Acquisition and pointing control for inter-satellite laser communications

Communications and range estimation between formation members are essential functions of formation flying. The inherent features of laser free-space link: small weight and mass, power efficiency, broadband, high precision, availability of state-of-the-art components, make it a very attractive candidate for micro-satellite formation flying missions. A single two-way laser link, performing simultaneously communication and range estimation functions between formation spacecraft is being investigated in the framework of Broadband Laser Inter-Satellite Link (BLSIL), a joint Israeli-German applied research project. A unique technique of pointing control system for an intersatellite laser communication and ranging link is presented. In this system, the same laser is used both as a beacon and as a transmitter. To maximize the traffic capacity of the link, the beamwidth is controlled from broad in the acquisition stage, to narrow in the tracking stage. Laser link pointing control presents difficult demands from the beam pointing precision. To establish and maintain the laser link between the satellites in orbit, pointing control must be fast and precise. A hierarchic two-level system control is proposed. The lower level controls the gimbals of the optical head of the electro-optical transceiver; the higher level is a fast closed loop that simultaneously controls the beamwidth and direction. It is the primary subject of the paper. Initial pointing acquisition assumes exchange of position-related information between the satellites to establish the initial line of sight (LOS). After initial acquisition a cooperative extremum-seeking algorithm is used for the fast higher level closed loop control. The paper gives a detailed description of the control algorithms. Representative simulations are presented.