Radar Cross Section Enhancement Research Articles

A Planar 2-D 8×8 Van Atta Retrodirective Array for RCS Enhancement in a Wide Angular Range

Large-scale 2-D Van Atta retrodirective arrays (RDAs) typically use coaxial lines to form feeding networks, resulting in a large 3-D structure. In this study, a planar 2-D Van Atta RDA with 64 antennas and 32 transmission lines is proposed. By adding an air layer between the feeding network and the antenna array, double-layer microstrip lines can be used to realize a planar feeding network. Different types of length compensation networks are designed in limited space to eliminate the length differences between the 32 microstrip lines. Based on measured results, the Van Atta RDA works at 8.9–10.4 GHz, and the −6 dB beamwidths of the monostatic radar cross section (RCS) at 9.65 GHz are 78° (E-plane) and 90° (H-plane). Compared with the same size metal plane, the proposed Van Atta RDA has RCS enhancement of about 10–20 dB in a wide angular range.

A novel retroreflective planar Luneburg lens for the radar cross section enhancement

A novel retroreflective planar Luneburg lens for the radar cross section (RCS) enhancement is proposed in this paper. The planar Luneburg lens consists of 10 layers of metasurfaces, which are used to realize the layered permittivity distribution of three-dimensional (3D) gradient-index Luneburg lens. First, a single-layer planar metasurface element is designed. The element consists of one single substrate layer and a metallic layer, and two metal square ring patches are etched on the substrate. Then, by using the transformation optics (TO) and the metasurface element, the equivalent layered permittivity distribution with 2–25 relative permittivity is realized, and elements of the planar Luneburg lens illuminated by the 30° oblique incident plane wave are designed. Finally, a 10-layer retroreflective planar Luneburg lens at 5 GHz is designed, fabricated and measured. At 5 GHz frequency, its measured RCS values in most of the incident angle domain from −60° to 60° are enhanced in different degrees. The most notable enhancements occur in the incident angle domain around −27° and 27°, while the maximum measured RCS enhancement is more than 25 dB. This is the first time that the planar Luneburg lens is used to increase its RCSs with many incident angles.

Electromagnetic Backscattering Mechanisms of Frequency Selective Surfaces for Effective Analysis of Radar Cross Section by the First-Order Approximation of Floquet Modes

Full-wave analysis of finite but electrically large frequency selective surfaces (FSSs) substrates for the radar cross section (RCS) estimation is too time-consuming and unbearable for practical applications. Such FSS substrates can be either absorbing or reflective types and are widely used as radomes of antennas for stealth applications, where the electromagnetic (EM) backscattering is of much concern. This large EM problem is simplified by applying Floquet mode decomposition on the scattering fields, where the Kirchhoff approximation is employed to find the backscattering fields for RCS estimation. The resulted formula allows one to explore the scattering mechanism for RCS enhancement or reduction. Besides, the approximation is used to resemble the FSS radome by equivalent substrates. Thus, the transmission and reflection coefficients of the equivalent dielectric substrates are similar to those from FSS elements under an infinite array assumption. Afterward, the resulting EM problems of antenna radiations and scattering can be easily analyzed using the equivalent substrates of EM materials. Numerical examples of planar FSSs to form radomes demonstrate the feasibility of numerical simulations’ proposed work. The presented work can also be directly applied for fast analysis of EM shielding effectiveness by FSS substrates.

Research on conformal reflectarray for RCS enhancement in specific angular domain

In this paper, a new application scenario for the conformal reflectarray is proposed. A conformal reflectarray element composed of a split ring and two arcs acting as phase shift lines is adopted. The phase shift between 0° and 360° is achieved by adjusting the length of the two arc lines. A new phase distribution calculation method is proposed to achieve a 10-dB enhancement of monostatic radar cross section (RCS) at 15° to 30° of the tail of the missile. Projectiles (missile body) with and without the conformal reflectarray are fabricated and measured. Measurement results showed that the projectile with the proposed conformal reflectarray achieves RCS enhancement in the angular range of 15° to 30°. Specifically, RCS enhancements of 17.6, 10.43, and 29.02 dB are achieved at 15°, 30°, and the design angle of 22.5°, respectively. This work is the first to apply a conformal reflectarray to RCS enhancement in a specific angular domain.

