On-Orbit Servicing is meant to revolutionize the aerospace industry, focusing on the sustainability of space activities. Such missions rely on robotic manipulators allowing to perform life-extending operations on existing assets or to mitigate the problems related to space debris. In recent years, the idea of a Space Tug, a multi-purpose vehicle capable of operating in complete autonomy, was developed in forecast of the importance that on-orbit servicing will play as a consequence of the increment of Earth orbiters. The aim of this work is the developement of a simplified Functional Engineering Simulator and the design of a Guidance, Navigation and Control system for a Space Tug mission concerning the active removal of a piece of space debris.