Abstract:In recent years, the scanning SISO(single-input-single-output) array near-field imaging technology has attracted widespread attention in civilian fields, such as security inspection and medical treatment. However, during the echo processing of traditional range migration algorithm, the multi-step approximation and interpolation operation cannot be avoided, and the adverse effect caused by signal propagation attenuation is ignored. Therefore, this research proposed a FFT (fast Fourier transform)-based scanning SISO array near-field imaging technology in which the amplitude attenuation factor in the echo model was preserved. The convolution operation on azimuth and altitude dimensions was performed according to the characteristics of the target echo equation. The target image could be obtained by applying FFT and coherent accumulation steps. Simulation analysis and experimental results show that the proposed method can not only improve the target image quality while ensuring the efficiency of image reconstruction, but also effectively reduce the impact of signal spatial propagation loss on the imaging quality.

Abstract:Millimeter wave has great potential in the field of nondestructive testing due to its unique advantages of high resolution, high penetration and low photon energy. For the application of millimeter wave nondestructive testing, a three-dimensional imaging algorithm for nonuniform planar array was proposed by combining the idea of the range migration algorithm and the nonuniform fast Fourier transform. The proposed algorithm uses nonuniform fast Fourier transform to reconstruct the signal spectrum in wavenumber domain, which overcomes the limitation of nonuniform spatial sampling on traditional imaging algorithms based on Fourier transform. The proposed algorithm is aimed to eliminate the complicated interpolation operation and improve the imaging efficiency. In order to verify the effectiveness of the proposed algorithm, the back projection algorithm was used as a comparison, the simulation and experiment was carried out, respectively. Simulation and experimental results all show that the proposed algorithm can achieve high-efficiency imaging of dielectric targets with high imaging accuracy and fast speed.

Abstract:To address the problems of poor robustness and high computational complexity of some existing coded aperture imaging algorithms, an enhanced terahertz coded aperture imaging method based on convolutional neural networks was proposed in this paper. The method realized implicit modeling of the imaging system by constructing an end-to-end neural network that exploited the network′s strong inversion ability and noise immunity for target reconstruction at low SNRs (signal-to-noise ratios). The simulation experiments show that the proposed method can achieve the reconstruction of targets with different sparseness at different SNRs. Moreover, the proposed method can achieve a higher resolution reconstruction of the target at low SNRs in comparison with to the classical optimization iterative algorithm.

Abstract:Aiming at the new requirements of antenna for low-observable aircraft platform, metasurface antenna with microwave and infrared cross-spectrum and low-observable performance was proposed and designed. To enable the antenna to possess normal radiation properties and absorbing characteristics simultaneously, a novel idea that metasurface design thought integrated into the antenna design was proposed. In this way, radar cross section of the antenna can be reduced without additional loading of the absorbing metasurface, and only by using absorbing characteristics of the antenna structure. On this basis, to realize infrared low-observable of the antenna, metasurface with microwave transparent characteristics and low emissivity characteristics in the infrared band was designed, and it was loaded above the antenna to reduce antenna emissivity. Experimental results show that the novel designed metasurface antenna operates from 2.72 GHz to 2.82 GHz and the maximum gain reaches 6.72 dBi, which indicates the antenna possesses good radiation properties. The reflection magnitudes of the electromagnetic metasurface antenna are all less than -10 dB from 2.70 GHz to 2.80 GHz. The minimum reflection magnitude is -12.49 dB. The emissivity of the antenna is 0.212 by loading the metasurface, which is 0.334 (61.2%) lower than the antenna without metasurface. From the above results, it can be concluded that the novel designed antenna achieves low-observability of both microwave and infrared simultaneously while maintaining normal radiation properties. The novel metasurface antenna provides a novel approach for antenna design of low-observable aircraft platforms.

