A method for detecting perturbation of a physical system from a reference state associated with a reference parameter (.omega..sub.0) to a perturbed state associated with a perturbed parameter (.omega.) includes firstly deriving the reference parameter (.omega..sub.0). A reference vector (F) is then derived which describes the system's state at the reference parameter (.omega..sub.0). A measurement-related vector (Z) associated with a perturbed state of the system is then subtracted from the reference vector (F) to provide an error vector (E). The error vector members are summed and normalised by division of a summation of elements of a vector (F') representing a derivative (f(.omega..sub.0.delta..omega.).sub.e) of a reference itself represented by the reference vector (F), the derivative (f(.omega..sub.0.delta..omega.).sub.e) being evaluated at the reference parameter (.omega..sub.0). This provides a result equal to the difference (.omega.-(.omega..sub.0) between the perturbed parameter (.omega.) and the reference parameter (.omega..sub.0).