## 4 Results and Analysis
### Technical Theme Intensity Evolution  
The evolution of technical theme intensity can fully demonstrate changes and development trends over time. We calculate the intensity value of each technical topic in different years based on Formula (4) using the 35 identified technical topics, and draw a technical topic heat map, as depicted in Figure 5. The ordinate on the right is the intensity value; yellow indicates lower intensity, and red indicates higher intensity. The vertical axis on the left represents each technical theme. The horizontal axis represents the time window period.  
It can be seen from the technical theme heat map that the intensity of most technical themes fluctuates, which shows that most technical topics in the field of unmanned ship technology are not fixed. Still, over time, the research interest in related technologies has been adjusted to adapt to the development trends of the times. As can be seen from Figure 5, Hull structure (Topic #4) and Remote control (Topic #26) are hot-spot technologies. It has received widespread attention in various periods, and the importance of technical topics has always remained high. Further, a line chart is drawn to show the evolution trends of growth and decline in technology topics over time. Figure 6 reports the evolution trend of growth technology topics. It can be seen that Path planning (Topic #3), Surveying and mapping technology (Topic #27), Motion optimization (Topic #28), Target detection (Topic #31), Garbage cleaning (Topic #32), Water quality/water area detection (Topic #33) are growing. The intensity of these six technical themes shows an upward trend. The technical theme intensity is at a relatively high level. This indicates that these six technical topics are hot technologies in the current field of unmanned ship research. Related technologies are developing rapidly, and many related patents have been generated. Figure 7 reports the evolution trend of declining technology topics. Wireless communication (Topic #1), Automatic recovery system (Topic #9), Remote control system (Topic #11), Attitude control system (Topic #20), Acquisition technology (Topic #22), Image processing (Topic #25) showed fluctuating trends in different periods. This shows that these six technical topics are relatively mature technologies currently being researched in unmanned ships, and the research interest is gradually decreasing.  
\begin{table}
\begin{tabular}{c c} \hline Topic & Topic \\ \hline Topic \#24. & Drives, Motors, Propellers, Power, Farming, Feeding, Shafts, Transmission, Fields, Feeding \\ Topic \#25. & Image, Ultrasound, Camera, Receiver, Image-processing, Pixel, Classification, Area, Object, Body \\ Topic \#26. & Equipment, Operation, Monitoring, Remote, Water, Area, Technology, Field, Communication, Method, Work \\ Topic \#27. & Surface, Case, Screw, thread, Slide, Motor, Map, Gear, Structure, Frame, Shaft \\ Topic \#28. & Model Move Trajectory Parameter Problem Environment Predict Optimization Algorithm State \\ Topic \#29. & Antenna Chassis Test Shell Cable Catamaran Connector Circuit Board Rotating Shaft Structure \\ Topic \#30. & Solar, Energy, Battery, Power, Generation, Electricity, Electric, Battery, Panel, Generator, Wind, Wave, Utilisation \\ Topic \#31. & Target, Water, surface, Detection, Radar, Features, Vision, Area, Cohesion, Data, Utilization \\ Topic \#32. & Mechanisms, Garbage, Surface, Float, Regulating, Drive, Field, Technology, Floaters, Dynamics \\ Topic \#33. & Detection, Monitor, Water, quality, Water, body, Waters, Analyze, Watery, Sensor, Automation, Pollution \\ Topic \#34. & Area, Guidance, Water, Preset, Bathymetry, Terrain, Technology, Surface, Water, Sample, Operations \\ \hline \end{tabular}
\end{table}
Table 3: Cont.  
Figure 5: Technology topic heat map.  
Figure 6: Evolutionary trends of growth technology topics.  
Figure 7: Evolution trend of declining technology topics.