What is the micro-Doppler effect and how can a radar use it to classify different types of targets?

The micro-Doppler effect in radar refers to the small Doppler frequency modulations created by the mechanical vibrations, rotations, or articulations of a target's components, superimposed on the target's bulk Doppler shift from its overall translational motion. The micro-Doppler signature is unique to the type of target and its motion dynamics, enabling target classification. Examples: a walking person generates micro-Doppler from the swinging arms and legs; the arm swing creates symmetric Doppler modulation at approximately ±1-3 m/s relative to the body velocity; the leg motion creates a distinctive periodic pattern. A helicopter's rotating blades create a strong periodic micro-Doppler signature; the blade tip velocity (200-300 m/s) creates Doppler shifts far exceeding the helicopter's translational Doppler; the blade rate and number of blades are identifiable. A vehicle with rotating wheels and engine vibrations produces micro-Doppler corresponding to the wheel rotation frequency and engine RPM. A drone's propellers create a high-frequency micro-Doppler (blade tip speeds of 30-100 m/s at rotation rates of 3000-10,000 RPM). The micro-Doppler is extracted by: time-frequency analysis of the radar return (Short-Time Fourier Transform, Wigner-Ville distribution, or wavelet transform). The resulting time-frequency spectrogram shows the micro-Doppler modulation pattern, which is used for: target classification (distinguishing people from vehicles, helicopters from fixed-wing aircraft, drones from birds), activity recognition (walking vs. running, crawling, carrying objects), and intent analysis (detecting concealed weapons from gait anomalies).