Abstract
Remote sensing satellites are often built with payloads that do not include line of sight steering mechanisms, such that pointing their payloads requires rotation of the whole satellite. In cases, when frequent line of sight retargeting is required, there is a need for efficient actuators and control schemes that would support rapid attitude maneuvering together with adequate pointing accuracy and stability between the maneuvers. These control schemes shall accommodate a variety of realistic conditions, such as general three dimensional maneuver direction, existence of initial and/or final angular rates, non zero net angular momentum and various actuators constraints. Within this frame, this research develops the Backwards Planning approach as one of the possible control methods for rapid maneuvering. The method is based on state feedback and combines time efficiency together with straight forward computation flow. Novel efficient methods to execute the Backwards Planning Control in the 3D attitude space are proposed here. The methods refer both for the first saturated control phase of the maneuver and for the last braking phase. The actuators used for the spacecraft control in this research are either Reaction Wheels (RWs) or Single Gimbal Control Moment Gyros (SGCMGs) or both of them together. The advantage of the SGCMG is in rapid rotational maneuvering, but their application for high quality pointing requires very accurate gimbal mechanisms. On the other hand, RWs are usually more suitable for accurate pointing, but their torque to power performance is inferior in maneuvering. It is shown that the coordination of SGCMGs and RWs together enables to draw more performance from the SGCMGs in terms of agility and meet the pointing requirements between maneuvers where only the RWs are used. Novel SGCMG steering laws are suggested as well. While the steering laws determine the required angular rate for each gimbal, most steering laws are defined in the angular momentum domain and output the gimbals angular rates to produce a given required torque or angular momentum increment. This research however, practices a novel steering law in the gimbal angles domain. While both steering laws turn to be dynamically equivalent for small control signals, as in the steady state, it is shown that the steering in the gimbal angles domain is more effective in maneuvering with the Backwards Planning control logic.