1. An apparatus for use in estimating an actual magnitude of a physical parameter, where at least three redundant signals are input to the apparatus, each input signal is indicative of a sensed magnitude of the parameter, the apparatus comprising: a differencer providing in response to the at least three redundant input signals at least three difference signals indicative of the relative differences between the sensed magnitudes; an evaluator for computing in response to said at least three difference signals, a plurality of evaluation signals; and a synthesizer for generating in response to said evaluation signals, an estimate signal indicative of an estimate of the actual magnitude of the physical parameter, wherein said estimate signal has a magnitude indicative of a summation of the product of the magnitude indicated by each redundant input signal and an associated proportionality factor, wherein each proportionality factor has a value equal to one of a plurality of possible values for that proportionality factor, the value of each proportionality factor depending upon said magnitudes of said difference signals.