Low Radar Cross Section Research Articles

Tracking and Track Management of Extended Targets in Range-Doppler Using Range-Compressed Airborne Radar Data

Ship tracking facilitates a comprehensive insight into maritime traffic situations and ensures its safety and security. However, with the current operational surveillance systems, detecting several maritime threats is still a major challenge. In this article, we propose a supportive ship tracking concept using an airborne-based radar sensor. The proposed tracking algorithm is suitable for dense multitarget scenarios. Tracking is performed in the range-Doppler domain. The primary advantage of using the range-Doppler domain is that ships even with low radar cross section moving with certain line-of-sight velocity appear out of the clutter region, thus improving their detectability. In addition, a powerful track management system is also developed to handle false targets. The simulated and real experimental results from the DLR’s airborne radar sensors, F-SAR and DBFSAR, are presented to prove the concept.

DIAT-μSAT: Small Aerial Targets’ Micro-Doppler Signatures and Their Classification Using CNN

Protective measures against small unmanned aerial vehicles (UAVs) are vital from a national security perspective. As a result, the importance of surveillance systems that automatically identify and classify low radar cross section (RCS) aerial targets increases. In this work, an indigenously developed continuous wave (CW) (X-band: 10 GHz) radar is used to build a diversified “DIAT- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu $ </tex-math></inline-formula> SAT” dataset comprising 4849 micro-Doppler signature images of five different small aerial targets. We also proposed a transfer learning-based deep convolutional neural network (DCNN) approach for classifying low RCS aerial targets. We demonstrated the classification accuracy of 95% and 97%, with VGG16 and VGG19 as feature extractors, respectively, with minimal false-negative and -positive results. The open-field experimental classes covered in this work are: 1) a two-blade rotor; 2) a three-short-blade rotor; 3) a three-long-blade rotor; 4) a quadcopter; 5) a bionic bird; and 6) a two-blade-rotor and bionic bird. We also observed a good classification accuracy (>97%) when more than one target is operated simultaneously.

Ultra-thin ultra-wideband tunable radar absorber based on hybrid incorporation of active devices

Structural radar absorber has important application in stealth field for its ability to effectively absorb incoming radar wave and to bear the load at the same time. Metasurface absorbers can achieve nearly-perfect absorption of radar wave, and have characteristics of light weight and thin structure, but their bandwidth are usually narrow. To solve this problem, a new method of broadening the bandwidth of metasurface absorber is proposed in this work. With varactor and PIN diode integrated in a hybrid manner, the continuous tunning and discrete switching are combined together to broaden the effective absorption bandwidth of the absorber. Using this method, an ultra-wideband tunable metasurface absorber is designed and the absorbing mechanism is analyzed in depth. By changing the bias voltages of PIN diodes and varactors, the absorbing frequency can be continuously tuned within a wide band from 4.57 GHz to 8.51 GHz. Measured results verify the low radar cross section characteristics of the absorber and the effectiveness of the design method. The proposed method is simple and feasible, and can be extended to other broadband structure design.

Low RCS Dual-Polarized Crossed Dipole Antenna Co-Designed With Absorptive Frequency- Selective Reflection Structure

This article proposes a novel strategy for realizing a reduction in radar cross-section (RCS) of dual-polarized crossed dipole antenna. At first, a compact and polarization-insensitive absorptive frequency-selective reflection (AFSR) structure is proposed by incorporating a bandstop ring resonator within the circular-cross based broadband absorber. The bandstop ring resonator is designed on the backside of the resistive layer due to which a reflection window is realized at a frequency of 8.2 GHz between the two broadband absorptions (4.2-7.0 GHz and 9.2-11.5 GHz). A dual-polarized crossed dipole antenna is designed with operating frequency lying within the reflecting notch of the AFSR structure. A 6 x 6 AFSR structure array is truncated at the center where from which the crossed dipole is connected through a feed substrate. The AFSR structure enacts as a modified ground plane to the crossed dipole antenna. The proposed AFSR integrated antenna achieves an average mono-static RCS reduction of 12.51 dB and 12.62 dB for the TE and TM incident waves, respectively. Further, the AFSR based antenna is also measured for the bi-static RCS, wherefrom the average total RCS reduction of 80% for TE and TM incidence is attained in the frequency band of 4.2 to 11.5 GHz.

