KEYWORDS: Acquisition tracking and pointing, Sensors, Staring arrays, Free space optical communications, Laser applications, Control systems, Signal processing, Free space optics, Interfaces, Optical tracking
The overall architecture and the performances of the pointing, acquisition, and tracking (PAT) subsystem of the European SILEX program are examined. The mission constraints are described, and attention is given to the retained baseline built around a two-stage beam steering system, an open loop point-ahead control, and a full digital implementation. Performance prediction is based on signal processing, control algorithms, and detailed modeling of equipment and flexible structures. The way in which an accurate performance of the order of a microradian can be achieved with a correct allocation of the performance for each instrument of the system is shown. The rejection of the microvibrations coming from the platform when the terminal is operating is singled out as a key feature of the design. The PAT equipment and softwares - sensors, electromechanisms, and control units - are presented.
A description of the simulation model and the on-ground performances validation of the Acquisition and Tracking (PAT) subsystem and of the European SILEX program is presented. The SILEX scenario is composed of two terminals mounted on LEO orbit (SPOT 4) and geostationary orbit (ARTEMIS) spacecrafts. The PAT subsystem is in charge of the initialization of the link, of the tracking of the incoming beam and of the pointing of the emitted laser beam. A short description of the retained strategies (phases of the PAT mission and operating modes) and functional architecture (control laws and sampling frequencies) is presented. In order to validate the PAT performance assessment, a complete simulation software is developed. The model (equipment modeling, dynamical simulation of the terminal structure, and operating modes management) and the associated outputs are examined in detail. The chosen design and architecture of the PAT validation on a system testbed (STB) are described. The validation of a pointing system for optical intersatellite communication, thanks to the combination of simulations and STB measurements, is shown.
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