Abstract: An airborne passive SAR radar imaging algorithm is proposed,which makes use of the regular circular motion by an airplane to form a synthetic aperture in the case of one external illuminator available.Because the collected data are non-uniform and sparse in the Fourier space,the target scattering function can be reconstructed from the echo data through coordinate conversion,interpolation and two-dimensional Fourier transform.Theory deduction and computer simulation confirm the effectiveness of the proposed algorithm.