Reduction of the radar cross-section(RCS) is the key to stealth technology. To improve the RCS reduction effect of the designed checkerboard metasurface and overcome the limitation of thinlayer plasma in RCS reduction...Reduction of the radar cross-section(RCS) is the key to stealth technology. To improve the RCS reduction effect of the designed checkerboard metasurface and overcome the limitation of thinlayer plasma in RCS reduction technology, a double-layer-plasma-based metasurface—composed of a checkerboard metasurface, a double-layer plasma and an air gap between them—was investigated. Based on the principle of backscattering cancellation, we designed a checkerboard metasurface composed of different artificial magnetic conductor units;the checkerboard metasurface can reflect vertically incident electromagnetic(EM) waves in four different inclined directions to reduce the RCS. Full-wave simulations confirm that the doublelayer-plasma-based metasurface can improve the RCS reduction effect of the metasurface and the plasma. This is because in a band lower than the working band of the metasurface, the RCS reduction effect is mainly improved by the plasma layer. In the working band of the metasurface,impedance mismatching between the air gap and first plasma layer and between first and second plasma layers cause the scattered waves to become more dispersed, so the propagation path of the EM waves in the plasma becomes longer, increasing the absorption of the EM waves by the plasma. Thus, the RCS reduction effect is enhanced. The double-layer-plasma-based metasurface can be insensitive to the polarization of the incoming EM waves, and can also maintain a satisfactory RCS reduction band when the incident waves are oblique.展开更多
The present paper deals with the method for the radar cross-section (RCS)computations of arbitrarily complicated targets based on the work by D. Klement et al.(1988).This method is convenient in use, fast in operatio...The present paper deals with the method for the radar cross-section (RCS)computations of arbitrarily complicated targets based on the work by D. Klement et al.(1988).This method is convenient in use, fast in operation and precise in calculating RCS of a complicatedtarget. With this method, the RCS of classic scatterers, for example, a cone and a cylinder, arecomputed with the result of good agreement with experimental data. Furthermore, the RCS’of an aircraft model at various attitudes are calculated with the result of good agreement withexperimental data also.展开更多
A new method called multi-frequency holography (MFH) for two-dimensional radar cross-section imaging of rotating objects is introduced, in which a constant coherent reference signal from transmitted signal is added in...A new method called multi-frequency holography (MFH) for two-dimensional radar cross-section imaging of rotating objects is introduced, in which a constant coherent reference signal from transmitted signal is added into received signal over certain frequency-width. With the MFH only the intensity of received composite signals needs to be measured. Both imaging situations of far field and near field are considered in details. Special restrictions about the MFH are also discussed and simulated by numerical computation. Examples of numerical simulation show that the method is effective, applicable and perspective.展开更多
An ultra-wideband 2-bit coding metasurface is designed for radar cross-section(RCS) reduction. The design process is presented in detail, in which a polarization conversion metasurface(PCM) is first proposed. The prop...An ultra-wideband 2-bit coding metasurface is designed for radar cross-section(RCS) reduction. The design process is presented in detail, in which a polarization conversion metasurface(PCM) is first proposed. The proposed PCM can realize ultra-wideband circular polarization(CP) maintaining reflection. Moreover, Pancharatnam–Berry(PB) phase will be generated in the co-polarized reflection coefficient by rotating the metallic patches in its unit cells. Thus, based on the PCM, the four coding elements of a 2-bit coding metasurface are constructed using PB phase, and an ultra-wideband PB 2-bit coding metasurface is proposed according to an appropriate coding sequence. The simulated and experimental results show that the coding metasurface has obvious advantages of wideband and polarization-insensitivity. Compared to a metallic plate of the same size, it can achieve more than 10 dB RCS reduction in the frequency band from 9.8 GHz to 42.6 GHz with a relative bandwidth of 125.2% under normal incidence with arbitrary polarizations.展开更多
