Constellation of Satellites
The satellites' constellation (SC) domain consists of a group of low-Earth orbiting
satellites that have cross-link communication capability, each carrying nearly the
same suite of instruments. Each satellite receives high-level goals from ground station
operators, or other satellites. Then, it will perform its own planning by decomposing
a goal into a set of sub-goals to be achieved with its onboard resources and in cooperation with other satellites.
We intend to use the proposal of hierarchical coalitions, discussed through this work, to specify a possible configuration for this domain. For that, the principal issues and features that we are investigating are:
An interesting advantage of the SC domain is that it can be used to validate collaborative
planning and schedule approaches for space, being a first step for the development of
more challenging applications as the scenario discussed in follow.
- Architectures for SC are likely to employ a dynamic hierarchical organisation to combine the efficiency of hierarchical activity delegation with the robustness afforded by dynamic reorganisation. To include dynamic organisations in our approach, we should extend it so that the organisation model could be represented via constraints, and agents could be able to reason on relations and their features;
- Resources such as onboard memory, downlink accesses and bandwidth capabilities, are elements that in fact limit the number of experiments that can be performed. Our model supports this idea, providing a specific kind of constraint to represent the satellites' resources and their features. Thus, specific resource constraint manager modules can be implemented to reason on such constraints according to the
requisites of the application;
- Time is also fundamental to SC. In a general way, satellites whose missions involve observations
and/or communication with the ground must take into account access windows, which are intervals of opportunity defined by when the satellite is in view of the target on the ground. In fact, for SC the problem is more complex because the relative position of individual satellites must be taken into account when assessing viewing geometry (satellites need to be in a specific position during a specific time);
- Satellites are already being developed to store information and to support reasoning about commitments of other satellites. Together with commitments we must also stress the importance of reports. Consider the situation where all satellites commit on the observation activity. However
when one of them reaches the pre-defined position, it notes that its vision is blocked by
clouds. In this case, such a satellite must report this fact so that the leader decides what
must be done, such as changing (replanning) or cancelling the previous commitments;
- The idea of mutual support can also be used in this domain to generalise the concept
of collaboration. For example, in situations where a satellite moves out of the range
of a ground station, however the required messages may be relayed through the system
using other satellites in the constellation to still allow messages to reach their destination ;
- A last issue is associated with the autonomy of SC control systems. In general,
important uses of autonomy in SC are: to enable that satellites fly within specified tolerance
levels; avoid collisions; address fault detection, isolation and resolution; share
knowledge, and plan and schedule activities.
The principal aims of this domain is to exemplify the use of our approach in a futuristic one-sol (one Martian day) mission, where human users and agents (robots) are actively involved during the process of planning and execution.
The mission has several decision-making levels. The ground team on Earth sets the
macro goals, sharing the activities with the Mars-Habitats. Each Mars-Habitat has one
or more exploration teams, which are composed of a lander, two rovers (r1 and r2) and two astronauts (a1 and a2). Orbiters provide some auxiliary services. In our example there are two principal objectives for each exploration team: studying the surface of
Mars (activity assigned to rovers) and looking for some sign of life (activity assigned to astronauts). The lander provides a higher bandwidth communication channel (e.g., for high-quality images transmission) and a mobile micro-laboratory.
Centre for Intelligent Systems and their Applications
School of Informatics, The University of Edinburgh
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Last updated: Fri Sep 9 15:59:50 2005