phenomena that cannot be studied in any other way (e.g., a range of possible battles and wars), (4) to use high-resolution information, which is sometimes quite tangible (e.g., weapon performance), and (5) to help calibrate lower-resolution models.

This need for models at different levels of resolution will not change merely because computers become more capable. Thus, we also need to understand the relationships among phenomena at the different levels, which in practice means understanding how models at those levels should relate to each other.

Reasons for Interest in Connecting Models of Different Resolution

It is also necessary to connect models of different resolution. Connections may be in software, so that one model takes data electronically from another, or “offline ” (by what is humorously known as “sneakerware”), where humans take data from one model and then feed it to another, often massaging it during the transfer.

If the only purposes were analytical, then it might be sufficient and desirable to work with model families—when good ones existed. From time to time, one would cross-calibrate the models to ensure consistency with all known information. Most of the time, however, one would use a specific model tailored to the problem.

With the advent of distributed simulation, however, much is changing. The need now exists to connect a variety of models, often with different resolutions, and to do so at run time. Further, as computing power has increased, some workers have become interested in doing analysis with models that normally operate at one level of resolution, but occasionally call higher-resolution subroutines.

There are many reasons for operating at multiple levels in an advanced distributed simulation (ADS) environment. One objective is to avoid high resolution except when needed with the purposes of (1) conserving network and CPU resources; (2) simplifying and accelerating scenario setup; (3) reducing the number of simulation operators; (4) speeding simulation execution; and (5) simplifying setup and execution of low-priority “context” segments of a simulation while allowing detailed and authoritative representation of high-priority segments. Another purpose is connecting legacy simulations written at different levels of resolution.

Scope of the Challenge

Table E.1 reminds us of the basic levels at which military issues must be studied. 2 Work at these levels requires different models, but a planner at one level (e.g., a joint task force (JTF) commander) cares whether his planning frame-


Adapted substantially from a briefing by Robert Lutz of Johns Hopkins University's Applied Physics Laboratory, a briefing given at the MORS minisymposium referred to in footnote 1.

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