  In this paper, we consider a new unmanned aerial vehicle (UAV)-assisted
oblique image acquisition system where a UAV is dispatched to take images of
multiple ground targets (GTs). To study the three-dimensional (3D) UAV
trajectory design for image acquisition, we first propose a novel UAV-assisted
oblique photography model, which characterizes the image resolution with
respect to the UAV's 3D image-taking location. Then, we formulate a 3D UAV
trajectory optimization problem to minimize the UAV's traveling distance
subject to the image resolution constraints. The formulated problem is shown to
be equivalent to a modified 3D traveling salesman problem with neighbourhoods,
which is NP-hard in general. To tackle this difficult problem, we propose an
iterative algorithm to obtain a high-quality suboptimal solution efficiently,
by alternately optimizing the UAV's 3D image-taking waypoints and its visiting
order for the GTs. Numerical results show that the proposed algorithm
significantly reduces the UAV's traveling distance as compared to various
benchmark schemes, while meeting the image resolution requirement.
