Bridging the Modelling Gap: Examining the Expressiveness of Planning Domain Description Languages

Research in planning has advanced rapidly over recent years, spurred partly by a series of international planning competitions and the convergence on empirical evaluation that this has encouraged. One of the most important contributions made by the competition sequence is the development of a community standard modelling language, PDDL. This language has been extended from a relatively simple action description language to include constructs to express the use of resources and other metrically measured quantities and time. The traditional boundaries between planning and scheduling have become increasingly blurred, so that planning research is now regularly considered in application to problems that include management of resources, time, and the combined planning and scheduling of complex concurrent activities. In this paper we examine the extent to which PDDL2.1, the most recent development of the community standard modelling language, supports expression of realistic features of complex application domains. This examination is carried out with particular reference to two domains inspired by space applications: satellite observation planning and planetary rover exploration planning. Versions of these domains were used in the 3rd International Planning Competition (2002). The limitations of these versions with respect to the application problems and the gap between the modelling language and the demands of these applications is explored.