Josephine San

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

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

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

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 spacecrafts 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

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

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

8
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

9
Optics - Science (cont)

• 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

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

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

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

13
Component (wheel option)