Scattering and absorption characteristics of non-spherical cirrus cloud ice crystal particles in terahertz frequency band

We used discrete dipole approximation(DDA)to examine the scattering and absorption characteristics of spherical ice crystal particles.On this basis,we studied the scattering characteristics of spherical ice crystal particles at different frequencies and non-spherical ice crystal particles with different shapes,aspect ratios,and spatial orientations.The results indicate that the DDA and Mie methods yield almost the same results for spherical ice crystal particles,illustrating the superior calculation accuracy of the DDA method.Compared with the millimeter wave band,the terahertz band particles have richer scattering characteristics and can detect ice crystal particles more easily.Different frequencies,shapes,aspect ratios,and spatial orientations have specific effects on the scattering and absorption characteristics o f ice crystal particles.The results provide an important theoretical basis for the design of terahertz cloud radars and related cirrus detection methods.

The microwave scattering characteristics of sea ice in the Bohai Sea

Microwave remote sensing has become the primary means for sea-ice research, and has been supported by a great deal of field experiments and theoretical studies regarding sea-ice microwave scattering. However, these studies have been barely carried in the Bohai Sea. The sea-ice microwave scattering mechanism was first developed for the thin sea ice with slight roughness in the Bohai Sea in the winter of 2012, and included the backscattering coefficients which were measured on the different conditions of three bands（L, C and X）, two polarizations（HH and VV）, and incident angles of 20° to 60°, using a ground-based scatterometer and the synchronous physical parameters of the sea-ice temperature, density, thickness, salinity, and so on. The theoretical model of the sea-ice electromagnetic scattering is obtained based on these physical parameters. The research regarding the sea-ice microwave scattering mechanism is carried out through two means, which includes the comparison between the field microwave scattering data and the simulation results of the theoretical model, as well as the feature analysis of the four components of the sea-ice electromagnetic scattering. It is revealed that the sea-ice microwave scattering data and the theoretical simulation results vary in the same trend with the incident angles. Also, there is a visible variant in the sensitivity of every component to the different bands.For example, the C and X bands are sensitive to the top surface, the X band is sensitive to the scatterers, and the L and C bands are sensitive to the bottom surface, and so on. It is suggested that the features of the sea-ice surfaces and scatterers can be retrieved by the further research in the future. This experiment can provide an experimental and theoretical foundation for research regarding the sea-ice microwave scattering characteristics in the Bohai Sea.

Experimental study of C-band microwave scattering characteristics during the emulsification process of oil spills

In this study,oil spill experiments were performed in a water tank to determine changes in the surface scattering characteristics during the emulsification of oil spills.A C-band fully-polarimetric microwave scatterometer and a vector network analyzer were used to observe films of the following oils:crude oil with an asphalt content below3%that is prone to emulsification(type A),fresh crude oil extracted from an oilfield(type B),and industrial crude oil that was dehydrated and purified(type C).The difference in the backscatter results between the emulsified oil film and the calm water surface under C-band microwaves and the influence of the emulsification of the oil film on the backscatter were analyzed in detail.The results demonstrate that under a low-wind and no-waves condition(the maximum wave height was below than 3 mm),the emulsification of crude oil could modulated the backscatter through changes in the surface roughness and the dielectric constant,where the surface roughness had the dominant effect.The surface backscatters of the type B oil were greater than that of the type C oil in both the emulsified and non-emulsified states.In the non-emulsified state,the average differences in the backscatter between the type B and C oils were 2.19 dB,2.63 dB,and 2.21 dB for the polarization modes of VV,HH,and HV/VH,respectively.Smaller corresponding average differences of 0.98 dB,1.49 dB,and 1.5 dB were found for the emulsified state with a 20%moisture constant for the oil film.The results demonstrated that the surface roughness of the different oil films could vary due to the differences in the oil compositions and the oil film properties,which in turn affect the backscatter of the oil film surface.

