Nonlinear Radar Research Articles

넓은 입력전력 범위를 갖는 S-대역 광대역 고조파 응답기

This paper presents a through-type harmonic transponder for S-band nonlinear radar. To increase the operating frequency range, a 2-stage L-shaped matching network was configured for the input and output matching networks. In previous studies, conventional harmonic transponders were configured with λ/4 stubs in the input and output matching networks to minimize the influence of these networks because the impedance mismatch suppresses the second harmonic and fundamental signals. However, in this design, a λ/4 open stub was utilized at the output and configured with the output matching network to alleviate the insertion loss of the additional open stub. For verification, the proposed circuit was implemented with the circuit size of 37.6 mm×16.1 mm. The measurement results show that the conversion gain is maintained over −15 dB for frequencies between 2.8~3.8 GHz at −5.5 dBm and −17.5~11 dBm at 3.1 GHz of input power.

A Dual-Band Patch Antenna Employing a Folded Probe Feed for Nonlinear Radar Applications

In this work, we design a novel, integrated transmit (TX)/receive (RX) dual-band folded-probe-fed patch (DFPFP) antenna to improve nonlinear radar (NLR) system performance. Designed for down-looking, airborne NLR applications, the DFPFP antenna occupies an improved form factor ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.9\lambda \times 0.9\lambda \times 0.1\lambda $ </tex-math></inline-formula> , 1.8 kg) over existing commercial-off-the-shelf (COTS) designs and additionally offers minimal drag in airborne applications due to its low profile. The DFPFP antenna is simulated, fabricated, and tested, and excellent gain (9 dBi) and bandwidth (15%) performance are demonstrated. Iterations of the design from single, standalone TX and RX antennas to the final, integrated design are presented and discussed. The final DFPFP is compared to a pair of COTS antennas that are currently used for NLR system testing. Evaluated in the context of an NLR system, the custom DFPFP antenna significantly outperforms the COTS antennas, with signal-to-noise ratio (SNR) improvements as high as 17 dB, while offering significantly improved size, weight, and power (SWaP) performance.

О возможности реализации нелинейного радиолокатора с обработкой на частоте зондирующего сигнала

The conditions for the implementation of a nonlinear radar using a probing signal having a form as a periodic sequence of pairs of radio pulses with different amplitudes and equal energies, are considered. The energies of the reflected pulses are compared in the locator receiver for each pair, which makes it possible to detect reflections from nonlinear devices at the probing signal frequency by violating the equality of these energies. A numerical experiment was performed to evaluate the dependence of the current form through germanium and silicon diodes and their amplitude parameters at the frequency of the acting sinusoidal voltage with variable amplitude. As a result, it was found that the amplitude nonlinearity can be detected before the diode is completely "opened" if the two voltage amplitudes on the diode differ from each other by at least one and a half to two times. Since the pulse energies are proportional to their powers, and the pulse durations are unchanged, it should be possible to detect non-linearity by determination of a ratio between the radar cross-sections of non-linear objects for different pulse amplitudes. To obtain quantitative estimates, a simulation of the incidence of a flat, linearly polarized wave on a half-wavelength dipole and a half-wavelength folded dipole loaded with a diode was performed. It was found that the model type and parameters significantly affect both the values of the radar cross-sections and the degree of influence of nonlinearity on these values, so that the detection of non-linearity requires significantly higher power levels of the probing signal than the theory predicts.

Characterization and Detection of RF Electronics Using Localized Conjugate Component

Nonlinear radar has a unique advantage in finding nonlinear targets, like RF electronics and harmonic tags, under clutter circumstances, among which intermodulation-based schemes are usually favored for their resistance to self-harmonics interference. Currently, intermodulations produced by multiple large signals are dominantly used for nonlinear target characterization and detection. However, this work proposes to exploit the intermodulation generated by the large and small signals, referred to as localized conjugate component (LCC). We observe that LCC describes the localized nonlinear structure, which contains discriminative representation exclusive to nonlinear targets. Compared with traditional intermodulation detection, LCC is independent of the small signal's waveform and dependent on the power-dependent scattering parameter, which makes it flexible for waveform design and cost-effective for performance estimation. Benchtop measurement results validate above derived properties. Furthermore, an over-the-air experiment confirms the feasibility of nonlinear target detection at a stand-off distance using LCC. Its application prospects in target identification are also presented.

