  We study the problem of covering an environment using an Unmanned Aerial
Vehicle (UAV) with limited battery capacity. We consider a scenario where the
UAV can land on an Unmanned Ground Vehicle (UGV) and recharge the onboard
battery. The UGV can also recharge the UAV while transporting the UAV to the
next take-off site. We present an algorithm to solve a new variant of the area
coverage problem that takes into account this symbiotic UAV and UGV system. The
input consists of a set of boustrophedon cells -- rectangular strips whose
width is equal to the field-of-view of the sensor on the UAV. The goal is to
find a coordinated strategy for the UAV and UGV that visits and covers all
cells in minimum time, while optimally finding how much to recharge, where to
recharge, and when to recharge the battery. This includes flight time for
visiting and covering all cells, recharging time, as well as the take-off and
landing times. We show how to reduce this problem to a known NP-hard problem,
Generalized Traveling Salesperson Problem (GTSP). Given an optimal GTSP solver,
our approach finds the optimal coverage paths for the UAV and UGV. Our
formulation models multi-rotor UAVs as well as hybrid UAVs that can operate in
fixed-wing and Vertical Take-off and Landing modes. We evaluate our algorithm
through simulations and proof-of-concept experiments.
