## 2 Design of Real-Time Communication Subsurface Mooring System
### Hydrodynamic Analysis of SCB
As shown in Figure 5, the inclination angle of the SCB changed periodically in the range of \(-25^{\circ}\)-\(20^{\circ}\), and the variation period was about 4 s [18]. The test results show that the communication of the satellite module was normal when the swing angle was less than 30\({}^{\circ}\) in a non-occluded environment [19; 20]. Therefore, as can be seen in the simulation results, the SCB realized real-time data transmission under the condition of no higher than the fourth-level sea state. In order to verify the effectiveness of the above simulation analysis of the SCB, the sea trial was carried out in the sea near Hainan Island, and the sea state during the offshore sea trial was about level 3-4, which was close to the above simulation environment conditions. As shown in Figure 6, the tiltmeter sensor was installed at the center of gravity of the SCB and kept parallel to it. The tiltmeter used the MAT-1 Data Logger from  
Figure 4: The height of the antenna above the periodical sea surface changes. Figure 5: The inclination angle of periodical SCB changes. Lowell Instruments, which contains an integrated three-axis magnetometer and three-axis accelerometer. Through the coordinate transformation, the varied history of the SCB could be obtained by recording the three-axis angle value through the sensor data with the sampling frequency set at 1 Hz. Figure 7 shows that the inclination angle of SCB changes within 60 s. The swing period of the SCB under the influence of the wind and waves was 4 s with an average change amplitude of 18\({}^{\circ}\). The average maximum inclination angle was 20\({}^{\circ}\), and the inclination angle mostly varies periodically from \(-25^{\circ}\) to 20\({}^{\circ}\), which was consistent with the change period and maximum inclination angle of the SCB in the numerical simulation results. During this test, the real-time communication subsurface mooring system was deployed about 42 h, and the data return interval was 15 min. During the experiment, the SCB should have transmitted168 data packets, while the final number of data packets received by the shore station was 160. The success rate of the data transmission was over 95%. A preliminary analysis found that the period of data packet loss was concentrated between 1:00 and 6:00 on the second day. During this period, the SCB was affected by the severe sea state at night, which increased the inclination angle and reduced the water outlet height, causing the success rate of the data transmission to be slightly reduced. Figure 6: Tiltmeter sensor installation location on the SCB. Figure 7: The inclination angle of SCB changes within 60 s.  
In summary, by comparing the simulation results of the SCB with the sea trial results, it was found that the motion data variation of the SCB was consistent, which verified the correctness of the simulation results. The sea trial results show that the SCB could maintain a relatively stable attitude and water exit height under the severe sea state. This indicates the stable and reliable transmission of the submerged sensors data, and the communication success rate exceeded 95%. Combined with the above numerical simulation and sea trial results, the structural shape of the SCB can be further optimized to reduce its swing angle, increase the water outlet height of the antenna, improve the ability of the SCB body to resist wind and waves, and further improve the data transmission stability during long-term operation.