# Marine UAV-USV Marsupial Platform: System and Recovery Technic Verification
###### Abstract  
Heterogeneous unmanned systems consisting of unmanned aerial vehicles (UAVs) and unmanned surface vehicles (USVs) have great application potential in marine environments. At present, the fully autonomous recovery of UAVs is a key problem that restricts any significant application of a heterogeneous unmanned system. This paper presents a novel fully autonomous recovery system, covering the entire process of recovery of small fixed-wing UAVs on mobile platforms at sea. We describe methods or solutions for the key problems encountered by the current system, including active modeling of the UAV-USV heterogeneous platform motion model, accurate estimation of the highly dynamic relative motion of the heterogeneous platform, dynamic analysis of the arresting cable system, and compliance control of the manipulator recovery system. Based on these methods, a physical simulation platform for the fully autonomous recovery system, including an actively adjustable arresting cable, manipulator compliance recovery system, and other subsystems, is developed and verified through experiments. The experiments show that the system proposed in this study can achieve full autonomous recovery of a small ship-based fixed-wing UAV with a high success rate in a short period. This system is the foundation for practical applications of UAV-USV heterogeneous unmanned systems inas a large-scale fast and efficient search operation tool, which will greatly expand the application of UAV-USV marsupial platform in marine environment. The marsupial unmanned platform is a multi-robot system composed of a mother robot and one or more child robots that has been studied and applied in some scenarios. Anderson developed a ground-ground marsupial platform MACS for ground information collection [4]. Corke developed a ground-air marsupial unmanned platform for the automatic laying of small sensing devices [5]. Fletcher developed an air-ground marsupial unmanned platform [6]. At present, air-to-water surface marsupial unmanned platforms are mainly composed of rotary UAVs and USVs [1; 2; 3; 7; 8]. For a marsupial platform consisting of fixed-wing UAVs and USVs, due to the difficulty of recovering fixed-wing UAVs on USVs, it is necessary to design an efficient and reliable autonomous recovery system for small fixed-wing UAVs. This is the key to the application of fixed-wing UAVs in marine environment. There are many types of ship-based fixed-wing UAV landing or recovery methods, as shown in Figure 1. Several typical landing or recovery methods include runway arrested landing (Figure 1a) [9], net recovery (Figure 1b) [10], parachute/parafoil recovery (Figure 1c) [11], cable-hook recovery (Figure 1d) [12], skyhook recovery, post-stall landing, bio-inspired perched landing, wind-sock recovery, and trapeze recovery [13]. Among these, runway arrested landing is mainly used for large ships, because large ships have large decks, their mass and size are large, and they are not sensitive to the movement of waves. The net recovery system requires high-end guidance accuracy and energy absorption buffer level. If a recovery error occurs, the UAV may hit the ship's equipment. Generally, this kind of dangerous recovery method is not used in such scenarios. In the method of parachute/parafoil recovery, the parachute needs to occupy the space of the fuselage with limited space. The UAV descends quickly, and the body is easily damaged by the stronger water impact. UAVs must have sufficient water resistance and need to be salvaged with professional marine salvage equipment. Skyhook recovery requires the use of a certain length of vertical suspension wire, which is difficult to achieve on small unmanned ships. Cable hook recovery is an effective means to realize automatic recovery under conditions such as large waves, wind, and turbulence in the open sea. It mainly works well with small fixed-wing UAVs [13]. However, a cable-hook recovery system with weak attitude adjustment capability will produce a situation similar to the net recovery.