1. An apparatus for monitoring the operating condition of a system, comprising: sensor means for acquiring a time-varying signal characterizing operation of the system; means for decomposing said time-varying signal into a plurality of components; a memory for storing a plurality of reference snapshots of component values for known operating conditions; and processor means responsive to the decomposing means, disposed to generate estimates of the components using a similarity operation on the component values from the decomposing means with reference to the component values in each reference snapshot in the memory, according to: y -> expected = D · ( D T ⊗ D ) - 1 · ( D T ⊗ y -> input ) where y(input) is a vector of said plurality of components, y(expected) is a vector of said estimates, D is a matrix of said reference snapshots, and ® is said similarity operation, and wherein said processor means is further disposed to generate residual values by differencing the component values and the estimates thereof, for determination of deviating operating conditions of the system.