Josephine San Dave Olney 18 August, 1999

15 July 1999NASA/GSFC/IMDC2  Appears to be Feasible  Requirements  Coarse Pointing baselined on NGST  Future technology  Control modes  Area of concerns

15 July 1999NASA/GSFC/IMDC3  Pointing Requirements on Optical Spacecraft ACS  Accuracy (1  ) : 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  ) :Pitch/Yaw ± 20 arcsec; Roll 20 arcsec  Lateral Stability (1  ) : 3 mm

15 July 1999NASA/GSFC/IMDC4  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

15 July 1999NASA/GSFC/IMDC5 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

15 July 1999NASA/GSFC/IMDC6 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

15 July 1999NASA/GSFC/IMDC7 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

15 July 1999NASA/GSFC/IMDC8 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

15 July 1999NASA/GSFC/IMDC9 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

15 July 1999NASA/GSFC/IMDC10 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

15 July 1999NASA/GSFC/IMDC11 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

15 July 1999NASA/GSFC/IMDC12  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

15 July 1999NASA/GSFC/IMDC13 Component (wheel option)