This study presents an investigation of the scattering and backscattering properties of the particulates in three Chinese inland lakes(the Taihu Lake, the Chaohu Lake and the Dianchi Lake) based on in situ measurement...This study presents an investigation of the scattering and backscattering properties of the particulates in three Chinese inland lakes(the Taihu Lake, the Chaohu Lake and the Dianchi Lake) based on in situ measurements taken at 119 sites. We modeled the particulate scattering spectra using a wavelength-dependent power-law function, finding that the power-law exponents in the Taihu Lake and the Chaohu Lake differ from those in the Dianchi Lake but are similar to the values in the U.S. coastal waters. In contrast to the open ocean, the backscattering properties in the three lakes can not be determined only from chlorophyll-a concentration. The backscattering ratio spectra exhibit a wavelength dependence feature in all three lakes, generally decreasing with the increasing wavelength. Analysis results of the correlations between the backscattering ratio and the individual water quality parameters clearly show that there are distinctive relations among the three lakes, attributed primarily to different compositions of optically active materials in the three lakes. Analysis of the mass-specific scattering and backscattering coefficients shows that the coefficients at wavelength 532 nm in the Taihu Lake and Chaohu Lake are similar, but they are apparently different from those in the Dianchi Lake. Lastly, Model I multiple linear regressions were adopted to partition the mass-specific cross-sections for scattering and backscattering into organic and inorganic cross-sections to further interpret the scattering and backscattering properties. The relative contribution of organic and inorganic particulates to scattering and backscattering is clearly different among the three lakes. The scattering and backscattering properties of the particulates in the three inland lakes vary significantly based on our collected data. The results indicated that the existing semi-analytical water quality retrieval models of the Taihu Lake can not be applied perfectly to the Chaohu Lake and the Dianchi Lake.展开更多
The backscattering of light wave from arbitrarily convex dielectric objects withrough surface is investigated and formulas for calculating the backscattering cross-section of bothcoherent and incoherent fields are obt...The backscattering of light wave from arbitrarily convex dielectric objects withrough surface is investigated and formulas for calculating the backscattering cross-section of bothcoherent and incoherent fields are obtained.In the infrared wave-band,the influence of the ge-ometry,permittivity and statistical characteristics of the rough surface on LRCS is analyzed,byusing rough sphere and ellipsoids as examples.展开更多
Using experimental data reflected by the sea on specific radar cross-section (SRCS) at millimeter and centimeter waves, the approximations of the wind speed, angle of the sea surface radiation and polarization of th...Using experimental data reflected by the sea on specific radar cross-section (SRCS) at millimeter and centimeter waves, the approximations of the wind speed, angle of the sea surface radiation and polarization of the incident field can be calculated. The simulation model of the scattered signal has been proposed on the basis of the semi-Markov nested processes. For the first time it has been proved that for the description of reflections at spikes and pauses, it is possible to use finite atomic functions. The proposed model allows us to estimate the baekscatter intensity of millimeter and centimeter radio waves by the sea at grazing angle of surface radiation, as well as to simulate scattered signal.展开更多
In this paper, it is proved the ability of quantity reconstruction, amplitudes and coordinates of metallic strip local scattering sources from the backscattering pattern. They are performed as the results of numerical...In this paper, it is proved the ability of quantity reconstruction, amplitudes and coordinates of metallic strip local scattering sources from the backscattering pattern. They are performed as the results of numerical solution for the infinite perfect conducting strip in case of E-polarization of the incident plane electromagnetic wave. In this case it is necessary to fulfill the following conditions. The local sources amplitudes should be the same order, in transverse and longitudinal directions the local sources should be separated into distances more than apparatus resolution, and the object maximum size does not have to be more than approximately 50λ. It was shown the limit and ability of the further development of the offered method.展开更多