Abstract:Aiming at the problem the traditional voice acquisition method is difficult to correctly detect the voice activity at the desired voice source when multiple voice sources exist simultaneously, a multi-voice activities detection method based on terahertz radar was proposed. The short wave length of terahertz radar was utilized to improve the detection sensitivity for micro vibration. At the same time, the high range resolution brought by large broadband was used to distinguish different voice sources. After obtaining the voice signal, the signal quality was further improved for signal activities detection by spectral subtraction wavelet joint denoising method. The experimental results show that, when there are interfering voice source on the transmission path, the proposed method can more effectively distinguish multiple voice sources and obtain the voice activity at the desired voice source than the microphone-based method.

Abstract:Based on the strong nonlinear characteristics of terahertz schottky diode, a compact 220 GHz transceiver front-end for continuous wave frequency modulation radar detection was developed by using multiple harmonic frequency doubling and mixing method. In order to realize the transmitter with high power output, the power amplifier driver of 220 GHz frequency multiplier adopted 4-way power synthesis method to realize 70 GHz 300 mW high-power power amplifier module, and 70 GHz high suppression 7th order cavity bandpass filter to suppress high-order harmonic signal. In order to achieve a high sensitivity receiver, the 220 GHz harmonic mixer adopted hammer-head suppression structure and diode precise noise model to design the circuit structure, and the IF link adopted switch control to achieve dynamic gain control with dynamic range greater than 60 dB. The 220 GHz transceiver front-end achieves a maximum power output of more than 10 mW in the range of 215~225 GHz. The minimum noise figure of the receiver is less than 7 dB, and the gain dynamic range is greater than 60 dB (-7~54 dB).

Abstract:The realization of high sensitivity terahertz detector which can be operated at room temperature poses difficulty in this field. The terahertz detector based on the principle of thermoelectricity provides the posibility for wide detection frequency range and high sensitivity detection at room temperature. By utilizing the high carrier mobility and excellent thermoelectric properties of new micro-nano materials such as graphene and perovskite, the high-performance terahertz photodetectors operating at room temperature were prepared, the detection of the photoelectric response can be up to 271 mA/W, and the response time is less than 20 ms. Results show that photothermoelectric detectors prepared by graphene and perovskite are expected to be a new generation of high performance detectors in terahertz frequency band.

Abstract:Research on the modulation of the surface roughness to the RCS (radar cross section) has vital importance to the RCS scaled measurement for rough metal targets in the terahertz band. The changing laws of specular RCS of rough metal target with different surface roughness were studied. Based on the method of Kirchhoff approximation, combined with the theory of coherent and incoherent scattering, the method for predicting the specular RCS of rough metal target was proposed under the condition of small surface roughness. The effectiveness of the prediction method was validated by simulation experiments. The proposed method can achieve fast estimation of specular RCS for metal target with small surface roughness in the low terahertz band, and further predict the specular RCS for the scaled model with rough surface. This method also can provide effective RCS value for the comparison with the measurement result, which theoretically supports the study of RCS scaled measurements for rough targets in the terahertz spectrum.

Abstract:Plasma sheath formed during the reentry of near space hypersonic vehicle will interfere the electromagnetic wave detection. In order to solve this problem, the model of the typical near space vehicle was built, and the flow field distribution under different flight conditions during the reentry process was simulated. Based on the flow field distribution, the plasma parameter distribution was modeled, and the transmission characteristics of terahertz wave in the plasma sheath were theoretically calculated by using the scattering matrix method. The results show that high frequency terahertz wave can penetrate plasma sheath effectively. In addition, terahertz active imaging experiments were carried out in the laboratory environment. The experimental results show that the plasma exerts little effect on the terahertz active imaging results. The simulation and experimental results preliminarily prove the potential of terahertz technology for hypersonic vehicle detection, which is of great significance for national defense.

Abstract:The generalized finite integral transform method was applied to get the dynamic characteristic of a nonlocal thin plate based on the stress gradient nonlocal theory. The integral transform pairs were established by using the integral kernels which can satisfy the boundary conditions. The higher-order partial differential equation of the nonlocal thin plate was transformed in to a series of linear algebraic equations through the integral transform, and the nature frequencies of the governing equations can be solved exactly. To confirm the validity of the simulated, the results were compared with the finite element method and the existing ones. The effects of the nonlocal parameter and the dimension of the plate on the nature frequency were studied. The proposed method provides a new way to solve the mathematical problem of the nonlocal plate problem.