An Improved Kalman Filter Based on Long Short-Memory Recurrent Neural Network for Nonlinear Radar Target Tracking

The target tracking of nonlinear maneuvering radar in dense clutter environments is still an important but difficult problem to be solved effectively. Traditional solutions often rely on motion models and prior distributions. This paper presents a novel improved architecture of Kalman filter based on a recursive neural network, which combines the sequence learning of recurrent neural networks with the precise prediction of Kalman filter in an end-to-end manner. We employ three LSTM networks to model nonlinear motion equation, motion noise, and measurement noise, respectively, and learn their long-term dependence from a large amount of training data. They are then applied to the prediction and update process of Kalman filter to calculate the estimated target state. Our approach is able to address the tracking problem of nonlinear maneuvering radar target online end-to-end and does not require the motion models and prior distributions. Experimental results show that our method is more effective and faster than the traditional methods and more accurate than the method with LSTM network alone.

Detection of Electronic Devices Using FMCW Nonlinear Radar.

Nonlinear radars can be utilized to detect electronic devices, which are difficult to detect with conventional radars due to their small radar cross sections (RCS). Since the receiver in a nonlinear radar is designed to only receive harmonic or intermodulated echoes from electronic devices, it is able to separate electronic devices from non-electronic scatters (clutter) by rejecting their echoes at fundamental frequencies. This paper presents a harmonic-based nonlinear radar scheme utilizing frequency-modulated continuous-wave (FMCW) signals for the detection of various electronic devices at short range. Using a laboratory experiment setup for FMCW radar at S-band for Tx (C-band for Rx), measurements are carried out to detect electronic devices of various sizes. The results show that the detection of small electronic devices is possible with nonlinear FMCW radar when appropriate system parameters are selected. Furthermore, we also discuss the maximum detectable range estimation for electronic targets using the radar range equation for FMCW nonlinear radar.

Measurement of Nonlinear RCS of Electronic Targets for Nonlinear Detection

The conventional radar technology is based on linear detection—i.e., the same transmit and receive frequencies are used. However, with linear radars, difficulties arise when detecting electronic objects with relatively small radar cross section (RCS). To overcome these limitations, a nonlinear radar that can detect nonlinear responses (i.e., harmonic and intermodulation) scattered by electronic devices due to nonlinear interaction can be utilized. Nonlinear radars require a different analysis from linear radars for analyzing RCS. In this paper, we present an experimental analysis of the nonlinear RCS of various electronic devices. Unlike linear radars, RCS in nonlinear radars is determined by the amount of nonlinear responses backscattered to the radar. Therefore, we derive a radar equation accustomed to harmonic radars that consists of nonlinear RCS. We then obtain and analyze the nonlinear RCS of various targets from the measured harmonic responses of the targets based on the nonlinear radar equation.

SVM 분류기를 이용한 비선형 FMCW 레이다 표적 인식

In this paper, a method for nonlinear target recognition via machine learning is presented. The nonlinear radar environment used in this study was a frequency-modulated continuous-wave (FMCW) nonlinear radar with a transmit frequency band of 3.0~3.2 GHz and received a frequency band of 6~6.4 GHz corresponding to the second harmonics. Nonlinear radar measurements were performed using four types of electronic devices as nonlinear targets. Statistical parameters were extracted from the measured amplitude spectrum of the received harmonic responses for each target to successfully construct a classification algorithm. The extracted characteristic data were then used to construct and verify a support vector machine (SVM) classifier. The accuracy of the target classification by the trained SVM classifier was confirmed through verification data, and an accuracy of 85 % with 10-fold cross-validation was demonstrated.

