## 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
In cases where a spoofing source is identified within the VDES data link, a spoofing source identification algorithm is employed to determine the position or range of the deceptive entity. Thus, through the described methodology, accurate identification and localization of spoofing sources can be achieved. #### 2.2.1 Method for Calculating VDES Signal Propagation Distance and Ship-to-Shore Distance

VDES is a maritime communication and data exchange system operating within the VHF frequency band. When VHF signals propagate over the sea surface, they typically follow a two-ray model. However, the complexity of the marine environment introduces various factors that influence the transmission loss of VHF signals, including sea surface wind speed, tidal effects, and evaporation ducting. These factors lead to varying degrees of attenuation during signal propagation, subsequently affecting the transmission distance. Therefore, this section considers the complexity of VHF signal propagation in maritime environments and utilizes a model for VHF signal propagation loss under complex sea surface conditions to calculate the transmission distance of VDES signals [36]. Sea surface wind speed primarily affects the transmission loss of VHF reflected signals by altering the sea surface reflection coefficient. When considering sea surface wind speed, the transmission loss of VHF signals is determined as follows [40]:

Figure 6.2

In the equation, \(r_{1}\) represents the length of the direct wave propagation path; \(f\) denotes the frequency in MHz; \(\Gamma_{\text{R}}\) stands for the reflection coefficient; \(h_{t}\) and \(h_{r}\), respectively, indicate the heights of the transmitting and receiving antennas in meters; \(d_{1}\) is the horizontal distance from the transmitter to the reflecting point; \(d_{2}\) is the horizontal distance from the receiver to the reflecting point; \(E_{0}\) represents the electric field strength at the transmitter; R is the radius of the Earth; \(d\) is the propagation distance of the VHF signal; and \(\lambda\) is the wavelength of the VHF signal in meters. Tidal effects impact the transmission loss of VHF signals by altering both the reflection path length of these signals and the antenna heights of vessels. When considering tidal effects, the transmission loss of VHF signals can be expressed as follows [41]:

Figure 6.3

In the equation, \(h_{t}\) and \(h_{r}\) represent the heights of the transmitting and receiving antennas, respectively, in meters; \(f\) denotes the frequency in MHz; \(\Delta h\) is the height changecaused by tidal effects; \(d\) is the propagation distance of the VHF signal; and \(c\) is the speed of light. Evaporation ducts affect the transmission loss of VHF signals by capturing electromagnetic waves within the duct layer. When considering evaporation ducts, the transmission loss of VHF signals can be expressed as follows [42]:

Figure 6.4

In the equation, \(\lambda\) represents the wavelength of the VHF signal in meters; \(d\) denotes the propagation distance of the VHF signal in meters. The expression of \(\Delta\) in the equation is as follows [42]:

Figure 6.5

In the equation, \(h_{t}\) and \(h_{r}\) represent the heights of the transmitting and receiving antennas, respectively, in meters; \(h_{e}\) is the effective duct height; and \(d\) is the propagation distance of the VHF signal. Therefore, considering the influence of hydro-meteorological factors such as sea surface wind speed, tidal water level changes, and evaporation duct height on the transmission of VDES signals, the expression for the transmission loss of VHF signals can be formulated as follows:

Figure 6.6

In the equation, \(\text{P}=[p_{1},p_{2},\cdots,p_{n}]\) represents the environmental factor vector, \(p_{1},p_{2},\cdots,p_{n}\) describes the various parameters of the environmental factors, and \(L_{bu}(d,u)\) signifies the transmission loss influenced by sea surface wind speed. \(u\) denotes the sea surface wind speed in meters per second.