Abstract
High resolution radar image is always demanded. To achieve high resolution, wideband signal and longer imaging time are required. However, due to time-varying behavior of returned radar signals and due to multiple backscattering behavior of targets, radar image resolution can be significantly degraded and images become blurred. The conventional radar processor uses the Fourier transform to retrieve Doppler information. In order to use the Fourier transform adequately, some restrictions must be applied: the scatterers must remain in their range cells and their Doppler frequency contents should be stationary during the entire imaging time duration. However, due to the target's complex motion, the Doppler frequency contents are actually time-varying. Therefore, the Doppler spectrum obtained from the Fourier transform becomes smeared, and, thus, the resolution of the radar image is degraded. However, the restrictions of the Fourier processing can be lifted if the Doppler information can be retrieved with a method which does not require stationary Doppler spectrum. Therefore, the image blurring caused by the time-varying Doppler spectrum can be resolved without applying sophisticated motion compensation. By replacing the conventional Fourier transform with a time-frequency transform, a 2-D range-Doppler Fourier frame becomes a 3- D time-range-Doppler cube. By sampling in time, a time sequence of 2-D range-Doppler images can be viewed. Each individual time-sampled image from the cube provides superior image resolution and also enhanced signal-to-noise ratio. When the target contains cavities or duct-type structures, these scattering mechanisms appear in radar images as blurred clouds extended in range dimension. It is very useful to combine adaptive time-frequency wavelet transform with the radar imaging technique so that the 'clouds' can be removed and their resonance frequencies can be identified. By applying time-frequency processing for each cross-range lines of radar image, a 3-D range-Doppler-frequency cube is generated. The frequency slices of the cube provide information for identifying scattering centers as well as resonance frequencies.
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