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

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

3. 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

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

5. 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

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

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

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

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

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

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

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

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

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

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

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

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

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

19. 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

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

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

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

23. 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