Abstract:Based on precise ephemeris provided by Wuhan University, BDS(BeiDou navigation satellite system) broadcast ephemeris errors were computed over the past three years from January 2013 to September 2015. The orbit errors, clock errors, and SISREs (signal-in-space user range errors) are presented and analyzed in different time periods for assessment. Results show that the radial accuracy of overall BDS satellites is less than 0.7 m and the normal accuracy of that is less than 1.4 m. Additionally, no obviously long term trends are found in both radial and normal errors. In term of along-track errors, the accuracies of IGSO(inclined geo synchronous orbit) and MEO(medium earth orbit) satellites are all less than 2.1 m and the along-track accuracy of GEO(geosynchronous earth orbit) is improved from 14 m to 8 m. The clock accuracies of GEO, IGSO and MEO are 6.3 ns, 4.7 ns and 4.3 ns respectively, and the clock accuracies of all satellites are more than 6 ns. The SISREs of all satellites are generally less than 2 m, and the SISREs of MEO satellites are relatively stable over the past 3 years. However, IGSO SISREs and GEO SISREs have some fluctuations in certain level.

Abstract:In order to accurately simulate the Doppler characteristics of high dynamic GNSS signals, a method for designing any-order DDS (direct digital synthesis) signal synthesizer was proposed. An any-order DDS synthesizer structure was designed; a formula for calculating the initial phase of every accumulator in DDS synthesizer was derived by using theoretical analysis; a technique to determine the DDS word length was developed. Finally, the simulation accuracy of the proposed method for the Doppler characteristics of GNSS signals was validated through simulations. In addition, since the proposed method for designing DDS synthesizer has no order limitation, it can be widely used in different types of signal simulators.

Abstract:The presence of the new frequency introduces more observations and degrees of freedom in data combination, which is beneficial for cycle-slip detection and repair. A novel real time cycle-slip detection and repair method based on undifferenced triple-frequency observations was proposed. This method employed two groups of geometry-free and ionosphere-free combinations of code and carrier phase observables, and constructed the third linearly independent detector for cycle-slip by eliminating the ionospheric delay variation between two consecutive epochs. The performance of the proposed method was validated with BeiDou triple-frequency observations with 30 s sampling interval. Results show that this method can effectively detect and correct the cycle slips on each frequency, even under high ionospheric activity condition. 

Abstract:A dual-mode reconfigurable GPS L1/BDS B1 radio frequency front-end adopting low intermediate frequency architecture was realized in 0.18μm CMOS process. An auto-calibrating circuit was used to adjust the intermediate frequency filter′s time constant and to reduce frequency uncertainty. A 4-bits capacitors array was designed to widen the frequency tuning range of the voltage controlled oscillator and to improve phase noise performance. The system power consumption was reduced by hardware reuse technique. Test results show that the power consumption is 37.8mW with 1.8V voltage supply, and the voltage gain is 103dB, while the noise figures are less than 3.2dB in both GPS L1 and BDS B1.

Abstract:Ionospheric scintillation is a rapid change in the phase and amplitude of navigation signals, resulting in degraded accuracy and robustness of signal tracking loops. Compared with the frequency tracking loop and delay tracking loop, carrier tracking loop is much weaker, and will lose lock much more continually in scintillation. Therefore, a carrier tracking algorithm for BeiDou signals scintillation mitigation based on adaptive EKF (extended Kalman filter) was proposed. The integration of in-phase and quadrature channels was used to estimate the phase lock indicator values which are the control parameter to adjust the measurement vector of EKF adaptively with different scintillation scenarios. Through the adaptive measurement vector, the accuracy and robustness of the tracking can be improved and the probability of filter divergence can be decreased. Experimental results prove the validity of the analysis and the proposed carrier tracking algorithm.

Abstract:Carrier phase double difference GNSS spoofing detection technique is an effective one of those methods based on phase measurement, but the high false alarm probability problem is caused by angle ambiguity for double-element antenna-array. Therefore, a technique based on antenna-array direction change was presented. The carrier phase double differences were estimated in different antenna-array directions, and then the carrier phase double differences were used to discriminate the spoofing signals. In this way, the angle ambiguity can be eliminated, and the false alarm probability was reduced. Simulation results verified the effectiveness of the proposed spoofing detection technique.