LOW RCS MODIFIED DESIGN OF EXTERNAL CARRIAGE AND ANALYSIS OF ITS SCATTERING CHARACTERISTIC

An external carriage and a wing have a high radar cross section (RCS) level in a wide range of angles of pitch because they form an efficient corner reflector. Coupling scattering between the external carriage and the wing is controlled by designing a saw tooth structure of the external carriage on condition that the wing structure is not changed. A low RCS saw tooth scheme of the external carriage is given in the paper. The ray tracing method and the physical theory of diffraction are used to calculate the radar scattering characteristics of low RCS structures of the external carriage and wing. The results show that the saw tooth scheme of the external carriage is feasible and effective for low RCS design.

Distribution characteristic of scattering field for an ellipsoidal target irradiated by an electromagnetic wave from an arbitrary direction

It is of great importance for engineering applications to obtain the expression of scattering field for an ellipsoidal target irradiated by an electromagnetic wave from an arbitrary direction. Literature relevant to this problem is seldom found. In this paper, the scattering field for an ellipsoidal target is presented by utilizing the scale transformation of electromagnetic field and the rotation of coordinate system, with an electromagnetic wave projecting on the target from an arbitrary direction. The obtained result is in good agreement with the solution available from the literature if we consider the scale factors to be unity. Taking a conducting ellipsoidal target for sample, we perform the partial simulations of the ellipsoidal model and a plant leaf model by choosing different scale factors. The obtained results show that the distribution characteristic of scattering field is sensitively affected by the polarization of the incident wave and varies not much with the incident wave angle but changes with the observation point. At some points the scattering energy arrives at its maximum.

REBUILDING THE SEA WAVE HEIGHT SPECTRUM BY USING MICROWAVE SCATTERING CHARACTERISTICS

The microwave back-scattering characteristics are numerically analysed by the elec-tromagnetic scattering theory,and the sea wave height spectrum that reflects the wind field isrebuilt by comparing the calculated data with experimental data which were measured by airbornescatter.According to the wave height spectrum,the relationship between the back-scattering co-efficients and wind speeds can be obtained.Once the back-scattering coefficients of the sea surfaceare measured,then the wind speeds over the sea surface can approximately be determined.It isuseful for the microwave remote sensing in ocean.

Multilevel Characteristic Basis Function Method with ACA for Accelerated Solution of Electrically Large Scattering Problems

The multilevel characteristic basis function method(MLCBFM)with the adaptive cross approximation(ACA)algorithm for accelerated solution of electrically large scattering problems is studied in this paper.In the conventional MLCBFM based on Foldy-Lax multiple scattering equations,the improvement is only made in the generation of characteristic basis functions(CBFs).However,it does not provide a change in impedance matrix filling and reducing matrix calculation procedure,which is time-consuming.In reality,all the impedance and reduced matrix of each level of the MLCBFM have low-rank property and can be calculated efficiently.Therefore,ACA is used for the efficient generation of two-level CBFs and the fast calculation of reduced matrix in this study.Numerical results are given to demonstrate the accuracy and efficiency of the method.

Scattering Characteristics of Metallic Plates in Radial Waveguide

Mode matching is a useful method to analyze the mode characteristics during the section change of a waveguide. By applying this method to the radial waveguide, and solving specific problems due to the radial transmission of electromagnetic wave, this presents a calculating method in radial waveguide based on mode matching. The scattering characteristics of metallic plates in the radial waveguide are calculated, and verified qualitatively using an electromagnetic simulator, which confirmed the validity of this method.

