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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

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

NASA/GSFC/IMDC


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


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


Detector acs mode scenarios
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


Actuator selection criteria
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


Optics sun acq rate null
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


Optics science

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


Optics science con t
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


Optics slew
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


Technology
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


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


Component wheel option
Component (wheel option)

NASA/GSFC/IMDC


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