Time-frequency analysis of vibrating targets in airborne SAR systems

Vibrating targets may cause azimuth smearing in airborne synthetic aperture radar (SAR) images. Any motion of a reflector during the coherent integration time of a SAR image causes a phase modulation in the corresponding reflector phase history. For a sinusoidal motion, the phase modulation causes sidebands, often indistinguishable from stationary reflectors. The modulation is described in terms of a time-dependent Doppler frequency. Time-frequency methods are useful tools for such cases. Among the many possible time-frequency methods the authors chose the adaptive optimal kernel (AOK) method for the purposes of the present paper. With the AOK method they analysed an oscillating reflector within an airborne SAR image. The oscillation was clear and the oscillation parameters were estimated from the time-frequency distribution. The parameters agreed well with ground truth. In particular, the oscillation frequency agreed well, while the amplitude was less accurate, but still reasonable.