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Integrated Orbit and Attitude Control for a Nanosatellite with Power Constraints

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Integrated Orbit and Attitude Control for a Nanosatellite with Power Constraints

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    1. Integrated Orbit and Attitude Control for a Nanosatellite with Power Constraints Bo Naasz Matthew Berry Hye-Young Kim Chris Hall 13th Annual AAS/AIAA Space Flight Mechanics Meeting February 9-13, 2003, Ponce, Puerto Rico

    2. Overview ION-F and HokieSat Orbit & Attitude Coupling Dynamics Control Simulation/Software Results

    3. Ionospheric Observation Nanosatellite Formation (ION-F) Three of 10 student-built spacecraft in AFOSR/DARPA University Nanosatellite Program, also sponsored by NASA Goddard Space Flight Center Three-satellite stack will launch from Shuttle Hitchhiker Experiment Launcher System Mission goals Formation flying demonstration Distributed ionospheric measurements

    4. HokieSat DCS Hardware Orbit control UW/Primex Pulsed Plasma Thrusters (PPT) Impulse bit per thruster: 56 mN No radial thrust Paired thrusters cannot fire simultaneously Attitude control Magnetic torque coils Interact with Earth’s magnetic field Provide < 5 x 10-5 N-m Torque PPTs for limited yaw steering

    5. Maneuver Modes “Normal” mode Slew as required to point thrusters Negligible thrust torque 180 degree slews required “Sideways” mode Allow thrust torque Frequent control interruption No slews required

    6. Sources of Orbit-Attitude Coupling Natural dynamics: Attitude dependent orbit perturbations Atmospheric drag Solar radiation pressure Orbit dependent attitude perturbations Magnetic field variation Gravity gradient torque Dynamical coupling (very weak) Natural dynamics Attitude dependent orbit perturbations Atmospheric drag Solar radiation pressure Orbit dependent attitude perturbations Magnetic field variation Gravity gradient torque Dynamical coupling (very weak) Guidance Navigation & Control (GNC) System Shared resources Actuators (thrusters for orbit and attitude control, momentum dumping) Sensors (star trackers for attitude determination, celestial navigation) Power and spacecraft consumables Actuator induced disturbances Non-coupled thrusters Thruster disturbance torques Subsytem inter-dependencies Drag/SRP control Thruster pointing Natural dynamics Attitude dependent orbit perturbations Atmospheric drag Solar radiation pressure Orbit dependent attitude perturbations Magnetic field variation Gravity gradient torque Dynamical coupling (very weak) Guidance Navigation & Control (GNC) System Shared resources Actuators (thrusters for orbit and attitude control, momentum dumping) Sensors (star trackers for attitude determination, celestial navigation) Power and spacecraft consumables Actuator induced disturbances Non-coupled thrusters Thruster disturbance torques Subsytem inter-dependencies Drag/SRP control Thruster pointing

    7. Dynamics Orbit Two body motion Control forces from thrusters Perfect state knowledge Attitude External torques from gravity gradient, thrusters Control torques from magnetic torque coils Perfect state knowledge

    8. Orbit Control

    9. Thrust On/Off Logic Normal mode

    10. Thrust On/Off Logic (cont’d)

    11. Attitude Control LQR Torque perpendicular to magnetic field direction only Desired attitude set by maneuvering mode and desired thrust direction Assume torque is throttleable, with a maximum of ~ 5 x 10-5 N-m Torque

    12. Simulation Reference orbit: Semi-major axis: 6770 km Circular (e ? 0) Inclination: 52? Spacecraft initial conditions: 700m leader follower 700m same ground track Propagation: 1 second time step Runge-Kutta integration for Orbit and Attitude Software written in C++ Prototype of flight code 4 processes Orbit determination Orbit control Attitude determination Attitude control

    13. Results – Leader Follower, Normal Mode

    14. Results – Same Ground Track, Normal Mode

    15. Results – Same Ground Track, Sideways Mode

    16. Summary

    18. Control orbital 6DOF as two systems First System: First five elements (size, shape, orientation of orbit) Orbital Control

    19. Orbit Control Second system (a feedback phasing maneuver): Sixth element (angular position within the orbit)

    20. Orbit Dynamics

    23. Spacecraft Formation Flying

    24. Problem Statement Control the motion of formation-flying spacecraft using integrated nonlinear orbit and attitude feedback control laws to achieve a predefined target orbit.

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