  This letter addresses the problem of trajectory planning in a marsupial
robotic system consisting of an unmanned aerial vehicle (UAV) linked to an
unmanned ground vehicle (UGV) through a non-taut tether withcontrollable
length. To the best of our knowledge, this is the first method that addresses
the trajectory planning of a marsupial UGV-UAV with a non-taut tether. The
objective is to determine a synchronized collision-free trajectory for the
three marsupial system agents: UAV, UGV, and tether. First, we present a path
planning solution based on optimal Rapidly-exploring Random Trees (RRT*) with
novel sampling and steering techniques to speed-up the computation. This
algorithm is able to obtain collision-free paths for the UAV and the UGV,
taking into account the 3D environment and the tether. Then, the paper presents
a trajectory planner based on non-linear least squares. The optimizer takes
into account aspects not considered in the path planning, like temporal
constraints of the motion imposed by limits on the velocities and accelerations
of the robots , or raising the tether's clearance. Simulated and field test
results demonstrate that the approach generates obstacle-free, smooth, and
feasible trajectories for the marsupial system.
