Non-intrusive soil carbon content quantification methods using machine learning algorithms:A comparison of microwave and millimeter wave radar sensors

Agricultural and forestry biomass can be converted to biochar through pyrolysis gasification,making it a significant carbon source for soil.Applying biochar to soil is a carbon-negative process that helps combat climate change,sustain soil biodiversity,and regulate water cycling.However,quantifying soil carbon content conventionally is time-consuming,labor-intensive,imprecise,and expensive,making it difficult to accurately measure in-field soil carbon’s effect on storage water and nutrients.To address this challenge,this paper for the first time,reports on extensive lab tests demonstrating non-intrusive methods for sensing soil carbon and related smart biochar applications,such as differentiating between biochar types from various biomass feedstock species,monitoring soil moisture,and biochar water retention capacity using portable microwave and millimeter wave sensors,and machine learning.These methods can be scaled up by deploying the sensor in-field on a mobility platform,either ground or aerial.The paper provides details on the materials,methods,machine learning workflow,and results of our investigations.The significance of this work lays the foundation for assessing carbon-negative technology applications,such as soil carbon content accounting.We validated our quantification method using supervised machine learning algorithms by collecting real soil mixed with known biochar contents in the field.The results show that the millimeter wave sensor achieves high sensing accuracy(up to 100%)with proper classifiers selected and outperforms the microwave sensor by approximately 10%–15%accuracy in sensing soil carbon content.

Microwave method based on curve fitting method for high-precision collision frequency diagnosis

In this work,the results of plasma microwave transmission diagnosis were analyzed.According to the attenuation and phase shift of the electromagnetic wave propagating in the plasma,the electron density and collision frequency of the plasma can be diagnosed.Since part of the electromagnetic wave is reflected or diffracted when propagating in the plasma,and is not absorbed by the plasma,and this part of the attenuation is still included in the measured attenuation,the attenuation is distorted.Therefore,a curve fitting method is proposed to remove the attenuation caused by the plasma reflection,thereby improving the accuracy of the diagnosis of the collision frequency.The calibration effect of this method on plasmas with different electron densities and collision frequencies is analyzed,and a diagnostic frequency band with good calibration results is given.The curve fitting method is verified by experiment and simulation.After adopting the newly proposed method,the diagnosis accuracy of collision frequency can be increased by 30%.This method can be widely used in various types of plasma diagnosis and provides a new idea for plasma diagnosis.

Multifunctional MXene/C Aerogels for Enhanced Microwave Absorption and Thermal Insulation

Two-dimensional transition metal carbides and nitrides(MXene)have emerged as promising candidates for microwave absorption(MA)materials.However,they also have some drawbacks,such as poor impedance matching,high self-stacking tendency,and high density.To tackle these challenges,MXene nanosheets were incorporated into polyacrylonitrile(PAN)nanofibers and subsequently assembled into a three-dimensional(3D)network structure through PAN carbonization,yielding MXene/C aerogels.The 3D network effectively extends the path of microcurrent transmission,leading to enhanced conductive loss of electromagnetic(EM)waves.Moreover,the aerogel’s rich pore structure significantly improves the impedance matching while effectively reducing the density of the MXenebased absorbers.EM parameter analysis shows that the MXene/C aerogels exhibit a minimum reflection loss(RL_(min))value of−53.02 dB(f=4.44 GHz,t=3.8 mm),and an effective absorption bandwidth(EAB)of 5.3 GHz(t=2.4 mm,7.44–12.72 GHz).Radar cross-sectional(RCS)simulations were employed to assess the radar stealth effect of the aerogels,revealing that the maximum RCS reduction value of the perfect electric conductor covered by the MXene/C aerogel reaches 12.02 dB m^(2).In addition to the MA performance,the MXene/C aerogel also demonstrates good thermal insulation performance,and a 5-mm-thick aerogel can generate a temperature gradient of over 30℃ at 82℃.This study provides a feasible design approach for creating lightweight,efficient,and multifunctional MXene-based MA materials.

