Acquisition and Fine-pointing Control for a 400-Mbps Link Between a Low-Earth Orbiter and a Geostationary Satellite

Abstract

The possibility of using laser systems for space-based communications is attracting considerable attention in Europe in connection with the future European data relay satellite. The choice of a semiconductor laser as transmitter is only workable if a high-accuracy control system can be developed for precision beam pointing. The accuracy requirement is sub-microradian. Another major challenge is the acquisition of the line of sight (between the two spacecraft) over a relatively large cone of uncertainty (apex angle of 0.4 °). This paper gives details of a proposed definition for an acquisition and tracking subsystem using semiconductor technology. For a link between a spacecraft in low-Earth orbit and a geostationary relay satellite, the separation is taken as being 45,000 km. Two different versions of the acquisition and tracking subsystem are considered, depending on the mission specifications: - version 1 uses a pulsed diode laser and is suitable for applications where no significant volume of data is transmitted over the forward link (GEO -- LEO direction according to the established terminology); - version 2 uses a continuous-wave diode laser and can accommodate a 25 Mbps forward link. In both cases, the return link is rated to carry 400 Mbps. A different optical subsystem is proposed for each version. To demonstrate the feasibility of the pulsed version, a breadboard model of the LEO acquisition and tracking subsystem has been built. Acquisition time of around 4 s have been measured for an initial beam pointing error of 0.25 prod. Aspects of the GEO optical beacon have also been investigated using an optical fiber bundle. This permits the illumination of the entire GEO cone of uncertainty while maintaining speckle modulation at an acceptable level.