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.