CHOOSING A COORDINATE FRAMEWORK FOR SIMULATIONS Ralph M. Toms Paul A. Birkel SRI International The MITRE Corporation 333 Ravenswood Ave. 1820 Dolly Madison Blvd., M/S W649 Menlo Park, CA 94025 McLean, VA 22102 650-859-8592 703-883-6399 ralph_toms@sri.com pbirkel@mitre.org Keywords: Spatial Reference Model, Coordinate Systems, Dynamics Models, Vehicle Models, Environmental Data ABSTRACT: For good reasons many different coordinate systems and associated earth reference models are in use in C 4 ISR systems. The same is true for M&S applications in the DoD community. However, it is not clear that the choice of a coordinate system for near-Earth simulation applications has always been either well informed, or especially rational. In this paper, we review the generic spectrum of available coordinate systems and earth- reference models from the perspective of the simulation model developer, whose intuitive knowledge of the world sometimes assumes a "flat Earth" with the gravity vector pointed downwards. Given that perspective (and its interesting range of appropriateness), for many simulation applications (and models) a rigorous geodetic coordinate space—with its accompanying requirement for relatively expensive coordinate-space calculations on the surface—looks unnecessarily complex and computationally expensive. We examine the primary assumptions that environment and military modelers make about their coordinate-space, under what conditions those assumptions lead to errors, and how those errors can best be ameliorated. We highlight the impact of coordinate system selection on both kinetic and kinematic dynamics formulations. applications involving armor-anti-armor 1. I ntroduction configurations must now contend with an extended Legacy models used for simulating combat battle space in which organic anti-armor weapons operations are often simplified in their will operate at ever increasing ranges. As a result, representation of the battlefield environment (i.e., many of the assumptions made to restrict the the natural geophysical factors that influence application domains in legacy models are no longer battlefield operations). These encompass the shape appropriate. In addition, some of the simplifications of the earth, the earth density distribution, the oceans make it virtually impossible to federate across and other bodies of water, the terrain, the dissimilar domains and still maintain a “level atmosphere, the radiation environment caused by the playing field.” In the context of environmental sun, and a host of other physical phenomena. In the interactions, this means that one simulation element simulation domain, this collection of geophysical does not gain an unrealistic advantage over an data is often called the environmental database. In opposing simulation element due to having a legacy simulations, numerous simplifications have different perception of the environment. been made in depicting the battlefield environment to reduce both military and environmental model Meaningful interoperability of combat-related complexity, particularly computational complexity. simulations over a joint confederation is a Conse-quently, these simulations only apply to very challenging problem with a number of different limited domains and do not include the majority of aspects to it. Hierarchies of models have been environmental elements. proposed and employed for both analysis and training support applications for a number of years, New DoD Modeling and Simulation (M&S) sometimes with limited success. These usually requirements have made such simplifications less involve what is popularly called “swivel chair acceptable, if not untenable. Now and in the future, interfaces.” That is, during the execution of the we must be able to simulate joint combat operations simulation, humans interpret the output of one class that involve increasingly precise weapon delivery of simulations to develop inputs to another level over extended ranges. Even land combat (higher or lower class of simulations) in the
hierarchy. Such configurations tend to be very Standards Organization (SISO) Simulation expensive in terms of personnel requirements and Interoperability Workshops (SIWs) and at the risky in terms of consistent interpretations. Recent predecessor DIS workshops. Even as early as the developments in automation have addressed some second DIS workshop, these papers were the subject aspects of these problems by use of either the of much discussion. Some of the seminal Aggregate Layer Simulation Protocol (ALSP) [1] or observations made at the second DIS workshop are Distributed Interactive Simulation (DIS) Protocol. in references [3,4,5,6,7]. As the field of distributed interactive simulation has matured—and with the Simultaneous networking between simulations at increasing requirement for joint, allied, and coalition differing levels of representation has been applications—a more standardized approach has demonstrated using ALSP and DIS. However, many evolved. open questions remain concerning the validity and efficacy of this approach for general applications. Of particular importance to the portrayal of the Such endeavors have been only partially successful, geophysical environment of the earth is a in part because the legacy simulations involved were standardized Spatial Reference Model (SRM) and not designed to interface with each other, either at associated coordinate reference frameworks [8,9]. the same or different levels of aggregation. Some The SRM includes earth reference models (ERMs) demonstrations have been likened to a person who that represent the geometrical shape of the earth and speaks only English having a telephone conversation the gravitational potential associated with the earth. with a person who speaks only German—they may These ERMs are coupled with appropriate coordinate have demonstrated the ability to network and systems to enable proper determination of the interchange data, but it is questionable whether they location, both statically and temporally, of had meaningful interoperability. environmental elements and battlefield elements. Fortunately the interoperability situation is One of the larger impediments to a common improving. Introduction of the High Level understanding of spatial reference is semantics. The Architecture (HLA) with its concept of a Federation SRM defines standard terms so that there is a Object Model (FOM) forces those who are common understanding of the elements of the SRM. attempting to interoperate to define common Some examples where misunderstandings have interfaces and semantics [2]. The parallel occurred include confusing the earth with a development of the Runtime Infrastructure (RTI) particular ERM. These are clearly not the same. encourages standard use of networking services. If Another example that leads to representational the federation is operating at the same aggregation problems is confusing geodetic height with mean sea level with the same or very similar representations of level height or with orthometric height. These are the real world, then the problem is simplified and all different measures of the vertical. The reasonably consistent interoperablility is achievable. uninformed use of terminology can only be mitigated Consistency in this context does not necessarily through standards and education. imply validity, particularly if the portrayal of critical A common view of location, both static and models like the environment is oversimplified or temporal, is essential for simulating a battlefield. In missing. In essence, the HLA concept addresses stand-alone simulations, the environment must be critical federation issues in a top-down fashion. portrayed in a manner that properly simulates the However, if the component simulations in the interactions of modern weapons and combat systems federation are at differing levels of representational with the environmental elements. A standard accuracy, other critical and very difficult modeling portrayal of the environment is needed to provide considerations are left to the federation developers to valid comparisons, both between different address. In this paper, we address one of those simulations and with expected real-world behavior. issues: the influence of spatial reference models on This conclusion is equally applicable to simulation adequate portrayal of both the geophysical nodes in federations. In addition, in distributed environment and military systems. applications a standardized portrayal of the environment is essential to providing a valid, level playing field. 2. Spatial Reference Several diverse papers on the subject of spatial Initial efforts in networking distributed simulations referencing and environmental modeling issues have have uncovered a number of impediments to been presented at the Simulation Interoperability interoperability that are related solely to spatial
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