Microwave Photonics for Modern Radar Systems

The emerging new concepts and technologies based on microwave photonics have led to an ever-increasing interest in developing innovative radar systems with a net gain in functionality,bandwidth /resolution,size,mass,complexity and cost when compared with the traditional implementations. This paper describes the techniques developed in the last few years in microwave photonics that might revolutionize the way to design multifunction radar systems,with an emphasis on the recent advances in optoelectronic oscillators( OEOs),arbitrary waveform generation,photonic mixing,phase coding,filtering,beamforming,analog-to-digital conversion,and stable radio-frequency signal transfer. Challenges in implementation of these components and subsystems for meeting the technique requirements of the multifunction radar applications are discussed.

Numerical simulation and inversion of offshore area depth based on x-band microwave radar

A detection method of offshore area depth utilizing the x-band microwave radar is proposed. The method is based on the sea clutter imaging mechanism of microwave radar, and combined with dispersion equation of the liner wave theorem and least square method （LSM）, consequently get the inversion results of water depth in the detected region. The wave monitoring system OSMAR-X exploited by the Ocean State Laborato-ry, Wuhan University, based on a microwave radar has proven to be a powerful tool to monitor ocean waves in time and space. Numerical simulation and inversion of offshore area depth are carried out here; since JONSWAP model can give description of stormy waves in different growth phase, it is suitable for simulation. Besides, some results from measured data detected by OSMAR-X x-band radar located at Longhai of Fujian Province, China, validates this method. The tendency of the average water depths inferred from the radar images is in good agreement with the tide level detected by Xiamen tide station. These promising results suggest the possibility of using OSMAR-X to monitor operationally morphodynamics in coastal zones. This method can be applied to both shore-based and shipborne x-band microwave radar.

An approach to a non-contact vital sign monitoring using dual-frequency microwave radars for elderly care

This study aimed to produce a prototype system for non-contact vital sign monitoring of the elderly using microwave radar with the intention of reducing the burdens on monitored individuals and nursing caregivers. In addition, we tested the ability of the proposed prototype system to measure the respiratory and heart rates of the elderly in a nursing home and discussed the systems effectiveness and problems by examining results of real-time monitoring. The prototype system consisted of two 24-GHz microwave radar antennas and an analysis system. The antennas were positioned below a mattress to monitor motion on the body surface for measuring cardiac and respiratory rates from the dorsal side of the subjects (23.3 ± 1.2 years) who would be lying on the mattress. The heart rates determined by the prototype system correlated significantly with those measured by electrocardiography (r = 0.92). Similarly, the respiratory rates determined by the prototype correlated with those obtained from respiration curves (r = 0.94). Next, we investigated the effectiveness of the prototype system with 7 elderly patients (93.3 ± 10.56 years) at a nursing home. The proposed system appears to be a promising tool for monitoring the vital signs of the elderly in a way that alleviates the need to attach electrodes overnight to confirm patient safety.

Noncontact Monitoring of Relative Changes in Blood Pressure Using Microwave Radar Sensors

This study aims to confirm whether noncontact monitoring of relative changes in blood pressure can be estimated using microwave radar sensors. First, an equation to estimate blood pressure was derived, after which, the effectiveness of the estimation equation was confirmed using data obtained by a noncontact method while inducing variations in blood pressure. We considered that the Bramwell-Hill equation, which contains some parameters that directly indicate changes in blood pressure, would be an appropriate reference to construct an estimation equation for the noncontact method, because measurements using microwave radar sensors can measure minute scale motion on the skin surface induced by the pulsation of blood vessels. In order to estimate relative changes in blood pressure, we considered a simple equation including the pulse transit time (PTT), amplitude of signals and body dimensions as parameters. To verify the effectiveness of the equation for estimating changes in blood pressure, two experiments were conducted: a cycling task using an ergometer, which induces blood pressure fluctuations because of changes in cardiac output, and a task using the Valsalva maneuver, which induces blood pressure fluctuations because of changes in vascular resistance. The results obtained from the two experiments suggested that the proposed equation using microwave radar sensors can accurately estimate relative changes of blood pressure. In particular, relatively favorable results were obtained for the changes in blood pressure induced by the changes in cardiac volume. Although many issues remain, this method could be expected to contribute to the continuous evaluation of cardiac function while reducing the burden on patients.

