  In this paper, we consider the cellular Internet of unmanned aerial vehicles
(UAVs), where UAVs sense data for multiple tasks and transmit the data to the
base station (BS). To quantify the "freshness" of the data at the BS, we bring
in the concept of the age of information (AoI). The AoI is determined by the
time for UAV sensing and that for UAV transmission, and gives rise to a
trade-off within a given period. To minimize the AoI, we formulate a joint
sensing time, transmission time, UAV trajectory, and task scheduling
optimization problem. To solve this problem, we first propose an iterative
algorithm to optimize the sensing time, transmission time, and UAV trajectory
for completing a specific task. Afterwards, we design the order in which the
UAV performs data updates for multiple sensing tasks. The convergence and
complexity of the proposed algorithm, together with the trade-off between UAV
sensing and UAV transmission, are analyzed. Simulation results verify the
effectiveness of our proposed algorithm.
