I. Introduction
In the analysis of radar signatures from complex targets, joint time-frequency distributions can provide significant insights into the physics present in the signal [1]. Joint time-frequency distributions have been used to analyze the backscattering phenomenology from frequency dispersive targets such as pipes and cavities [2]–[4], as well as time-varying structures including wind turbines [5]–[7] and moving humans [8]–[10]. For example, the modal dispersion present in pipes and cavities is clearly seen in a time-frequency distribution. In the study of wind turbine scattering, the positive and negative Doppler features from turbine blade flashes can be readily observed in the joint (dwell time)-(Doppler frequency) plane. Similarly, the time-varying Doppler features from the limbs of a moving human are manifested as “microDoppler” trajectories. Significant effort has been devoted to identifying human activities based on these unique features [11]–[13].