Environmentally Friendly and Multifunctional Shaddock Peel-Based Carbon Aerogel for Thermal-Insulation and Microwave Absorption

Eco-friendly electromagnetic wave absorbing materials with excellent thermal infrared stealth property,heat-insulating ability and compression resistance are highly attractive in practical applications.Meeting the aforesaid requirements simultaneously is a formidable challenge.Herein,ultra-light carbon aerogels were fabricated via fresh shaddock peel by facile freeze-drying method and calcination process,forming porous network architecture.With the heating platform temperature of 70℃,the upper surface temperatures of the as-prepared carbon aerogel present a slow upward trend.The color of the sample surface in thermal infrared images is similar to that of the surroundings.With the maximum compressive stress of 2.435 kPa,the carbon aerogels can provide favorable endurance.The shaddock peel-based carbon aerogels possess the minimum reflection loss value(RLmin)of−29.50 dB in X band.Meanwhile,the effective absorption bandwidth covers 5.80 GHz at a relatively thin thickness of only 1.7 mm.With the detection theta of 0°,the maximum radar cross-sectional(RCS)reduction values of 16.28 dB m^(2) can be achieved.Theoretical simulations of RCS have aroused extensive interest owing to their ingenious design and time-saving feature.This work paves the way for preparing multi-functional microwave absorbers derived from biomass raw materials under the guidance of RCS simulations.

Ultrabroad Microwave Absorption Ability and Infrared Stealth Property of Nano-Micro CuS@rGO Lightweight Aerogels

Developing ultrabroad radar-infrared compatible stealth materials has turned into a research hotspot,which is still a problem to be solved.Herein,the copper sulfide wrapped by reduced graphene oxide to obtain three-dimensional(3D)porous network composite aerogels(CuS@rGO)were synthesized via thermal reduction ways(hydrothermal,ascorbic acid reduction)and freeze-drying strategy.It was discovered that the phase components(rGO and CuS phases)and micro/nano structure(microporous and nanosheet)were well-modified by modulating the additive amounts of CuS and changing the reduction ways,which resulted in the variation of the pore structure,defects,complex permittivity,microwave absorption,radar cross section(RCS)reduction value and infrared(IR)emissivity.Notably,the obtained CuS@rGO aerogels with a single dielectric loss type can achieve an ultrabroad bandwidth of 8.44 GHz at 2.8 mm with the low filler content of 6 wt%by a hydrothermal method.Besides,the composite aerogel via the ascorbic acid reduction realizes the minimum reflection loss(RL_(min))of−60.3 dB with the lower filler content of 2 wt%.The RCS reduction value can reach 53.3 dB m^(2),which effectively reduces the probability of the target being detected by the radar detector.Furthermore,the laminated porous architecture and multicomponent endowed composite aerogels with thermal insulation and IR stealth versatility.Thus,this work offers a facile method to design and develop porous rGO-based composite aerogel absorbers with radar-IR compatible stealth.

Ultralight Magnetic and Dielectric Aerogels Achieved by Metal-Organic Framework Initiated Gelation of Graphene Oxide for Enhanced Microwave Absorption

The development of a convenient methodology for synthesizing the hierarchically porous aerogels comprising metal–organic frameworks(MOFs)and graphene oxide(GO)building blocks that exhibit an ultralow density and uniformly distributed MOFs on GO sheets is important for various applications.Herein,we report a facile route for synthesizing MOF/reduced GO(rGO)aerogels based on the gelation of GO,which is directly initiated using MOF crystals.Free metal ions exposed on the surface of MIL-88A nanorods act as linkers that bind GO nanosheets to a three-dimensional porous network via metal–oxygen covalent or electrostatic interactions.The MOF/rGOderived magnetic and dielectric aerogels Fe_(3)O_(4)@C/rGO and Ni-doped Fe_(3)O_(4)@C/rGO show notable microwave absorption(MA)performance,simultaneously achieving strong absorption and broad bandwidth at low thickness of 2.5(-58.1 dB and 6.48 GHz)and 2.8 mm(-46.2 dB and 7.92 GHz)with ultralow filling contents of 0.7 and 0.6 wt%,respectively.The microwave attenuation ability of the prepared aerogels is further confirmed via a radar cross-sectional simulation,which is attributed to the synergistic effects of their hierarchically porous structures and heterointerface engineering.This work provides an effective pathway for fabricating hierarchically porous MOF/rGO hybrid aerogels and offers magnetic and dielectric aerogels for ultralight MA.

