appears that only a single action is selected at any one time frame and not a sequence of actions, as one would expect in any sort of plan that spanned a finite time window (from current situation to future goal). What has been implemented is, in effect, a situation-driven decision-theoretic reflexive action generator, operating over a planning horizon of a single time frame. It may be possible to convert the current planner into a full-fledged multistep plan generator, although it is unclear how this would be achieved in the current implementation.

Considerable effort has been devoted to applying the Soar cognitive architecture (Laird et al., 1987; Laird and Rosenbloom, 1990) to the modeling of human behavior in fixed-wing tactical missions across three services: the Air Force, the Navy, and the Marines (see Chapter 2). Initial efforts have led to a limited-scope demonstration of feasibility for FWA-Soar (Tambe et al., 1995); more recently, FWA-Soar was used in the synthetic theater of war (STOW)-97 large-scale warfighting simulation (Laird, 1996).

The standard Soar approach to hierarchical goal decomposition is taken, so that a high-level goal (e.g., "conduct intercept") is broken down into its component subgoals (e.g., "adjust radar," "achieve proximity"). Each of these subgoals is then successively broken down until some implicit ''atomic-level" goal is reached (e.g., "press trigger"), beyond which no further subgoals or activities are generated. The successful achievement of any given goal in the hierarchy typically requires the satisfaction of one or more associated subgoals, so that a tree structure naturally arises. Thus in the tactical air domain, one might see the following tree structure (modified from Laird, 1996):

Intercept

Search

Select threat

Achieve proximity

In the above, indenting is used to indicate the level in the tree, and at each level, only one node (shown in bold) is expanded. Satisfaction of any given goal may require the satisfaction of all its subgoals (for example, the launch missile goal shown above) or of only some subset of its subgoals (for example, perform