Enhanced interfacial polarization of biomass-derived porous carbon with a low radar cross-section

Ultra-thin microwave absorbers have been urgently demanded for electromagnetic applications in recent years. Herein, porous carbon with a “flower cluster” microstructure was synthesized from biomass waste (mango seeds) by a facile activation and carbonization method. The novel structure reduced the density and also improved the impedance matching, dipole polarization, and provided many carbon matrix-air interfaces for interfacial polarization, resulting in superior microwave absorption performance. At an ultra-thin thickness of 1.5 mm, extraordinary microwave absorption was achieved, with a reflection loss (RL) of −42 dB. The effective absorption bandwidth reached 4.2 GHz. The RL can be further improved to −68.4 dB by adjusting the amount of activator to manipulate the structure of porous carbon. In addition, from the simulated radar scattering results, the maximum reduction in the radar cross-section (RCS) reached 30.4 dBm2, which can greatly reduce the probability of equipment being detected by radar. This work provides a low-cost and high-performance microwave absorber for electromagnetic stealth technologies.

Detection and Tracking of UAV Targets Using Deep Learning

In recent years, the use of Flying drones and modern Unmanned aerial vehicles (UAVs) with the latest techniques and capabilities for both civilian and military applications growing sustainably on a large scope, Drones could autonomously fly in several environments and locations and could perform various missions, providing a system for UAV detection and tracking represent crucial importance. This paper discusses Designing Detection and Tracking method as a part of Aero-vehicle Defense System (ADS) for UAVs using Deep learning algorithms. The small Radar cross-section (RCS) foot-print makes a problem for Traditional methods and Aero-vehicle Defense systems to distinguish between birds, stealth fighters, and UAVs incomparable of size and RCS characteristics, the detection is a challenge in low RCS targets because the chance of detection is incredibly less moreover, in the existence of interference and clutter which reduce the performance of detection process rapidly.

A low‐radar cross section Vivaldi antenna array based on reflection cancelation

In this article, a low radar cross section (RCS) Vivaldi antenna array based on reflection cancelation is proposed, which is composed of two antenna elements. By using different feed structures and loading metal strips of different lengths, the two antenna elements have approximately 180° reflection phase difference from 5.1 to 11.9 GHz under y-polarized incident waves and good impedance matching performances from 8 to 12 GHz. Then the different antenna elements are fed by power dividers to form different subarrays. Furthermore, shape modification is applied to the array, which is composed of different subarrays. Finally, RCS reduction of proposed array is achieved in 5.2–12 GHz under y-polarized incident waves compared with reference array, with a maximum reduction value of 16 dB. At the same time, the proposed array has good radiation characteristics. The prototype of proposed array is fabricated and measured to verify the design. The simulated results and measured results are in good agreement.

A small vessel detection using a co-located multi-frequency FMCW MIMO radar

Small vessels detection is a known issue due to its low radar cross section (RCS). An existing shore-based vessel tracking radar is for long-distance commercial vessels detection. Meanwhile, a vessel-mounted radar system known for its reliability has a limitation due to its single radar coverage. The paper presented a co-located frequency modulated continuous waveform (FMCW) maritime radar for small vessel detection utilising a multiple-input multiple-output (MIMO) configuration. The radar behaviour is numerically simulated for detecting a Swerling 1 target which resembles small maritime’s vessels. The simulated MIMO configuration comprised two transmitting and receiving nodes. The proposal is to utilize a multi-frequency FMCW MIMO configuration in a maritime environment by applying the spectrum averaging (SA) to fuse MIMO received signals for range and velocity estimation. The analysis was summarised and displayed in terms of estimation error performance, probability of error and average error. The simulation outcomes an improvement of 2.2 dB for a static target, and 0.1 dB for a moving target, in resulting the 20% probability of range error with the MIMO setup. A moving vessel's effect was observed to degrade the range error estimation performance between 0.6 to 2.7 dB. Meanwhile, the proposed method was proven to improve the 20% probability of velocity error by 1.75 dB. The impact of multi-frequency MIMO was also observed to produce better average error performance.

