# Presentation: 116768

## Micro-Arcsecond X-ray Imaging Mission Pathfinder (MAXIM-PF)

**Eric Stoneking **
**Paul Mason**
**May 17, 2002**

**ACS**

## 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

## 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

## 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

## “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)		

**“****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**

## 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

## 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

## Backup Slides

**Sensor Configurations**
**Components**
**Trade Studies**

## Hub/Detector Sensor Configuration

## Hub/Freeflyer Sensor Configuration

## Attitude/Translation Requirements and Sensors: Optics Hub

## Attitude/Translation Requirements and Sensors: Detector

## Attitude/Translation Requirements and Sensors: Freeflyer

## ACS Components Optical Hub

## ACS Components Detector

## ACS Components Free Flyer

## ROM ACS Labor Cost

- Note: 1) Estimated cost derived from MAP  cost in $K

## 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

## 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

## 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)

## 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

## 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

## 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