Continual Coordination of Spacecraft through Shared Activities Brad Clement & Tony Barrett Artificial Intelligence Group Jet Propulsion Laboratory California Institute of Technology {bclement, barrett}@aig. jpl. nasa. gov http: //www-aig. jpl. nasa. gov/ All rights reserved, California Institute of Technology © 2002
Shared Activity Coordination (SHAC) • SHAC – Framework for defining and implementing automated interactions between planning systems (a. k. a. coordination protocols) – Continual coordination algorithm – Testbed for evaluating protocols • Prior state-of-art – scientists meet to negotiate mission plan(s) – distributed planning treated as a collaborative problem • planners collaborate on resolving state conflicts • space mission planning is decentralized; requires complex resources • Main Capabilities – Spacecraft react as team to unexpected events. – Missions can evaluate distributed planning protocols in real-time environment. – Alternative protocols easy to design and implement. All rights reserved, California Institute of Technology © 2002
Motivation Over 40 multi-spacecraft missions proposed! – Autonomous single spacecraft missions have not yet reached maturity. – How can we cost-effectively manage multiple spacecraft? Earth Observing System Origins Program NMP Sun-Earth Connections NMP Structure & Evolution of the Universe Mars Network All rights reserved, California Institute of Technology © 2002
Shared Activity Coordination Planner Executive Shared activities implement team plans, joint actions, and shared states/resources All rights reserved, California Institute of Technology © 2002
Shared Activity Model • parameters (string, integer, etc. ) – constraints (e. g. agent 4 allows start_time [0, 20], [40, 50]) • decompositions (shared subplans) • permissions - to modify parameters, move, add, delete, choose decomposition, constrain • roles - maps each agent to a local activity • protocols - defined for each role – change constraints – change roles • changes sharing agents and protocol assignments – handle changes received from other agents All rights reserved, California Institute of Technology © 2002
SHAC Algorithm Given: 1. 2. 3. 4. 5. 6. 7. a plan with multiple activities, including a set of shared_activities with constraints, and a projection of plan into the future. Revise projection using the currently perceived state and any newly added goal activities. Alter plan and projection while honoring constraints. Release relevant near-term activities of plan to the real-time execution system. For each shared activity in shared_activities – if outside consensus window, • apply each associated protocol to modify the activity – else • apply simple consensus_protocol Communicate changes in shared_activities and constraints. Update shared_activities and constraints based on received communications. Go to 1. All rights reserved, California Institute of Technology © 2002
Protocol Capabilities Default protocol class • joint intention • mutual belief • resource sharing • active/passive roles • master/slave roles Extending protocol classes 1. modify permissions 2. modify local parameter constraints 3. add/delete sharing agents 4. change roles of sharing agents All rights reserved, California Institute of Technology © 2002
Delegation Protocol Delegation: : modify. Roles() – if roles does not contain exactly 1 subordinate • choose a subordinate to whom to delegate the activity • add subordinate to roles Subordination: : modify. Roles() – if cannot resolve conflicts involving activity • remove self from roles All rights reserved, California Institute of Technology © 2002
Asynchronous Weak Commitment AWC: : modify. Permissions() – if have highest priority • remove self’s modification permissions (add, move, delete) – else • give self modification permissions AWC: : modify. Constraints() – if cannot resolve local conflicts and conflicts with constraints of higher ranking agents • set own rank to highest rank plus one • generate parameter constraints (no-good) describing locally consistent values All rights reserved, California Institute of Technology © 2002
Mars Scenario • • MER depends on orbiters to route data Orbiters have their own agendas, but agree to relay MER data based on priority. Orbiters schedule comm requests and respond with downlink times. MER delegates to orbiter with best MGS downlink time or plans to send directly to Earth. SHAC MER A B. Clement MEX Odyssey MER B All rights reserved, California Institute of Technology © 2002