Coastal winds are strongly influenced by topology and discontinuity between land and sea surfaces. Wind assessment from remote sensing in such a complex area remains a challenge. Space-borne scatterometer does not pro...Coastal winds are strongly influenced by topology and discontinuity between land and sea surfaces. Wind assessment from remote sensing in such a complex area remains a challenge. Space-borne scatterometer does not provide any information about the coastal wind field, as the coarse spatial resolution hampers the radar backscattering. Synthetic aperture radar (SAR) with a high spatial resolution and all-weather observation abilities has become one of the most important tools for ocean wind retrieval, especially in the coastal area. Conventional methods of wind field retrieval from SAR, however, require wind direction as initial information, such as the wind direction from numerical weather prediction models (NWP), which may not match the time of SAR image acquiring. Fortunately, the polarimetric observations of SAR enable independent wind retrieval from SAR images alone. In order to accurately measure coastal wind fields, this paper proposes a new method of using co-polarization backscattering coefficients from polarimetric SAR observations up to polarimetric correlation backscattering coefficients, which are acquired from the conjugate product of co-polarization backscatter and cross-polarization backscatter. Co-polarization backscattering coefficients and polarimetric correlation backscattering coefficients are obtained form Radarsat-2 single-look complex (SLC) data.The maximum likelihood estimation is used to gain the initial results followed by the coarse spatial filtering and fine spatial filtering. Wind direction accuracy of the final inversion results is 10.67 with a wind speed accuracy of 0.32 m/s. Unlike previous methods, the methods described in this article utilize the SAR data itself to obtain the wind vectors and do not need external wind directional information. High spatial resolution and high accuracy are the most important features of the method described herein since the use of full polarimetric observations contains more information about the space measured.This article is a useful addition to the work of independent SAR wind retrieval. The experimental results herein show that it is feasible to employ the co-polarimetric backscattering coefficients and the polarimetric correlation backscattering coefficients for coastal wind field retrieval.展开更多
The calculation formulas of monostatic radar cross-section (RCS) of arbitrary re-flectors with arbitrarily polarized plane-wave incidence are derived, where the spicular field isobtained by geometrical optics (GO) and...The calculation formulas of monostatic radar cross-section (RCS) of arbitrary re-flectors with arbitrarily polarized plane-wave incidence are derived, where the spicular field isobtained by geometrical optics (GO) and the edge-diffracted field is calculated by the method ofequivalent currents (MEC). Some typical calculated results are given by means of RCS spatialgraphs. For both horizontal and vertical polarizations, the theoretical results obtained in thispaper agree very well with the experimental results as well as the results from uniform theory ofdiffraction.展开更多
Manipulation of electromagnetic waves is essential to various microwave applications,and absorbing devices composed of low-pressure gas discharge tubes and radar-absorbing materials(RAM)can bring new solutions to broa...Manipulation of electromagnetic waves is essential to various microwave applications,and absorbing devices composed of low-pressure gas discharge tubes and radar-absorbing materials(RAM)can bring new solutions to broadband electromagnetic stealth.The microwave transmission method is used to measure the physical parameters of the plasma unit.The designed structure exhibits superior absorption performance and radar cross-section(RCS)reduction capability in the 2–18 GHz band,with unique absorption advantage in the S and C frequency bands.It is found that the combination of the plasma and the RAM can significantly broaden the absorption frequency band and improve the absorption performance with excellent synergistic stealth capability.Experimental and simulation results present that broadband,wide-angle,tunable electromagnetic wave absorption and RCS reduction can be achieved by adjusting the spatial layout of the combined plasma layer and the type of RAMs,which creates opportunities for microwave transmission and selective stealth of equipment.Therefore,the wave manipulation by combined plasma array and RAM provides a valuable reference for developing numerous applications,including radar antenna stealth,spatial filter,and high power microwave shielding.展开更多
