Abstract:To study the development status and technical problems of the emerging spacecraft cluster network, this paper analyzes the latest literatures in this field. Spacecraft cluster is a kind of novel multiple spacecraft collaborative work framework, which performs information interaction via wireless links and serves to the increasingly complex and diverse space exploration missions in the future. First, this paper makes the concept of spacecraft cluster network clear by involving the definition of satellite cluster. Then, it analyzes the current status and development course of the distributed space exploration missions as well as describes the spacecraft cluster network from the perspective of theory and orbital dynamics. Finally, it summarizes the key technical problems when studying spacecraft cluster network, including topology control strategies, data routing algorithm, space inter-communication protocol, cross-layer resource allocation algorithms. It also puts forward some corresponding suggestions of development. The result shows that technologies in spacecraft cluster network are still at the initial stage, and several key technical problems need to be solved. Some mature networks, e.g., Internet and Mobile Sensor Network, can serve as technical reference models for the breakthrough of these problems.

Abstract:When the chaser spacecraft closely tracks and observes the non-cooperative target in space, it should approach to and keep pointing to the non-cooperative target from the particular direction. For the problem that the chaser spacecraft keeps closely tracking and pointing to the non-cooperative target, in the case of the target with the attitude motion and the unknown orbit maneuver, based on the relative orbit dynamics and the attitude dynamics which are described in the line-of-sight coordinate frame and the body coordinate frame, respectively, the six-degree-of-freedom model of orbit and attitude simultaneously control is proposed. The RBF neural network is employed to adaptively estimate and compensate the system uncertainties and the unknown motion parameters of the target. Using the backstepping method, a controller which can control the chaser spacecraft to converge to the desired relative orbit and attitude in finite time is proposed. Considering the nonlinearity of the control input, such as saturation and dead zone, an improved control algorithm is developed. The simulation results are provided to show the effectiveness of the control model and the control algorithms. Moreover, the improved control method has higher control accuracy.

Abstract:To research the problem of designing hypersonic glide vehicle's no-fly zone evasion trajectory, a maneuver evasion trajectory optimization design method based on joint point is proposed. The hypersonic glide vehicle's trajectory is divided into several stages by joint points. Based on this concept, the design of hypersonic glide vehicle's maneuver evasion trajectory based on joint is implemented utilizing adaptive pseudospectral method's ability of solving multi-stage optimization problems and adjusting collocation points' distribution. The simulation results get maneuver evasion trajectories that can meet varieties of constraints and evade no-fly zones under different circumstances. It is demonstrated that this method is able to be used for the substantial lateral maneuver trajectory's design. Moreover, the joint points' positions can control the maneuver evasion trajectory's shape.

Abstract:To improve the quality of the reconstruction for high resolution astronomical image, a compressed sensing denoising and reconstruction algorithm, which combines wavelet with wiener filtering, is proposed based on the traditional compressed sensing (CS) iterative wavelet thresholding algorithm. The design method for this algorithm is that: a predesigned wavelet wiener filtering operator is used to replace the traditional wavelet threshold operator to select the wavelet coefficient of astronomical image in each iteration, thus the pseudo-gibbs phenomenon caused by the threshold denoising method in the reconstructed image can be suppressed effectively, and then the total variation method is used to adjust the reconstructed image for improving its quality. The experimental results show that the proposed algorithm can achieve better denoising and reconstruction performance, and can effectively protect the detailed feature information of high resolution astronomical image, compared with the traditional iterative wavelet thresholding algorithm. In addition, when the compression ratio is higher, the proposed algorithm can also help to the relatively higher visual quality and peak signal to noise ratio.

