A fast method for circular SAR three-dimensional imaging system by near-field elevation scanning is proposed in this paper. It is based on cylindrical spectrum theory which exploits the Fourier decomposition of the targets distribution instead of point by point imaging in earlier works. The proposed method sets up the relationship between the target image and the scattering field in spatial frequency domain. This leads to overcome the problem of computational inefficiency which was observed previously in projection-slice theorem. The near-field scattering is firstly analyzed by relating the return signal to the near-field focus function. The near-field focus function is then transformed to spatial frequency domain and evaluated by the method of stationary phase. Finally, the imaging result is given by three-dimensional inverse Fourier transformation from spatial frequency domain of targets. The proposed method is validated by the simulation results of distributed targets. In addition, experimental validation was also achieved in microwave chamber at X-band with targets placed on the turntable.
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