Radiative transfer models have been widely used to simulate the radar backscattering from forested areas. A three-dimensional radar backscatter model of forest canopy developed in previous studies takes full account o...Radiative transfer models have been widely used to simulate the radar backscattering from forested areas. A three-dimensional radar backscatter model of forest canopy developed in previous studies takes full account of spatial position of trees in a forest stand, and the interactions among crown, trunk and ground surface. The model predicted well for the co-polarized backscatter measurements, but underestimated the backscattering for cross-polarization, primarily because only the first-order scattering within tree crowns was considered in the model. The backscattering at cross-polarization depends strongly on multiple scatter- ing within tree crowns. To produce good estimations for cross-polarized component, the matrix-doubling method is employed here to compute multiple-scattering within the crown. The modified model is compared with the original model, and the field forest measurements and AIRSAR data are used for validation of the modified model. The cross-polarization backscattering is improved in different degrees for different crown structures and at different bands.展开更多
The development of 3D structural composites with electromagnetic(EM)wave absorption could attenuate EM waves.Herein,magnetized flower-like Cu_(9)S_(5)/ZnFe_(2)O_(4)composites were fabricated through a multistep hydrot...The development of 3D structural composites with electromagnetic(EM)wave absorption could attenuate EM waves.Herein,magnetized flower-like Cu_(9)S_(5)/ZnFe_(2)O_(4)composites were fabricated through a multistep hydrothermal method.The crystallographic and surface phase chemical information,morphological structure,and magnetic and EM parameters of the composites were analyzed.The prepared Cu_(9)S_(5)/ZnFe_(2)O_(4)composites have multiple loss paths for EM waves and present an overall 3D flower-like structure.The Cu_(9)S_(5)/ZnFe_(2)O_(4)composites exhibit a minimum reflection loss of-54.38 dB and a broad effective absorption bandwidth of 5.92 GHz.Through magnetization,ZnFe_(2)O_(4)particles are self-assembled and grown on the surfaces of Cu_(9)S_(5).Such a modification is conducive to the generation of additional cross-linking contact sites and the effective introduction of a large number of phase interfaces,crystalline defects,special three-dimensional flower-like structures,and magneto-electrical coupling loss effects.Moreover,the synergistic effect of multiple loss strategies effectively improves EM wave absorption by the material.This work can provide a strategy for the use of magnetizationmodified sulfide composite functional materials in EM wave absorption.展开更多
许多研究已表明合成孔径雷达(SAR)对水稻识别及作物长势监测很有潜力。但是,以往的研究多是采用单极化多时相SAR数据进行水稻监测的。该文本着探讨多极化方式的优势以及降低数据购买成本和减少数据处理量的目的,对单时相双极化的ENV ISA...许多研究已表明合成孔径雷达(SAR)对水稻识别及作物长势监测很有潜力。但是,以往的研究多是采用单极化多时相SAR数据进行水稻监测的。该文本着探讨多极化方式的优势以及降低数据购买成本和减少数据处理量的目的,对单时相双极化的ENV ISAT A SAR APP数据的水稻识别能力进行了评价。在水稻生长季节,获取了覆盖江苏洪泽县的A SAR APP时间序列数据。首先,分析比较不同地物的后向散射系数,选择出最能区分水稻与非水稻的单时相数据;然后,采用决策阈值法将水稻信息从图像中提取出来;最后,利用DGPS实测的样地数据对水稻识别进行精度验证。结果表明,利用水稻齐穗期至近成熟期的HH和VV极化的ENV ISAT A SAR APP图像能较好区分水稻与非水稻,水稻识别精度可达86%以上。展开更多
基金supported in part by the China Postdoctoral Science Foundation (No. 2020M673341)in part by the Natural Science Basic Research Program of Shaanxi (No.2023-JC-YB-549)+1 种基金in part by National Natural Science Foundation of China (Nos. 62371375 and 62371372)Innovation Capability Support Program of Shaanxi (No. 2022TD-37)。
文摘Reduction of the radar cross-section(RCS) is the key to stealth technology. To improve the RCS reduction effect of the designed checkerboard metasurface and overcome the limitation of thinlayer plasma in RCS reduction technology, a double-layer-plasma-based metasurface—composed of a checkerboard metasurface, a double-layer plasma and an air gap between them—was investigated. Based on the principle of backscattering cancellation, we designed a checkerboard metasurface composed of different artificial magnetic conductor units;the checkerboard metasurface can reflect vertically incident electromagnetic(EM) waves in four different inclined directions to reduce the RCS. Full-wave simulations confirm that the doublelayer-plasma-based metasurface can improve the RCS reduction effect of the metasurface and the plasma. This is because in a band lower than the working band of the metasurface, the RCS reduction effect is mainly improved by the plasma layer. In the working band of the metasurface,impedance mismatching between the air gap and first plasma layer and between first and second plasma layers cause the scattered waves to become more dispersed, so the propagation path of the EM waves in the plasma becomes longer, increasing the absorption of the EM waves by the plasma. Thus, the RCS reduction effect is enhanced. The double-layer-plasma-based metasurface can be insensitive to the polarization of the incoming EM waves, and can also maintain a satisfactory RCS reduction band when the incident waves are oblique.