Polarization characteristics and controllability mechanism of passive scattering elements

Polarization feature is one of the important features of radar targets,which has been used in many fields.In this paper,the grid models of some typical foreign moving targets are constructed on the simulation platform,such as glider,cruiser,fixed wing aircraft,and rotorcraft.The electromagnetic scattering characteristics of the moving platforms under the incidence of circular polarization waves are calculated.The typical polarization characteristics which the orthogonal and in-phase components have in the echoes are analyzed and proved.Based on the polarization scattering matrix(PSM)theory,from the point of view of the physical reproduction,the technical status quo that the existing technical approaches are difficult to realize the passive simulation of polarization characteristic of the target is summarized.To solve this problem,combined with the vector synthesis law,the realization mechanism of controllable polarization characteristic of target echoes is proposed,the analytical expressions of polarization control matrix and polarization ratio are deduced,and the controllability of polarization ratio feature in the case of circular polarization is verified by simulation calculation.

Extraction of Echo Characteristics of Underwater Target Based on Cepstrum Method

The analysis and characteristic extraction of target echo characteristics are important in underwater target detection and recognition. Rigid acoustic scattering components are generally used as major echo contributors with relatively stable characteristic information. Previous studies focus on echo characteristics from a single angle, thereby limiting the amount of extracted characteristic information. This paper aims to establish a full-angle rigid echo components model and overcome the difficulty of the extraction of time delay characteristics of narrow-band acoustic scattering echoes. On the basis of the analysis of the target echo highlight model, the echo characteristics of rigid acoustic scattering components are extracted in the cepstrum domain, and a wavelet process is proposed to enhance the effect of time delay estimation. Experimental data indicate that the extracted time delay characteristics accord with the rigid echo characteristics of underwater target, thereby validating the effectiveness of the cepstrum method.

Bubble acoustical scattering cross section under multi-frequency acoustic excitation

The acoustical scattering cross section is usually employed to evaluate the scattering ability of the bubbles when they are excited by the incident acoustic waves. This parameter is strongly related to many important applications of performance prediction for search sonar or underwater telemetry, acoustical oceanography, acoustic cavitation, volcanology, and medical and industrial ultrasound. In the present paper, both the analytical and numerical analysis results of the acoustical scattering cross section of a single bubble under multi-frequency excitation are obtained. The nonlinear characteristics（e.g.,harmonics, subharmonics, and ultraharmonics） of the scattering cross section curve under multi-frequency excitation are investigated compared with single-frequency excitation. The influence of several paramount parameters（e.g., bubble equilibrium radius, acoustic pressure amplitude, and acoustic frequencies） in the multi-frequency system on the predictions of scattering cross section is discussed. It is shown that the combination resonances become significant in the multi-frequency system when the acoustic power is big enough, and the acoustical scattering cross section is promoted significantly within a much broader range of bubble sizes and acoustic frequencies due to the generation of more resonances.

Elastic Characteristic Extraction Method of Underwater Target Based on Adaptive Filtering

In this paper, a kind of elastic characteristic extraction method of underwater targets based on adaptive filtering is introduced. The kernel of the method is the recursive least square （RLS） algorithm. Firstly, the geometric scattering signal is fit by using the echo signal and the incident signal. Then, the frequency spectrum of the echo signal and the geometric scattering signal are calculated. At last, the frequency spectrum of the elastic scattering signal is obtained. The research of the simulation and lake experiment is carried on. As the results show, the formants of the elastic signal frequency spectrum can be precisely extracted by the algorithm, and the extraction algorithm can apply to extracting the elastic characteristic from the echo signal in a real underwater acoustics environment.

