The System Visualization Tool (SVT) was developed to provide systems engineers with a means of graphically representing networks. The SVT generates diagrams that illustrate the structure and state of user-defined networks. The program provides systems engineers with a powerful tool that simplifies requirements analysis, testing, and maintenance of complex software-controlled systems. The tool employs visual models that support the analysis of chronological requirements sequences, simulation data, and related software functions. It currently employs these models to support analysis of the Space Shuttle's OMS and RCS propellant distribution systems; however, the tool can be applied to other pneumatic, hydraulic, and propellant distribution networks.

The SVT is used to define and view arbitrary configurations of major system hardware components such as propellant tanks, valves, propellant lines, and engines. In addition, it graphically displays the status of each component. One of the major advantages of SVT is that it utilizes visual cues to represent the configuration of each component within a network. In its application to the Space Shuttle program, the SVT visually represents the state of a system by using a color scheme to indicate the presence or absence of fuel and oxidizer throughout each leg of a network. Systems engineers are able to distinguish the state of the network models by interpreting the color of the network's components.

• Red is used to present information about fuel.
• Green is used to present information about oxidizer.
• Blue is used when information about both fuel and oxidizer is to be displayed.
• A ------ dashed line anywhere within the network indicates that the presence of at least one propellant is unknown.

A unique feature of the SVT is its ability to control and monitor userdefined valves by associating switches and status indicators (known as Measurement/Stimulus Identifiers (MSIDs) or discretes) to them. Systems engineers may assign up to twelve discretes to control or monitor each valve. In addition, the rules of the network are determined by logical combinations of up to four discrete inputs for each of the valves defined in the network.