Large and expensive systems are often best built by creating a high fidelity look and feel simulator.  Military systems are often prototyped this way and actually gamed to see if the mission profile will really work.  The FullFunctionalSimulator is part of a HierarchyOfPrototypes starting with PaperPrototype and working up the fidelity chain.

Above FullFunctionalSimulator is still another level which is FullyStimulatedEndToEndPrototype -- we did at least an approximation to that on the LAMPS MkIII Lifecycle Support Laboratory at the Naval Air Development Center.  In a fully stimulated system all the environmental interfaces are hit with signals they think are real so that you stimulate actual hardware systems, sensor and the like with real signals.  It is a cool concept, but it is expensive.  When your life depends on getting it right though, you tend to want to go the distance.  --RaySchneider

P.S. I was the Lifecycle Software Support Engineer during the Concept Development Phase.  It was my last job before I decided to try my hand in industry instead of government.
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