Pointing, Acquisition And Tracking (PAT) Subsystems And Components For Optical Space Communication Systems

Abstract

One important subsystem for a transceiver package useable for optical space communication is the so-called Pointing, Acquisition and Tracking (PAT) Subsystem. Due to the highly collimated laser beams very precise beam steering is required. The PAT Subsystem is mainly involved in the acquisition procedure and has to guarantee the proper tracking of the line of sight so that the communication requirements, e.g. the bit error rate, can be met during the complete tracking phase. Furthermore the PAT Subsystem must be able to cover an angular range of more than a hemisphere if it is used on a low earth orbit (LEO) satellite which communicates with a geostationary (GEO) one. To meet the requirements of the PAT with respect to its dynamic and kinematic behaviour it is composed of three mechanisms which are the Coarse Pointing Assembly (CPA), the Fine Pointing Assembly (FPA) and the Point-Ahead Assembly (PAA). The interactions of these mechanisms are controlled by appropriate electronic and computing units which include the Control Law Electronics (CLE). This paper describes a complete PAT Subsystem for a CO2 laser communication package. All mechanisms and electronic units of this subsystem have been developed under ESTEC and DFVLR contracts. The most important features and test results are presented. Furthermore, design approaches of specific PAT mechanisms are illustrated. These mechanisms are developed within the scope of the SILEX project (Semi-conductor Laser Intersatellite Link Experiment) which is part of ESA's PSDE (Payload and Spacecraft Development and Experimentation) programme and aims at hardware which is different from the CO2 hardware.