*Article* **Underwater Acoustically Guided Docking Method Based on Multi-Stage Planning**

**Hongli Xu \*, Hongxu Yang , Zhongyu Bai and Xiangyue Zhang**

Faculty of Robot Science and Engineering, Northeastern University, Shenyang 110819, China; yanghongxu@stumail.neu.edu.cn (H.Y.); zhongyubai@stumail.neu.edu.cn (Z.B.);

zhangxiangyue@mail.neu.edu.cn (X.Z.)

**\*** Correspondence: xuhongli@mail.neu.edu.cn

**Abstract:** Autonomous underwater vehicles (AUVs) are important in areas such as underwater scientific research and underwater resource collection. However, AUVs suffer from data portability and energy portability problems due to their physical size limitation. In this work, an acoustic guidance method for underwater docking is proposed to solve the problem of persistent underwater operation. A funnel docking station and an autonomous remotely operated vehicle (ARV) are used as the platform for designing the guidance algorithms. First, the underwater docking guidance is divided into three stages: a long-range approach stage, a mid-range adjustment stage and a shortrange docking stage. Second, the relevant guidance strategy is designed for each stage to improve the docking performance. Third, a correction method based on an ultra-short baseline (USBL) system is proposed for the ARV's estimate of the depth, relative position and orientation angle of the docking station. To verify the feasibility of the docking guidance method, in this work, tests were performed on a lake and in a shallow sea. The success rate of autonomous navigation docking on the lake was 4 out of 7. The success rate of acoustic guidance docking on the lake and in the shallow sea were 11 out of 14 and 6 out of 8, respectively. The experimental results show the effectiveness of the docking guidance method in lakes and shallow seas.

**Keywords:** underwater docking; acoustic guidance; autonomous navigation; autonomous underwater vehicle; autonomous remotely operated vehicle; docking station