Circularly PolarizedS/CDual-Band Antenna for Nonlinear Detection

Nonlinear radars detect nonlinear (harmonic or intermodulation) responses excited from semiconductor junctions within electronic targets, allowing for the detection of small targets that are otherwise difficult to detect. To properly detect the nonlinear responses from targets, which are typically much lower than the responses at the fundamental frequency, the transmit, and receive antennas in a nonlinear radar, must ensure high isolation between the transmit and receive ports to suppress unwanted harmonics from the transmit side interfering with target responses. In this letter, a circularly polarized dual-band shared-aperture antenna, applicable to nonlinear radar, is presented. The <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S</i> -band part of the antenna has a center frequency of 3 GHz for transmit, while the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</i> -band part is designed to operate at the second-harmonic band of 6 GHz for receive. The proposed antenna is designed to exhibit high isolation between the transmit and receive ports in both bands. The characteristics of the proposed antenna are demonstrated via simulation and measurement, where it shows good circular polarization (CP) characteristics at both <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S</i> -and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</i> -bands with high isolation between ports. Furthermore, the performance of the proposed antenna for nonlinear detection is verified through an experiment where a nonlinear target is detected.

Synthesizing of images at combinational frequencies in a nonlinear radar

Synthesizing of images at combinational frequencies in a nonlinear radar

Long-range nonlinear radar for surface monitoring of territories and objects

Long-range nonlinear radar for surface monitoring of territories and objects

Convolutive Complex-Valued Independent Component Analysis for Nonlinear Radar Signal Processing and Maritime Weak Target Detection

This paper first establishes a new complex independent component analysis (cICA) algorithm based on the spatiotemporal extension of complex-valued entropy bound minimization (CEBM) to separate received complex-valued radar signals. Next, we propose a new cICA-based detector with an open structure to find Swerling model targets, lognormal targets, and sea-surface small floating targets in coherent high-resolution maritime surveillance radars. The detector encountered three major problems when adopting cICA for detection and solved them using three effective suggestions. After performing cICA on the time series received by the radar, we obtained two different sources. Using the first and second theoretical and empirical moment estimates of the K-distribution, the target was selected between these two output source signals. Detector performance was verified quantitatively and qualitatively using the real-life IPIX radar database. Comprehensive experiments on this database with synthetic injected targets showed promising results. The computational time and sample size dependency of the proposed cICA algorithm were also discussed. Finally, a comparison of the detector with several novel detectors for detecting sea-surface floating small targets of the IPIX radar database demonstrated the proposed detector’s superiority.

Експериментальні дослідження процесу виявлення нелінійних переходів вибухових пристроїв з неконтактними датчиками цілі

Кардинальні зміни характеру та значне збільшення динаміки ведення мінної війни у воєнних конфліктах сучасності стали результатом зміщення пріоритетів у бік застосування новітніх зразків мінної зброї та саморобних вибухових пристроїв переважно з неконтактними датчиками цілі. Такий стан питання викликав загострення проблеми протидії вибухонебезпечним загрозам, а поряд з цим потребу у забезпеченні потрібного рівня ефективності, безпеки та собівартості процесів пошуку і виявлення вибухових пристроїв з неконтактними (електронними) датчиками цілі. Існуючі технічні засоби пошуку та виявлення вибухонебезпечних предметів, як правило, у більшості випадків ґрунтуються на використанні активних електромагнітних методів. Наближення таких пошукових пристроїв до вибухових пристроїв із магнітними (неконтактними) датчиками цілі або тих, що керуються по радіо, досить часто призводить до несанкціонованого вибуху і як наслідок до втрат.&#x0D; Встановлено, що майже усі неконтактні датчики цілі поєднує спільна ознака – наявність напівпровідникових елементів у їхньому складі, або переходів типу метал-окисел-метал. Вказана особливість покладена в основу сучасних засобів виявлення вибухових пристроїв з неконтактними датчиками цілі методом нелінійної радіолокації. Однак, при цьому проблемними досі залишаються вимоги щодо безпеки використання пошукових пристроїв даного типу у ручному варіанті або на наземних транспортних засобах. Одним із шляхів розв’язання даного протиріччя є встановлення пошукових пристроїв на дистанційно-керовану платформу. При чому, виникає нагальна потреба обґрунтування тактико-технічних вимог до засобів даного типу при зміні умов та способів їх застосування.&#x0D; На основі проведеного аналізу попередніх досліджень, як вітчизняних, так і закордонних фахівців, стосовно використання електромагнітних методів пошуку і виявлення рукотворних предметів в укриваючих середовищах (ґрунт, сніг, вода тощо), запропоновано науково-методичний підхід щодо планування експериментальних досліджень процесу виявлення нелінійних переходів вибухових пристроїв з неконтактними датчиками цілі. Експериментальні дослідження, призначені для перевірки і уточнення розроблених теоретичних положень щодо створення перспективних дистанційно керованих радіолокаційних комплексів пошуку та виявлення вибухових пристроїв з неконтактними датчиками цілі за допомогою методу нелінійної радіолокації, а у випадку їх підтвердження – технічного обґрунтування вимог до пошукових пристроїв нелінійної радіолокації та дистанційно керованих радіолокаційних комплексів в цілому.

