Gravity-gradient stabilization

Abstract: A near-minimum-t ime (near-maximum-acceleration/deceleration) control technique is presented to rotate a spacecraft around its eigenaxis (shortest angular path). Practical control constraints such as wheel torque and wheel speed are also taken into account. In this paper a nominally nadir pointing satellite in a circular low Earth orbit under passive gravity gradient stabilization and active reaction wheel control will be used as an example. A standard linear feedback quaternion regulator will be used to three-axis stabilize the spacecraft before and after each large angular rotation. Robustness against inertia modeling errors is ensured by tracking the reference maneuver. The algorithm presented can easily be adapted to be used on inertially stabilized spacecraft.

Abstract: OF THESIS PASSIVE ATTITUDE STABILIZATION FOR SMALL SATELLITES This thesis addresses the problem of designing and evaluating passive satellite attitude control systems for small satellites. Passive stabilization techniques such as Gravity Gradient stabilization, Passive Magnetic Stabilization, and Aerodynamic stabilization in Low Earth Orbit utilize the geometric and magnetic design of a satellite and the orbit properties to passively provide attitude stabilization and basic pointing. The design of such stabilization systems can be done using a high fidelity simulation of the satellite and the environmental effects in the orbit under consideration to study the on-orbit behavior and the effectiveness of the stability system in overcoming the disturbance torques. The Orbit Propagator described in this thesis is developed to include models for orbit parameters, Gravity Gradient torque, Aerodynamic Torque, Magnetic Torque, and Magnetic Hysteresis Material for angular rate damping. Aerodynamic stabilization of a three-unit CubeSat with deployable side panels in a “shuttlecock” design is studied in detail. Finally, the Passive Magnetic Stabilization system of KySat-1, a one-unit CubeSat, is also described in detail and the simulation results are shown.

Abstract: A spacecraft is adapted for orbital flight in two alternative modes, referred to as the earth-oriented and quasi sun-oriented modes. The spacecraft comprises a spacecraft body, at least one solar array extendible outwardly from the spacecraft body, passive attitude control means utilizing gravity gradient stabilization means which is extendible and retractable relative to the spacecraft body, and active attitude control means. The gravity gradient stabilization means is effective when extended to stabilize the spacecraft in the earth-oriented mode, and the active attitude control means is effective to stabilize the spacecraft in the quasi sun-oriented mode.