Abstract:To study the effects of different ways of importing the membrane stress and different stress values on diagonal extendable solar sail craft, the finite element simulation analysis is carried out. In detail, the necessity of importing the membrane stress and its reasonable value range are overviewed and three ways of importing the membrane stress are introduced. Based on the finite element software ABAQUS, the model of the 20 m×20 m solar sail craft made by European Space Agency (ESA) and German Aerospace Center (DLR) is established. The structure analysis of the model is performed with static nonlinear method considering the three ways of importing the membrane stress. Furthermore, comparative analysis is made on the membrane stress distribution and the effects of membrane stress on the structural rigidity as well as sail state of solar sail are evaluated. In general, the observations in this paper are significant for further research on space solar sail prototype. The results show that stress concentration occurs on the solar sail corner of membrane surface under different ways of importing the membrane stress, while the fold distribution of the membrane surface is different, frequency increases with the increasing of the importing membrane stress, which is beneficial to overall movement of the solar sail craft.

Abstract:To identify multiple damage locations in the structure, a method of multiple damage locations identification and imaging was proposed. In this method, the damage index is calculated by using an effective Lamb wave response automatic extraction algorithm. The detected key area in the structure is divided into several subregions, and the effective response signals containing the structural damage information are automatically extracted from the entire Lamb wave responses received by the piezoelectric sensors. Subsequently, the damage indexes of every subregion based on the correlation coefficient are evaluated by using the effective Lamb wave response automatic extraction algorithm. Eventually, the damage locations identification and imaging are performed using the reconstruction algorithm for probabilistic inspection of damage (RAPID). The experimental verification research is conducted using an aeronautic aluminum plate. The experimental results show that the method proposed in this research is capable of identifying multiple damage locations quickly and effectively, as well as imaging the damages in detected area clearly.

Abstract:Using the fracture mechanics and finite element analysis theory, the effect of section types of the ribs and multiple-site damage on the crack turning behavior and fracture characteristics of the integral stiffened panel is investigated. First the crack growth model of the integral stiffened panel was established, and the reliability of the established model was proved in comparison with the test results of the crack growth rates. Based on the above model, the influence of section types of the ribs on the crack fracture parameters and crack turning was discussed. Finally the multiple cracks problem was investigated. The study showed that the effect of the sectional types of the ribs on the fracture parameters was not significant and the crack turning was more likely to occur when the crack was growing far away from the ribs. In the case of multiple cracks, cracks will not turn, but grow straightly and connect to form a large crack. Moreover, the strong interference of adjacent crack tips can lead to the severe increase of stress intensity factor and acceleration of the crack growth. Furthermore, the crack propagation life will be seriously shortened. Therefore, effective repairing methods are needed to improve the residual strength and service life of the structure.

Abstract:To investigate the effect of flap deflection on aerodynamic performance of the parafoil, CFD models are established for different flap deflections. By using the finite volume method, the spatial discretization is carried out and the RANS equation is solved. The aerodynamic performance of the parafoil in the stage of turning and flaring is simulated, and then combining with the least square procedure, parameter identification is carried out to implement the modification of the parafoil aerodynamic model. Simulation results show that the flap deflection can cause the change of the parafoil pressure distribution, the decrease of the stall angle of attack, the sudden increase of the lift drag coefficient, and the complex effect on aerodynamic performance of the parafoil. Compared with the traditional aerodynamic model, the modified parafoil aerodynamic model can better describe the regulation of the flap deflection and the aerodynamic performance of the parafoil, and the calculation precision is improved effectively. It provides a theoretical reference for precise modeling of the parafoil in the stage of turning and flaring.

Abstract:To improve the coverage accuracy and efficiency of the satellite, the geometric characteristic related to satellite coverage of a ground region is analyzed, and the division method for space geometric configuration of constellation is mainly studied based on the properties of the Delaunay triangulation and the Voronoi Diagram. Subsequently, the constellation coverage problem is simplified into multiple one-satellite coverage problems by defining the coverage area of each satellite. Furthermore, an accurate and effective approach to solve the satellite constellation coverage problem for any type of coverage area is put forward. Finally, the comparison of accuracy and efficiency between the proposed approach and the classic Grid-point method is given for different target areas. The test result shows that the proposed method is valid and more efficient. The proposed method can also be used in satellite constellation optimization design and performance evaluation.

