Some far-field radar imaging techniques (like spotlight-mode synthetic aperture radar and holography) are shown to have a tomographic formulation. For short radar-target distance (much shorter than the distance required in the far-field condition) the far-field computer aided tomography algorithm (CAT) may blur the reconstructed image. After studying first the near- field theory of some microwave imaging techniques, this paper shows that a unified near-field microwave tomography algorithm is possible. The received signal in SAR, which is a convolution between the spherical projection of the target and the emitted signal, can be interpreted as a Fourier transform of the spherical projection of the image as in near-field holography and tomography. Reconstruction of an SAR image may then be accomplished using a back projection algorithm. This study is an interchange of ideas between these different disciplines and it is a step towards a unified near-field algorithm. An optimal method can be used for combining the four polarimetric images (hv basis) to reduce speckle compared with the span image and single channel image.
Safwan El Assad,
"Analog reconstruction methods for radar imaging", Proc. SPIE 1748, Radar Polarimetry, (12 February 1993); doi: 10.1117/12.140634; https://doi.org/10.1117/12.140634