文摘The present paper deals with the method for the radar cross-section (RCS)computations of arbitrarily complicated targets based on the work by D. Klement et al.(1988).This method is convenient in use, fast in operation and precise in calculating RCS of a complicatedtarget. With this method, the RCS of classic scatterers, for example, a cone and a cylinder, arecomputed with the result of good agreement with experimental data. Furthermore, the RCS’of an aircraft model at various attitudes are calculated with the result of good agreement withexperimental data also.
文摘A new method called multi-frequency holography (MFH) for two-dimensional radar cross-section imaging of rotating objects is introduced, in which a constant coherent reference signal from transmitted signal is added into received signal over certain frequency-width. With the MFH only the intensity of received composite signals needs to be measured. Both imaging situations of far field and near field are considered in details. Special restrictions about the MFH are also discussed and simulated by numerical computation. Examples of numerical simulation show that the method is effective, applicable and perspective.
基金Project supported by the National Natural Science Foundation of China (Grant No. 62072378)the Natural Science Foundation of Shaanxi Province, China (Grant No. 2019JM077)the Xi’an Science and Technology Plan Project, China (Grant No. GXYD20.4)。
文摘An ultra-wideband 2-bit coding metasurface is designed for radar cross-section(RCS) reduction. The design process is presented in detail, in which a polarization conversion metasurface(PCM) is first proposed. The proposed PCM can realize ultra-wideband circular polarization(CP) maintaining reflection. Moreover, Pancharatnam–Berry(PB) phase will be generated in the co-polarized reflection coefficient by rotating the metallic patches in its unit cells. Thus, based on the PCM, the four coding elements of a 2-bit coding metasurface are constructed using PB phase, and an ultra-wideband PB 2-bit coding metasurface is proposed according to an appropriate coding sequence. The simulated and experimental results show that the coding metasurface has obvious advantages of wideband and polarization-insensitivity. Compared to a metallic plate of the same size, it can achieve more than 10 dB RCS reduction in the frequency band from 9.8 GHz to 42.6 GHz with a relative bandwidth of 125.2% under normal incidence with arbitrary polarizations.
基金Under the auspices of National Natural Science Foundation of China(No.41171269,41101378)
文摘This study presents an investigation of the scattering and backscattering properties of the particulates in three Chinese inland lakes(the Taihu Lake, the Chaohu Lake and the Dianchi Lake) based on in situ measurements taken at 119 sites. We modeled the particulate scattering spectra using a wavelength-dependent power-law function, finding that the power-law exponents in the Taihu Lake and the Chaohu Lake differ from those in the Dianchi Lake but are similar to the values in the U.S. coastal waters. In contrast to the open ocean, the backscattering properties in the three lakes can not be determined only from chlorophyll-a concentration. The backscattering ratio spectra exhibit a wavelength dependence feature in all three lakes, generally decreasing with the increasing wavelength. Analysis results of the correlations between the backscattering ratio and the individual water quality parameters clearly show that there are distinctive relations among the three lakes, attributed primarily to different compositions of optically active materials in the three lakes. Analysis of the mass-specific scattering and backscattering coefficients shows that the coefficients at wavelength 532 nm in the Taihu Lake and Chaohu Lake are similar, but they are apparently different from those in the Dianchi Lake. Lastly, Model I multiple linear regressions were adopted to partition the mass-specific cross-sections for scattering and backscattering into organic and inorganic cross-sections to further interpret the scattering and backscattering properties. The relative contribution of organic and inorganic particulates to scattering and backscattering is clearly different among the three lakes. The scattering and backscattering properties of the particulates in the three inland lakes vary significantly based on our collected data. The results indicated that the existing semi-analytical water quality retrieval models of the Taihu Lake can not be applied perfectly to the Chaohu Lake and the Dianchi Lake.
文摘The backscattering of light wave from arbitrarily convex dielectric objects withrough surface is investigated and formulas for calculating the backscattering cross-section of bothcoherent and incoherent fields are obtained.In the infrared wave-band,the influence of the ge-ometry,permittivity and statistical characteristics of the rough surface on LRCS is analyzed,byusing rough sphere and ellipsoids as examples.
