Space Systems Laboratory Massachusetts Institute of Technology

SPHERES

Development of Formation Flight and Docking Algorithms Using the

SPHERES Testbed

Prof. David W. Miller

MIT Space Systems Laboratory

Allen Chen, Alvar Saenz-Otero, Mark Hilstad, David W. Miller

Space Systems Laboratory Massachusetts Institute of Technology

SPHERES IntroductionSPHERES:

Synchronized Position Hold Engage Reorient Experimental Satellites

SPHERES is a cost-effective, risk-tolerant, interactive testbed operated inside the ISS for the development and maturation of formation flight and autonomous rendezvous and docking technologies (autonomy, control, and metrology).

• Multi-vehicle Testbed - long duration micro-gravity environment allows 6DOF per vehicle and close proximity maneuvers representative of envisioned missions: TPF, Starlight, TechSat 21, Orbital Express, and Mars Sample Return.

• Cost-effective - replenishable consumables, IFM and observations by crew reduce operation costs; data (up)downlink and video coverage expedites algorithm design; reconfigurable and upgradeable hardware accommodates new technologies.

• Risk-tolerant - ill-behaved algorithms can be stopped and corrected without affecting future operations. Allows software to mature from conception to flight quality without danger of mission failure.

Space Systems Laboratory Massachusetts Institute of Technology

SPHERES Design Philosophy

• Experimental Research in a micro-Gravity Environment Allows:

– Demonstration and validation.– Repeatability and reliability.– Determination of simulation accuracy.– Realization of performance limitations.– Discovery of new physical phenomena.– Understanding of operational drivers.

• Laboratory Attributes:– Delineates between generic and test-

specific equipment.– Allows reconfiguration of hardware for

mission-specific equipment.– Supports extended investigations with

multiple experiment iterations.– Accommodates unforeseen failures in

a risk tolerant environment.– Provides software reconfiguration for

rapid changes of algorithms.– Enhances the iterative research

process via interaction and observation by humans.

– Supports multiple investigators.

If one cannot simulate the space environment in the laboratory, simulate the laboratory

environment in space.

Space Systems Laboratory Massachusetts Institute of Technology

SPHERES SPHERES Testbed

• SPHERES free-flier units– Up to 3 units can be used simultaneously (with possible upgrade to more units).– Each independent unit contains propulsion, power, wireless communications,

metrology, and processing sub-systems.– Sensors and actuators provide full state vector (6DOF, 12 states) of each unit.

• Laptop unit– ISS standard PC laptop serves as a base station for software reconfiguration and

telemetry downloads.

STG

Metrology

STS

– Provides access to the Ku-band for ISS to ground communications.

• Communications– Satellite-to-satellite (STS) for formation/docking

control.– Satellite-to-ground (STG) for telemetry download

and software upload.

• Metrology– Four external metrology transmitters, mounted

to the body of the ISS, create frame of reference.

Space Systems Laboratory Massachusetts Institute of Technology

SPHERES SPHERES Testbed

• Structures– Aluminum internal frame, covered with

Lexan panels.

• Propulsion– Cold-gas thruster system using CO2 at

70psig.– Micro-machined custom nozzles

attached to electric solenoid valves.

• Metrology– IR/Ultrasound ranging system resembles

GPS within ISS environment (1Hz).– 6DOF IMU system for high frequency

data (50Hz).

• Avionics– C40 DSP Main Processor (will be

upgraded to a C6701)

• Power– 16 AA batteries

Diameter 0.25 mMass 3.1 kgMax Linear Acceleration 0.17 m/s2

Max Angular Acceleration 3.5 rad/s2

Battery Life ~90 minCommunications Data Rate 19200 kbpsPower 7 WMetrology Resolution 2 mm

0.5Tank Life ~10 min

Space Systems Laboratory Massachusetts Institute of Technology

SPHERES Testbed Validation

• KC-135 Reduced Gravity Airplane– Full 6DOF dynamics– Short duration

• Tests Performed (2/01 and 3/01)– System checkouts– Single SPHERE attitude control– Master/Slave formation flight

• 2D Laboratory Experiments– Long duration 2D tests (3DOF)– Can use fixed supplies instead of

consumables– Preliminary low-cost testing prior to

KC or ISS deployment

• Tests Performed (ongoing)– Master/Slave formation flight– Docking algorithms

Space Systems Laboratory Massachusetts Institute of Technology

SPHERES Formation Flight

• KC-135 Frame Follower– Master unit attached to KC-135 frame.– Slave commanded to follow the rotation of the Master: should remain oriented with

frame.– 10Hz STS communications of full attitude state (3 angles and 3 angular rotations).– Slave must recover from initial deployment.

Space Systems Laboratory Massachusetts Institute of Technology

SPHERES Docking Research

• 2D Docking Demonstrations– Cooperative docking: Master unit awaits the arrival of the slave with full actuation to

align docking port.– Slave unit starts with initial attitude offset, aligns itself with the master, and then

translates to perform the docking.– 10Hz STS communications of full 2D state (4 position, 2 attitude).

Space Systems Laboratory Massachusetts Institute of Technology

SPHERES Team Capability

• MIT Space Systems Laboratory– David W. Miller

• Formation flight, rendezvous and docking research in support of Techsat21, ST-3, Terrestrial Planet Finder

• Design and PI of 0-g dynamics and controls laboratories

– MODE STS-40, 48, 62– DLS on MIR– MACE STS-67, 106, ISS

– Jonathan P. How• Formation flight, differential GPS,

robust control

– Brian Williams• Spacecraft autonomy, remote

agent, Livingstone autonomous model-based diagnosis on DS-1

• Payload Systems Incorporated– Developer and integrator of

experiments in human-rated space platforms (Shuttle, MIR, ISS)

• The fact that our facilities have more reflights & first flights is testimony to the versatility of, and demand for, our dynamics and controls laboratories

Space Systems Laboratory Massachusetts Institute of Technology

SPHERES Conclusions

• SPHERES has demonstrated operation as a Formation Flight and Docking Algorithm testbed in 2D and KC-135 environments.

– KC-135 experiments validated the operation of SPHERES in a 6DOF environment.– 2D Laboratory testing provided initial insight into communications requirements for

Formation Flight systems– 2D Demonstrations of both Formation Flight and Docking Algorithms are ongoing.

• ISS Deployment in early 2003– Manifested in Flight 12A.1 of the ISS for April 2003 (with possible 11A launch)– Minimum mission span of 6 months.

• Programmatic Status– DARPA (AF) Orbital Express Program

• In-space assembly, upgrade, and servicing

– NASA Goddard Space Flight Center Formation Flight Algorithms.

– NASA Starlight, ST6, and Terrestrial Planet Finder missions.

– Capture of Mars sample return vehicles.