Abstract:In order to solve the attitude maneuver problem for flexible spacecraft with time-varying parameters, the variable amplitudes zero vibration derivative shaper with stronger robustness was developed based on the variable amplitudes zero vibration shaper. The robust attitude maneuver strategy, in which its objective was to maneuver the spacecraft to the desired attitude state and cancel the residual vibration of the rotating flexible appendages at the same time, was proposed. Simulations were conducted by means of a spacecraft with rotating solar wings which undergoes three-axis maneuvers. The results indicate that this strategy is robust to frequency variations, uncertainties of damping coefficient and other uncertain factors.

Abstract:Aiming at the compatibility analysis between GSO (geostationary satellite orbit) satellite systems, uplink and downlink interference scenarios were designed between single-beam and multi-beam GSO satellite systems, and GSO satellite systems interference was evaluated on multiple dimensions, such as satellite orbital position, system link availability, and interference system′s earth stations location. Recommendations were proposed on the nearest location of interference system′s earth stations at different orbital intervals, which met International Telecommunication Union limit. The granularity of each dimension was refined. Different dimensions were compared horizontally, and interference variation characteristic curves and the influence level of each dimension were analyzed. When link availability was fixed, the interference value changed slowly with the orbital interval at >2°, the interference value changed rapidly with the interval at ≤2°, especially when the interval was at≤0.1°, the interference value rose sharply. Taking network data of the CHNSAT-81.5 and INSAT-KA82.5E satellites registered by the International Telecommunication Union as examples, the interference-to-noise ratio and the carrier-to-interference ratio were compared with Visualyse, the result error was kept in the range of 0.7 dB with an effective evaluation performance.

Abstract:Aiming at the passive boundary layer control methods of COIL (chemical oxygen-iodine lase) cavity, three types of experiment rigs were designed. The upper and bottom walls of laser cavity are removable, so the boundary layer control effect with different experiment rigs can be compared. The experiment results indicate that the thickness of boundary layer in laser cavity can be reduced by both slotted wall, mainstream ejected slot and perforated wall. The pressure distribution in cavity, especially in the latter of cavity, is improved. The boundary layer thickness can be reduced further by increasing the sucking rate in a proper range, meanwhile the cavity pressure can be dropped and the COIL power can be increased. But the COIL power also can be dropped with the overlarge sucking rate. Among the three experiment rigs, the mainstream ejected slot is the most sensitive to sucking rate, that when the sucking rate improved to 5%, the COIL power drops obviously. The perforated wall is the most insensitive to sucking rate, and the COIL power has little change while the sucking rate improved from 1% to 7%.

Abstract:The numerical method was used to study the aerodynamic flow field characteristics of hypervelocity projectiles, especially in the reentry phase. The prediction accuracy of S-A and k－ω SST turbulence model was verified by wind tunnel test data. The calculation results show that the prediction accuracy of the two turbulence models is higher than 2% in normal force prediction. In axial force prediction, the S-A turbulence model has a high prediction accuracy of 4.6%. When the projectile reenters at a large angle of attack, the transverse flow effect of projectile is more obvious. The shock wave makes the surface pressure of the projectile increase sharply on the windward side, while the large-scale streamwise vortex structure formed on the leeward side reduces the pressure, and the increase of the pressure on the windward side has a greater impact on the aerodynamic coefficient of projectile. The aerodynamic drag and lift coefficient at high angle of attack are obviously nonlinear, the drag coefficient is obviously increased, and the static stability margin is also sharply reduced, which makes the convergence characteristics of projectile worse, and that is treated as the main reason for the velocity attenuation during the reentry process of projectile.

