1. Method for estimating the equivalent radar cross section of an object on the basis of a first far-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, said method comprising: carrying out a modal decomposition of said first and second matrices and selecting the most significant modal vectors (V, V′) of the first and second matrices thus decomposed in order to obtain a first base and a second base of modal vectors; establishing a correspondence between modal vectors of the first base and modal vectors of the second base; determining an operator (F) for passing from second modal values (λ′ s ) associated with the modal vectors of the second base to first modal values (λ s ) associated with the modal vectors corresponding to the first base; making a plurality (N μ ) of measurements of complex diffracted wave and incident wave amplitude ratios in near field 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; projecting the vector (a′ μ ), formed by the ratios thus measured, onto said modal vectors of the second base, in order to obtain second modal components (λ′ μ ) on this base; obtaining first modal components (λ μ ) on the first base by applying said operator to the second modal components; generating from the first modal components and the modal vectors of the first base a reconstructed vector (ã), the components of which represent the complex incident wave and diffracted wave amplitude ratios for said plurality of pairs of directions and polarisations; determining the equivalent radar cross section from at least one component of the reconstructed vector.