*4.3. Acoustic Guidance Docking Tests in the Shallow Sea*

In the shallow sea acoustic guidance docking tests, the ARV started the docking task with a random initial heading angle at a random position between 20 m and 500 m from the docking station. The long-guidance waypoints were set at 220 m, 180 m, 120 m and 80 m ahead of the docking station, and the sailing speed was set at 2 knots for the first waypoint path mode and 1 knot for the remaining stages.

The docking station was deployed 1.5 m to 2.5 m underwater, and the position and the orientation angle of the docking station were measured and input to the ARV autonomous docking module as preset values.

Real images and video screenshots of the autonomous docking tests in the shallow sea are shown in Figure 14. A total of eight acoustic guidance docking tests were performed, of which six docking tests were successful. The records of the docking tests are shown in Table 5.

**Table 5.** Records of acoustic guidance docking tests in the shallow sea.


**Figure 14.** Autonomous docking tests in the shallow sea.

The track and the deviation curve of the seventh acoustic guidance docking test are shown in Figure 15. To overcome the influence of the complex underwater environment on ARV navigation and USBL reception, more waypoints were set in the long-range approach stage, adjustments were made from a longer distance to reduce lateral deviation during docking and more USBL positioning information was received to locate the relative position of the docking station more accurately. It can be seen that the ARV has tracked to the centreline of the docking station when it sailed to 180 m ahead of the docking station. During the approach, as the position and orientation angle of the docking station are updated, the centreline of the docking station will also change, resulting in an increase in the lateral deviation of the ARV. Through continuous adjustment, the ARV can track the centreline of the docking station in real time and complete the docking successfully.

**Figure 15.** (**a**) Track of acoustic guidance docking test in the shallow sea. (**b**) Deviation curve of acoustic guidance docking test in the shallow sea.

We have compared the success rates of some classical docking tests in the sea, as shown in Table 6. It can be seen that the docking algorithm in this paper has improved the docking success rate, demonstrating the reliability of the docking algorithm. The heading adjustment algorithm in the mid-range adjustment stage effectively reduced the lateral deviation during docking. The real-time adjustment of the depth, relative position and orientation of the docking station using USBL effectively overcame the depth and orientation changes of the docking station in the water as well as the cumulative errors generated by the ARV inertial navigation. All these were helpful for the improvement of docking success rate.


**Table 6.** Comparison of success rates of sea docking tests.

After analysis, the following reasons are summarised for the failures of the acoustic guidance docking tests in the shallow sea:


#### **5. Conclusions**

Aiming at persistent operation of autonomous underwater vehicles, this paper proposes an underwater docking method based on acoustic guidance. According to the structural properties of the funnel docking station, the underwater docking is divided into three stages. To solve the problem of autonomous navigation deviation in underwater docking, a USBL-based correction method for the docking station depth, relative position and orientation angle is proposed. Autonomous navigation docking tests on a lake were first performed, with a docking success rate of 4 out of 7. Next, acoustic guidance docking tests on the lake were performed, and the docking success rate was 11 out of 14. Finally, through shallow sea tests, acoustic guidance docking tests were performed, with a docking success rate of 6 out of 8. The following conclusions can be drawn from the analysis of the test data: the average lateral error of the autonomous navigation docking was 0.077 m, which met the required accuracy for docking, however, due to the cumulative error of inertial navigation, the docking success rate was low; acoustic guidance docking tests on the lake improved the docking success rate by 17.9% by correcting the ARV's estimate of the depth, relative position and orientation angle of the docking station, verifying the reliability of the acoustic guidance algorithm on the lake; finally, in acoustic guidance docking tests in a shallow sea, due to the complexity of the ocean environment, the success rate of the docking was reduced. Therefore, improving the docking success rate of acoustic guidance in the sea and increasing the stability of the docking guidance algorithm are the focus of future research.

**Author Contributions:** Conceptualisation, H.X.; methodology, H.X., Z.B., X.Z. and H.Y.; software, Z.B. and H.Y.; validation, H.X., Z.B., X.Z. and H.Y.; formal analysis, H.X. and H.Y.; data curation, H.Y.; writing—original draft preparation, H.Y.; writing—review and editing, Z.B.; project administration, H.X.; funding acquisition, H.X. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was supported by the National Defense Preliminary Research Project under grant No. 50911020604, by the Fundamental Research Funds for the Central Universities under grant N2126006, by the Fundamental Research Funds for the Central Universities under grant N2326004 and by the National Natural Science Foundation of China under grant 62203099.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflicts of interest.

#### **Abbreviations**

The following abbreviations are used in this manuscript:


#### **References**


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