1. A method of generating a broadband transfer function of complex characteristics of a linear time-invariant system within an apparatus, comprising: using a user input/output interface unit to acquire measured data characterising properties of the system; using a processing unit having a central processing unit and a memory to define a set of poles in the complex plane to characterise the system; using the processing unit to iteratively perform the following: define a set of orthonormal rational basis functions incorporating the defined poles, use the orthonormal rational basis functions to estimate transfer function coefficients, and derive revised values for the complex poles, until a desired level of accuracy of the transfer function coefficients is attained; using the revised complex poles to determine parameters of the broadband transfer function with the processing unit; and using the broadband transfer function to identify a mapping between inputs and outputs of the time invariant system.