Field discrete Monte Carlo simulation of electromagnetic scattering characteristics of plasma sheath

The electromagnetic scattering of an ultra-high-speed plasma covered target is simulated by the field discrete Monte Carlo method. The flow field network is used to simulate the trajectory of electromagnetic wave particles according to the electromagnetic distribution parameters, and the electromagnetic characteristics are calculated according to the statistical parameters of the reflected particles. The computational time-consumption is less correlated with the incident electromagnetic wave frequency [except for the Radar Cross Section (RCS) valley characteristics], and it is convenient for parallel calculation, which realizes the fast calculation of the electromagnetic scattering of the electrically large plasma-covered target. On this basis, the electromagnetic scattering characteristics of the blunt cone hypervelocity target are calculated, and the main RCS reduction and enhancement mechanisms are given.

Planar microwave retroreflector based on transmissive gradient index metasurface

In this paper, a novel planar microwave retroreflector based on a transmissive gradient metasurface combined with a curved metal mirror is proposed and demonstrated. The transmissive metasurface can efficiently converge a wide-angle incident wave to a pre-designed curved metal mirror behind it with a proper distance, which acts as an effective reflective surface that can greatly enhance the backscattering of the incident wave with a wide-angle view. According to the full-wave simulations, the proposed metasurface retroreflector can perform an excellent retroreflective effect for incident microwaves of angle view between −30° and 30° range. A prototype was fabricated and the experimental results verify that the metasurface retroreflector can realize the monostatic radar cross section (RCS) enhancement with a continuous wide incident angle view from −30° to 30° at 10 GHz within a stable 3 dB RCS level. It is further demonstrated that the excellent wide-angle backscattering performance (absolute RCS enhancement value, operational bandwidth and/or incident angle view) of the proposed microwave metasurface retroreflector is competitive against the traditional trihedral corner reflector with comparable dimensions, thus opening up new possibilities to substitute the traditional bulky radar retroreflector by using a planar compact metasurface structure for microwave engineering. The presented microwave metasurface retroreflector is promising to develop into a low-profile, light weight and planar radar retroreflector which possesses tremendous RCS backscattering enhancement and wide-angle view operation range.

Wide-angle flat metasurface corner reflector

In this paper, a wide-angle flat corner reflector based on the multiple phase gradient modulated metasurface is proposed and demonstrated. Through designing the related gradient phase, the propagation direction of the reflected electromagnetic wave is just opposite to the propagation of the incidence wave based on the generalized version of the reflection law. This guaranteed a radar cross section (RCS) enhancement in the vicinity of this incidence direction. To derive a uniform mono-static RCS in a wide incidence angle domain, the flat metasurface corner reflector is decomposed into several regions with different areas, which are designed for RCS enhancing under wave incidence in different incidence directions. As an example, the metasurface corner reflector consisted of two different phase gradients operating over the X-band is designed to realize wide-angle RCS enhancement. The simulated results indicate that in the designed incidence directions, the reflected waves are all opposite to the incidence wave for both TE and TM polarized waves. To validate the RCS enhancement performances, the mono-static RCS of the designed metasurface corner reflector is measured. The measured results reveal good accordance with the simulations, and both demonstrated that the designed metasurface corner reflector obtains tremendous RCS enhancement performances in a wide-angle domain (−50°–50°) for both TM and TE polarized waves.

Research of a wide-angle backscattering enhancement metasurface

To enhance backscattering, corner reflector and Luneburg lens are usually used. They can operate effectively in a broad angle range and also in a quite wide band. However, corner reflector as a typical structure of backscattering enhancement device, has obvious disadvantages in practical application. For example, it is usually made of metal material, which causes it to be too heavy and bulky. Luneburg lens is generally made of dielectric with strong loss and high cost, which is unfavorable for applications. Thus, it is necessary to explore a new way to realize wide-angle backscattering enhancement. In this paper, a phase gradient metasurface with wide-angle radar cross section (RCS) enhancement property is proposed and demonstrated, which consists of two phase gradients with equal magnitude but in opposite directions. Through designing a reflective phase profile along the surface, an equivalent wave vector can be generated, with doubled magnitude but in an opposite direction to the parallel component of the wave vector of the incident wave. At the incidence angles =-45 and 45, electromagnetic (EM) waves are reflected to the directions just opposite to the directions of incident waves. And at incidence angle =0, the incident EM wave is coupled into spoof surface wave and then guided to another region to decouple into a free space wave. These guarantee RCS enhancement property in a related angular domain. The polarization independent Jerusalem cross unit is used to design the phase gradient, and a wide-angle RCS enhancement metasurface is designed. The simulated results indicate that at the designed incidence angles, directions of the reflected waves are all opposite to the directions of incidence waves for both x and y polarized wave. In order to evaluate the RCS enhancement performances, the mono-static RCS of the designed wide-angle RCS enhancement metasurface is measured. Both the simulations and experiments are in good agreement with each other, and show that the designed metasurface obtains tremendous RCS enhancement performances in a wide-angle domain (-45-45) for both x and y polarized wave with frequencies ranging from 9 GHz to 12 GHz.

