1. An apparatus for detecting an operational state transition in a system instrumented with sensors, comprising: a model for generating estimates of sensor values in response to receiving actual sensor values characterizing the operation of the system; a global similarity engine for generating a global similarity score for a comparison of a set of estimated sensor values from said model and a related set of actual sensor values from said system; and a state transition detector disposed to receive global similarity scores from said global similarity engine and indicate an operational state change when a global similarity score falls outside of a selected range.