DOCKS: docking system for microsatellites

The Space Rider Observer Cube (SROC) aims at demonstrating enabling technologies for inspection and docking. SROC will be transported into orbit by the reusable Space Rider (SR) vehicle, will perform inspection manoeuvres on SR, will dock back onto SR and will re-enter Earth's atmosphere inside the SR cargo bay. To perform the docking, SROC is provided with a docking system (DOCKS), whose key characteristic is the high level of autonomy from the other SROC systems. DOCKS is composed by a docking mechanism, a set of relative pose sensors and a local computer that executes a navigation algorithm to compute the relative pose between SROC and SR through model-based estimation and sensor fusion. To perform the approach and docking manoeuvre, the estimation of the pose is transmitted to the SROC attitude and orbital control system that applies control forces and torques required. The actual docking is achieved in three steps: 1) a self-alignment and damping process is obtained through a compliant probe-drogue mating configuration, 2) a first non-rigid connection is provided by an electromagnet (soft-docking), and finally 3) the rigid connection is achieved by a three-claw mechanism. The SROC mission development is ongoing and DOCKS is a key subsystem that has been realized in two prototypes involved in experimental campaigns. The first is a laboratory prototype aimed at the functional and kinematic testing of the probe-drogue configuration. The second is an advanced breadboard finalized at the dynamic estimation of the docking performance by 1) assessing the acceptable envelope of misalignments (lateral and angular) and approach velocities for a successful docking, 2) verifying the capabilities of the damping system by measuring contact loads, and 3) verifying the soft- and hard-docking capabilities of the system. Performance tests have been executed in a representative laboratory environment that features a low friction table, a floating satellite mock-up and a vehicle approaching a fixed docking port with a velocity of approximately 5 mm/s. Additionally, the navigation algorithm has been tested through a dedicated simulator to verify its performance and the compatibility of the realtive pose estimation error with the maximum accepted misalignments. The estimation algorithm was tested during approach manoeuvres of the vehicle on the low friction table, moving under the actions of cold-gas thrusters. This paper describes the consolidated design of DOCKS, and its functional and performance verification through laboratory experiments.