Space structures dynamic identification through output only approach

A suitable approach capable to identify the dynamic model of the structure, in terms of the modal model, during the on-orbit-life time of the spacecraft is proposed. This approach is intended to help engineers to validate the analytical dynamic model of a space structure with the aid of the actual dynamic parameters directly estimated during the flight, and to monitor the behavior of the modal parameters during the whole space mission for a possible identification of structural damages and for the definition of a suitable control law when space manoeuvres are required. In the developed methodology, the modal parameters - natural frequencies, damping ratios, and mode shapes - are estimated by the analysis of the response of the structure only as a result of its "natural" loading environment. These modal parameters could be estimated assuming that the output response is due to a white noise excitation in the frequency range of interest. The Frequency Domain Decomposition, FDD, is the core of the proposed approach. Starting from the calculation of the Power Spectral Density, PSD, matrix of the output responses, natural frequencies and the corresponding mode shapes could be estimated by taking advantages of the Singular Value Decomposition, SVD, of the PSD matrix. Furthermore, the damping ratios are also available from the free decay of the system achieved by inverse Fourier transforming the PSD matrix across the previous identified natural frequencies. In this paper, a methodology aimed to identify dynamic parameters from Output-Only data is presented and applied to different spacecraft structures. The time responses of the considered structures during their normal orbit operations are first evaluated, eventually they are used for a subsequent output-only modal parameter identification. Moreover, the capability to identify structural changes by the measurement of the natural frequency shifts, is also investigated.