Abstract:To mitigate the cost of additional area and peripheral circuit, which is caused by conventional dimension adjusting way in the subthreshold region, the reverse short-channel and the reverse narrow-channel effect are applied to improve the conventional way. Hence, the additional cost of area and peripheral circuit is reduced, and noise margin of the Static Random Access Memory is also enhanced. In addition, the reading and writing performance is simultaneously optimized. A 10-T Static Random Access Memory cell is fabricated in 130 nm process, and test results show that the effects implemented in the cell can save about 76% of area consumption, and facilitate 30.5% enhancement in the noise margin compared with the conventional way. Consequently, the SRAM can function steadily under the voltage of 0.32 V.

Abstract:The recognition of the guidance law of a non-cooperative target vehicle is very useful for the counteraction design of the pursuing vehicle. A generic implicit guidance function is presented to describe all known guidance laws, and a practical model with time-varying guidance parameters is used to approximate the generic implicit guidance function. Based on this model, a LPF enhanced Adaptive Kalman Filter is proposed to recognize the guidance law in real-time. Simulation results show that the proposed algorithm can recognize the guidance law fast and effectively when the non-cooperative vehicle employs either the Proportional Navigation guidance law or the Bang-Bang guidance law.

Abstract:A validated numerical simulation modeling of flapped wing wake is carried out by using RANS discretion. The flapped wing wake region is divided into three regions with significant different dynamic features. Three regions including co-rotation field, merging field and axial symmetry far field are induced. A non dimensional parameter of vortex interval is used to describe above regions. The Reynolds number, vortex interval, flap angle are used as control variables to change ratio of circulation of flap vortex and tip vortex. Reynolds effect is found as prolonged co-rotation field and enhanced rotation motion in wake. Decreased vortices interval leads an accelerated merging in near field but a limited dimension of resulted vortex. Small flap angle leads a weak flap vortex. The dominated tip vortex stretches and filament flap vortex in free share layer. The merging process is totally changed. The tip vortex trajectory does not expand. Reynolds number effect is the enhancement of induction in tip vortices merge. Induction of vortices with low intensity is not obvious in the wake.

Abstract:To decrease the bank angle reversal frequency in the skip phase and suppress the aerodynamic load effectively in the final phase of the numerical predictor-corrector guidance for lunar return capsule, an improved guidance scheme is proposed by searching for the energy point of the only one bank reversal in the skip phase and compensating the guidance command with drag acceleration feedback in the final phase. Firstly, the secant method is used to search for the bank angle reversal energy point, so that the bank angle is just reversed once in the skip phase. Secondly, based on the exponential atmosphere assumption, the derivative of the drag acceleration is obtained, and a reference drag acceleration profile is determined according to the load constraint. Finally, the errors of the drag acceleration and its derivative between the reference profiles and the actual profiles are used to compensate the basic bank angle magnitude in the final phase to relieve the aerodynamic load. The testing results demonstrate that the proposed algorithm can reduce the bank angle reversal frequency in the skip phase and suppress the aerodynamic load effectively with strong robustness, which show that the problems have been successfully solved and the scheme can provide a reference for practical guidance system design.

Abstract:To improve the accuracy of the integrated navigation system INS (Inertial Navigation System)/CNS (Celestial Navigation System)/GNSS (Global Navigation Satellite Systems) integrated navigation system for long-time working, this paper establishes the integrated navigation system state equation and the observation equation based on quaternion errors, and analyzes the observability and observable degree of the system states by applying Piece-Wise Constant System (PWCS) theory for different input conditions. Based on the analysis of the observability and observable degree, using the measurement information of attitude and position vectors from star sensor and GNSS, this paper proposes a method of on-line calibration for the error parameters of gyros, accelerometers and star sensor. The observability analysis and simulation results show that this method can precisely estimate the scale factor, zero, and installation error of gyros, accelerators and star sensor. This method can improve the long-term working accuracy of INS/CNS/GNSS integrated navigation system and has an important engineering application value.