基金National Academy of Sciences of Ukraine(NASU)and Russian Foundation for Basic Research(RFBR)2012-2013(Project #12-02-90425)
文摘Using experimental data reflected by the sea on specific radar cross-section (SRCS) at millimeter and centimeter waves, the approximations of the wind speed, angle of the sea surface radiation and polarization of the incident field can be calculated. The simulation model of the scattered signal has been proposed on the basis of the semi-Markov nested processes. For the first time it has been proved that for the description of reflections at spikes and pauses, it is possible to use finite atomic functions. The proposed model allows us to estimate the baekscatter intensity of millimeter and centimeter radio waves by the sea at grazing angle of surface radiation, as well as to simulate scattered signal.
文摘In this paper, it is proved the ability of quantity reconstruction, amplitudes and coordinates of metallic strip local scattering sources from the backscattering pattern. They are performed as the results of numerical solution for the infinite perfect conducting strip in case of E-polarization of the incident plane electromagnetic wave. In this case it is necessary to fulfill the following conditions. The local sources amplitudes should be the same order, in transverse and longitudinal directions the local sources should be separated into distances more than apparatus resolution, and the object maximum size does not have to be more than approximately 50λ. It was shown the limit and ability of the further development of the offered method.
基金The National Natural Science Foundation of China under contract Nos 41306186,41076012 and 41276019the Youth Science Fund Project of State Oceanic Administration of China
文摘Coastal winds are strongly influenced by topology and discontinuity between land and sea surfaces. Wind assessment from remote sensing in such a complex area remains a challenge. Space-borne scatterometer does not provide any information about the coastal wind field, as the coarse spatial resolution hampers the radar backscattering. Synthetic aperture radar (SAR) with a high spatial resolution and all-weather observation abilities has become one of the most important tools for ocean wind retrieval, especially in the coastal area. Conventional methods of wind field retrieval from SAR, however, require wind direction as initial information, such as the wind direction from numerical weather prediction models (NWP), which may not match the time of SAR image acquiring. Fortunately, the polarimetric observations of SAR enable independent wind retrieval from SAR images alone. In order to accurately measure coastal wind fields, this paper proposes a new method of using co-polarization backscattering coefficients from polarimetric SAR observations up to polarimetric correlation backscattering coefficients, which are acquired from the conjugate product of co-polarization backscatter and cross-polarization backscatter. Co-polarization backscattering coefficients and polarimetric correlation backscattering coefficients are obtained form Radarsat-2 single-look complex (SLC) data.The maximum likelihood estimation is used to gain the initial results followed by the coarse spatial filtering and fine spatial filtering. Wind direction accuracy of the final inversion results is 10.67 with a wind speed accuracy of 0.32 m/s. Unlike previous methods, the methods described in this article utilize the SAR data itself to obtain the wind vectors and do not need external wind directional information. High spatial resolution and high accuracy are the most important features of the method described herein since the use of full polarimetric observations contains more information about the space measured.This article is a useful addition to the work of independent SAR wind retrieval. The experimental results herein show that it is feasible to employ the co-polarimetric backscattering coefficients and the polarimetric correlation backscattering coefficients for coastal wind field retrieval.
基金This work is supported by Chinese Research Institute of Electronic Science National Commission of Education, respectively
文摘The calculation formulas of monostatic radar cross-section (RCS) of arbitrary re-flectors with arbitrarily polarized plane-wave incidence are derived, where the spicular field isobtained by geometrical optics (GO) and the edge-diffracted field is calculated by the method ofequivalent currents (MEC). Some typical calculated results are given by means of RCS spatialgraphs. For both horizontal and vertical polarizations, the theoretical results obtained in thispaper agree very well with the experimental results as well as the results from uniform theory ofdiffraction.
基金supported by National Natural Science Foundation of China(No.51907198)Natural Science Research Foundation of Anhui Province(No.1908085MF205)+1 种基金Director Fund of State Key Laboratory of Pulsed Power Laser Technology(Nos.SKL2021ZR07,SKL2021ZR06)China Postdoctoral Science Foundation(No.2021MD703944).
