  This article presents a three-dimensional nonlinear trajectory tracking
control strategy for unmanned aerial vehicles (UAVs) in the presence of spatial
constraints. As opposed to many existing control strategies, which do not
consider spatial constraints, the proposed strategy considers spatial
constraints on each degree of freedom movement of the UAV. Such consideration
makes the design appealing for many practical applications, such as pipeline
inspection, boundary tracking, etc. The proposed design accounts for the
limited information about the inertia matrix, thereby affirming its inherent
robustness against unmodeled dynamics and other imperfections. We rigorously
show that the UAV will converge to its desired path by maintaining bounded
position, orientation, and linear and angular speeds. Finally, we demonstrate
the effectiveness of the proposed strategy through various numerical
simulations.
