I. Introduction
Computer generated forces (CGFs) are autonomous or semiautonomous entities that represent military units, such as tanks, soldiers and combat aircrafts, in simulation software for military operations. CGFs are similar to non-player characters in computer games and are used in military simulationbased training and decision support applications. CGFs enable simulating large military operations as one operator is able to control several military units. The behaviour of the CGFs, E.g. how they move, where they look, when they shoot etc., should represent the behaviour of corresponding human soldiers or manned systems as accurately as possible. Ideally, a soldier training with a virtual simulator should not notice whether his teammates or opponents are human controlled entities or CGFs. Realistic CGF behaviour also makes it possible to simulate various plans or courses of actions to improve the situation awareness and get a good understanding of how a situation could play out [1] . Simulations can help build and train the mental model of the trainee by practising situation comprehension and projection, situation awareness level two and three in Endsley's model of situation awareness [2] . In aviation, around 20% of the errors are related to problems with the mental model according to Endsley and Garland [3] . Given that errors made by soldiers in a stress situation are similar to those made in aviation, improving their mental model is of high importance. This requires the CGFs to behave in a natural way, as their behaviour affects the situation comprehension and projection of the trainee.