Command level maneuver optimization for the International Space Station
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This paper presents a maneuver commanding optimization tool for minimizing fuel requirements during International Space Station (ISS) guidance, navigation, and control (GN&C) maneuvers. The dynamic behavior of the ISS varies significantly throughout its construction, as mass properties and external aerodynamic shape change with the addition of new components and the attachment and movement of various earth-to-orbit vehicles and payloads. Performance of the GNC system is similarly affected by the vehicle configuration as well as the sequence and timing of commands. In particular, fuel required to achieve and maintain desired attitude depends on the configuration, the environment, the controller used, and the GN&C command sequence. GN&C operations for the ISS are constrained by the ability to issue a specific set of commands, which are limited to certain mode changes, controller parameter updates, attitude commands, and control moment gyro (CMG) momentum commands. An approach for optimizing the command sequence was developed with the objective of minimizing fuel utilization. A high-speed and variable fidelity simulation for the ISS dynamics, environment, and GNC system, was developed and integrated into an optimization code that includes a complete model for the operator commanding capability. Commands, parameters, and time-tags can all be treated as optimization variables. An initial guess for the optimal solution can be supplied by a simplified model using the differential inclusions method. Optimal solutions for the full nonlinear problem are obtained via 3D visualizations of the search space combined with local gradient techniques. This tool will provide ground-based flight controllers with an automated command sequence optimization capability for any GNC operations scenario. Ultimately, this tool could be extended to provide the crew with an onboard autonomous capability for attitude control planning, optimization, and operation.
