  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.