Abstract:As one of the active cooling methods, the opposing jet shows its advantages on the aerodynamic thermal reduction for hypersonic flight. To accurately predict the thermal protection properties of the opposing jet, the numerical investigation on the flow and heat transfer characteristics for the opposing jet of nosecone at Mach 6 was conducted by using a fluid-thermal coupling strategy. The related turbulent model and fluid-thermal coupled method were validated by the comparison of the numerical and experimental results. Flow patterns of the opposing jet with different total pressure ratio were gained, and the influence of the total pressure ratio of the opposing jet on the heat transfer between fluid and solid structure was discussed. Besides, the influences of attack angle and solid materials on structural heat reduction were also investigated. The numerical results indicate that:the increase of total pressure ratio of the opposing jet brings obviously further heating reduction, and accordingly the solid structure temperature presents more uniformity; calculation of the heating in 60 s indicates that the positive attack angle enlarges the temperature difference in the solid structure, which induces the lower temperature air flowing to the leeward side furtherly, so the cooling for the leeward side of nosecone is enhanced; for alloy IN718 and C-103 two different materials, their discrepancy of heating can be neglected at the cooling recirculation region of nosecone and the area where the recompression shock influence obviously; in comparison, the use of C-103 shows its advantage of even distribution of temperature in the internal system of structure.

Abstract:In order to study the structural integrity analysis of finocyl grain for solid rocket motor under the ignition loading, a numerical simulation method with considering the three-dimensional damage viscoelastic constitutive was proposed. The commercial finite element software was taken to the secondary development, and the user subroutine was written. Therefore, the numerical simulation was carried out. The distribution of stress, strain and displacement filed of finocyl grain were obtained under the condition of the temperature loading and ignition pressure. Results show that the deformation mode of inner surface of grain is different under the temperature loading and ignition loading. However, regardless of the temperature loading or ignition loading, both the Von Mises stress and Von Mises strain in the fin-slot region of grain are higher than those in other parts. This method can be applied to the security assessment of structural integrity analysis of solid rocket motor.

Abstract:Compared with single-layer attacks, cross-layer attacks in wireless ad hoc networks can better conceal attack behavior, achieve better attack effects, or reduce the cost of attack. In order to detect cross-layer attacks in wireless ad hoc networks, a detection model based on game theory was proposed. As the attack will inevitably affect the parameters of each protocol layer, the corresponding strategy matrix and payoff matrix was built from the aspect of attack-defense game of protocol layer, and the mixed strategy Nash equilibrium solution was obtained by equilibrium analysis. Simulation experiments results show that the detection system adopting mixed strategy can achieve a better detection performance and save the energy consumption significantly, compared with the traditional detection system.

Abstract:For the difference between modalities, a cross-modality person re-identification algorithm which based on symmetric network was proposed. The network combined the modal confusion based on probability distribution with adversarial learning, and generated modal-invariant features through symmetric network to achieve modal confusion. To deal with appearance differences and intra-modality differences, the network constructed a mixed-triplet loss using convolution features of different hidden layers, which can improve the characterization capability of the network. Numerous experimental results on the RegDB and SYSU-MM01 datasets demonstrate the effectiveness of the method.

Abstract:When the traditional Voronoi diagram divides a large number of point sets into Voronoi, there are too many Voronoi cells, which makes it difficult to apply to such fields as geographic information systems and biomedicine. In order to solve this problem, a Voronoi diagram based on adaptive DBSCAN (density-based spatial clustering of applications with noise) was proposed. The phenomenon of Voronoi unit merge was explained. The necessary and sufficient conditions for its occurrence were proved. The algorithm for generating the Voronoi diagram was proposed and simulated. In order to verify its effectiveness, the algorithm was applied to neutrophils under the microscope and fire point data on the surface of China. The results show that the algorithm can effectively solve the problem that Voronoi diagram is too meticulous when the point set size is large, which breaks through the single point to single point division form of the traditional Voronoi diagram. In addition,the algorithm broadens the application of Voronoi diagrams in the fields of graphic image processing,biomedicine and geographic information systems.

