<Mission> <Discipline> - NASA

&lt;Mission&gt; &lt;Discipline&gt; - NASA

Micro-Arcsecond X-ray Imaging Mission Pathfinder (MAXIM-PF) Final Version ACS Eric Stoneking Paul Mason May 17, 2002 ACS Drivers Final Version 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 Attitude Control Page 2 Technologies Final Version 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 Attitude Control Page 3 ACS Control Modes Final Version 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

Attitude Control Page 4 ACS Requirements Imposed On Other Sub-Systems Final Version 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) MAXIM-PF, May 13-17, 2002 Goddard Space Flight Center Attitude Control Page 5 ACS Concerns and Comments Final Version 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 Attitude Control Page 6 Future Studies Final Version 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 Attitude Control Page 7 Backup Slides Final Version Sensor Configurations Components Trade Studies

MAXIM-PF, May 13-17, 2002 Goddard Space Flight Center Attitude Control Page 8 Hub/Detector Sensor Configuration Final Version Hub S/C Laser Beacon illuminates Detector S/C Super Star Tracker centers on Laser Beacon Detector S/C Super gyros hold inertial attitude Laser Detector measures range by time-of-flight of reflected laser beam Normal Star Tracker places Laser Beacon against fixed stars Reflector Cube reflects laser beam back to Hub for ranging Coarse Ranging by omni RF comm link MAXIM-PF, May 13-17, 2002 Goddard Space Flight Center Attitude Control Page 9 Hub/Freeflyer Sensor Configuration Final Version Freeflyer S/C

Hub S/C Reflector Cube Swarm Sensor measures range by bouncing RF, laser off Hub Small Laser Beacon Normal Star Tracker places Hub beacon against fixed stars Coarse Ranging by omni RF comm link MAXIM-PF, May 13-17, 2002 Goddard Space Flight Center Attitude Control Page 10 Attitude/Translation Requirements and Sensors: Optics Hub Final Version Optics Hub Axis Roll Pitch Yaw X Trans Y Trans Z Trans Optics Hub Axis Roll Pitch Yaw X Trans Y Trans Z Trans Phase 1

Phase 2 Control Reqt Knowledge Reqt Control Reqt Knowledge Reqt TBD arcmin 1 arcsec TBD arcmin 1 arcsec 1 arcsec 1 arcsec 1 arcsec 1 arcsec 1 arcsec 1 arcsec 1 arcsec 1 arcsec 5 m TBD cm 5 m TBD cm TBD cm 30 um 10 um 10 um TBD cm 30 um 10 um 10 um Rate Sensor Gyro Gyro Gyro Accel Accel, 1E-9 m/s^2 Accel, 1E-9 m/s^2 MAXIM-PF, May 13-17, 2002 Goddard Space Flight Center Position Sensor Star Tracker (X-Y) Star Tracker (X-Y) Star Tracker (X-Z) Time-of-flight laser ranging to detector None None

Attitude Control Page 11 Attitude/Translation Requirements and Sensors: Detector Final Version Detector Axis Roll Pitch Yaw X Trans Y Trans Z Trans Detector Axis Roll Pitch Yaw X Trans Y Trans Z Trans Control Reqt 0.5 arcmin 0.5 arcmin 0.5 arcmin 5 m 5 cm 5 cm Phase 1 Knowledge Reqt 1 arcsec 1 arcsec 1 arcsec TBD cm 15 um 15 um Rate Sensor Gyro Gyro Gyro Accel

Accel, 1E-9 m/s^2 Accel, 1E-9 m/s^2 MAXIM-PF, May 13-17, 2002 Goddard Space Flight Center Control Reqt 0.5 arcmin 0.5 arcmin 0.5 arcmin 5 m 5 cm 5 cm Phase 2 Knowledge Reqt 1 arcsec 1 arcsec 1 arcsec TBD cm 15 um 15 um Position Sensor Star Tracker (X-Y) Star Tracker (X-Y) Star Tracker (X-Z) Time-of-flight laser ranging from hub Laser beacon tracker, Gyro ( <1 uas/day) Laser beacon tracker, Gyro ( <1 uas/day) Attitude Control Page 12 Attitude/Translation Requirements and Sensors: Freeflyer Final Version Freeflyer Axis Roll Pitch Yaw X Trans Y Trans LOS to hub Control Reqt N/A N/A N/A N/A N/A N/A Phase 1 Knowledge Reqt N/A N/A N/A N/A N/A N/A Control Reqt 1 arcsec 1 arcsec 1 arcsec 0.5 mm [1] 10 um 10 um Phase 2 Knowledge Reqt 1 arcsec 1 arcsec 1 arcsec 0.5 mm [1] 10 um 10 um [1] 2 arcsec @ 100 m Freeflyer Axis Roll Pitch Yaw X Trans Y Trans LOS to hub Rate Sensor Gyro

