## 1 Introduction
### Radar Micro-Doppler as a Potential Solution  
Micro-Doppler radar is a technology which revolutionizes the perception and analysis of motion on a minuscule scale while benefiting from radar's inherent ability to operate continuously in all weather conditions. By leveraging the principles of radar technology and the Doppler effect, this advanced system uniquely detects, isolates, and interprets micro-motions within larger objects. This capability enables an unparalleled level of detail in analyzing complex moving targets. By concentrating on these subtle movements, micro-Doppler radar unlocks vast potential across diverse applications, including military and defense, biomedical research, and autonomous systems. Radar vibrometry is a rapidly growing field, particularly in infrastructure monitoring, automotive systems, and defense applications, as it enables stand-off, non-contact assessment of object vibrations. When an object vibrates back and forth, its velocity follows a sinusoidal pattern relative to the radar's line of sight, reaching zero at the extremes of its trajectory and peaking when moving directly toward or away from the radar. Since an object's linear velocity correlates with its vibration frequency and amplitude, analyzing micro-Doppler signatures reveals critical information about its vibrational characteristics and potential causes. High-frequency radars, which operate at shorter wavelengths, offer enhanced sensitivity due to the inverse relationship between Doppler frequency and wavelength. These frequencies also produce narrower beamwidths, minimizing the need for large antenna apertures and allowing for compact, precise radar systems. This combination of sensitivity and efficiency positions radar vibrometry as a powerful tool for analyzing dynamic systems across a range of applications. Early experiments in remote observation and measurement of mechanical vibrations were conducted using lasers [6; 7]. While laser sensing is highly effective at close range and under ideal conditions, its performance diminishes in long-range applications where barriers or atmospheric turbulence are present. In such scenarios, radar vibrometry offers a compelling alternative, combining advantages such as low cost, eye safety, barrier penetration, and portability, making it a practical solution for diverse and challenging environments. Mechanical vibrations or rotations of targets, or structural components on a target, introduce additional frequency modulations into the reflected radar signal. These modulations create sidebands around the target's primary Doppler frequency, which corresponds to its overall linear motion [8]. This phenomenon, known as the micro-Doppler effect, becomes particularly prominent when the target is stationary, as the absence of a main Doppler frequency leaves only the micro-Doppler modulations for analysis. Radar vibrometry at microwave frequencies has primarily been used to monitor the structural integrity of assets such as bridges and overhead cables, by analyzing their vibration responses to wind and traffic loads [9; 10]. Bridges are subject to continuous dynamic loads from traffic, wind, and thermal fluctuations. Traditional contact-based sensors (e.g., accelerometers and strain gauges) require maintenance and may degrade over time. Micro-Doppler radar offers a remote, non-invasive alternative. The use of synthetic aperture radar to measure vertical vibrations of a suspension bridge has been demonstrated [11]. Wind turbine blades are exposed to complex aerodynamic loads, making their structural integrity crucial. Micro-Doppler radar systems can remotely capture blade vibrations without the need for instrumenting rotating parts and characterize the vibrational modes of wind turbine blades during operation [12]. Preserving historical structures requires non-destructive, reversible techniques. Micro-Doppler radar is ideal for monitoring subtle oscillations in walls, columns, or domes without direct contact.