  The classical Doppler shift originates from the movement of a target's center
of mass, but it does not hold information about the internal dynamics of the
scattering object. In contrast, micro-Doppler signatures contain data about the
micro-motions that arise from internal degrees of freedom within the target
(such as rotation and vibration), which can be remotely detected by careful
analysis of the scattered field. Here we investigate, both theoretically and
experimentally, how coupling between a pair of closely situated targets affects
the resulting micro-Doppler signatures. The presented model considers a pair of
near-field coupled resonators with dynamically reconfigurable scattering
properties. Voltage controlled varactor diodes enable modulating the scattering
cross-section of each target independently, mimicking rotational degrees of
freedom. As a result, coupled micro-Doppler combs are observed, containing new
frequency components that arise from the near field coupling, making it
possible to extract information about the internal geometry of the system from
far-field measurements. From a practical point of view, micro-Doppler
spectroscopy allows remote classification of distant objects, while deep
understanding of the coupling effects on such signatures in the low frequency
regime can provide valuable insight for radar and sonar systems, as well as
optical and stellar radio-interferometry, among many others.
