  In this correspondence, 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.
