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Josephine San Dave Olney 18 August, 1999

MAXIM. Josephine San Dave Olney 18 August, 1999. Attitude Control System. Assessment. Appears to be Feasible Requirements Coarse Pointing baselined on NGST Future technology Control modes Area of concerns. Science Requirements. Pointing Requirements on Optical Spacecraft ACS

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Josephine San Dave Olney 18 August, 1999

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  1. MAXIM Josephine San Dave Olney 18 August, 1999 Attitude Control System

  2. Assessment • Appears to be Feasible • Requirements • Coarse Pointing baselined on NGST • Future technology • Control modes • Area of concerns NASA/GSFC/IMDC

  3. Science Requirements • Pointing Requirements on Optical Spacecraft ACS • Accuracy (1 s) : • Course (with tracker) Pitch/Yaw ± 2.0 arcsec; Roll 20 arcsec • Fine (long term drift) pitch/yaw ± 0.5 marcsec • Jitter • Coarse ± 0.1 arcsec • Fine ± 0.1 marcsec • Pointing Requirements on Detector Spacecraft ACS • Stability (1 s) : Pitch/Yaw ± 20 arcsec; Roll 20 arcsec • Lateral Stability (1 s) : 3 mm NASA/GSFC/IMDC

  4. Optics ACS Mode Scenarios • Rate null/Sun Acquisition: null rates, point arrays normal to sun • Sensors : Coarse Sun Sensor, Gyro • Actuator: options: large wheel/small wheel + good PAF/thruster • Acquisition: acquire stars to establish reference attitude • Sensors: HD301 Star tracker(ST), Fine Sun Sensor (FSS), gyro (ring laser) • Actuator: same as rate null/sun acquisition • Science: inertial pointing • Sensors: coarse pointing - ST, FSS, gyro; Fine pointing - instrument • Actuator: options:/large wheel with thruster/PPT/small wheel with PPT • Slew: eigenaxis rotation capability • Sensor: gyro • Actuator: options: wheel/PPT • Safehold: independent safe mode, same as sun acquisition NASA/GSFC/IMDC

  5. Detector ACS mode scenarios • Rate null/Sun Acquisition: null rates, point arrays normal to sun • Sensor same as Optics spacecraft; actuator can be wheel or thruster • Initial Acquisition: acquire stars to establish reference attitude • Sensors same ad Optics spacecraft’s acquisition; actuator same as sun acquisiton • Science: inertial pointing • Pointing: same as optics spacecraft coarse pointing • Lateral control: laser and PPT • Slew: Acquire optics spacecraft, acquire new target • Sensor: gyro • Actuator: wheel or thruster • Delta V - Re-acquire new target position • Safehold: independent safe mode, same as sun acquisition NASA/GSFC/IMDC

  6. Actuator Selection Criteria • Quantization - Science requirement • Disturbance torque - Science requirement • Torque Capability - slew and solar torque • Momentum capability • Tip off rate • Solar pressure at drift orbit • Solar force is about 0.2 m N • Assuming 0.1 meter cp offset for optics spacecraft, Solar torque is 20 micro Nm • In one day the momentum build up is about 1.8 Nms NASA/GSFC/IMDC

  7. Optics - sun acq/rate null • Large wheel (80 Nms) • Tip off rate less than 0.05 deg/sec • Imbalance torque disturbance • Weight and Power • Small wheel (40 Nms)+ PAF • Assuming with good PAF, tip off rate less than 0.01 deg/sec • 40 Nms wheel • Weight and Power • Thruster • PPT is not sufficient to null the rate (0.01 deg/sec) and acquire the sun • hydrazine - Sloshing problem • Cold gas - only choice NASA/GSFC/IMDC

  8. PPT only Better quantization* No need to unload momentum* No heritage yet, EO1 will have one axis PPT control as test Need 12 PPT with no redundancy* Mass, power, cost Limited number (10 million) of firing (fire every 3 s for 1 year) Plume impingement Electro-magnetic contamination Need to further investigate items without ‘*’ Wheel with isolation; thruster for momentum unloading Quantization Need to unload momentum* Has heritage* Longer life time* With four wheel provides redundancy* Mass, power, cost Imbalance torque disturbance Same as PPT last item Optics - SCIENCE NASA/GSFC/IMDC

  9. Optics - Science (con’t) • Small wheel with isolation /PPT • Wheel for pointing, PPT for momentum unloading • Finer quantization of wheel • Extend PPT life time • Wheel for coarse pointing, PPT for fine pointing and momentum unloading • Better quantization for fine pointing • Extend PPT lifetime • Depend on the actuator induced disturbance and other studies NASA/GSFC/IMDC

  10. Optics - Slew • Wheel • Less than 6 hours to slew 45 degree • Remain a zero-momentum system • PPT • 12 hours to slew 45 degree • After the slew, the system momentum may not be zero NASA/GSFC/IMDC

  11. Technology • New Generation Integrated Wheel • Wheel and electronic all integrated • Low noise, low imbalance torque, low power • Spartan 400 series and Triana heritage • New Generation Star Tracker • NGST heritage • Accuracy 1.35 arcsec accuracy per star • Pulse Plasma Thruster • As three axis fine control actuator NASA/GSFC/IMDC

  12. Concerns • PPT concerns • Plume impingement, EMI, Life time • Maintain a zero momentum system • A better solar torque estimation • Tracking strategy of optics and detector spacecraft • Fine pointing Strategies • Operation scenarios post separation • Null rate before separate two spacecraft • Rate after two spacecraft separation • What is the rate after two spacecraft separation NASA/GSFC/IMDC

  13. Component (wheel option) NASA/GSFC/IMDC

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