МАТЕМАТИЧНА МОДЕЛЬ ПОШУКУ ТА ВИЯВЛЕННЯ ВИБУХОВИХ ПРИСТРОЇВ З НЕКОНТАКТНИМИ ДАТЧИКАМИ ЦІЛІ МЕТОДОМ НЕЛІНІЙНОЇ РАДІОЛОКАЦІЇ

Radical changes in the nature and significant increase in the dynamics of mine warfare in modern military conflicts are the result of shifting priorities towards the use of the latest models of minesweepers and improvised explosive devices with mostly non-contact target sensors. This state of affairs has exacerbated the problem of countering explosive threats, and with it the need to ensure the required level of efficiency, safety and cost of search and detection of explosive devices with non-contact (electronic) target sensors. Existing technical means of search and detection of explosive objects, as a rule, in most cases are based on the use of active electromagnetic methods. The approach of such search devices to explosive devices with magnetic (non-contact) target sensors or those controlled by radio, often led to an unauthorized explosion and as a consequence to losses among the sappers.It is established that almost all non-contact sensors of the target are united by a common feature - the presence of semiconductor elements in their composition, or transitions such as metal-oxide-metal. These features are the basis of modern means of detecting explosive devices with non-contact target sensors by nonlinear radar. However, the requirements for the safety of using search engines of this type in the manual version or on land vehicles are still problematic. One way to resolve this contradiction is to install search engines on a remote-controlled platform. Moreover, there is an urgent need to substantiate the tactical and technical requirements for this type of equipment when changing the conditions and methods of their application.Based on the analysis of previous studies, both domestic and foreign experts, on the use of electromagnetic methods of search and detection of man-made objects in sheltered environments (soil, snow, water, etc.), proposed an improved mathematical model of search and detection of explosive devices with non-contact target sensors nonlinear radar. This model, in contrast to the existing ones, takes into account the probabilistic nature of the search process and the plane indicators of the guaranteed detection zone of devices with non-contact target sensors by nonlinear radar, functional indicators of electromagnetic wave propagation in free space and shelter. In the field of nonlinear radar, the concept of "zone of guaranteed detection of explosive devices with non-contact target sensors" was introduced for the first time, the essence of which is to ensure the implementation of the requirements for equivalent probability of detection at a level not lower than specified. In addition, the proposed mathematical model, among other technical characteristics, allows to set the performance parameters of the remote-controlled radar complex search and detection of explosive devices with non-contact target sensors by nonlinear radar. The proposed improvements of the mathematical model and the sequence of calculations will increase the accuracy of the results of modeling the search and detection of explosive devices with non-contact target sensors in sheltered environments when substantiating the requirements for demining of this type.