Abstract:The traditional multipath mitigation algorithms for high-order BOC (binary offset carrier) signals have a narrow effective convergence region, which is a weakness for stably tracking, therefore, a multi-region discriminator for multipath mitigation of high-order BOC signals was proposed. The optimized QStrobe discriminator was applied when code error was in the near zero region, which improved the multipath mitigation performance, while the QBOC discriminator was applied in the other regions which had an excellent convergence performance. The Kalman filter was utilized to estimate the tracking error with open-loop method for real-time switch of discriminator. Simulation Results of BOC(15, 2.5) and BOC(14, 2) signals show that the multi-region discriminator algorithm can eliminate the tracking ambiguity. In addition, compared with the traditional QStrobe algorithm, the 6 dB attenuated multipath error envelope area is improved by 51% and 57% respectively.

Abstract:The spoofing interference can mislead target receiver in resulting in wrong position and time, which is a serious threat to the security of GNSS applications. An anti-spoofing method using the pseudo-range single-differences of two receivers was proposed. Using the variance analysis technique, the optimal spoofing detection variable based on the pseudo-range single-differences was deduced and the statistical character of the detection variable was analyzed. After analyzing, the parameters such as the receiver noise, the receiver baseline, and the satellite number have a large influence on the detection performance; as the receiver noise and satellite number are uncertain, the detection performance can be improved by increasing the length of baseline. When the length of baseline is 10 meters, the simulation results illustrate that the spoofing detecting probability is up to 98% if the false alarm rate is 0.01.

Abstract:Considering that the global navigation satellite system signal is buried in the strong interference and thermal noise, a novel anti-jamming scheme which utilizes subspace projection and beamforming method was proposed. The data vector received by antenna array was projected onto the noise subspace which is orthogonal to interference subspace to achieve interference suppression. When the local code was synchronized with the reference antenna jammer-free data, beamforming was performed according to the minimum mean square error criterion. Simulation results show that the proposed method can steer the main beam to the direction of global navigation satellite system signal wave with effective interference suppression, and the output signal-to-noise ratio can be significantly increased while the correlation peak curve and the carrier tracking performance are not corrupted.

Abstract:The analytical evaluation indexes for three signal distortion errors of BOC(Binary Offset Carrier) signals were developed. Then the proposed evaluation indexes were used to evaluate the distortion errors of BOC(14,2) and MBOC(6,1,1/11) signals, which were the advised basis signals of BeiDou global satellite navigation system. The proposed distortion error evaluation indexes can be used to evaluate the distortion errors of modern global navigation satellite system signals, and it has directive significance to modern signal design and integrity monitoring.

Abstract:Building a guide star catalog which has the characteristics of completeness, simplicity and query rapidity is an important prerequisite and guarantee to realize the SINS/CNS(strapdown inertial navigation system/celestial navigation system). The Tycho-2 catalog in astronomy was selected as the initial catalog of the guide star catalog. The ball rectangle partition method was improved on the basis of the Tycho-2 catalog. It included how to divide the partition and the sub partition. The selection strategy of the alternative guide star was also creatively proposed. The experimental results show that the number of the alternative guide stars using the improved ball rectangle partition method most accounts for 19.37% of the guide stars used for the declination zone method and only accounts for 3.12% of the guide stars used for the traverse method. Therefore, the improved ball rectangle partition method is more accurate and rapid, and is more suitable for the strapdown inertial navigation system/celestial navigation system.

