Final Version Micro-Arcsecond X-ray Imaging Mission Pathfinder (MAXIM-PF) Eric Stoneking Paul Mason May 17, 2002 ACS

Final Version MAXIM-PF, May 13-17, 2002 Goddard Space Flight Center Attitude Control Page 2 ACS Drivers  Very tight attitude and translation control requirements  1 arcsec is limit of existing state of the art  Subarcsec attitude, sub-millimeter translation control to be achieved through technology under development  “Super star tracker”  Very stable gyros  Micro-thrusters  Swarm sensors  Formation Flying  Requires inter-spacecraft sensors and communication  Requires distributed control laws, fault detection, safing algorithms

Final Version MAXIM-PF, May 13-17, 2002 Goddard Space Flight Center Attitude Control Page 3 Technologies  Key Hardware Technologies  Sensors  “Super Star Tracker”  Quad cell laser beacon tracker  Very low-drift gyros ( < 1 uas/day)  Swarm Sensor  Low bias Accelerometer  Micro-Newton Thrusters  Formation Flying Algorithms  Formation acquisition and maintenance  Micro-thrust Control  Disturbance estimation and rejection  Parameter estimation and adaptation  CG migration/fuel usage  Bias/drift estimation

Final Version MAXIM-PF, May 13-17, 2002 Goddard Space Flight Center Attitude Control Page 4 ACS Control Modes  Coarse Formation Acquisition  Omni RF ranging with small programmed maneuvers to solve “Lost in Space”  Maneuver to assigned positions in formation (within meters)  Fine Formation Acquisition  Acquire laser beacons in star trackers  For Phase 2, freeflyers acquire swarm sensors  Maneuver Detector to acquire science target  Science  Hold attitude and relative position  Maneuver  Execute commanded attitude/translation maneuver while maintaining formation  Translation requirements relaxed from Science mode  One day in Phase 1, ~ 1 week in Phase 2, dependent on thrust level  Safehold  Point solar arrays to Sun  Collision avoidance

Final Version MAXIM-PF, May 13-17, 2002 Goddard Space Flight Center Attitude Control Page 5  “Super Star Tracker” (Laser beacon tracker + low-drift gyros) needed for detector control(Instrument)  Thruster impulse bit < 20  N-sec(Propulsion)  Omni RF used for coarse formation acquisition (Comm)  Lowest structure mode should be > 10 Hz, to minimize interaction with attitude control loop (Mechanical) ACS Requirements Imposed On Other Sub- Systems

Final Version MAXIM-PF, May 13-17, 2002 Goddard Space Flight Center Attitude Control Page 6 ACS Concerns and Comments  Technologies, while not “miracles”, still carry significant development risk  Concerns  Contamination due to thruster firing  Lost in Space problem  Misalignment of Star Trackers, gyros, optics  Due to tolerances of the Phase 1 S/C connections  If impulsive thrusters are used, drive frequencies must be chosen to stay from structural resonant frequencies  Tight control and knowledge requirements  Requires higher control bandwidths  Ensure quiet motion in formation mode  Advanced estimation and control techniques are needed  Trade bandwidth against estimator complexity  Control authority levels should overlap  During retargeting coarse control is utilized  Settling times  Maintaining the formation control during retargeting will help to provide a quiet structure

Final Version MAXIM-PF, May 13-17, 2002 Goddard Space Flight Center Attitude Control Page 7 Future Studies  Expansion to full MAXIM mission architecture  Several freeflyers will have the capability to lead a subgroup  ACE and C&DH should be developed to handle an increase in the number of S/C  Tighter safehold and collision avoidance constraints  Direct inter-FF ranging?  Higher Formation and individual S/C Bandwidth  Increase the number of reference fiducials on Hub

Final Version MAXIM-PF, May 13-17, 2002 Goddard Space Flight Center Attitude Control Page 8 Backup Slides  Sensor Configurations  Components  Trade Studies

Final Version MAXIM-PF, May 13-17, 2002 Goddard Space Flight Center Attitude Control Page 9 Hub/Detector Sensor Configuration Super Star Tracker centers on Laser Beacon Normal Star Tracker places Laser Beacon against fixed stars Laser Beacon illuminates Detector S/C Laser Detector measures range by time-of-flight of reflected laser beam Reflector Cube reflects laser beam back to Hub for ranging Hub S/C Detector S/C Super gyros hold inertial attitude Coarse Ranging by omni RF comm link

Final Version MAXIM-PF, May 13-17, 2002 Goddard Space Flight Center Attitude Control Page 10 Hub/Freeflyer Sensor Configuration Small Laser Beacon Normal Star Tracker places Hub beacon against fixed stars Swarm Sensor measures range by bouncing RF, laser off Hub Reflector Cube Hub S/C Freeflyer S/C Coarse Ranging by omni RF comm link