Abstract:To verify the short take-off and large-load characteristics of the fan-wing aircraft, a 6 kg fan-wing aircraft is studied. Based on the analysis of its structure characteristics and flight principle, the longitudinal model is built according to the force analysis on the ground and in the air, which effectively connects the ground running and the aerial flying. Meanwhile, four factors (elevator deflection, position of center of gravity, fan wing rotation speed, and airplane load) influenced the take-off running distance are analyzed and compared through numerical simulations. The simulation results show that: the no-load flight leads to the shortest take-off distance; the elevator deflection is located as large as it can be during the take-off process; as the center of gravity closer to the force application position of the cross-flow fan, the take-off distance remains shorter; when the factors are fixed, the shortest take-off distance, less than 9 m, is obtained with an optimal rotation speed. The allocation of the four factors is needed to gain a better take-off performance through the comparative analyses.

Abstract:Global Navigation Satellite System (GNSS) is now widely used in transportation, and the users have shown growing concerns with the service reliability of GNSS. However, due to the factors such as ionospheric scintillation, multi-path effect, and electromagnetic interference, GNSS has inherent vulnerability, which may damage the quality of GNSS service, and even cause enormous risks. Assessing GNSS vulnerability and releasing alarms to users when vulnerability factors happen are the valid way to consolidate GNSS utilization. To assess GNSS vulnerability under urban roads, this paper proposes three proper vulnerability assessment indexes and conducts vulnerability assessment by the utilization of entropy weight method. Road experiments showed that tunnel, urban canyon, avenue and open road are decreasingly vulnerable, and the algorithm proposed is capable of assessing GNSS vulnerability of urban roads.

Abstract:An adaptive backstepping sliding mode control method is adopted to solve the control problem of uncertain nonlinear systems with unknown actuator failures. A model for the nonlinear actuator is developed which includes hysteresis nonlinearity, partial loss of effectiveness and total loss of effectiveness. Radial basis function neural network is employed to approximate the unknown nonlinear functions, and the parameters of neural network are tuned on-line by adaptation laws to improve the effect of approximation. The dynamic surface control is combined with backstepping control to avoid the explosion of complexity in the traditional backstepping design method. The influence of modeling error and uncertain disturbances is suppressed by introducing the adaptive compensation term. The closed loop system is proved to be semi-globally uniformly ultimately bounded by theoretical analysis. Simulation results are presented to demonstrate the effectiveness of this method.

Abstract:In terms of the uncertainty measurement and the constraint on the attitude quaternion, this paper proposes a quaternion constrained cubature Kalman filter based on uncertain measurements (UCCKF) for the spacecraft attitude estimation system. The proposed filter algorithm overcomes the limitation of the quaternion-constrained cubature Kalman filter. An independent Bernoulli random process is introduced to describe the measurement uncertainty, and the three degree spherical radial cubature rule is adopted to computer the posterior mean and covariance of the system state. To deal with quaternion normalization problem, the two-step projection theory is applied to solve the quaternion constraint. Simulation results show that, compared with constrained cubature Kalman filter and unscented mixture filter, the proposed UCCKF can help to implement the better convergence and higher estimation accuracy in the case of measurement uncertainty. This illustrates that the proposed algorithm is effective and feasible for the nonlinear attitude estimation system.

Abstract:To avoid the root lost problem in the former methods, a new method of solving the eigenproblem of Rayleigh wave in multilayered viscoelastic medium. The reason of root-lost and pseudo-root phenomenon is analyzed. From this, a series of different initial imaginary parts are employed, and the roots whose imaginary part is larger than real part are omitted. A flowchart of solving the eigenproblem is presented based on this, and the eigenproblem of Rayleigh wave in multilayered viscoelastic medium in an example is successfully solved with the method. The results show that there is no root-lost and pseudo-root phenomenon when solving the eigenproblem with the method. In the eigenproblem of Rayleigh wave in multilayered viscoelastic medium, root-lost and pseudo-root phenomenon can be avoided by employing a series of different initial imaginary parts and omitting the roots whose imaginary part is larger than real part.

