Simulation Categories

[NOTE: Freewriting first draft for the "Military Modeling" chapter of the upcoming book, Dynamic Modeling, edited by Paul Fishwick, to be published by CRC Press in 2006.]

Over the past several decades, a number of different types of models have been developed for representing a military system or mission. These have gradually converged into commonly recognized categories of representation. These categories have significantly improved the ability of military modelers to communicate with each other and to exchange models without misunderstanding the differences between the products being created.

Engineering

Engineering models focus on the details of what a system does. These capture the physical properties of materials, liquids, aerodynamics, servomechanisms, and computer control of specific systems. They also include interactions between two physical objects or between an object and its environment. An engineering model attempts to understand the physical capabilities of the system at a level that is accurate enough to be used to design the system. Historically, physical prototypes were used to conduct these experiments. However, advanced computer technologies and modeling techniques have allowed us to create digital models of systems that are nearly as predictive as are live physical tests. These models offer many advantages over their physical counterparts. They are almost infinitely malleable so that experiments can be conducted on many thousands of variations rather than just a few physical prototypes. They are nearly infinitely instrumentable. It is possible to collect data from all points in space and time around the event of interest. When using physical prototypes we are often limited by our ability to place sensor, communication, and recording equipment at the precise place and time of interest.

Virtual

A “virtual model” often refers to a three-dimensional representation of a system that is operating in a digital three-dimensional environment. The focus is usually on the visual appearance of the object and the environment, more than on the properties of physics that were the focus of engineering models. Because of its visual focus, the objects most often represented are military vehicles and humans that would appear on a battlefield. This category is closely aligned with the more popularly recognized term, “virtual reality”.

A virtual model and environment are usually constructed to simulate individual soldiers who are immersed in a system that generates visual, aural, and tactile stimuli. The goal is usually to train, test, or measure the ability of the human to respond in a desirable manner to the stimuli. Flight simulators are the most popularly recognized form of these models and systems.

Constructive

A “constructive model” represents an accumulation or aggregation of a number of objects, behaviors, and properties. In order to deal with the incredibly large and complex missions of the military, a very structured organizational hierarchy has evolved. To represent the information that is available at the different levels in this hierarchy and to represent the functions of the hierarchy itself, constructive models have been created.

A constructive model may represent a flight of four aircraft as a single item in the simulation. It may also group several hundred vehicles, humans, and equipment into a single object model. This model must then represent the aggregated behaviors of its many different constituent parts. There are a number of motivations for this type of modeling. First, it allows the simulation system developers to capture the operations of a much broader battlefield in a form that can be run on a reasonable computer suite. Second, in many cases the behavior of groups of objects are not understood at the engineering or virtual level, but can be represented as a higher-level aggregate. Third, this type of model mimics the organization, representation, and information that are used in the real military organizational hierarchy.

Very basic constructive models of military operations can be seen in many board and computer games, such as Chess, Stratego, and Risk. Constructive simulation systems differ from virtual systems in that the human operator or player is often positioned outside of the battle. Engagements are not usually targeted at the human player, so they are in a position to think more strategically about the situation and are not required to react to individual events that appear to threaten them personally, as would occur in a virtual system.

Live

Though a “live model” appears to be an inappropriate description, the term has been adopted to refer to activities in which live humans, vehicles, and equipment engage in mock combat. The combat events do not involve real munitions and attempt to avoid situations that could have lethal outcomes. Using computer, communication, navigation, and laser technologies, training areas have been constructed in which combatants can use their real weapons in a form that is as physically realistic as possible. Laser beams often replace bullets and computer messages indicate where bombs are dropped.

Live modeling allows humans to train in the real environment, to experience the physical hardships of traversing rough terrain, operating in the desert sun, and experiencing the effects of dirt and water on the equipment. The humans and vehicles become living models in a living simulation. In many cases, these live participants are also supplemented with virtual and constructive models to enrich the entire training experience.

Environment

The model of the environment has historically been a static representation of terrain, vegetation, roads, rivers, wind, clouds, rain, ocean waves, salinity, ocean bottom, and any number of other features. This environment has provided a medium within which the above models operated. The environment impeded the movement of objects, obstructed sensor visibility, and changed the outcomes all types of operations. However, in the midst of all of this activity, the environment itself remained static and unchanged. A bomb dropped on a truck may destroy the truck, but make no change to the underlying terrain or the surrounding vegetation.

Recently, this has been changing. Military simulation systems have included dynamic models of the interaction between military systems and environmental features. Simulated objects are able to knock down trees, crater roads, dig holes, build barriers, and destroy buildings. To support this, a new form of environmental model has evolved which understands the physical effects of vehicles and weapons on dirt, trees, and masonry block structures. Environmental modeling is no longer limited to static data structures, but includes dynamic models that respond to military operations.