Techniques for Microwave Imaging.

This dissertation presents a number of techniques for obtaining high-resolution microwave images. The development of these methods is directed to imaging with radars using wavelengths of 5 to 50 millimeters however, the results apply to any form of wave propagation involving reflections of coherent signals. A system for obtaining two-dimensional images is described which combines pulse compression and synthetic aperture processing of signals reflected from rotating objects to achieve resolution in range and cross range. Both the pulse compression and synthetic aperture processing are accomplished by discrete Fourier transforms DFTs implemented by fast Fourier transform FFT algorithms. Examples of images are presented which demonstrate resolution of three wavelengths using a signal bandwidth of approximately 1.5 GHz and an object rotation of 10 degrees. The principal limitation of this technique is the image degradation which results from focal aberrations in the unfocused synthesized aperture. These limit the achievable resolution and the maximum size of objects that can be imaged and produce space-variant, point-spread functions which are diffraction-limited only at the center of the object space. Focusing the synthetic aperture leads to a point-spread function which is diffraction-limited and space-invariant. The required focusing operation is implemented by mapping the recorded signals through a polar transformation and subsequently processing the data with a two-dimensional Fourier transform. By this method, the focused synthetic aperture can be extended to a full 360 degrees and provides resolution of less than one-quarter wavelength.