Abstract:The network of SSS (smart-satellites-swarm) was modelled in order to transfer the ICRPS (inter-satellite communication routing problem of SSS) into the least-delay path problem. And the ICRAS was developed to solve this problem. ICRAS used dynamic programming method to program the inter-satellite communication routing between two members of a SSS in multiple phases. The smart satellite which need transmit data adopted a static programming algorithm for inter-satellite communication routing to find its successor satellite during each programming phase. ICRAS was able to cope with the problem that data had to be delivered in pieces since the propagation capability of an inter-satellite link was restricted. A large quantity of simulation experiments were designed and conducted to examine and analyze the performances of ICRAS when different SSS utilized ICRAS to program inter-satellite communication routings.

Abstract:To solve the problem of non-linear gray level distortion and strong noise interference in remote sensing multi-modal image matching, a remote sensing multi-modal image matching algorithm based on PCA (principal components analysis) and HOG (histogram of oriented gradients) was proposed. This algorithm uses HOG to extract the common features of geometric structure between images, which can effectively overcome the problem of nonlinear grayscale distortion of multi-modal images. Besides, a fast multi-scale PCA algorithm was proposed to enhance the local gradient direction in HOG, so that it can accurately extract the structural features of the image under the condition of strong noise interference. In order to improve the calculation speed of the algorithm, the integrated image method was used to reduce the computational complexity of the feature extraction process, and the fast Fourier transform was used to achieve a highly efficient matching search. The experiment used a variety of remote sensing multi-modal images (including visible light images, synthetic aperture radar images, and infrared images) to verify the matching algorithm. The results show that, compared with existing algorithms, the proposed algorithm significantly improves the matching performance.

Abstract:Airborne network for aeronautic swarm is the information interaction bond between swarm members, whose routing strategy directly affects the real-time performance and reliability of information transmission, and then affects the combat effectiveness of networked swarm. Considering that there are many uncertainties in airborne network for aeronautic swarm, in response to the risk of route failure and avoid routing updates, a Failure-Oblivious routing method from the perspective of routing algorithm was proposed under software-defined network architecture. Unlike traditional routing strategies, this method can deal with routing failure problem without the burden of predicting routing failure by using random multipath routing algorithms. Theoretical derivation and simulation verification show that this routing method can reduce the risk of route failure in airborne network for aeronautic swarm, thus guaranteeing that communication delay cost is under control.

Abstract:In order to solve the problems of low feature differentiation of high-dimensional data in text feature extraction, poor self-learning performance of rule-based representation learning, and excessive pruning of variational autoencoder, a text feature extraction model based on SBVAE (sparse balanced variational autoencoder) was proposed. In order to eliminate noise interference and improve robustness of the text feature extraction model, a bidirectional noise reduction mechanism was designed for variational autoencoder in the input layer of the text feature extraction. A sparse balance method combined with simulated annealing algorithm of weights of KL (Kullback-Leibler) terms was proposed to alleviate the effect of excessive pruning caused by KL divergence, and forced decoders to make full use of the latent variables. The model improves the discrimination of high-dimensional data features. Experiments were carried out in several aspects, including comparative analysis of text feature extraction model, sparse performance and influence of sparse balance on the lower bound of variation in hidden space. The results show that the proposed model has good performance. The highest accuracy of the proposed model of Fudan and Reuters datasets is increased by 12.36% and 8.06% in comparison with that of PCA, respectively.

Abstract:In order to solve the problem of early warning aircraft′s airspace configuration in key target air defense operations, a model of multiple early warning aircraft′s array selection and optimal configuration of aircraft number is constructed. The specific tasks of the early warning aircraft′s air defense operations were determined by analyzing the division of early warning lines for the key targets, and the quantitative model of array selection was constructed. A quantitative model of airspace configuration for parallel and tandem routes of multiple early warning aircraft was proposed on the premise of ensuring its own safety and giving full play to the inherent detection power of early warning aircraft. The example operation results show that the model can compare and analyze the effects of key parameters from the cooperative detection of multiple early warning aircraft in airspace configuration, deployment forces and route pattern selection,which can provide theoretical basis for mission planning of early warning aircraft′s combat troops.