  For acquisition of narrow-beam free-space optical (FSO) terminals, a Global
Positioning System (GPS) is typically required for coarse localization of the
terminal. However, the GPS signal may be shadowed, or may not be present at
all, especially in rough or unnameable terrains. In this study, we propose a
lidar-assisted acquisition of an unmanned aerial vehicle (UAV) for FSO
communications in a poor GPS environment. Such an acquisition system consists
of a lidar subsystem and an FSO acquisition subsystem: The lidar subsystem is
used for coarse acquisition of the UAV, whereas, the FSO subsystem is utilized
for fine acquisition to obtain the UAV's accurate position. This study
investigates the optimal allocation of energy between the lidar and FSO
subsystems to minimize the acquisition time. Here, we minimize the average
acquisition time, and maximize the cumulative distribution function of
acquisition time for a fixed threshold. We learn that an optimal value of the
energy allocation factor exists that provides the best performance for the
proposed acquisition system.
