1. Method for estimating the equivalent radar cross section on the basis of a first near-field diffraction model of said object and a second near-field diffraction model of said object, the first and second models being able to be respectively represented by a first matrix (A b ) and a second matrix (A′ b ) of complex diffracted wave and incident wave amplitude ratios, for a plurality (4N 2 ) of pairs of respective directions and polarisations of the incident wave and observation of the diffracted wave, in which: a modal decomposition of said first and second matrices is carried out and the most significant modal vectors (V, V′) of the first and second matrices thus decomposed are selected ( 130 ) in order to obtain a first base and a second base of modal vectors; a correspondence is established ( 140 ) between modal vectors of the first base and modal vectors of the second base; an operator (F) for passing from the second modal vectors (λ′ s ) associated with the modal vectors of the second base to the first modal values (λ s ) associated with the modal vectors corresponding to the first base is determined ( 150 ); a plurality (N μ ) of measurements of complex diffracted wave and incident wave amplitude ratios in near field are made ( 160 ) for a plurality of pairs of directions and at least one pair of respective polarisations of the incident wave and observation of the diffracted wave; the vector (a′ μ ), formed by the ratios thus measured, is projected ( 180 ) onto said modal vectors of the second base, in order to obtain second modal components (λ′ μ ) on this base; first modal components (λ μ ) on the first modal base are obtained ( 190 ) by applying said operator to the second modal components; from the first modal components and the modal vectors of the first base a reconstructed vector (ã) is generated ( 193 ), the components of which represent the complex incident wave and diffracted wave amplitude ratios for said plurality of pairs of directions and polarisations; the equivalent radar cross section is determined ( 195 ) from at least one component of the reconstructed vector.