Imaging of micromotion targets with unknown number of rotating parts based on time-frequency analysis

In inverse synthetic-aperture-radar (ISAR) imaging, the target can be viewed as a turntable model after the translational motion compensation, which includes range alignment and auto-focusing. The rigid body that rotates a small angle uniformly during the imaging time has a constant Doppler frequency. Therefore, the well-focused image can be obtained by the range-Doppler (RD) algorithm. For a micromotion target, however, its micro-Doppler (m-D) induced by the vibrating or rotating part is a time-varying function. In such situation, the ISAR image of the main body may be shadowed, particularly when the m-D interference is emphatic. This would bring great difficulties to the image interpretation and feature extraction for both the main body and the rotating parts. The problem is more accentuated when there are multiple vibrating or rotating parts, and the number of these latter is unknown. This article presents a solution to the problem of estimating the number of signal component based on the short-term Rényi entropy.