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.© (1991) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.