This thesis is framed within the context of the ESA Space Rider Observer Cube (SROC) mission. In this in-orbit servicing mission, a CubeSat is deployed from the Space Rider’s cargo bay to carry out a visual inspection of the vehicle, followed by a rendezvous and docking maneuver to return to the cargo bay. This work covers the development of an Extended Kalman Filter (EKF) to be implemented in the Navigation system of SROC for the phases of relative navigation (i.e under 1 km). The filter shall estimate the relative position, velocity, attitude and angular velocity between the Chaser and Target spacecraft.