Integrated Low-Profile Low Radar Cross Section Circularly Polarized Dipole Antenna Array

In this article, an integrated low-profile low radar cross section (RCS) circularly polarized (CP) antenna array based on multifunctional modes generation method is proposed, which can realize good in-band radiation performance and wide out-of-band RCS reduction characteristics on the same layer without additional subassembly except for resistors. By embedding a resistor in the center of a metallic strip, the half and one and a half-wavelength resonant modes are changed into the absorption modes, while the one wavelength resonant mode is excited as the radiation mode when offset feeding at an appropriate location. Furthermore, the CP radiation performance is achieved with four bent and center-rotating-symmetric metallic strip and the sequentially rotated feeding network. The simulation results show that the designed integrated antenna array generates the same left-hand CP radiation performance as the array without embedding resistors, and the monostatic RCS of the array is significantly reduced in the lower and upper bands, with an average reduction value of 10 dB. Prototypes of the designed antenna array with <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$4 \times 4$ </tex-math></inline-formula> elements are fabricated and measured to validate the simulation results.

Low-RCS and Gain-Enhanced Antenna Using Absorptive/Transmissive Frequency Selective Structure

A gain-enhanced antenna consisting of a bandpass frequency selective surface (FSS) and a patch antenna is initially proposed in this communication. By utilizing the reflection characteristic of the FSS, a quasi-Fabry–Perot (F-P) cavity is formed between the FSS and antenna’s ground, which considerably improves the gain of the antenna. Meanwhile, the bandpass FSS has a little phase compensation for oblique transmitting waves, which also slightly enhances the gain. Then a low-radar cross section (RCS) and gain-enhanced antenna is proposed by using absorptive/transmissive frequency selective structure (ATFSS). It is implemented by mounting the ATFSS above a patch antenna. The ATFSS is constructed by stacking a lossy layer above a bandpass FSS. Both antennas are fabricated, assembled, and measured. They achieve a gain-enhancement of 5.27 and 3.84 dB, respectively. Besides, the low-RCS antenna achieves 10 dB monostatic RCS reduction from 5 to 5.33 GHz and from 9 to 13.4 GHz under the TE-polarization, and from 8.4 to 13.5 GHz under the TM-polarization, respectively. Simulated results show it also achieves a good improvement in bistatic RCS reduction as well.

적응형 압축센싱 기반의 드론 SAR 시스템을 이용한 저피탐 은폐 표적물 탐지

Synthetic aperture radar (SAR) uses radio waves to detect targets in all-weather conditions. The latest SAR development enables to operate drone-based SAR systems that can detect low radar cross section (RCS) targets hidden around foliage areas in mountain sides. In drone SAR equipment, low weight constraints limit the battery capacity and pulse sampling rate, leading to image quality degradation. In this study, we attempt to enhance the image quality of power limited drone SAR system by applying compression sensing algorithms. After conducting on-site field experiments, an adaptive target detection process is adopted toward low RCS targets concealed in vegetation areas. It is shown that drone SARs can be used to obtain images of various underground targets or targets hidden under foliage using enhanced compressive sensing algorithms.

Novel strategy to design a metasurface with integrated radiation and a broadband low radar cross section

In this paper, a novel design strategy that integrates good radiation and broadband low radar cross section (RCS) characteristics based on the concept of metasurface is proposed. The metasurface element adopts an etched cross patch and it directly behaves as a radiating structure. After that, a metasurface-based thinned array antenna A1 and a checkerboard metasurface antenna A2 are designed. The -10 dB operating bandwidth of these two antennas is 13.08–14.92 GHz (13.1%). Compared with the conventional rectangular grid array, A1 and A2 have similar radiation performance along with in-band and out-of-band RCS reduction (RCSR) in any polarized normal incidence. Reasons and merits of different arrangements are analyzed. Simulated and measured results verify the effectiveness of the design strategy.

