  In this paper, we consider a scenario where an unmanned aerial vehicle (UAV)
collects data from a set of sensors on a straight line. The UAV can either
cruise or hover while communicating with the sensors. The objective is to
minimize the UAV's total flight time from a starting point to a destination
while allowing each sensor to successfully upload a certain amount of data
using a given amount of energy. The whole trajectory is divided into
non-overlapping data collection intervals, in each of which one sensor is
served by the UAV. The data collection intervals, the UAV's speed and the
sensors' transmit powers are jointly optimized. The formulated flight time
minimization problem is difficult to solve. We first show that when only one
sensor is present, the sensor's transmit power follows a water-filling policy
and the UAV's speed can be found efficiently by bisection search. Then, we show
that for the general case with multiple sensors, the flight time minimization
problem can be equivalently reformulated as a dynamic programming (DP) problem.
The subproblem involved in each stage of the DP reduces to handle the case with
only one sensor node. Numerical results present insightful behaviors of the UAV
and the sensors. Specifically, it is observed that the UAV's optimal speed is
proportional to the given energy of the sensors and the inter-sensor distance,
but inversely proportional to the data upload requirement.
