A formal description of tactical plan recognition

Abstract

Plan recognition can roughly be described as the problem of finding the plan(s) underlying the observed behaviour of agent(s). Of course, usually, the observed behaviour and available background knowledge does not determine the underlying plan, and therefore one can typically at best generate (reasonable) plan hypotheses. Traditionally, plan recognition has been studied, formalized and implemented in areas like story understanding and user modelling. In this paper, we propose a formal definition of tactical plan recognition, i.e. the recognition of enemy plans. We will focus on military applications, where this task of tactical plan recognition is crucial, but this task is relevant for every application where one has to deal with intelligent adversial agents. Tactical plan recognition differs from traditional plan recognition in a number of ways. For example, an enemy will often try to avoid making his plans known. We will not pay much explicit attention to this feature. We will focus on another important characteristic feature of tactical plan recognition, namely that the identity of the observed enemy objects, for which plans are to be recognized, may be unknown. A consequence of this is that it is typically not known which observations originate from the same objects. Our formalization of plan recognition is based on classical abduction. The concepts of classical abduction can readily be applied to plan recognizers for identified observations, as has been done by Lin and Goebel [18] and Bauer and Paul [7] . However, for tactical plan recognition some adaptations have to be made. Here the plan recognizer will not only have to generate plan hypotheses, but also assignment hypotheses, which correspond to formal links of objects to observations. A choice for an assignment is essentially a decision concerning the question which observations originate from the same objects. For observations with stochastic variables the probability of an assignment hypothesis is calculated, rather than the probability of the plan hypotheses. For this, Reid’s multiple hypothesis tracking formula can be adapted to calculate the assignment hypothesis probability.