Simultaneous data collection of small maritime targets using multistatic radar and forward scatter radar

Radar detection of small maritime targets is of great interest in the context of coastal and port security for prevention of activities such as smuggling and piracy. Multistatic radar and forward scatter radar offer detection advantages compared with conventional monostatic systems, such as advantageous multi-perspective target view for the former and target radar cross-section enhancement for the latter. In this study, experimental results are presented to characterise simultaneous measurements investigating the detection of a small inflatable boat by a multistatic radar and a forward scatter radar. These results are believed to be the first example of simultaneous experimental comparison of such systems.

Amplitude scintillation effects on SAR

Space-based low-frequency synthetic aperture radar (SAR) is affected by the ionosphere, which induces both phase and amplitude fluctuations, known as scintillation, into the radar signal. This paper describes the effect of amplitude scintillation on SAR imagery. The two-way amplitude and intensity probability density functions (pdf) for both monostatic and bistatic SAR are derived from the one-way Nakagami-m distribution. The moments are then used to determine the SAR radiometric calibration error and image contrast from the one-way S 4 index. It is also shown that monostatic SAR experiences an S 4-dependent radar cross-section (RCS) enhancement that is not experienced by bistatic SAR. The anisotropy of the ionospheric irregularities strongly affects the degree to which amplitude scintillation will be visible in SAR imagery. The description of anisotropic effects is reviewed and extended to cover SAR. The variation over the Earth is illustrated, showing that a sun-synchronous satellite will experience the strongest effect near Brazil. Two PALSAR images of the same area of Brazilian rainforest are compared, one of which shows azimuthal streaking, corresponding to an amplitude modulation of ± 1 dB. The one-way S 4 index is determined from this imagery using both the RCS enhancement and image contrast measures of S 4, which produce similar results.

Backscattering enhancement analysis for targets in continuous random media based on wave polarization

Owing to the double passage effect, the phenomenon of backscattering enhancement arises in which the radar cross-section (RCS) in a random medium is twice that in free space. In a previous study, it was proved that the enhancement in radar cross-section (ERCS) deviates from two and has large and anomalous fluctuations, sometimes as a result of the wave polarization and other parameters, especially for targets in strong random media. Linear, including horizontal and vertical, polarizations were considered. In this paper, a numerical analysis is presented to show that the fluctuations can be reduced and make ERCS dependent almost only on the double passage effect under certain conditions. Therefore, we will have a better detection technique of targets of large sizes in continuous random media. In doing that, the linear and circular polarizations of incident waves are considered. We assume the case where a directly incident wave is produced by a line source in the far field distributed uniformly along the axis parallel to the conducting cylinder (target) axis.

Radar cross-section enhancement of dihedral corner reflector using fractal-based metallo-dielectric structures

Effective use of fractal-based metallo-dielectric structures for enhancing the radar cross-section (RCS) of dihedral corner reflectors is reported. RCS enhancement of about 30 dBsm is obtained for corner reflectors with corner angles other than 90°. This may find application in remote sensing and synthetic aperture radar.

Strong backscattering enhancement for partially convex targets in random media

Earlier work pointed out that the radar cross section (RCS), owing to the double-passage effect on waves propagating through random media, is enhanced by a factor ranging from one up to three. However, our study has manifested numerically a strong enhancement in RCS of targets with concave-convex surfaces in continuous random media, by taking account of boundary conditions of waves on targets. We have found that when a plane wave illuminates a convex portion of the concave–convex conducting target in a random medium, the factor of enhancement oscillates about two with target size. Moreover, this enhancement becomes quite large at certain target shapes and also under a specific condition of target size and spatial coherence length of the incident H-wave. The large enhancement is considered as an anomalous feature in the behaviour of backscattered waves in random media. By analysing the scattering problem for beam-wave incidence on the same target in free space, we manifest the mechanism of such anomalous enhancement.