1. A method of generating a broadband transfer function of complex characteristics of a linear time-invariant (LTI) system, comprising: acquiring data characterising properties of the system; defining a set of poles in the complex plane to characterise the system; iteratively defining a set of orthonormal rational basis functions incorporating the defined poles, using the orthonormal rational basis functions to estimate transfer function coefficients, and deriving revised values for the complex poles, until a desired level of accuracy of the transfer function coefficients is attained; and using the revised complex poles to determine parameters of the broadband transfer function.