APPLICATIONS OF TIME-FREQUENCY PROCESSING TO RADAR IMAGING

Due to the time-varying behavior of the Doppler frequency of radar ret urns, and due to the multiple backscattering behavior of radar targets , the resolution of radar images can be significantly degraded and tho se images may be blurred. Conventional radar processors use the Fourie r transform to retrieve Doppler information. To use the Fourier transf orm properly, some restrictions must be applied: the scatterers must r emain in their range cells and their Doppler frequency shifts should b e stationary during the entire imaging time. However, due to a target' s complex motion, the Doppler frequency shifts will be time-varying. T herefore, the Doppler spectrum obtained from the Fourier transform wil l be smeared, and, the radar image will be blurred. However, the restr ictions of the Fourier transform can be lifted if the Doppler informat ion is retrieved with a time-frequency transform that does not require a stationary Doppler spectrum. The image blurring problem caused by t ime-varying Doppler frequency shifts can be solved without resorting t o sophisticated motion-compensation techniques. By replacing the conve ntional Fourier transform with a time-frequency transform, a 2-D range -Doppler Fourier frame becomes a 3-D time-range-Doppler cube. By sampl ing in time, a time sequence of 2-D range-Doppler images can be viewed . Individual, time-sampled images from the cube provide superior image resolution. When targets contain cavities or duct-type structures, th ese structures' scattering mechanisms appear in radar images as blurre d ''clouds'' extending in range. It is beneficial to incorporate the t ime-frequency transform into range profiles of the radar image. By so doing ''clouds'' can be removed and structure resonance frequencies id entified. (C) 1997 Society of Photo-Optical Instrumentation Engineers.