Abstract: This paper discusses a multiple-input modal identification method in the frequency domain. The frequencyresponse function (FRF) is expressed as a rational fraction formulation with matrix coefficients. The Forsytihe orthogonal polynomial is exploited to reduce the ill-conditioning of the equations and to decouple the system matrix.Taking use of the eigenvalue decomposition and singular value decomposition, the poles and residues of the systemare solved. An aircraft model called GARTEUR, which is widely used in European aerospace community, is takento test this algorithm. It was proved that this method is good for heavily coupled modes even polluted by high noiselevels (up to 30 %) . Good modal identification results are obtained, which can be further improved while higher order of the denominator polynomials is used.

Key words: modal identification, multiple input multiple output, orthogonal polynomials, frequency response function, rational fraction