Broadband Low-RCS and High-Gain Microstrip Antenna Based on Concentric Ring-Type Metasurface

A novel integration strategy of microstrip antenna and metasurface is proposed to achieve low-radar cross section (RCS) and improved radiation performance simultaneously. Aimed at the applications in stealth cylindrical carrier, the antenna has a circular configuration, and the metasurface cells are closely arranged around the radiation patch in a concentric ring-type manner. Moreover, the mode analysis method is fully used to analyze the antenna's working mechanism and guide the optimization process. Owing to the addition of metallic vias, the original quasi-TM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">30</sub> mode is altered to three series-TM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10</sub> modes and thus enhanced gain is achieved. Interestingly, the addition of vias also generates a new resonance which widen the antenna's impedance bandwidth. In another aspect, both in-band and out-of-band RCS reductions are obtained simultaneously. Finally, two prototypes are fabricated to validate the proposed design strategy. Good agreements between measurements and simulations are obtained. Measured results show that the gain of the proposed antenna is at least 3 dB higher than the conventional antenna in the entire operating band, and obvious RCS reduction is achieved from 4.0 to 8.5 GHz concurrently.

A novel polarization converter based on the band-stop frequency selective surface

A dual-passband single-polarized converter based on the band-stop frequency selective surface (FSS) with a low radar cross-section (RCS) is designed in this article. The unit cell of the proposed converter is formed by a polarization layer attached to the band-stop frequency selective surface. The simulation results reveal that the co-polarization reflection coefficients below −10 dB are achieved in 3.82–13.64 GHz with a 112.4% fractional bandwidth (the ratio of the signal bandwidth to the central frequency). Meanwhile, a polarization conversion band is realized from 8.14 GHz to 9.27 GHz with a polarization conversion ratio which is over 80%. Moreover, the 1 dB transmission window is obtained in two non-adjacent bands of 3.42–7.02 GHz and 10.04–13.91 GHz corresponding to the relative bandwidths of 68.9% and 32.3%, respectively. Furthermore, the radar cross-section of the designed structure can be reduced in the wideband from 2.28 GHz to 14 GHz, and the 10 dB RCS reduction in the range of 4.10–13.35 GHz is achieved. In addition, the equivalent circuit model of this converter is established, and the simulation results of the Advanced Design System (ADS) match well with those of CST Microwave Studio (CST). The archetype of the designed converter is manufactured and measured. The experiment results match the simulation results well, which proves the reliability of the simulation results.

Investigations and prospects of Fabry-Perot antennas: A review

Fabry-Perot (FP) antennas have characteristics of planar structures combined with high gain, and they have been widely used in wireless communications. With the progress of ongoing research, FP antennas have achieved various capabilities, but many of them are still under development, such as low-profile, wideband, circular polarization, multi-band, low-radar cross section (RCS) and reconfigurable features. This paper discusses the theoretical analysis methods and research progress of FP antennas, and explains the realization methods of different features of FP antennas. In order to indicate different technologies for realizing various capabilities, the key technologies and features of some of the latest designs are described. Finally, the research situation and prospects of FP antennas are summarized to guide their research directions in the future.

A metasurface-enabled Fabry–Pérot (FP) circularly polarized antenna with continuous wideband polarization conversion purity

Abstract In this paper, a wideband continuous pure right hand circularly polarized (RHCP), high gain, and low-radar cross-section (RCS) array antenna is proposed. A linear-to-circularly polarization conversion (LCPC) Metasurface (MS) is employed as the superstrate of the Fabry–Pérot (FP) resonator antenna, consisting of two oblique slits etched patches located at top and bottom, respectively, and a metal ring with corner-cutting patch inside, that ensure a wideband transmission and reflection LCPC frequencies ranging from 9 to 22 GHz, and 7–13.5 GHz, respectively. While, a pure RHCP LCPC frequency band of 9–12 GHz is produced by adopt the proposed MS that is benefit from the design of etched oblique slits and corner-cutting patch surrounded by the metal ring, where the magnitude and phase difference can be kept in the variation of ±3 dB and 10°, respectively. Then, a rectangle patch-fed MS FP antenna is designed by an arrangement of 5 × 5 MS unit cells. Following this, the sequence rotated technique is utilized to arrange the array antenna by 2 × 2 units, ensuring a wide band RCS frequency band. The proposed array antenna is fabricated and measured, which indicated the correctness of this design for performance of high gain, low RCS, and wideband pure RHCP. Compared with recent reported MS-based FP works, a wideband LCPC frequencies purity is obtained, and a good radiation and scattering performance is obtained in the design.