文摘Manipulation of electromagnetic waves is essential to various microwave applications,and absorbing devices composed of low-pressure gas discharge tubes and radar-absorbing materials(RAM)can bring new solutions to broadband electromagnetic stealth.The microwave transmission method is used to measure the physical parameters of the plasma unit.The designed structure exhibits superior absorption performance and radar cross-section(RCS)reduction capability in the 2–18 GHz band,with unique absorption advantage in the S and C frequency bands.It is found that the combination of the plasma and the RAM can significantly broaden the absorption frequency band and improve the absorption performance with excellent synergistic stealth capability.Experimental and simulation results present that broadband,wide-angle,tunable electromagnetic wave absorption and RCS reduction can be achieved by adjusting the spatial layout of the combined plasma layer and the type of RAMs,which creates opportunities for microwave transmission and selective stealth of equipment.Therefore,the wave manipulation by combined plasma array and RAM provides a valuable reference for developing numerous applications,including radar antenna stealth,spatial filter,and high power microwave shielding.
基金supported by National Basic Research Program of China (Grant No. 2007CB714404)Hi-tech Research and Development Program of China (Grant No. 2006AA12Z114)National Natural Science Foundation of China (Grant Nos. 40701124, 40734025)
文摘Radiative transfer models have been widely used to simulate the radar backscattering from forested areas. A three-dimensional radar backscatter model of forest canopy developed in previous studies takes full account of spatial position of trees in a forest stand, and the interactions among crown, trunk and ground surface. The model predicted well for the co-polarized backscatter measurements, but underestimated the backscattering for cross-polarization, primarily because only the first-order scattering within tree crowns was considered in the model. The backscattering at cross-polarization depends strongly on multiple scatter- ing within tree crowns. To produce good estimations for cross-polarized component, the matrix-doubling method is employed here to compute multiple-scattering within the crown. The modified model is compared with the original model, and the field forest measurements and AIRSAR data are used for validation of the modified model. The cross-polarization backscattering is improved in different degrees for different crown structures and at different bands.
基金This work was supported by the National Natural Science Foundation of China(No.51477002)the University Synergy Innovation Program of Anhui Province,China(No.GXXT-2019-028).
文摘The development of 3D structural composites with electromagnetic(EM)wave absorption could attenuate EM waves.Herein,magnetized flower-like Cu_(9)S_(5)/ZnFe_(2)O_(4)composites were fabricated through a multistep hydrothermal method.The crystallographic and surface phase chemical information,morphological structure,and magnetic and EM parameters of the composites were analyzed.The prepared Cu_(9)S_(5)/ZnFe_(2)O_(4)composites have multiple loss paths for EM waves and present an overall 3D flower-like structure.The Cu_(9)S_(5)/ZnFe_(2)O_(4)composites exhibit a minimum reflection loss of-54.38 dB and a broad effective absorption bandwidth of 5.92 GHz.Through magnetization,ZnFe_(2)O_(4)particles are self-assembled and grown on the surfaces of Cu_(9)S_(5).Such a modification is conducive to the generation of additional cross-linking contact sites and the effective introduction of a large number of phase interfaces,crystalline defects,special three-dimensional flower-like structures,and magneto-electrical coupling loss effects.Moreover,the synergistic effect of multiple loss strategies effectively improves EM wave absorption by the material.This work can provide a strategy for the use of magnetizationmodified sulfide composite functional materials in EM wave absorption.
文摘许多研究已表明合成孔径雷达(SAR)对水稻识别及作物长势监测很有潜力。但是,以往的研究多是采用单极化多时相SAR数据进行水稻监测的。该文本着探讨多极化方式的优势以及降低数据购买成本和减少数据处理量的目的,对单时相双极化的ENV ISAT A SAR APP数据的水稻识别能力进行了评价。在水稻生长季节,获取了覆盖江苏洪泽县的A SAR APP时间序列数据。首先,分析比较不同地物的后向散射系数,选择出最能区分水稻与非水稻的单时相数据;然后,采用决策阈值法将水稻信息从图像中提取出来;最后,利用DGPS实测的样地数据对水稻识别进行精度验证。结果表明,利用水稻齐穗期至近成熟期的HH和VV极化的ENV ISAT A SAR APP图像能较好区分水稻与非水稻,水稻识别精度可达86%以上。