# Research on Laser Dual-Mode Fusion Detection Method of Ship Wake Bubbles
## 5 Experiment and Analysis of Ship's Wake Dual-Mode Fusion Detection
### Image Detection Module Results  
The molybdenum wire plate is placed at a distance of 10 m from the detection system, and the current is constant at 0.35 A. After a period of time, the ceramic tube is placed near the molybdenum wire plate, and the ceramic tube is used to simulate the false wake flow for about 20 s, then the ceramic tube is closed. The results collected before electrification are shown in Figure 17a, the results obtained after electrification are shown in Figure 17b, the results of generating simulated false wakes are shown in Figure 17c, and the results obtained after Gaussian filtering and adaptive threshold segmentation are shown in Figure 17d. By comparing the three images, it is found that when there are weak bubbles in the field of view, due to the influence of the optical characteristics of the bubbles themselves, the laser echo will be enhanced, thus showing an increase in the gray value on the image. In order to further verify the feasibility of the method, multi-frame images are processed, and the changes in the maximum and minimum gray values in the image are counted. The results are shown in Figure 17. At this time, the SBR can reach up to about 16, and the detection SBR is nearly 5 times higher than that of the laser adaptive detection mode. The up and down beats of the maximum and minimum gray values are caused by the fact that there are not always target bubbles at the focus, and the reliability is worse than that of the laser APD detection mode. When the false wake occurs, the detection SBR can reach up to 31, and the detection SBR is 10 times higher than that of the laser adaptive detection mode. When the false wake occurs, the detection SBR is nearly 2 times higher than that of the simulated wake detection SBR, and the change is obvious. After passing through the junction of the simulated wake and the false wake, the signal amplitude gradually decreases and slowly recovers to the simulated wake signal change. According to [1], when the detection system is gradually close to the ship, the stronger the laser echo signal is, the overall signal change is similar to a slow upward trend. When there is a false wake, there will be an obvious 'bump' in the signal change. It is verified that the detection mode has a good detection SBR and can effectively resist the existence of false wakes, which reduces the false alarm rate of laser tracking detection of ship's wakes.  
Through the analysis of simulation and outdoor experimental data, the multi-timescale hierarchical signal processing method can simultaneously detect and identify remote sparse, discrete microbubbles and short-range dense, continuous large bubbles at the millisecond timescale. Through multi-period cumulative processing analysis at the second timescale, the detection SBR is further improved, and it has strong environmental adaptability and real-time processing capability. The results obtained by the image processing algorithm based on time difference accumulation can also identify the existence of false wakes and improve the detection SBR. Through the analysis of simulation and outdoor experimental data:  
* the multi-timescale hierarchical signal processing method can simultaneously detect and identify remote sparse, discrete microbubbles and short-range dense, continuous large bubbles at the millisecond timescale;
* through multi-period cumulative processing analysis at the second timescale, the detection SBR is further improved, and it has strong environmental adaptability and real-time processing capability;
* the results obtained by the image processing algorithm based on time difference accumulation can also identify the existence of false wakes and improve the detection SBR.  
By combining the advantages of the two methods, the detection SBR of the ship's wake can be further improved and the false alarm rate of the laser detection of the ship's wake can be reduced.