A novel Fabry–Pérot (FP) antenna with high gain, low radar-cross-section (RCS), and wideband circular polarization

AbstractWe propose a novel Fabry–Pérot (FP) antenna consisting of a checkered polarization-conversion metasurface (PCM), corner-cut square patch antennas, and sandwiched compounds. The proposed antenna achieves low radar-cross-section (RCS), high gain, and wideband circular polarization (CP). The corner-cut square patch antennas facilitate high reflectivity, satisfactory transmission magnitude, and the desired phase difference. An embedded metal between two rings of substrate contributes to reducing cross-polarization, improving transmission efficiency, enhancing bandwidth, and reducing RCS. Following simulations, we fabricated a prototype of the proposed antenna and tested its performance. Measurements from the simulation and prototype tests were similar within a reasonable margin of error. Compared with alternative antennas, our proposed FP antenna offers high gain, wideband CP, low cost, a low RCS, and a lower profile.

In-band RCS reduction antennas using an EBG surface

AbstractThe paper has proposed a multilayer, polarization rotation featured, low radar cross-section (RCS) antenna using electromagnetic band-gap (EBG)-based frequency selective surface (FSS) at 8.25 GHz. Cross-shaped EBG unit cells offer zero reflection phase and −25 dB reflection magnitude at 8.25 GHz. The FSS layer consists of eight cross-shaped EBG unit cells sandwiched between two substrates to offer high absorptivity at the desired band. The circular patch antenna resonating at 8.25 GHz is placed on the top substrate having a lower dielectric constant. Four circular-shaped patches are etched at the four corners of the top layer and are coupled with two feed lines which are aligned 90° to each other at the bottom layer and interconnected diagonally to achieve polarization rotation. The proposed antenna offers a gain of 6.72 dB and an in-band RCS of −21.4 dBsm. Incident energy is backscattered into eight directions separated by angle ϕ = 45°. The proposed antenna has the RCS reduction band of 7.7–9.4 GHz. It offers normalized polarization rotation ratio more than 0.8 within the −40° to 40° angular region at the frequency band 8–8.5 GHz. The measured result using the fabricated prototype agrees well with the simulated one.

Superstrate-based patch antenna array with reduced in-band radar cross section

AbstractTo design a low radar cross section (RCS) antenna, the major concern is not only to reduce scattering, but also to maintain its radiation parameters, viz. gain, voltage standing wave ratio (VSWR), etc. This paper presents a simple configuration of low RCS microstrip patch array with a periodic structure-based superstrate. The ground of the array is designed as reduced ground plane with high impedance surface elements, viz. rectangular patch and Jerusalem cross. The configuration of superstrate consists of multilayered, viz., two-layered and three-layered structures having partially absorbing and reflecting surfaces. In both the proposed configurations, the array gain of 12.5 dB is maintained with reduced structural RCS over the entire in-band frequency range. The reflection coefficient (~ −20 dB) and VSWR (~ 1.1) of the array are maintained. It is shown that the proposed superstrate-based patch array design has significantly reduced in-band RCS (−18 dBsm) at its resonant frequency.

Design of a miniaturized ultrawideband and low scattering antipodal vivaldi antenna array

This paper presents a miniaturized ultra-wideband (UWB) antipodal Vivaldi antenna (AVA) array with low-scattering characteristics integrated a hybrid diffusive-absorptive metasurface. Periodic elliptical slots at the outer edges and a dielectric lens are utilized for antenna element to improve performances including miniaturized size, wide bandwidth, and high gain. The optimized element is fabricated and measured, the results demonstrate that the − 10 dB impedance bandwidth is 4.5–50 GHz with a ratio bandwidth (fH/fL) of 11.1:1, and the maximum gain at 35 GHz is 12.7 dBi, which are in good agreement with simulation. By loading an optimized Minkowski-shaped metasurface as the ground reflector, which combines the multielement phase cancellation (MEPC) and EM absorption technology, the 4 × 4 array realizes a low radar cross section (RCS) without the radiation performance degradation. Simulated and measured results show that the proposed low-scattering array has a 10-dB RCS reduction band ranging from 5 to 50 GHz at normal incidence for both polarizations. Furthermore, the array structure shows extremely low-observable capability, which is larger than 15 dB of the RCS reduction from 7.1 to 50 GHz with a ratio bandwidth of 7.0:1. The results verify the feasibility of improving the performance of antenna and the UWB low-scattering functionality.