Evolution Characteristics of a Local Strong Hailstorm in Shandong Peninsula

On May 28,2018,a strong hailstorm accompanying gale(with speed of 17 m/s)and hail(with the maximum diameter of 42 mm)occurred in Yantai in Shandong Peninsula.Based on the conventional weather observation,Doppler weather radar,and automatic weather station data,the evolution characteristics of the strong hailstorm in Shandong Peninsula were analyzed.The results show that this multi-cell strong storm occurred under the background of high-altitude cold vortex.The strong cold advection at the middle and high altitudes and the weak warm advection at the low altitude formed a strong unstable stratification,and also led to cooling at the middle altitudes at the same time.The height of 0 and-20℃dropped significantly,which provided better environmental conditions for the growth of hail in the air.The lower temperature 0℃also ensured less melting of hail during falling,and the middle dry layer was conducive to the formation of thunderstorm gale.The vertical wind shear above medium intensity was the development and maintenance mechanism of thunderstorm cells gradually evolving into a strong hailstorm.From the analysis of radar echo characteristics,it is found that it was a strong hailstorm weather process caused by the linear multi-cell storm.The thunderstorm cells that could produce large hail occurred and developed,and the whole process lasted for 2 h under the triggering of the convergence line formed by the interaction of the sea breeze front and ground cold front.It had obvious characteristics of hail such as three-body scattering spikes,side-lobe echoes,high-hanging strong echoes,as well as the characteristics of thunderstorm gale such as arcuate echoes,middle-level radial convergence,and back side inflow.The storm development process was accompanied by backward propagation,but the long axis of the line storm was basically perpendicular to the movement direction of the storm,so there was no short-term heavy precipitation.The stratification instability conditions,water vapor conditions and ground convergence conditions in the eastern part of the peninsula were significantly worse than those in the western part.As a result,the thunderstorm cells first appeared in Laizhou,and then developed into the linear multi-cell storm in the process of eastward movement.Finally,it was rapidly weakened and disappeared after passing through Yantai.

Analysis of echo signal modulation characteristic parameters on aerial and space targets

Based on the scattering characteristic,the comparison of RCS(radar cross-section)at different positions of a target in the same direction of incidence can be obtained first by extruding or deleting part of the entity.A simulation method of aerial&space targets echo characteristics(A&STEC)is proposed that is universal to aerial and space targets.We utilize a fixed-wing UAV(unmanned aerial vehicle)and typical missiles in simulation.The echo signal modulation characteristic parameters are calculated theoretically by the atmospheric attenuation model,the finite element method and a MUMPS solver.The verification simulations show that this method can analyze the influence of the target shape,incident direction,detection position and detection frequency on echo waveform,intensity and energy distribution.The results show that the profile of echo waveform can invert the general shape of the target.The relationship between time and intensity can determine whether the target is moving towards or away from the detector in addition.These conclusions can provide a reference for the ballistic missile target tracking and the defense against UVA intrusion in theory.

Influence of aircraft surface distribution on electromagnetic scattering characteristics

In this paper,two typical stealth aircraft concepts（wing fuselage blended and flyingwing） were designed.Then three gradually changed surface distribution models with the same planform for each concept were created.Based on the multilevel fast multipole algorithm（MLFMA）,the vertical polarization transmitting/vertical polarization receiving（VV） and horizontal polarization transmitting/horizontal polarization receiving（HH） radar cross section（RCS） characteristics were simulated with five frequencies between 0.1 and 1.0 GHz.The influences and mechanisms of aircraft surface distribution on electromagnetic scattering characteristics were investigated.The results show that for the wing fuselage blended concept,the W RCS of this frequency range is higher than the HH RCS in most cases,while it is just the opposite for the flying-wing concept.As for the two aircraft concepts,the RCS levels of HH and W both decrease with the frequency increasing,but the HH RCS has a faster downward trend.The surface distribution has little influence on HH RCS characteristics.On the contrary,it has a significant impact on W RCS characteristics,and the amplitude of the VV RCS increases with the surface thickness.

Special characteristics of fluorescence and resonance Rayleigh scattering for cadmium telluride nanocrystal aqueous solution and its interactions with aminoglycoside antibiotics

