Current navigation systems on satellites base their measurements on synchronized signals from MEO constellations. The employment of Free-Space Optical (FSO) Communication for next-generation KEPLER navigation constellation would bring several advantages, such as precise global synchronization, sub-millimetre ranging accuracy and high data-rate communication to allow a fast measurement exchange. The core of this technology is represented by optical terminals, which are characterised by extremely narrow laser beams, with a divergence in the order of micro-radians. Hence, pointing requirement is critical for the payload performance. Extreme levels of required pointing accuracy bring new challenges in the design of the spacecraft, such as micro-vibrations management.