Tracking System Design For Small Aperture Multiple Access Laser Communications Terminals

Abstract

Current activities in the field of pointing, acquisition and tracking (PAT) system design for free space laser communications generally have as their main drivers a bit rate greater than 10 Mb/s and the low laser power now available from suitable laser diodes (typically 50 mW). These have resulted in large terminals with apertures of 20 to 35 cm and telescopes around one metre long. This paper presents the results of a study performed by British Aerospace which looked at a system using much smaller apertures (around 5 cm) and which would result in more compact terminals. Such volume efficiency introduces the possibility of a single 'central node' satellite communicating simultaneously with multiple user satellites in different orbits. Links at 2 Mb/s with up to 8 users was the design case. This paper shows that the PAT system for a multiple access node is not simply a miniaturised version of one for a large aperture single access system..The design features low friction actuators, acquisition and tracking functions combined in one sensor and a robust solution to the problem of host satellite vibration. Acquisition times of 2 seconds are anticipated provided the central node is already tracking at least one user.