# Deep-Sea Underwater Cooperative Operation of
## 2 Related Studies on Cooperative Operations
### Unmanned Surface Vehicles and Autonomous Underwater Vehicles  
The cooperative operation of unmanned surface vehicles (USVs) and AUVs can be generally divided into two modes, single USV to single AUV and single USV to multiple AUVs. The role of the USV is to act as a communication relay station between the support (mother) ship and the AUVs, which in turn are responsible for transmitting data and issuing instructions to realize cooperative operation of multi-AUV cluster operations to promote the formation of a three-dimensional grid and a large-scale ocean observation system. WHOI proposed a three-phase vision for deep-sea operations without requiring a research vessel R/V [29]. The first phase was unmanned by supporting the deployment of the USV and the AUV on the research vessel R/V. However, there was a restriction that the research vessel R/V must remain within the ultrashort baseline (USBL) operational range when the AUV mission is launched by the USV, which acts as an intermediary. Nonetheless, the research vessel R/V was required for all launch and recovery operations, with a repeat cycle of one or more days. The second phase realized the ridge segment's large-scale autonomous exploration with the USV and AUG. The research vessel R/V was needed only to complete the initial launch of the USV and AUGs and the recovery after the entire detection mission was  
Figure 3: Cooperative operation of Canghai Lander and Fendouzhe HOV. The Chinese characters on HOV is Fendouzhe, which is the name of HOV. completed by the USV and AUG, autonomously. The third phase adopted a new generation of USV to realize oceanic basin's large-scale autonomous exploration and monitoring. In addition to undertaking AUV navigation and communication between the surface-based center and AUV, this type of USV can perform an autonomous launch as well as charge and recover unmanned submersibles, which significantly improves the marine endurance and investigation range and reduces operating costs. Ocean Infinity proposed a wide-area ocean exploration system based on pairwise matching of USVs and AUVs [30]. The entire system comprised of a single research vessel R/V and a fleet of USV-AUV pairs. After launching the USVs and AUVs, the AUVs shook hands with their corresponding USVs before entering the mission phase. During the mission, each AUV communicated with its paired USV through underwater high precision acoustic positioning system (HiPAP) only, and the AUV sent the status data to the USV, which in turn transmitted the data to the research vessel R/V and returned the command of the research vessel R/V (change direction or terminate the mission) to the AUV simultaneously. The USV therefore tracked the AUV and communicated with the research vessel R/V as an information relay. The \"Zipangu of the sea\" project proposed a regional underwater positioning and communication system comprising a semi-submersible USV, a hovering AUV, and three surface buoys to support multi-AUV cluster operations [31,32]. The semi-submersible USV was used as a moving surface relay system for positioning and communication. It obtained its own position data using global navigation satellite system (GNSS) antenna, established communication with the research vessel R/V through a wireless local area network or iridium communication and provided positioning and communication for unmanned submersibles through the USBL and underwater acoustic communication systems (UACS) [33,34]. The hovering AUV was located at any depth and acted as an underwater communication relay system between the research vessel R/V and the unmanned submersibles. Therefore, it maintained acoustic communication with the research vessel R/V, semi-submersible USV, and AUVs without being affected by the sea surface or the sea floor [35,36]. Three buoys formed a sea surface long baseline system. In recent years, with the main focus on the marine stereoscopic observation network, China has successively launched AUGs, AUVs, and USVs; buoys; and other marine observation equipment networking operations.