Modal parameter estimates of a structure, vibrating under actual loadings and operational conditions, have addressed the interest of researchers during the last years. For all the developed approaches, structures need not be excited by expensive devices, because the modal signature of the structure can be achieved from the knowledge of the system time responses only, derived from the natural excitation due to the ambient loads. Thus natural frequencies, damping ratios, and mode shapes can be estimated using techniques both in the frequency and in the time domain only by the measured responses of the dynamic system. In this paper, a methodology to estimate the modal model of a dynamic system is proposed. The biased frequency response functions could be identified from the measured time responses by a developed technique based on the properties of the Hilbert transformation applied to the auto power spectral densities of the output signals. Then, trough the identification of the state space matrix, the modal parameters could be derived. The accuracy of the proposed approach has been investigated through several experimental investigations carried out on different aerospace structures. © (2006) by the Katholieke Universiteit Leuven Department of Mechanical Engineering All rights reserved.