Patch antenna for nonlinear radar

The design of a dual-band patch antenna for a nonlinear radar operating at combinational frequencies has been developed. The antenna has a rectangular emitter surrounded by dielectric layers over a metal screen. The emitter has a U-shaped slot. The results of simulation and measurements of the antenna sample parameters are presented. The antenna is matched with the feeder at the frequencies near 1.3 and 1.6 GHz, and in the range beyond these frequencies the antenna operates as a notch-filter. The antenna has a unidirectional pattern. The radiation maximum is directed perpendicular to the plane of the antenna.

Parameter optimization for the three-frequency feed for the mirror antenna of the non-linear radar

Analyzed below is a radiating structure represented by a flat self-complementary three-sector structure fed by three segments of the microstrip line and located above the stepped metal screen, and to be used as an exciter for the non-linear radar antenna. When proper matching with the feeding coaxial cable is achieved, results of the numerical simulation proved that the antenna can provide near-circular polarization of radiation at the probing signal frequency, and linear polarization at the second and third harmonics of this signal.

Parametric Nonlinear Locator

The device relates to the field of radar techniques for locating or objects detecting using reflection or reradiation of radio waves. This device can be used to detect and identify the objects containing non-linear electrical contacts. The non-linear radar contains two sources of signals that generate the signals in close frequency rangers, and a signal combining scheme associated with the antenna. The source of the first signal is provided with a low-frequency modulator. A vector reflectometer is included between the source of the second signal and the signal combining circuit. The design is simplified and the detectability of the non-linear radar is improved due to these design features.

Nonlinear Radar for Finding RF Electronics: System Design and Recent Advancements

An extensive review of nonlinear radar systems is performed. Emphasis is placed on designs relevant to detecting RF electronics that were not intentionally manufactured as visible radar targets. The state of the art in nonlinear radar is conveyed by presenting high-level system architecture, explaining the rationale behind design decisions pertaining to that architecture, and listing the specifications that nonlinear radar designers have achieved. The authors’ recent advancements in nonlinear radar technology are summarized.

Static and Moving Target Imaging Using Harmonic Radar

Nonlinear radar exploits the difference in frequency between radar waves that illuminate and are reflected from electromagnetically nonlinear targets. Harmonic radar is a special type of nonlinear radar that transmits one or multiple frequencies and listens for frequencies at or near their harmonics. Nonlinear radar differs from traditional linear radar by offering high clutter rejection and is particularly suited to the detection of devices containing metals and semiconductors. Examples include tags for tracking insects, tags worn by humans for avoiding collisions with vehicles, or for monitoring vital signs. Such tags contain a radio-frequency (RF) nonlinearity, often a Schottky diode, connected to a suitable antenna. Targets with inherent nonlinearities, such as metal contacts, semiconductors, transmission lines, antennas, filters, and ferroelectrics, also respond to nonlinear radar. In this paper, the successful exploitation of harmonic radar for moving target imaging and synthetic aperture imaging of targets, while suppressing clutter signals from linear targets, are presented. Our results demonstrate some unique advantages of harmonic radar over its traditional linear counterpart.

The nonlinear radar testing on the reliability of the objects’ identification

The nonlinear radar testing using a single wideband simulator of the laying device is considered. It is shown that the simulator on the basis of the flat two-armed spiral antennas with semiconductor diode in the load has a thermal sensitivity to electromagnetic field. The energy dissipation of the probe signal leads to deformation of the current-voltage characteristics (CVC) of the diode composed of the simulator. Parameter $\gamma$ is introduced as the degree of closeness to the equilibrium state of the diode. It characterizes the degree of CVC deformation during sensing. According to the experimental data normalized approximating function (NAF) depending on the parameter $\gamma$ is obtained. The phenomenon of a lens identification attribute inversion using NAF is investigated. It consists in changing the ratio between the levels of second and third harmonics response signal to the opposite value. The acquisition of unmasking properties of the structure "metal-oxide-metal" by simulator is identified. The maximum frequency distribution of the unmasking sign inversion occurrence depending on the normalized voltage value of operating point displacement of the diode composed of the simulator is received. It is suggested to capture the unmasking sign inversion of the simulator to judge the reliability of embedded devices detection and identification by the ability of nonlinear radar.