Abstract:To implement fault diagnosis using the vibration signal of gearbox, the vibration signal is processed by the orthogonal matching pursuit (OMP) algorithm. The vibration signal of gearbox contains running characteristics and mixes a lot of noises, and presents non-stationary signal feature. It is important to remove redundant information, extract major information to express signal. Traditional frequency domain analysis method can only qualitatively judge fault from the frequency domain figure, cannot quantitatively compute the fault. Orthogonal matching pursuit is a quantitative method to extract characteristics. In this paper the OMP method is used to map time domain to frequency domain based on the Fourier orthogonal basis, and the major characteristics are obtained in the frequency domain. According to the principal component analysis (PCA), it extracts three sets of major characteristics, then the characteristics of known fault classification signal are compared with the characteristics of the detected signal. By comparing the location and the fault of the vibration state can be judged. The experiment results show that the method can accurately judge the characteristics of five different forms of the gearbox from the normal state to the 100% wear, and complete the fault diagnosis and classification of the gearbox.

Abstract:The state before and after water entry of high-speed projectiles was analyzed based on water entry ballistics and hydrodynamics and the dynamics model of projectile longitudinal motion was built. The trajectory simulation during water entry process of high-speed projectile with various turbulent angular velocity was estimated. The change rules of trajectory and velocity of focus, pitch angle, pitch angular velocity were obtained, and the influence of turbulent angular velocity was analyzed. The results show that with increasing of tail slapping frequency, the immersion depth of the trajectory tail increases and the continuous time increases. Moreover with tail slapping frequency increasing, the pitch angular velocity after tail slapping increases, then the increasing extent decreases or the pitch angular velocity decreases slightly. The bigger the turbulent angular velocity is, the more times the tail slaps in the same time happen, the earlier the tail slapping happens, and the shorter the moving time of the projectile is in cavitation chamber.

Abstract:To reduce the drag of underwater vehicle, experiment is carried out to study the turbulent drag-reducing of underwater vehicle injected with surfactant in a water tunnel. For the experimental work presented here, surfactant is injected from the head of the vehicle. The drag reduction (DR) of surfactant as a function of fluid velocity and surfactant concentration are investigated. The influences of the surfactant on the lift-force and drag-force with different attack angle are analyzed. Surfactant solutions at different concentrations, namely 200, 400, 600, 800 and 1000 mg/L, are used to examine the influence of surfactant on drag coefficient. The flow velocities are set between 4 and 10 m/s. The present results show the DR effect is weak when the surfactant concentration is 200 mg/L. With the increase of the surfactant concentration to 1 000 mg/L, the drag coefficient is reduced by about 40.8%. The DR ratio ranges between 12.5% and 40.8% at all flow velocities up to 10 m/s from 4 m/s when the surfactant concentration is 1 000 mg/L. With the increase of surfactant concentration, the drag coefficient of the vehicle with angle of attack is also reduced and there is slight effect on the lift-force coefficient.

Abstract:To simulate the ship advancing in large waves with large-amplitude motion, a CIP-based strongly-nonlinear seakeeping analysis tool is developed. The effects of viscosity and strongly-nonlinearity are considered in the numerical model, which is based on the viscous flow theory. First, convergence studies are carried out by calculating the radiation problems of S175 hull with forward speed. Then, the ship response of S175 hull including the heave and pitch motions in waves with amplitude and length of waves varied is calculated. Finally, strongly-nonlinear ship responses in large head waves are calculated and captured successfully. It is confirmed that the developed seakeeping analysis tool has numerical stability and accuracy, and has the capability of simulating the strongly-nonlinear seakeeping problem, such as, the phenomena of wave breaking, slamming, and green water on deck.

