## 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
When the wind speed increases to 5 m/s and the received signal power is \(-\)77 dBm, the propagation distance decreases to 30 km, resulting in the spoofing source being located within a circle of 30 km radius centered on the monitoring station. For a wind speed of 10 m/s and a received signal power of \(-\)69 dBm, the VDES signal propagation distance is 20 km, placing the spoofing source within a circle of 20 km radius centered on the monitoring station. Finally, when the wind speed reaches 15 m/s and the received signal power is \(-\)60 dBm, the propagation distance is 10 km, and the spoofing source is located within a circle of 10 km radius centered on the VDES monitoring station, maintaining an equal probability distribution. Figure 19b illustrates that, under different evaporating waveguide heights, the spoofing source is distributed with equal probability in a circle with varying propagation distances as the radius. It also shows the variation of the spoofing source location across different VDES signal propagation distances for evaporating waveguide heights of 1 m, 2 m, 3 m, and 4 m. When the evaporating waveguide height is 1 m and the received signal power is \(-\)83 dBm, the VDES signal propagation distance is 40 km; at this time, the spoofing source is located in a circle centered on the VDES monitoring station, with an equal probability distribution within a radius of 40 km. When the evaporating waveguide height is increased to 2 m and the received signal power is \(-\)77 dBm, the propagation distance decreases to 30 km, placing the spoofing source in a circle with a radius of 30 km centered on the monitoring station. At an evaporating waveguide height of 3 m and a received signal power of \(-\)69 dBm, the VDES signal propagation distance is 20 km, which means the spoofing source is located within a circle of 20 km radius centered on the VDES monitoring station. Finally, when the evaporating waveguide height is 4 m and the received signal power is \(-\)60 dBm, the propagation distance remains 10 km, positioning the spoofing source in a circle centered on the monitoring station with a radius of 10 km, maintaining an equal probability distribution. Figure 19c illustrates the equal probability distribution of the spoofing source within a circle defined by varying propagation distances under different tidal water level heights. It also shows the changes in the spoofing source location across different VDES signal propagation distances when the tidal water level is 1 m, 2 m, 3 m, and 4 m, respectively. When the tidal water level is 1 m and the received signal power is \(-\)60 dBm, the VDES signal propagation distance is 10 km; at this time, the spoofing source is located within a radius of 10 km, centered on the VDES monitoring station. When the tidal water level rises to 2 m and the received signal power is \(-\)69 dBm, the propagation distance increases to 20 km, placing the spoofing source in a circle of 20 km radius centered around the VDES monitoring station. For a tidal water level of 3 m and a received signal power of \(-\)77 dBm, the VDES signal propagation distance is 30 km, which means the spoofing source is located within a circle of 30 km radius centered on the monitoring station. Finally, when the tidal water level reaches 4 m and the received signal power is \(-\)83 dBm, the VDES signal propagation distance is 40 km, positioning the spoofing source within a circle of 40 km radius centered on the VDES monitoring station, maintaining an equal probability distribution. (2) Spoofing Source Localization Using Two Monitoring Stations

When two monitoring stations simultaneously receive signals from the spoofing source, the position trajectory of the spoofing source can be determined based on the positional information of both monitoring stations and the time difference in signal arrival. Assuming monitoring station A is located at (38 deg58.8' W, 117 deg22.8' E) and monitoringstation B is at (39 deg3' N, 117 deg10.8' E), the trajectory of the spoofing source can be established.