CdTe nanocrystals(CdTe NCs) were achieved by reaction of CdCl2 with KHTe solution and were capped with sodium mercaptoacetate.The product was detected by transmission electron microscopy(TEM),high-resolution transmission electron microscopy(HRTEM),energy dispersive spectroscopy(EDS),fluorescence spectra,ultraviolet-visible spectra and X-ray diffraction(XRD).The CdTe NCs are of cubic structure and the average size is about 5 nm.The fluorescence quantum yield of CdTe NCs aqueous solution increased from 37% to 97% after 20 d under room light.The maximum λem of fluorescence changed from 543 nm to 510 nm and the blue shift was 33 nm.CdTe NCs aqueous solution can be steady for at least 10 months at 4℃ in a refrigerator.The resonance Rayleigh scattering(RRS) of CdTe NCs in the aqueous solution was investigated.The maximum scattering peak was located at about 554 nm.The interactions of CdTe NCs with amikacin sulfate(AS) and micronomicin sulfate(MS) were investigated respectively.The effects of AS and MS on fluorescence and RRS of CdTe NCs were analyzed.It was found that AS and MS quenched the photoluminescence of CdTe NCs and enhanced RRS of CdTe NCs.Under optimum conditions,there are linear relationships between quenching intensity(F0-F),intensity of RRS(I-I0) and concentration of AS and MS.The detection limits(3б) of AS and MS are respectively 3.4 ng·mL-1 and 2.6 ng·mL-1 by the fluorescence quenching method,and 15.2 ng·mL-1 and 14.0 ng·mL-1 by the RRS method.The methods have high sensitivity,thus CdTe NCs may be used as fluorescence probes and RRS probes for the detection of aminoglycoside antibiotics.

An interpolating reproducing kernel particle method for two-dimensional scatter points

An interpolating reproducing kernel particle method for two-dimensional （2D） scatter points is introduced. It elim- inates the dependency of gridding in numerical calculations. The interpolating shape function in the interpolating repro- ducing kernel particle method satisfies the property of the Kronecker delta function. This method offers a mathematics basis for recognition technology and simulation analysis, which can be expressed as simultaneous differential equations in science or project problems. Mathematical examples are given to show the validity of the interpolating reproducing kernel particle method.

Raman scattering enhancement characteristic of NbCl_5-and Nb_20_5-doped silica fibers

Two kinds of Nb-doped silica fibers, an NbCl5-doped fiber and an Nb205-doped fiber, are fabricated and char- acterized in this Letter. First, the refractive index profiles of both fibers are obtained, and then their Raman spectra are measured with 785 nm exciting light. The Nb-doped fibers＇ Raman spectra are compared with a conventional GeO2-doped single-mode silica fiber that is prepared with the same method and under the same conditions. As a result, the Raman gain coefficients of the Nb-doped silica fiber core are obtained. The exper- imental results show that Nb2O5 doping can enhance the Raman scattering intensity of the optical fibers.

Doppler shift of a laser pulse beam scattered by a rotating cone and cylinder

Based on laser radar equations, a Doppler shift model of a laser pulse beam scattered by a rotating arbitrary convex target is reported in this paper. The boundary relations between an incident pulse beam and the detected area elements are analyzed by geometric methods. The Doppler shift characteristics of the rotating cone and cylinder are discussed and the difference between the laser pulse beam and the plane wave scattered from the same rotating target is compared accordingly. Numerical simulations show that the Doppler shift is tightly relevant to their dimensions, speeds, and so on. In the same incidence conditions, the pulse beam and plane wave have difference peak values and the same Doppler shift bandwidth. If the waist radius of the pulse beam is larger, the peak value is higher, and the Doppler shifts are proportional to the speed of the rotating target. By virtue of our theoretical model, we probe into the scattered characteristics of the Doppler shifts of a laser pulse beam, which would benefit target identification in national defense.

Regional characteristics of ultrasonic structural scattering in liver

In this paper, the ultrasonic spectra of liver's structural scattering wave obtained by autoregressive model and the spectral characteristics were analyzed statisticaJly The experimellt and analysis results show that a liver can be divided into three distingulshed scattering regions. The top layer region shows approkimate Rayleigh scattering characteristic, the central region shows diffosion scattering, and the region between the top layer and centre shows random scattering. These distribution characteristics correlate with the distribution of the microstructure characteristics of liver. The experiment results provide possibilities for tissue recognition using spectrum characteristics of the regional structure scattering.