Digital image reconstruction of microwave holograms

A system capable of recording the Fourier hologram of a reflecting target at various microwave frequencies is described. The assembly consists of a swept frequency X -band interferometer circuit that illuminates a given target and a detector that scans a large square receiving area. The system is supervised by a microprocessor which controls the scanning operation, data acquisition, and storage of the resulting holograms. Image reconstruction is accomplished by discrete Fourier inversion of the data, and the output format uses computer graphics. The system's performance is evaluated by the analysis of various metallic objects. The lateral resolution is determined by the operating wavelength, linear dimensions of the recording area, and the range of the target, and for the conditions of the experiment is approximately one wavelength. Adequate depth resolution is obtained by combining multifrequency holograms measured in a range from 8 to 12 GHz, thereby achieving a depth resolution of 4 cm. Some potential applications are demonstrated. These include the measurement of target shape and displacement using convolution and subtraction techniques.