Advanced Guidance, Navigation and Control strategies (in nominal and off-nominal conditions) of small satellites in soft-docking manoeuvers

Rendezvous and Docking maneuvers (RVD) always have been, and still are, one of the most delicate operations that have to be performed during a space mission. When two spacecrafts are only few meters away from each other, their relative position, attitude and velocity have to be controlled precisely in order to fulfill the end conditions of the maneuver and to avoid the collision between the two orbiting bodies. Within the context of this thesis, the satellites involved in the close-approach maneuver are SROC (Space Rider Observer Cube) and its mothercraft, Space Rider (SR). SROC is a 12U microsatellite designed by ESA in collaboration with Politecnico di Torino, Tyvak international and Universit`a di Padova, which will be part of the Space Rider mission. The goal of SROC is to acquire visual information of Space Rider while orbiting around it and to subsequently perform a controlled soft-docking. The results presented in this report are, in particular, focused on various radial docking scenarios involving SROC. All the simulations were executed in the MATLAB/Simulink environment through of a complex and realistic Guidance Navigation and Control (GNC) model which simulates the relative trajectory of the two spacecrafts. Concerning the Dynamic model of the system, besides the Clohessy-Wiltshire (CW) and Euler equations, various external disturbances were implemented. Regarding the control action, a Nonlinear Model Predictive Control (NMPC) is implemented. A particular focus was posed on the thruster model: a new thruster selection algorithm was coded in order to achieve a satisfactory output, following the control force given by the NMPC. Final approach maneuvers and Collision Avoidance Maneuvers (CAMs) were simulated both in nominal and off-nominal conditions. Subsequently, a retry point was defined few kilometers ahead of Space Rider: from this point a second attempt at close approach and docking can be performed. The retry point is reached through a two phase maneuver composed by an initial open loop maneuver followed by a Linear Quadratic Regulator (LQR) controlled close loop maneuver. To test the adequate robustness and sensitiveness to initial conditions of the system, the different maneuvers were analyzed through various Montecarlo campaigns, in accordance with ESA’s ECSS guidelines.