shared_activity comm_a { agent mera { datasize; planner = Aspen. Planner. Interface (10, comm_windows, critical_size; odyssey, odyssey_view_sv 0, in, high_size; mgs, mgs_view_sv 0, in, response_time; mex, mex_view_sv 0, in); start_time; communication = Socket. Communication (". . /dist-coord/mars-ports. txt"); roles = comm_orbiter_a by mera, communicator = Aspen. Communicator ; }; move_mer_data by mgs, Team Model File move_mer_data by odyssey, agent merb { move_mer_data by mex; planner = Aspen. Planner. Interface (10, comm_windows, protocols = odyssey, odyssey_view_sv 0, in, mera Mars. Delegation , mgs, mgs_view_sv 0, in, mgs Subordination, mex, mex_view_sv 0, in); mex Subordination, communication = Socket. Communication (". . /dist-coord/mars-ports. txt"); odyssey Subordination; permissions = communicator = Aspen. Communicator ; }; comm_orbiter_time_a (all), move_mer_data (place, detail, response_time); agent odyssey { }; planner = Aspen. Planner. Interface (10, 10); communication = Socket. Communication (". . /dist-coord/mars-ports. txt"); shared_activity comm_b { datasize; communicator = Aspen. Communicator ; }; critical_size; high_size; agent mgs { response_time; planner = Aspen. Planner. Interface (10, 10); start_time; communication = Socket. Communication (". . /dist-coord/mars-ports. txt"); roles = comm_orbiter_b by merb, communicator = Aspen. Communicator ; }; move_mer_data by mgs, move_mer_data by odyssey, agent mex { move_mer_data by mex; planner = Aspen. Planner. Interface (10, 10); protocols = communication = Socket. Communication (". . /dist-coord/mars-ports. txt"); merb Mars. Delegation , mgs Subordination, communicator = Aspen. Communicator ; }; mex Subordination, odyssey Subordination; permissions = protocol Mars. Delegation(); protocol Subordination(); comm_orbiter_time_b (all), move_mer_data (place, detail, response_time); }; All rights reserved, California Institute of Technology © 2002
Mars Scenario MER activities Odyssey activities through Odyssey critical comm pancam odyssey earth downlink critical data no pending request uplink from DSN direct critical pancam comm earth must-be wait request comm earth wait for uplink comm odyssey wait for odyssey no pending uplink received request no pending request wait for uplink comm earth wait for no pending uplink request All rights reserved, California Institute of Technology © 2002
Mars Scenario MER activities Odyssey activities science activities comm earth no pending request Odyssey science activities comm odyssey comm earth critical pancam MER A no pending request comm earth traverse comm earth must wait request wait for no pending request uplink All rights reserved, California Institute of Technology © 2002
Mars Scenario MER activities Odyssey activities science activities comm earth comm odyssey comm earth critical pancam comm earth must wait Odyssey no pending request wait for uplink traverse comm earth odyssey received no pending request comm earth science activities comm odyssey comm earth critical pancam comm earth traverse comm earth must wait MER A no pending request wait for uplink wait for no pending request uplink All rights reserved, California Institute of Technology © 2002
Mars Scenario MER activities Odyssey activities science activities comm earth comm odyssey comm earth critical pancam comm earth must wait Odyssey no pending request wait for uplink odyssey received traverse comm earth no pending request comm earth science activities comm odyssey comm earth critical pancam comm earth must wait MER A no pending request wait for uplink traverse comm earth no pending odyssey received reserved, Californiarequest All rights Institute of Technology © 2002
Communication Window-Based Protocols comm opportunities MEX MGS Odyssey DSN MER Planner Executive Problem: when should xmit commitments be made on communication? If too early, chances to optimize are lost. If too late, de-commitments could result in lengthy returns to Earth. All rights reserved, California Institute of Technology © 2002
Communication Window-Based Protocols MEX MGS Odyssey DSN MER Planner Executive Immediate requests may not be fulfilled during communication because replanning is needed. By starting request in prior comm window, consensus can be reached more easily because there is time to replan in between comm windows. Would a single consensus window be better because of dependencies among orbiters? All rights reserved, California Institute of Technology © 2002
SHAC for ground planning systems • Tech Sat 21 Mission adopts SHAC coordination system for use in coordinating multiple experiments for Mission – Technology enables automated conflict detection and resolution between distributed experiment operations teams when experiments require use of shared spacecraft resources SHAC Coordination De-conflicted Master TS-21 Schedule … Individual TS-21 Experiment Plans B. Clement All rights reserved, California Institute of Technology © 2002
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