Pointing Acquisition And Tracking System For Silex Inter Satellite Optical Link

This paper is concerned with the architecture and functional analysis of the Pointing Acquisition and Tracking (PAT) subsystem of the European Silex program (Semiconductor laser Intersatellite Link Experiment). The Silex scenario is composed of two optical terminals mounted on low earth orbit, and geosynchronous orbit spacecrafts. High rate communication between the two terminals requires very accurate tracking performances over a high bandwidth taking into account satellite vibrations as well as PAT electromechanical or optical noise. In a first part, a detailed presentation of the retained functional architecture in terms of control laws and strategies, steering algorithms and related signal processing is given, while a second part presents the design choices retained for the PAT equipments. Emphasis will be given on the different operating modes strategy and switching required by the different phases of the PAT mission : acquisition, tracking and pointing. Finally, a performance analysis is carried out that concerns the rejection capacity with respect to vibrations generated by the host spacecraft, as well as optical sensors noise and mechanism error effects. Spacecraft vibrations profile is derived from in flight measurements on SPOT1 satellite and on ground measurements on SPOT2. Performance analysis will be illustrated by time and frequency simulations based on equipment modelling, and actual performances measurements derived from current associated technological programs.