  The problem of navigating a formation of interconnected tethered drones,
named STEM (System of TEthered Multicopters), in an unknown environment is
considered. The tethers feed electrical power from a ground station to the
drones and also serve as communication links. The presence of more than one
interconnected drone provides enough degrees of freedom to navigate in a
cluttered area. The leader drone in the formation must reach a given point of
interest, while the followers must move accordingly, avoiding interference with
the obstacles. The challenges are the uncertainty in the environment, with
obstacles of unknown shape and position, the use of LiDAR (Light Detection And
Ranging) sensors, providing only partial information of the surroundings of
each drone, and the presence of the tethers, which must not impact with the
obstacles and pose additional constraints to how the drones can move. To cope
with these problems, a novel real-time planning algorithm based on numerical
optimization is proposed: the reference position of each drone is chosen in a
centralized way via a convex program, where the LiDAR scans are used to
approximate the free space and the drones are moved towards suitably defined
intermediate goals in order to eventually reach the point of interest. The
approach is successfully tested in numerical simulations with a realistic model
of the system.
