Page 90 - Innovations in Intelligent Machines

80 I.K. Nikolos et al.
search and rescue scenarios or exploration of unknown environments (space-
oriented applications). In order to establish a reliable and efficient frame-
work for the cooperation of a number of UAVs several problems have to be
encountered:
• UAV task assignment problem: a number of UAVs is required to per-
form a number of tasks, with predefined order, on a number of targets.
The requirements for a feasible and efficient solution include taking into
account: task precedence and coordination, timing constraints, and flyable
trajectories [8]. The task re-assignment problem should be also considered,
in order to take into account possible failure of a UAV to accomplish its
task. The task assignment problem is a well-known optimization problem;
it is NP-hard and, consequently, heuristic techniques are often used.
• UAV path planning problem: a path planning algorithm should provide
feasible, flyable and near optimal trajectories that connect starting with
target points. The requirement for feasible trajectories dictates collision
avoidance between the cooperating UAVs as well as between the vehicles
and the ground. The requirement of flyable trajectories usually dictates
a lower bound on the turn radius and speed of the UAVs [8]. Addition-
ally, an upper bound for the speed of each UAV may be required. The
path optimality can be defined in various ways, according to the mission
assigned. However, a typical requirement is to minimize the total length
of the paths.
• Data exchange between cooperating UAVs and data fusion: exchange
of information between cooperating UAVs is expected to enhance the
effectiveness of the team. However, in real world applications communica-
tion imperfections and constraints are expected, which will cause coordi-
nation problems to the team [9]. Decentralized implementations of the
decision and control algorithms may reduce the sensitivity to communi-
cation problems [10].
• Cooperative sensing of the targets: the problem is defined as how to
co-operate the UAV sensors in terms of their locations to achieve optimal
estimation of the state of each target [11] (a target localization problem).
• Cooperative sensing of the environment: the problem is defined as how
to cooperate the UAV sensors in order to achieve better awareness of
the environment (popup threats, changing weather conditions, moving
obstacles etc.). In this category we may include the coordinated search of
a geographic region [12].

1.3 Path Planning for Single and Multiple UAVs

Compared to the path-planning problem in other applications, path planning
for UAVs has some of the following characteristics, according to the mission
[13, 14, 15]: