface wind speed of 10 m/s, a tidal water level variation of 0.2 m, an evaporation duct height of 0.1 m, and received powers of \(-\)61 dBm, \(-\)68 dBm, \(-\)75 dBm, and \(-\)82 dBm, the positioning results of the spoofing source at VDES signal coverage ranges of 10 km, 20 km, 30 km, and 40 km calculated using Equation (8) are illustrated in Figure 21.

In Figure 11(a), the position of monitoring station C is (38 deg39.6' N, 121 deg8.04' E). With a VDES signal coverage range of 10 km, the location of the spoofing source determined based on the positions of each monitoring station and the signal coverage range is (38 deg41.8909' N, 121 deg8.1217' E). In Figure 11(b), with monitoring station C at (38 deg40.14' N, 121 deg8.64' E) and a VDES signal coverage range of 20 km, the spoofing source location is (38 deg42.2063' N, 121 deg8.1238' E). In Figure 11(c), when monitoring station C is at (38 deg37.74' N, 121 deg7.44' E) with a coverage range of 30 km, the spoofing source is located at (38 deg40.7501' N, 121 deg8.1145' E). Finally, in Figure 11(d), with monitoring station C at (38 deg36.54' N, 121 deg8.04' E) and a coverage range of 40 km, the spoofing source is determined to be at (38 deg40.1323' N, 121 deg8.1106' E). In summary, the method of spoofing source identification and localization based on distance matching analysis effectively detects threats from spoofing sources in the VDES

Figure 21: Results of spoofing source localization by a three monitoring stations. data link. It enables the determination of the range and location of spoofing sources based on the varying number of monitoring stations capable of receiving deceptive signals. ## 3 Design of Software for Monitoring VDES Data Link Credibility

Based on the analysis of the VDES data link integrity monitoring method, a VDES data link integrity monitoring system was constructed in this study. The implementation process of the system algorithm is illustrated in Figure 22, and the effectiveness of this system in practical scenarios was validated through various interference setups. According to Figure 22, the VDES data link integrity monitoring system initiates the initialization of system parameters. Subsequently, the system parses the messages received by the VDES base station and verifies the conformity of the message format with the VDES raw message database. If the received message format does not align with the stored formats in the database, leading to unsuccessful parsing, it is determined that the content of the VDES message is corrupted due to radio interference. This results in an output indicating an untrusted monitoring result for the VDES data link.