Satellite Formation Flying (SFF) and Proximity Operations are applications that have increasingly gained interest over the years. These applications foresee the substitution of a single spacecraft with a system of multiple satellites that perform coordinated position and attitude control maneuvers, which in turn results in higher accuracy of payload measurement, higher flexibility, robustness to failure, and reduction of development costs. These systems present however higher difficulties in their design since they have not only absolute but also relative state requirements, which make them also liable to higher control action expense with respect to (wrt) the single satellite systems. Moreover, applications like Automated Rendez-Vous and Docking (RVD) and in general close proximity maneuvers present a high risk of impact between the satellites, which must be treated with an appropriate design of the on board Guidance Navigation and Control (GNC) system. These aspects justify the development and employment of a ground hardware simulator representative of two or more satellites performing coordinate maneuvers, allowing the investigation of these problems with an easily accessible system.
The aim of my Ph.D. Activities has consisted in the development and testing of the cooperating SPAcecRaft Testbed for Autonomous proximity operatioNs experimentS (SPARTANS) hardware simulator, which is under development since 2010 at the Center of Studies and Activities for Space (CISAS) of the University of Padova. This ground simulator presents robotic units that allow the reproduction of the relative position and attitude motions of satellites in proximity or in formation, and can be therefore employed for the extensive study of control algorithms and strategies for these types of applications, allowing dedicated hardware in the loop to be tested in a controlled environment. At the beginning of my Ph.D., the testbed consisted in the first prototype of Attitude Module (AM), a platform with three rotational Degrees of Freedom (DOF) of Yaw, Pitch and Roll, controllable through a GNC system based on incremental encoders and air thrusters. A small contribution was initially given in support of the execution of a series of 3 DOF attitude control maneuvers tests with the AM. Subsequently, the first activity consisted in the design and development of the air suspension system that enables a low friction translational motion of the a whole Unit of the testbed over the test table, with the characterization of air skids available in laboratory. The subsequent activity consisted in the design and development of the Translation Module (TM), the lower section of the whole Unit, as modular structure supporting the air suspension system, the AM, and the on board localization system. After this activity the on board localization system for position and Azimuth estimation, based on Optical Flow Sensors (OFS), was developed and tested. The system was installed on a TM base prototype and it was calibrated and tested with the imposition of known motions through rotational and translational motorized stages wich were used in conjunction, presenting max deviations at the level of 0.1° for a total rotational range of 40°, and max deviations of 1 mm for a total translational range of 100 mm. Combined maneuvers, i.e. translational and rotational motions imposed in sequence, were subsequently performed, showing a drift trend, up to approximately 1 cm for a 90° rotation. Subsequently the OFS system was assembled in the TM and integrated with an external vision system, under development in parallel in the context of the SPARTANS project. Results showed a good general concordance between the two systems, but combined maneuvers with extended rotational range showed again a drift trend in the OFS system solution, not only in position but also in Azimuth.
A parallel activity consisted in the design and development of the levellable test table for the Units with a modular structure. Another activity consisted in the development of a Matlab Software Simulator for Units tests planning. A series of preliminary standard and optimal control maneuvers were planned with the software simulator.
The last activity of my Ph.D. consisted in the analysis of an inspection scenario for satellite removal purposes, with the goal of reproducing the relative dynamics in scale with the SPARTANS simulator. The chosen scenario foresaw the inspection, through a vision system on board an inspection satellite, of the currently freely tumbling Envisat spacecraft . The analysis performed with a Matlab software simulator was focused on the acquisition and maintainance of a circular relative orbit at close range starting from a flyaround orbit, through the employment of Model Predictive Control (MPC) and Linear Quadratic Regulator (LQR) optimal controllers. Simulations results showed a lower tracking error in position with the MPC controller wrt to the LQR controller, but with a higher control action expense: for a 6 hours inspection on a 41 m radius circular relative orbit, the max total delta-v component resulted of 3.3 m/s for MPC, while it resulted of 0.7 m/s for LQR.
In the present configuration the SPARTANS testbed presents a first complete Unit and test table to be assembled in the immediate future for the execution of the first position and attitude control maneuvers. The final configuration of the testbed will present a minimum of two Units allowing to perform coordinate control maneuvers for the investigation and study of problems and strategies related to SFF, Automated Rendez-Vous and Docking, and in general proximity manevuers
