## 2 VDES Data Link Credibility Monitoring Approach  
This paper examines two primary categories of vulnerability threats, as delineated in Figure 2, and proposes a collaborative monitoring strategy based on radio interference detection and spoofing source identification and localization to oversee the credibility status of data links within the outlined threat scenarios, as depicted in Figure 3. Radio interference threats targeting VDES data links originate from various sources, including atmospheric interference, adjacent channel interference, electromagnetic radiation interference, and pseudo-transmission interference from high-power devices. Detection of these interferences can be effectively carried out through several methodologies, including telegram format compliance verification, telegram parameter legitimacy verification, telegram content integrity verification, and verification of transmission and reception consistency. To elaborate, the process of telegram format compliance verification involves parsing received VDES messages. Successful parsing indicates compliance with the format; conversely, a failure to parse suggests the presence of radio interference, thereby compromising the credibility of the VDES data link. Additionally, telegram parameter legitimacy verification can utilize an AIS/VDES telegram distributed database, meticulously curated by the author's research team in collaboration with relevant maritime administrative entitiesfollowing extensive research and consultations. This database contains all legitimate telegrams previously dispatched by each shore station. Ensuring that the parameters within the VDES message align with those of legitimate telegrams from the specific shore station in the database is essential for passing the parameter legitimacy check; any misalignment signals potential radio interference, jeopardizing the credibility of the VDES data link. For a comprehensive understanding of the implementation methodologies related to these aspects, readers are referred to reference [36]. Furthermore, telegram content integrity verification employs the MD5 algorithm to assess the integrity of received VDES messages against legitimate telegrams stored in the original VDES telegram database. A match in the hash values of both messages indicates successful passage of the content integrity check; conversely, a mismatch signifies radio interference, thereby impacting the credibility of the VDES data link. VDES shore stations disseminate link management telegrams through TDB statements, which VDES ship stations then relay using VEM statements upon reception. After receiving this relayed message, VDES shore stations use the HMAC algorithm to validate the consistency between transmitted and received telegrams. If the HMAC values of both telegrams align, the transmission and reception consistency check is passed; any disparity indicates radio interference, casting doubt on the credibility of the VDES data link. Upon completing radio interference detection procedures, the VDES messages transition to the spoofing source identification and localization algorithm. Spoofing source interference targeting VDES data links originates from \"Spoofing\" base stations that threaten maritime safety by propagating false VDES messages to vessels within their signal coverage, often by misappropriating the Maritime Mobile Service Identity (MMSI) of genuine VDES shore stations. Detecting such interferences requires a comprehensive analysis of ship-to-shore distances and propagation distances to ascertain the presence of spoofing sources. VDES ship-based stations compute the ship-to-shore distance by comparing the vessel's position, relayed by the VDES ship-based station, with the base station's position, using a VHF signal propagation loss model and received power calculations to determine VDES signal propagation distances in complex maritime environments. Subsequent analysis evaluates whether the ship-to-shore distance falls within the acceptable propagation distance tolerance range.