Final Version MAXIM-PF, May 13-17, 2002 Goddard Space Flight Center Attitude Control Page 11 Attitude/Translation Requirements and Sensors: Optics Hub

Final Version MAXIM-PF, May 13-17, 2002 Goddard Space Flight Center Attitude Control Page 12 Attitude/Translation Requirements and Sensors: Detector

Final Version MAXIM-PF, May 13-17, 2002 Goddard Space Flight Center Attitude Control Page 13 Attitude/Translation Requirements and Sensors: Freeflyer

Final Version MAXIM-PF, May 13-17, 2002 Goddard Space Flight Center Attitude Control Page 14 ACS Components Optical Hub

Final Version MAXIM-PF, May 13-17, 2002 Goddard Space Flight Center Attitude Control Page 15 ACS Components Detector

Final Version MAXIM-PF, May 13-17, 2002 Goddard Space Flight Center Attitude Control Page 16 ACS Components Free Flyer

Final Version MAXIM-PF, May 13-17, 2002 Goddard Space Flight Center Attitude Control Page 17 ROM ACS Labor Cost Note: 1) Estimated cost derived from MAP cost in $K

Final Version MAXIM-PF, May 13-17, 2002 Goddard Space Flight Center Attitude Control Page 18 Phase 1 Command structure  ACE/C&DH in charge of the unit sensor/actuators  Receive measurements from freeflyer attitude sensors  Sends thruster commands to freeflyer Thrust commands Attitude and Position Information Thrust Attitude and position Thrust

Final Version MAXIM-PF, May 13-17, 2002 Goddard Space Flight Center Attitude Control Page 19 Formation  Configuration:  Expandable  Increase the number of free flyers with several acting as local leaders  Redundancy  For the full version local leaders can take the place of the hub or detector  Communication issues  Reduces communicate traffic  Improves local and global autonomy

Final Version MAXIM-PF, May 13-17, 2002 Goddard Space Flight Center Attitude Control Page 20 Formation Configuration  Detector:  Communicates with ground and Hub  Has more fuel and thrust authority for retargeting  Additional safehold communication/ranging capabilities can be utilized to provide position of self and hub (full mission)  Optical Hub:  Provides command for formation structure and retargeting  Safehold beacon used to keep free flyers near  In safehold sends detector updates on current estimated location of FF and self (full mission).  Freeflyers:  In Safehold, execute collision avoidance and stay close to Hub  Freeflyers can lead a subgroup as numbers of S/C grows (full mission)  Can replace some of the functionality of the Hub (full mission)

Final Version MAXIM-PF, May 13-17, 2002 Goddard Space Flight Center Attitude Control Page 21 High Accuracy Formation Control Technologies  External and Internal Disturbance estimation  Estimate fuel usage and CG migration  Sensor bias and drift  Uncertainty bounds  Localized disturbance levels  Other system parameters  Control  Utilize estimated states compensation scheme  Adaptive/Robust schemes can account for variations in parameters (Mass Properties, CP-CG offset, local variations in solar pressure)  Phase 2 may employ distributed control schemes to decentralize control  Reduces risk by S/C-level redundancy  May reduce computational load on Hub

Final Version MAXIM-PF, May 13-17, 2002 Goddard Space Flight Center Attitude Control Page 22 Trades performed  Reaction Wheels vs. Thrusters for Attitude Control  Reaction wheels would be jitter sources  Continuous micro-thrust needed for translation control  Recommendation: Use thrusters for attitude as well as translation control  Do Freeflyers talk to each other?  Inter-FF comm would simplify “Lost in Space” solution  Direct measurement of FF-FF ranges  Inter-FF comm complicates RF comm system  More channels required  Recommendation: No FF-FF comm  Avoids complicating RF comm system  “Lost in Space” may be solved with Hub-FF ranging, with small programmed maneuvers

Final Version MAXIM-PF, May 13-17, 2002 Goddard Space Flight Center Attitude Control Page 23 Sensor/Actuator Resolution  Minimum Impulse Bit = 20  N-sec achievable by PPTs or FEEPs  Assumes 100-sec limit cycle on 10  m translation control, and 100-kg S/C  PPTs provide 10  N-sec or less  FEEPs provide 1  N thrust resolution  Accelerometer Resolution Required ~= 1.0x10 -9 m/s^2  Acceleration “bit” is thruster resolution divided by S/C mass  FEEP thruster resolution = 1.0E-6 N, S/C mass < 1000 kg  Onera (GRACE) accelerometer resolution = 3.0x10 -9 m/s^2  Right order of magnitude