An Overview of Active Microwave Imaging for Early Breast Cancer Detection

First,this article reviews the background of microwave imaging for early breast cancer detection,with a focus on active methods.Then active approaches,namely microwave tomography and radar-based microwave imaging,to microwave breast cancer detection are overviewed briefly,where there are recent developments in imaging algorithms as well as antennas,models,phantom and experimental systems.Lastly,we give concluding remarks and future research.In a word,the main objective of this article is to provide an overview of the principles,development,and current research status of these approaches.

Microwave characteristic simulation research for a kind of novel electromagnetic structure

A kind of novel electromagnetic structure of Cassini cross section is proposed and simulation is implemented with business microwave soft CST based on finite integral technique （FIT）. The electromagnetic field mode type of Cassini wave-guide belongs to TE, and the electromagnetic field intensity is stronger near the neck region than at other areas. For Cassini electromagnetic patches and lumped elements, the radar cross section （RCS） is smaller around 7 GHz with -30.85 dBsm, and the absorbing property is better around 13 GHz with 4.56 dBsm difference of RCS from comparing of pure medium. For novel radiation structure of Cassini cross-section patches, the electromagnetic field value is larger in the neck areas of two half patches. At last, the potential application and development of Cassini oval structure are put forward in the electromagnetic stealth technology and antennae design.

MICROWAVE SENSOR DEVELOPMENT IN RECENT TWO YEARS IN CHINA

The development of microwave sensors in recent two years in China are in troduced with an emphasis on spaceborne sensors without the applications in cluded. The microwave sensors as the main payloads to be boarded on the future operational satellites, such as FY-3 meteorological satellites and HY-2 marine satellite are introduced with much in detail. Besides these, four new sensors are outlined, i.e. the imaging radar altimeter,synthetic aperture radiometer, and polarimetric radiometer. Two recently conducted flight experiment campaigns are also introduced with results shown.

Assessment of Non-Contact Measurement Using a Microwave Sensor to Jugular Venous Pulse Monitoring

The jugular venous pulse (JVP) waveform provides an insight into right heart function, and its assessment is important in patients with heart failure. However, the conventional pulse-transducer (contact) method for monitoring this waveform is not frequently used because it requires a high degree of skill. The aim of this study was to confirm the effectiveness of a prototype non-contact system that employs microwave radar (24 GHz, 7 mW;non-contact system) for JVP measurement. Experiments were conducted on eight healthy male volunteers (21.88 ± 0.99 years). JVP measurements were compared between the conventional contact method and the proposed non-contact method. Change in JVP waveform was measured in response to an angle of reclining in five steps from the supine position to 75° of elevation. The obtained JVP measurements were similar between the two methods. Because in the non-contact method the faint pulsation of the JVP is not suppressed by the pressure of a sensor placed on the skin, the prototype microwave radar system is particularly suitable for evaluating the JVP waveform.

High-Precision Vital Signs Detection Method Based on Spectrum Refinement and Extended DCMA

In this paper,the spectral estimation algorithm is extended to the detection of human vi-tal signs by mm-wave frequency modulated continuous wave(FMCW)radar,and a comprehensive algorithm based on spectrum refinement and the extended differentiate and cross multiply al-gorithm(DCMA)has been proposed.Firstly,the improved DFT algorithm is used to accurately obtain the distance window of human body.Secondly,phase ambiguity in phase extraction is avoided based on extended DCMA algorithm.Then,the spectrum range of refinement is determ-ined according to the peak position of the spectrum,and the respiratory and heartbeat frequency information is obtained by using chirp z-transform(CZT)algorithm to perform local spectrum re-finement.For verification,this paper has simulated the radar echo signal modulated by the simu-lated cardiopulmonary signal according to the proposed algorithm.By recovering the simulated car-diopulmonary signal,the high-precision respiratory and heartbeat frequency have been obtained.The results show that the proposed algorithm can effectively restore human breathing and heart-beat signals,and the relative error of frequency estimation is basically kept below 1.5%.

