# Marine UAV-USV Marsupial Platform: System and Recovery Technic Verification
###### Abstract
That is, if a recovery error occurs, the UAV may hit the ship's equipment. If the attitude active adjustment capability of the cable-hook recovery system can be greatly improved and coordinated in real time according to the flight conditions of the UAV, an efficient and safe autonomous recovery system can be possible. Figure 1: Several common small ship-based fixed-wing UAV recovery methods. (**a**) Runway arrested landing; (**b**) net recovery; (**c**) parachute/parafoil recovery; and (**d**) cable-hook recovery. In recent years, some novel UAV recovery methods have been developed. Zhang developed a hierarchical control structure for autonomous landing of fixed-wing UAVs [14]. Joo investigated a UAV navigation system with aid from an external camera for landing [15]. Kim developed an autonomous vision-based net recovery system for small fixed-wing UAVs [16]. Kong focused on the problem of landing an UAV in unknown and Global Navigation Satellite System (GNSS)-denied environments, based on an infrared stereo vision system [17]. Fan proposed a method to make UAV only use the airborne front-looking camera to find and align the runway [18]. Klausen used two multirotor-drone suspension nets to recover small fixed-wing UAVs [19; 20]. It coordinated the net-based recovery system and the fixed-wing UAV by constructing a virtual runway, and finally caused the fixed-wing UAV to crash into the net and land the net-based recovery system on the USV. However, these abovementioned methods and systems do not take into account the capture of the fixed-wing UAV from the net and the re-release of the UAV after landing on the USV, the fully autonomous recovery, and release of fixed-wing UAVs on USVs has not been achieved. In order to solve this problem, based on the analysis of existing methods, we have developed an adaptively adjustable cable hook recovery system, an autonomous flexible grasp-and-place UAV system, and realized full autonomous recovery of small fixed-wing UAVs on USV [21]. This system mainly includes an active adjusting cable hook system, a recovery net, and a compliant grasping and placing system. Its design diagram and physical simulation system are shown in Figure 2. The basic considerations of the proposed system design are motion compensation, wherein it compensates for USV lateral and longitudinal sway motions; UAV control deviations; and flexible mechanisms and ropes, where movement energy is quickly absorbed to reduce damage to the UAV. During the recovery process, the cable-hook system can actively adjust and maintain the attitude of the arresting cable according to the UAV's flight status, along with the attitude change of the USV and the undulating sea. At the same time, through coordinated adjustment with the UAV, its tail hook can be successfully hooked to the arresting cable, so that the speed of the UAV decreases rapidly and it is landed on the recovery net. The compliant grasping and placing system visually recognizes the position and attitude of the UAV and removes the UAV from the arresting rope and recovery net, using pneumatic suction cups, and places it in a storage box. The main recovery process is shown in Figure 3.  
The most significant contribution of this study is that, for the first time, a fully autonomous solution covering the entire process of using a small-scale fixed-wing UAVs from landing to recovery and placement is proposed. The key technology of this system is introduced, and the physical entity production and experimental verification of the system are carried out on this basis. This system can provide a very valuable reference for the fully autonomous recovery of small fixed-wing UAVs on USVs. A wide and thorough literature survey has revealed that, the study described here, has not been published yet. Figure 2: Full autonomous recovery system of ship-based small fixed-wing UAV. (**a**) Design diagram of the recovery system. (**b**) Physical simulation of the recovery system. The rest of this paper is organized as follows.