## 4 Experimental Results and Analysis
### Analysis of Ship Anomaly Detection Results  
From the above experimental analysis, it can be seen that ships with similar shapes and motion patterns are likely to cause classification confusion, such as cargo ships and tanker ships, fishing ships, passenger ships, and tug ships. However, there are also some cases in which the classification labels of some ships are quite different from their message types. For example, in Figures 4 and 5, the labels of ships in AIS messages are cargo ships, whereas the model gives fishing ship labels. This may be because these ships evade detection by forging and transmitting inappropriate AIS messages. As an important part of maritime surveillance, ship anomaly detection is of great significance for ensuring maritime safety. In this section, case studies are conducted on ships with anomalous classification results to verify the effectiveness of the proposed method.  
In this section, a ship with the MMSI code 367588710 was first selected from the anomalous classification results for anomaly detection analysis. In the AIS database, the type attribute field of its messages shows that it is a cargo ship, whereas the output of the classification model in this paper is a fishing ship. According to the trajectory drawn by its historical AIS messages, as shown in Figure 6, it can be seen that the ship's movement range is small, the longitude range is less than 5\({}^{\circ}\), and the movement trajectory is complicated, which is inconsistent with the trajectory characteristics of a cargo ship's linear long-distance navigation.  
The Marine Traffic website [41] is one of the most popular online ship tracking service websites in the world. It can provide a real-time chart of global maritime routes online, including ship pictures, ship origin, ship destination, estimated route, IMO number, etc. The registered photos and relevant information about this ship (MMSI: 367588710) on the Marine Traffic website are shown in Figure 7. It can be seen that this ship does not conform to the characteristics of general cargo ships in appearance, as there is a common fishing rod structure among fishing ships. In summary, the ship is likely an anomalous ship that is a fishing ship but broadcasts the wrong ship type.  
In this section, the ship (MMSI: 701006130), whose message attribute is displayed as a passenger ship in the AIS database, is selected for anomaly detection analysis. The classification model proposed in this paper classifies this ship as a fishing ship. Figure 8 shows the historical trajectory of this ship. It can be seen that the motion range of this ship is small, and the range of longitude and latitude does not exceed 5\({}^{\circ}\). In addition, the trajectory of this ship is staggered and complicated, which is inconsistent with the trajectory characteristics of the passenger ship; thus, this ship is also very likely to be an anomalous ship that is a fishing ship but broadcasts the wrong ship type.  
Figure 6: The historical trajectory of the ship (MMSI: 367588710).  
The registration information and photos of this ship retrieved from the Marine Traffic website are shown in Figure 9. It can be seen that the type of this ship is a fishing ship, which further proves the effectiveness of the proposed method in this paper.  
Figure 8: The historical trajectory of the ship (MMSI: 701006130).  
Figure 7: The registration information and photos of the ship (MMSI: 367588710) on the Marine Traffic website.  
Figure 9: The registration information and photos of the ship (MMSI: 701006130) on the Marine Traffic website.