微波视觉与SAR图像智能解译

高分辨率雷达成像技术和人工智能、大数据技术的快速发展,有力促进了雷达图像智能解译技术的进步。由于雷达传感器本身的特殊性和电磁散射成像物理的复杂性,雷达图像的解译缺乏光学图像的直观性,准确迅速识别分类的需求对雷达图像解译提出了迫切的挑战。在借鉴人脑光视觉感知机理和计算机视觉图像处理相关技术基础上,进一步融合电磁散射物理规律及其雷达成像机理,我们提出发展微波域雷达图像解译的“微波视觉”的新交叉领域研究。该文介绍微波视觉的概念与内涵,提出微波视觉认知模型,阐述其基础理论问题与技术路线,最后介绍了作者团队在相关问题上的初步研究进展。

UWB HPM对雷达方舱内电子设备的影响仿真分析

针对高功率微波辐照下雷达方舱内电子设备的响应和毁伤效应等实际问题,通过建立雷达方舱和舱内电缆模型进行了辐照仿真分析。结果表明,高功率微波信号在进入方舱时,方舱起到一定的滤波作用,类似带通滤波器;雷达方舱和舱内设备的反射会影响方舱内场强分布和极化,造成方舱内电场分布不均匀,各向异性降低,但尚未达到混响室程度;通过仿真概略计算了方舱内信号分布和传输特性;方舱内电缆耦合信号电压基本在同一个数量级,超宽谱信号辐照影响主要以扰乱为主。针对上述影响,提出了相应加固措施,可为雷达在高功率微波武器攻击下进行后门防护加固提供理论支撑。

雷达抗前门耦合攻击防护能力对比分析

针对雷达遭受高功率微波武器攻击时容易损伤的问题,理论分析了雷达最远防护边界和雷达天线增益及接收机限幅器能力的关系,从空间滤波的角度,对比了反射面雷达和有源相控阵雷达抗前门耦合攻击的能力。分析得出只有高功率微波武器处于反射面雷达天线的主瓣范围附近时,对其前门耦合攻击效果才强于有源相控阵雷达;反之,要弱于有源相控阵雷达。由于反射面天线雷达主波束很窄,造成攻击的时机很短,所以反射面雷达在面对高功率微波武器时防护能力更强。

微波光子信号同步及其在分布式相参雷达中的应用

分布式相参雷达技术通过多节点高性能信号同步实现跨平台的信号级相参融合,可大幅提升雷达探测、跟踪和抗干扰等能力,是雷达领域的重要发展方向之一。随着雷达频率范围的拓展、瞬时带宽的增大和搭载平台的多样化,分布式相参雷达技术对节点间信号的时间、空间、频率、相位同步性能提出了越来越高的要求,这使得传统电学信号同步技术面临巨大挑战。文中介绍了分布式相参雷达对信号时、空、频、相同步的性能要求和国内外相关技术的研究进展,重点总结了微波光子同步技术的基本原理和代表性成果;构建了微波光子分布式相参探测原理验证系统,验证了微波光子分布式相参的可行性,为推动分布式相参雷达的前沿发展与工程化应用提供了关键技术支撑。

基于薄膜铌酸锂的微波光子雷达芯片

微波光子雷达拥有分辨率高、处理速度快、同时多波段、多功能等优势,在航天系统中具有广阔的应用前景。为了满足航天应用对雷达系统体积、质量和功耗的严格要求,集成化已经成为微波光子雷达的重要发展方向。介绍了一款基于薄膜铌酸锂的微波光子雷达芯片,利用该芯片实现了雷达发射信号倍频产生和回波信号去斜接收,并对距离天线不同位置的目标进行了测距实验,得到的测量距离与实际距离基本相同,两者之间的误差小于4 mm。