Gyro Gyro Accel, 1E-9 m/s^2 Accel, 1E-9 m/s^2 Accel, 1E-9 m/s^2 MAXIM-PF, May 13-17, 2002 Goddard Space Flight Center Position Sensor Star Tracker (X-Y) Star Tracker (X-Y) Star Tracker (X-Z) Star Tracker sees hub laser beacon Star tracker sees hub laser beacon Swarm Sensor Attitude Control Page 13 ACS Components Optical Hub Final Version Components Model Coarse Sun Sensor Laser Beacons f or Freeflyers Laser Beacon f or Detector Accelerometer Star Tracker Gyro Adcole 11866 Cost Q uantity ($K) ACE (Like SDO & GPM) Onera Ball CT-602 Litton SI RU I ndependent processor, includes

EVD Dynamic Simulator Ground Support equipment f or I &T Dynamic Simulator Ground Support equipment f or I &T Peak (W) Saf ehold (W) 8 6 1 1 2 1 56 60 TBS TBD 1300 1000 0.04 6 TBS 6 11.8 5.44 0 6 TBS 2 18 22 0 6

TBS 2 18 40 0 0 TBS 2 0 22 1 300 6 11 26 11 50 N/ A N/ A N/ A N/ A 300 N/ A N/ A N/ A N/ A 35.3

59 92 35 Subtotal = 2716 MAXIM-PF, May 13-17, 2002 Goddard Space Flight Center Mass Orbit Avg (Kg) (W) Attitude Control Page 14 ACS Components Detector Final Version Cost Q uantity ($K) Components Model Coarse Sun Sensor Accelerometer Star Tracker Gyro ACE (Like SDO & GPM) Adcole 11866 Onera Ball CT-602 Litton SI RU I ndependent processor, includes EVD Dynamic Simulator Ground Support equipment f or I &T

Dynamic Simulator Ground Support equipment f or I &T Peak (W) Saf ehold (W) 8 1 2 1 56 TBD 1300 1000 0.04 6 11.8 5.44 0 2 20 22 0 2 20 40 0 2 0 22 1 300 6

11 26 11 50 N/ A N/ A N/ A N/ A 300 N/ A N/ A N/ A N/ A 29.3 55 88 35 Subtotal = 2656 MAXIM-PF, May 13-17, 2002 Goddard Space Flight Center Mass Orbit Avg (Kg) (W) Attitude Control Page 15

ACS Components Free Flyer Final Version Cost Quantity ($K) Components Model Coarse Sun Sensor Swarm Sensor Accelerometer Star Tracker Gyro ACE (Like SDO & GPM) Adcole 11866 MSTAR Onera Ball CT-602 Litton SI RU I ndependent processor, includes EVD Dynamic Simulator Ground Support equipment f or I &T Dynamic Simulator Ground Support equipment f or I &T Peak Saf ehold (W) (W) 8 1 1 2 1 56 TBD

TBD 1300 1000 0.04 10 6 11.8 5.44 0 15 2 20 22 0 15 2 20 40 0 15 2 0 22 1 300 6 11 26 11 50 N/ A N/ A

N/ A N/ A 300 N/ A N/ A N/ A N/ A 39.3 70 103 50 Subtotal = 2656 MAXIM-PF, May 13-17, 2002 Goddard Space Flight Center Mass Orbit Avg (Kg) (W) Attitude Control Page 16 ROM ACS Labor Cost Final Version GNC / Systems Engineering ACS Design & Analysis Labor ACS Hardware Labor ACE Hardware Labor Hybrid Dynamic Simulator (HDS)

Integration and Testing TOTAL Dynamic Simulator Hardware Labor GRAND TOTAL $1,529 $4,032 $4,762 $961 $2,111 $1,948 $15,343 $300 $15,643 Note: 1) Estimated cost derived from MAP cost in $K MAXIM-PF, May 13-17, 2002 Goddard Space Flight Center Attitude Control Page 17 Phase 1 Command structure Final Version ACE/C&DH in charge of the unit sensor/actuators Receive measurements from freeflyer attitude sensors Sends thruster commands to freeflyer Attitude and Position Information Attitude and position Thrust MAXIM-PF, May 13-17, 2002 Goddard Space Flight Center Thrust

commands Attitude and position Thrust Attitude Control Page 18 Formation Final Version 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 MAXIM-PF, May 13-17, 2002 Goddard Space Flight Center Attitude Control Page 19 Formation Configuration Final Version 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 Attitude Control Page 20 High Accuracy Formation Control Technologies Final Version 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 Attitude Control Page 21 Trades performed Final Version 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 Attitude Control Page 22 Sensor/Actuator Resolution Final Version 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 MAXIM-PF, May 13-17, 2002 Goddard Space Flight Center Attitude Control Page 23

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