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Attitude & Orbit Control Subsystem

Attitude & Orbit Control Subsystem. 26 April 2007. Contents. Key Requirements AOCS Design Description Functional block diagram AOCS modes AOCS Hardware Description Hardware Functions/ characterization Interface Summary (Power, Bi-level, Discrete, analog, serial bus)

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Attitude & Orbit Control Subsystem

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  1. Attitude & Orbit Control Subsystem 26 April 2007

  2. Contents • Key Requirements • AOCS Design Description • Functional block diagram • AOCS modes • AOCS Hardware Description • Hardware Functions/ characterization • Interface Summary (Power, Bi-level, Discrete, analog, serial bus) • AOCS Software Development

  3. Contents (cont’d) • Major Trade-offs • Star camera orientation • Thruster configuration • Jitter analysis (rigid body) • Sun Sensor configuration • Design and Analysis • ASH mode • Navigation filter • Attitude estimator • Off loading • Guidance • Normal mode

  4. AOCS Key Requirements • Orbit Altitude • Orbit Inclination • Equator Crossing Time • Attitude Control Accuracy • Attitude Control Accuracy Goal • Attitude Control Bandwidth • Attitude Knowledge

  5. AOCS Key Requirements (cont’d) • Attitude Maneuvers • Spacecraft Jitter • On-board Orbit Determination • Satellite Autonomous Operations • Over-sampling • Maneuver Agility

  6. Sun sensor GPS MAG Star camera IMU AOCS Design Description: Functional block diagram 1: Attitude acquisition and Safe-hold (ASH) sub-mode: Stabilize (STAB) Sun tracked (STRA) Sun locked (SLO) 2: Normal mode (NM) sub-mode: Geocentric attitude pointing (GAP) Maneuver (MAN) Fine imaging pointing (FIP) Sun pointing (SUP) 3: Orbit control mode (OCM) ASH Mode manager : STAB, STRA, SLO ASH Mode control - Reaction Whl command - Magnetoquer command satellite Telecommand Commanded NM Mode manager : quaternion GAP, MAN, FIP, SUP Attitude estimation Normal Mode control - Reaction Whl command Orbit - Magnetoquer command estimation 3 OCM Mode control - Thruster command other

  7. ASH Mode STRA A A STAB SLO AOCS Design Description: AOCS modes TC TC SUP A TC ARO GAP MAN TC OCM TC A TC FIP TC TC Normal Mode A : Automatic transition TC : Telecommanded transition ARO : Attitude Reconfiguration Order (from any submode)

  8. AOCS Hardware Description: • Sensors: • Sun Sensors • Magnetometers • Inertial Measurement Unit (IMU) • Star Camera with two Camera Heads • Actuators: • Reaction Wheels • 3 Magnetic Torquer • 1 RCS (cold gas) with 4 thrusters

  9. Attitude maneuver performed by a cluster of 4 whls • Wheel capacity • 20 deg/min for each axis based on current whl capacity • Possible to increase agility for specific axis from ( , ) • 25 % torque margin a b Major Trade-offs : Maneuver Agility

  10. Major Trade-offs : Magnetorquer sizing Wheel Control Preliminary analysis shows: • Wheel unloading control in NM mode, Maximum command magnetic command shall be able to retain wheels angular momentum variation induced by the environment disturbing torques • Detumbling control In ASH mode, maximum command magnetic command shall be able to stabilize the spacecraft within 2 orbits • Cross denote wheel control has been absent from the control loop and enforced S/C with nadir attitude in eclipse and sun pointing attitude in sunlight • H was calculated by integrating T off-loading + Tdist instead of feeding from wheel speeds S/C (nadir /Sun pointing) , wheel off-loading control law

  11. Earth Sun direction 7.5 deg -Zsc 39 deg Sun masking Available for roll maneuver: 59.8 deg 23 deg Earth limb masking 28.6 deg CHU los +Ysc Major Trade-offs : Star camera orientation • Sun is a point source, Sun masking angle: 39 deg • Earth is an extended source, Earth masking angle (from Earth limb): 23 deg CHU B los +Y CHU A los Xsc +Z CHU los +X Rr Ysc Rx Zsc

  12. Major Trade-offs : Star camera orientation (cont’d) Conclusion: • Based on the simulation results, at least one of the two CHUs will be always kept out from blinding. • To extend roll maneuver capacity from +/- 25 deg to +/- 35 deg, elimination of 10 deg either in Sun or Earth exclusion angle is needed

  13. COM y x 4 z 2 1 3 Major Trade-offs : Thruster configuration • Four thrusters configuration • Only one of the two thruster branches is used after 1 failure • Propulsion module is centred around centre of mass (COM), the thruster configuration cannot create any torque aligned on Y axis. • Orbit control • On Y axis: • No capacity around Y, Y axis is always controlled by wheels. • On X and Z axes: • In the nominal case, the thruster is performed by firing the 4 thrusters simultaneously. • In a degraded case (one thruster failure), the pair that includes the failure thruster is no longer used and the thruster is performed with the remaining thrusters. The X or Z axis is therefore control by wheels • Off-modulating Control. The pair (1,2) control Z axis, the pair (3,4) control X axis

  14. Argo PDR – AOCS Jitter Analysis

  15. Preliminary Performance Analysis: Jitter analysis (rigid body) Objective: Analyze whether pointing req. for 0.5” ∀ freq > 0.015 Hz is achievable. Method: Frequency domain analysis. Results: Normal Mode (FIP, MAN sub-modes) + time delay

  16. Jitter Conclusion Required specification achievable. Given 0.0061 Hz cl-BW, Relative Accuracy: 47.20” + 2nd order LPF with 4 Hz sampling rate  output: pointing error ~ 0.19”, for freq > 0.015 Hz .

  17. Argo PDR – AOCS Omni-directional Sun Sensor (OSS)

  18. OSS Conclusions • Maximum OSS sun direction error < 12 deg. • Sensitivity analysis will be done after PDR. Those including: variation of mean albedo, unequal cell degrade, mismatch of measurement resistors, head misalignment, and variation of backside radiation.

  19. Preliminary Performance Analysis: ASH mode • Objective: • To reduce the initial rate, after that to track Sun and control the solar array toward Sun while it is in eclipse or daylight. • To keep the satellite in safe state once any contingency or anomaly happened. • Method: Eclipse Sun presence automatic TC Normal Mode STAB STRA automatic SLO B-dot control law B-dot control law B-dot control law (X,Z) Sun acquisition control law (Y) Wheel off-loading control law (Y) ASH Mode

  20. Preliminary Performance Analysis: ASH mode (cont’d) Conclusion:Control law works. • The satellite spins down from the initial rate of 2.5°/s at each axis within 2 orbits, then transits from STAB to STRA. • STRA/SLO cyclic transition demonstrates Sun acquisition function well. • Angular momentum of each wheel is in the designed working range.

  21. Argo PDR – AOCS Navigation Filter Design (NAV)

  22. NAV requirement • Orbit determination (Normal mode) • Position: 25 m (3D-3s) • Velocity: 1.8 m/s (3D-3s),

  23. Argo PDR – AOCS Inertial Attitude Estimation (IAE)

  24. Inertial Attitude Estimation (IAE) • Hardware: • Star camera (ASC) • Gyro (IRU) • Measurements:

  25. IAE Conclusions • LPF is good enough + fast & easy to design/implement. • Angular error < 40 arc-second, rate error < 0.5 deg/hr. • Data fusion – camera head misalignment

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