A free-space laser communication system has been designed and partially developed as an alternative to standard RF
links from UAV to ground stations. This project belongs to the SINTONIA program (acronym in Spanish for low
environmental-impact unmanned systems), led by BR&TE (Boeing Research and Technology Europe) with the purpose
of boosting Spanish UAV technology.
A MEMS-based modulating retroreflector has been proposed as a communication terminal onboard the UAV, allowing
both the laser transmitter and the acquisition, tracking and pointing subsystems to be eliminated. This results in an
important reduction of power, size and weight, moving the burden to the ground station. In the ground station, the ATP
subsystem is based on a GPS-aided two-axis gimbal for tracking and coarse pointing, and a fast steering mirror for fine
pointing. A beacon-based system has been designed, taking advantage of the retroreflector optical principle, in order to
determine the position of the UAV in real-time. The system manages the laser power in an optimal way, based on a
distance-dependent beam-divergence control and by creating two different optical paths within the same physical path
using different states of polarization.