Automated Allocation of ESA Ground Station Network Services

The ESTRACK Planning System (EPS) operationally plans the use of the ESAs ESTRACK ground station network. It uses an incremental planning approach to successively build up the ESTRACK Management Plan using feedback provided by its user missions. The system is configured with a specification of the user missions needs and the networks constituents capabilities. The planning process matches both elements to create a plan that is free of conflicts and serves the needs of all missions. To build the plan, a constraint network is constructed from the dynamic input to the system and its configuration. The network is built incrementally exploiting the periodic nature of the communication request timings required by the user missions. The resulting CSP contains temporal binary constraints, linear constraints and disjunctions of binary constraints. Its consistency is checked in two steps: first the DTP (Disjunctive Temporal Problem) part is solved and then the remaining linear constraints are checked using Linear Programming algorithms. Future developments of EPS will include a more sophisticated resource modeling and the inclusion of more missions and external users. Advanced features like active constraints and global optimization are conceivable. The ESA ground station network (ESTRACK) The European Space Agency (ESA) runs a number of ground stations to support its own missions and the missions of industry contractors. 8 stations owned by ESA plus 3 cooperative stations form the basis of the Esa TRACKing network, ESTRACK. It also includes control and communication facilities. ESTRACK currently supports 10 operational ESA science missions. It provides services for data downlink and the uplink of commands to satellites in orbit. In addition to the regular ESA missions, ESTRACK supports requests from external users (e.g. NASA). Copyright © 2006, American Association for Artificial Intelligence (www.aaai.org). All rights reserved. The mission’s requests for satellite-to-ground communication are coordinated for the ESTRACK network as a whole. Until now planning and scheduling of ESTRACK was done manually, supported by a set of tools. The SCUT (Spacecraft Commitment Utilization Tool) is an analysis tool that was used to prove the capability of ESTRACK to support a set of missions. A set of rules defined the missions communication needs. The rules were applied to build a starting ground station allocation plan. They had to be configured to ensure the production of a conflict free plan. Automated conflict resolution or intelligent search algorithms were not used. The starting plan was loaded into a scheduling tool, where an operator had to edit it. The tool was then used to generate the station schedules. The planning as it was performed covered the needs of the ESTRACK network. In the future, more missions will have to be cared for and the network will grow by the number of stations. In order to coordinate this growing number of users and providers efficiently, an Automated Planning System is called for. An automated system using an intelligent planning algorithm is able to exploit a flexible assignment of communication services to user missions. In the following sections of this paper, a detailed problem description is presented. Once the scene is set, a model is demonstrated that supports the solution of the given planning problem. Algorithms that are used for the solution of the planning problem are discussed and first operational results are presented. Finally plans for the future development of the system are discussed. Planning the ESTRACK network An Automated Planning System can cope with the future demand of ground station planning. This planning system is called the ESTRACK Planning System (EPS). The planning system has to dynamically assign the missions requests to services that ground stations offer. Instead of assigning a ground station to a mission, the services, a ground station can offer, are identified and parameterized. Any mission that is using the ESTRACK network may request these services. This adds flexibility and robustness to the planning process. The system is flexible enough, so that a new mission or ground station can be added by changing the software’s configuration database. The software will find a ground station that fulfills the communication requirements of the new mission. Upon the unavailability of a ground station the system should be robust enough to find an alternative station with the required specification. The ESTRACK network supports different types of missions that have very different communication needs: • Earth observation missions with frequent short (several per day) communication periods. • Missions on highly elliptical orbits (astronomy) with long infrequent communication periods. • Missions situated in one of the sun-earth Lagrange points with daily repeated communication. • Interplanetary missions with very specific needs and features (long one way light time). Planning strategies have been developed that can cope with these very diverse communication requirements. EPS defines for each ground station a set of services it offers to the user missions. Some of the services may be mutually exclusive (they cannot be used at the same time) others may be used in one or several instances together with other services. The underlying resource model is part of a future development and is discussed in the last chapter of this paper. The first implementation of EPS assumes an exclusive usage of one ground station per mission in the same time slot.