A metasurface-based Direction of Arrival (DoA) estimation method is presented. The method exploits the properties of space-time modulated reflective metasurfaces to estimate in real-time the impinging angle of an illuminating monochromatic plane wave. The approach makes use of the amplitude unbalance of the received fields at broadside at the frequencies of the two first-order harmonics generated by the interaction between the incident plane wave and the modulated metasurface. Here, we first describe analytically how to generate the desired higher-order harmonics in the reflected spectrum and how to realize the breaking of the spatial symmetry of each order harmonic scattering pattern. Then, the one dimensional (1D) omnidirectional incident angle can be analytically computed using +1st and -1st order harmonics. The approach is also extended to 2D DoA estimation by using two orthogonally arranged 1D DoA modulation arrays. The accuracy of 1D DoA estimation is verified through full-wave numerical simulations. Compared to conventional DoA estimation methods, the proposed approach simplifies the computation and hardware complexity, ensuring at the same time estimation accuracy. The proposed method may have potential applications in wireless communications, target recognition, and identification.

This article presents a new design of diverse frequency time modulation for linear frequency-modulated (LFM) radar passive false target spoofing. To realize the passive diverse frequency time modulation, the modulated metasurface is designed accordingly, which is composed by a set of elements, each of which is independently time-modulated to realize specific false targets in the high-resolution range profile (HRRP) of the radar system. Due to the control of the modulation scheme in each element and the combined response of the overall surface, we demonstrate the capability to realize several false target patterns consisting of single and multiple targets symmetrically or, asymmetrically, distributed around the actual target. An HRRP general expression of the time-modulated diverse frequency metasurface at different observation angles is given to show the relation between the generated false targets and frequency components. Since having more flexible capacity of reflected signal manipulation compared to false target spoofing only through time modulation with consistent modulation frequency, the proposed method has enhanced performance on control of false targets at different observation angles. Both simulated and measured results demonstrate the validity of false target manipulation, demonstrating its effectiveness as electromagnetic countermeasure for achieving radar camouflaging and deception.

The phase information of the echo is essential for inverse synthetic aperture radar (ISAR) imaging. Time-modulated metasurface (TMM) generates time-varying reflection phase that can modulate the phase of echo, thus enabling jamming of ISAR imaging, which has great potential for real applications. In practice, covering the whole target with time-modulated metasurface is difficult to achieve. Consequently, the echo model of a radar target partially covered with a time-modulated metasurface is developed, which extends the model of ISAR jamming with TMM fully covering to a more general and near-realistic case in this letter. Then the echoes of each component of the target are obtained by electromagnetic (EM) simulation, and the time-varying reflection phase of the 1-bit time-modulated metasurface is obtained by experimental testing. Finally, using the above data, we validate that ISAR image of the component covered metasurface can be blurred using random time modulation sequence. Meanwhile, duplicated components can be produced in the ISAR image using periodic time modulation sequence. Experimental results of range profile jamming confirm the validity of the conclusions in this letter. The partial defocusing or duplicating of the radar target in the ISAR image can reduce the recognized possibility and enhance the jamming performance, effectively.

Space-time-modulated metasurfaces can manipulate electromagnetic waves in space and frequency domains simultaneously. In this article, an analytical design of space-time-modulated metasurfaces with modulation elements composed of two paths, In-phase (I) and Quadrature (Q), is proposed. The model is derived analytically, the space-/frequency-domain manipulations are achieved by designing the dimension and time sequence of I and Q paths. In the specular reflection direction, an objective frequency shift of the reflected first-order harmonic can be obtained. While, in other directions, the opposite first-order harmonic can be easily controlled by changing the dimension of I/Q paths, and the objective first-order harmonic remains unchanged. Furthermore, with a small dimension of I/Q paths, the first-order harmonic can be used for beam scanning by predesigning the start time of the modulation element. To realize the space-time-modulated metasurface with the required periodically time-varying responses, 2 bit unit cells loaded with dynamically switchable pin diodes are used as I/Q modulation. Both the analytical and numerical results demonstrate that space- and frequency-domain manipulations of the reflected fields by the first-order harmonics can be simultaneously obtained. The proposed designs have potential applications in wireless communications, radar camouflaging, and cloaking.

In this article, we propose a multifunctional space–time-modulated metasurface (STM-MTS) able to perform simultaneously a direction of arrival (DoA) estimation and radar cross section (RCS) manipulation in a single system. The metasurface consists of an alignment of unit cells, whose scattering response can be dynamically controlled to be transparent (full transmissive unit cell) or opaque (full reflective unit cell), according to the amplitude of the modulating signal applied to the loading p-i-n diodes. The DoA estimation is performed behind the metasurface, i.e., in the transmission region, where multiorder harmonics of the incident field whose complex amplitudes are proportional to the DoA are transmitted. The multiple signal classification (MUSIC) algorithm is employed to estimate DoA. Simultaneously, the monostatic/bistatic RCSs manipulation is performed in front of the metasurface, i.e., in the reflection region, where the reflected field is properly tailored by controlling the modulation scheme at unit-cell level. An $X$ -band STM-MTS is fabricated to experimentally verify the feasibility of the proposed design. A DoA estimation errors are less than 2 $^{\circ}$ in the incidence angular range of $-$ 50 $^{\circ}$ , $+$ 50 $^{\circ}$ is demonstrated, whereas monostatic and bistatic RCS manipulations of about 17 and 36 dB are achieved at operative frequency.

As engineered electromagnetic covers based on (space-) time modulated metamaterials and metasurfaces, Doppler cloaks are able to compensate the Doppler effect induced by the motion of a scatterer, making it appear as if it were at rest to a detecting radar system. Perfect Doppler compensation can be theoretically always achieved for any relative velocity and motion direction of the cloaked scatterer with respect to the detecting system. However, the motion can be still detected from the cross section variation of the Doppler cloaked scatterer, especially under oblique incidence illumination. The challenge is, therefore, to have a proper Doppler compensation and maintain the amount of scattered energy toward the detection system as much constant as possible with respect to the illumination angle. In this Letter, we propose the design of self-adaptive retro-reflective planar Doppler cloak composed of a pair of space-time modulated metasurfaces: the first metasurface focuses the incident field in a specific location on the second metasurface, which is designed for enabling retro-reflection and Doppler frequency shift compensation. Here, the self-adaptive Doppler cloak is applied to a metallic planar reflector, moving toward its normal direction, and illuminated by an oblique plane wave. We demonstrate that the proposed Doppler cloak can perform frequency conversion and simultaneously maintain the radar cross section of the reflector as much stable as possible within an angular range of about 60° centered at the normal direction. The self-adaptive Doppler cloak may enhance the undetectability of cloaked moving objects.