Abstract:To enhance the propulsive efficiency of ducted propeller for better endurance of underwater-vehicle, 19A/Ka4 series ducted propeller is studied through numerical simulation to analyze the factors of the duct and blade on the hydrodynamic performance by change the such parameter such as length-diameter ratio, diameter ratio, blade area ratio and pitch ratio under different mesh models. It is discovered that the length-diameter ratio has almost no influence of the open water performance, though, the cavitation performance is better with the increase of length-diameter ratio. The lesser duct diameter ratio and blade area ratio will increase the efficiency, and accordingly, the thrust coefficient and torque coefficient will decrease. The influence of the pitch ratio on ducted propeller is similar to the conventional propeller, with the increase of pitch ratio, the peak efficiency will increase until to a constant value. Based on aforementioned research in a specified range of blade area ratio and diameter ratio, it is found that the minimum blade area ratio and minor diameter ratio will generate the extreme efficiency.

Abstract:To study the effect of hydrodynamic performance on the new type vertical axis tidal turbine, using ANSYS-CFX software is adopted to simulate different working conditions for comparing the hydrodynamic performance between cross-type twin-turbine and H-type single turbine. Cross-type vertical axis tidal current twin-turbine is an improved form to traditional H-type turbine. The results show that: the efficiency of twin-turbine is about 10% higher than that of single turbine. The efficiency of twin-turbine changes little for ebbs and flows. The average values of thrust, lateral force and moment are equal between cross-type turbine and H-type turbine, but cross-type turbine's amplitude of fluctation is far less than that of H-type turbine, which can produce beneficial effects on the structure safety and fatigue life of turbines. Therefore, the cross-type vertical axis tidal current twin-turbine has obvious advantages compared to H-type single turbine in the efficiency and guarantying running life.

Abstract:Pressure coring is considered as one of the key techniques for the natural gas hydrate-bearing sediment coring. To overcome the problem of low successful rate of pressure coring for a pressure corer, an ice-valve-based pressure corer made out of in situ drilling fluid during coring was proposed. A series of experimental tests were conducted by using a specially designed experimental system. The formation process of the drilling fluid ice valves was studied by both experimental tests and computational fluid dynamics (CFD) simulation method. The pressure sustaining capacity of the ice valves was also measured experimentally. Results showed that the sustained pressure of the ice valve increased almost linearly with the increase of the ice valve length. However, the increasing length of the ice valve caused the extension of formation time of the ice valve. To save the coring time, the ice valve length for pressure coring should be as short as possible while satisfying the sealing requirement.

Abstract:Land suitability evaluation is an important tool for identifying ecological attributes and potential development of lands and guiding the development rationally. Taking wudalianchi town for instance, this paper constructed seventeen factors of resistance valuation system, standing on profit and loss analysis, using RS and GIS, and applying minimum cumulative resistance model in space region, physical geography, social economy and ecological protection four aspects. The results showed: type Ⅰ(highly suitable construction region), type Ⅱ(relatively suitable construction region), type Ⅲ(relatively unsuitable construction region), type Ⅳ(unsuitable construction region). The areas of these types are 41.01 km², 138.97 km², 387.85 km² and 492.26 km², and the percentages of these types are 3.86%, 13.11%, 36.59% and 46.44%. The results can provide a technology method for township construction land suitability and space regulating planning.

Abstract:Based on the analysis of foreign evaluation system of daily outdoor leisure sport space, the proper methods for developing the evaluation tool in China are investigated. In the case of Harbin, the initial indicator set is presented by focus group and the questionnaire survey is designed. The evaluation system of daily outdoor leisure sports space including 5 domains and 43 items is developed by classifying the initial indicators with Principal Components Analysis (PCA), calculating weight and giving the evaluation formula. The evaluation system is proved to be practical through the trial evaluation and countermeasure design in 8 target spaces.

Abstract:To make public space of winter cities with high activity, 28 activity influential factors were identified according to scene investigation and visit to the typical public space of winter city. Through the questionnaire survey, the related datum about the influence factors on the degree of people's activity was obtained, and 26 of these factors were retained with factor analysis calculation. These factors were classified into 5 activity factors, which were natural activity factor, location activity factor, space activity factor, facility activity factor and cultural activity factor, respectively. Based on the data, five classification evaluation criterions of 26 influential factors were developed, such as excellent, good, medium, qualified and unqualified. The analysis result shows that it is helpful to improve public space activity of winter cities with enhancing the evaluation level of the 26 factors.