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

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 MAXIM-PF, May 13-17, 2002 Goddard Space Flight Center

Technologies Formation Flying Algorithms Micro-thrust Control 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 MAXIM-PF, May 13-17, 2002 Goddard Space Flight Center

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 MAXIM-PF, May 13-17, 2002 Goddard Space Flight Center

ACS Requirements Imposed On Other Sub-Systems “Super Star Tracker” (Laser beacon tracker + low-drift gyros) needed for detector control (Instrument) Thruster impulse bit < 20 mN-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) MAXIM-PF, May 13-17, 2002 Goddard Space Flight Center

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 MAXIM-PF, May 13-17, 2002 Goddard Space Flight Center

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 MAXIM-PF, May 13-17, 2002 Goddard Space Flight Center

Backup Slides Sensor Configurations Components Trade Studies MAXIM-PF, May 13-17, 2002 Goddard Space Flight Center

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 Super gyros hold inertial attitude Coarse Ranging by omni RF comm link MAXIM-PF, May 13-17, 2002 Goddard Space Flight Center

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 MAXIM-PF, May 13-17, 2002 Goddard Space Flight Center

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

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

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

ACS Components Optical Hub MAXIM-PF, May 13-17, 2002 Goddard Space Flight Center

ACS Components Detector MAXIM-PF, May 13-17, 2002 Goddard Space Flight Center

ACS Components Free Flyer MAXIM-PF, May 13-17, 2002 Goddard Space Flight Center

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

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 MAXIM-PF, May 13-17, 2002 Goddard Space Flight Center

Formation Configuration: Expandable Redundancy Communication issues 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 MAXIM-PF, May 13-17, 2002 Goddard Space Flight Center

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) MAXIM-PF, May 13-17, 2002 Goddard Space Flight Center

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 MAXIM-PF, May 13-17, 2002 Goddard Space Flight Center

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 MAXIM-PF, May 13-17, 2002 Goddard Space Flight Center

Sensor/Actuator Resolution Minimum Impulse Bit = 20 mN-sec achievable by PPTs or FEEPs Assumes 100-sec limit cycle on 10 mm translation control, and 100-kg S/C PPTs provide 10 mN-sec or less FEEPs provide 1 mN 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 MAXIM-PF, May 13-17, 2002 Goddard Space Flight Center