  Marine waves significantly disturb the unmanned surface vehicle (USV) motion.
An unmanned aerial vehicle (UAV) can hardly land on a USV that undergoes
irregular motion. An oversized landing platform is usually necessary to
guarantee the landing safety, which limits the number of UAVs that can be
carried. We propose a landing system assisted by tether and robot manipulation.
The system can land multiple UAVs without increasing the USV's size. An MPC
controller stabilizes the end-effector and tracks the UAVs, and an adaptive
estimator addresses the disturbance caused by the base motion. The working
strategy of the system is designed to plan the motion of each device. We have
validated the manipulator controller through simulations and well-controlled
indoor experiments. During the field tests, the proposed system caught and
placed the UAVs when the disturbed USV roll range was approximately 12 degrees.
