The thesis work presented here deals with the control strategy adopted to maintain a space system in a desidered work condition during the observation phase of its mission. The system consists of three spacecrafts displaced in a triangular formation. In each of the them there are two floating test masses and it is equipped with sensors and actuators. The whole system is supposed to work as an interferometer. Its mission is to observe Gravitational Waves in a certain frequency range (0.1mHz-100mHz). The goal of the thesis is to design the MIMO controllers for the actuators, i.e. micro-thrusters and the electro-static suspensions, to maintain the two test masses at the center of the spacecraft by minimizing the effect of disturbance.