Coherent Doppler tomography for microwave imaging

A tomographic extension of the type of microwave Doppler imaging typified by synthetic aperture radar has recently been developed and shown experimentally to exhibit a high degree of spatial resolution. When CW irradiation is used, the sidelobes in the pointspread function are inherently high and tend to limit the dynamic range of the reconstructed images. The point-spread function of a system using CW irradiation and an aperture that completely surrounds the object has a central lobe of width of λ/5, but the first sidelobe is only 8 dB below the central peak. The limitation due to the high sidelobes can be partially overcome by using wide-band signals or bistatic diversity. One of the steps in reconstructing a coherent Doppler tomogram is to perform a two-dimensional Fourier transform. The ordinary two-dimensional discrete Fourier transform (DFT) produces points in the transform space on a Cartesian raster. In coherent Doppler tomography (CDT), however, the data are sampled on a polar raster. To diminish the computational burden associated with converting to the Cartesian raster and interpolating, we have developed an alternative algorithm which requires no interpolation and is based on interpreting the two-dimensional Fourier transform as a one-dimensional circular convolution integral. The quality of the images computed in this fashion compares favorably with that for the old method and the computational burden is greatly reduced.