Coherence Analysis of the Micro-Doppler Spectrum in Radar Echoes from Low-Altitude Rotor Targets

The small-amplitude motion of the target or its components, excluding the primary body translation, is referred to as micro-motion. This phenomenon reflects the subtle physical characteristics of the target, and its characterization holds significant importance in the feature extraction and the parameter estimation. This paper focuses on the micro-Doppler spectral characterization of a typical rotor target and systematically analyzes the radar echo modulation effects of the rotor. This is achieved by establishing a refined mathematical model of helicopter rotor echoes and deriving the frequency domain expression for the echoes produced by the rotating components. Through this derivation, a theoretical relationship between the target parameters and the micro-Doppler amplitude-phase characteristics is presented. The influence of the target parameters on the micro-Doppler characteristics is also analyzed, and the coherence of the micro-Doppler spectral lines of the target rotor echo is demonstrated. Finally, the validity of the theoretical analysis is confirmed through simulation experiments. This theory offers a novel approach for energy accumulation and feature extraction based on the coherence of the micro-Doppler spectrum.