  Concealing objects from interrogation has been a primary objective since the
integration of radars into surveillance systems. Metamaterial-based
invisibility cloaking, which was considered a promising solution, did not yet
succeed in delivering reliable performance against real radar systems, mainly
due to its narrow operational bandwidth. Here we propose an approach, which
addresses the issue from a signal-processing standpoint and, as a result, is
capable of coping with the vast majority of unclassified radar systems by
exploiting vulnerabilities in their design. In particular, we demonstrate
complete concealment of a 0.25 square meter moving metal plate from an
investigating radar system, operating in a broad frequency range approaching
20% bandwidth around the carrier of 1.5GHz. The key element of the radar
countermeasure is a temporally modulated coating. This auxiliary structure is
designed to dynamically and controllably adjust the reflected phase of the
impinging radar signal, which acquires a user-defined Doppler shift. A special
case of interest is imposing a frequency shift that compensates for the real
Doppler signatures originating from the motion of the target. In this case the
radar will consider the target static, even though it is moving. As a result,
the reflected echo will be discarded by the clutter removal filter, which is an
inherent part of any modern radar system that is designed to operate in real
conditions. This signal-processing loophole allows rendering the target
invisible to the radar even though it scatters electromagnetic radiation. This
application claims the benefit of patent number 273995, filed on April 16 2020.