Abstract:Nozzle is the crucial component used to generate high pressure water jet, and its structure form has large influence on dynamic performance of high pressure water jet. So the influence of nozzle structure on high pressure water jet was analyzed, and the optimization design of nozzle structure parameter was implemented. A two-phase flow computational fluid dynamics model was employed to analyze the flow field. 〖HJ*2〗The Kriging surrogate model was used to replace the computational fluid dynamics model in the process of optimization design for reducing the computational resources. The nondominated sorting genetic algorithm and multiobjective evolutionary algorithms based on decomposition were respectively employed to carry out single and multi objective optimization design. The research results show that the general capability of lineform nozzle is the best, the concavity-form nozzle is in the middle, and the protrudingform nozzle has the worst capability. The single and multi objective optimization design of lineform nozzle is implemented, in which the core zone length and the mass flux of water jet are taken as optimization objectives. Compared with the baseline, the two indexes increased by 14.71% and 27.56% respectively after the single objective optimization. The optimal semi-cone angle after multi objective optimization located on ［15.4°， 89.8 °］. The global optimization algorithm based on surrogate model and evolutionary algorithm is proved to be effective.

Abstract:To reduce calculation time and improve optimization result, a Gauss pseudospectral method was proposed. For the attack angle was regarded as a state variable and only bank angle was optimized, which was reasonable since attack angle profile was often predesigned in engineering. Down-range of vehicle was regarded as event constraint to obtain the maximum cross-range, while the flight path angle was regarded as path constraint to eliminate skips on the trajectory. Through simulation, the footprint of CAV(common aero vehicle) is generated, which is in accordance with the theoretical analysis of the indirect optimization method.

Abstract:In order to resolve the problem of relative navigation during the course approaching a non-cooperative space target, a highly accurate state propagation model of non-cooperative relative orbit motion was proposed on the basis of the relative orbit dynamics equations and the second order Runge-Kutta formula. Taking the J₂ perturbation into account and expanding the target orbit equations into second order Taylor series beside the reference orbit, the angular velocity and the angular acceleration of reference orbit were further deduced and the relative orbit dynamics differential equation was built. After giving the initial value, the second order Runge-Kutta method was used to propagate the relative orbit motion. With the use of numerical integral method, the model can be widely used without any limited condition in relative orbit dynamics model forms; with the use of second order Runge-Kutta method, the computing efficiency is improved while the calculation accuracy is guaranteed. Two simulation scenarios, a low Earth orbit scenario and a high Earth orbit scenario, were designed to testify the generality and precision.

Abstract:A research was presented to solve the problem of dynamics modeling and characteristics analysis in the case of the mass distribution of the spacecraft changes obviously during the on-orbit propellant refueling process. The variable-mass vehicle was considered to consist of two parts: a rigid platform and the liquid propellant confined in tanks. The liquid in tanks was regarded as a finite number of lumped masses with time-varying mass, configuration and position, so that, the actual physics could be abstracted as a set of variable mass particles with fixed border. Then, the dynamics model of refueling spacecraft was established by using the general equations of mass-varying particles set. Based on some reasonable assumptions about the structure of vehicle, the misalignment moment and the reactive force were eliminated from equations. It is remarkable that the established model include time-varying parameters and damping terms. Besides, the dynamics characteristics of the model were analyzed by using the Lyapunov′s stability criterion. Simulation results demonstrate that the difference structure of the vehicle effect the state trajectories dramatically, and prove that theoretic analysis is correct.

Abstract:The robust flight control based on the generalized extended state observer was investigated to address the problems of parameter uncertainties, external disturbances and flight control with measurement noise. The nonlinear longitudinal dynamics were transformed into the linear-like structure state-space equations of state-dependent coefficients on the basis of the state-dependent Riccati equation control method. According to the control method based on generalized extended state observer, the disturbance compensation gain was designed to estimate and attenuate the external disturbances. Consequently, the state feedback gain and disturbance compensation gain were derived to track the desired angle of attack by solving the state-dependent coefficients and the algebraic Riccati equation on-line. Compared with other control methods, the proposed approach not only is robust with the parameter uncertainties and external disturbances, but also remains perfect in tracking performance with the measurement noise. It has high engineering application value.

Abstract:In order to solve the configuration divergence problems of formation satellites under the complex mechanics environment in near-earth space, especially under the non-spherical perturbation influence, a formation satellites configuration control method based on improved LQR (linear quadratic regulator) was presented. The method estimated the maximum bounded range of error caused by an un-modeled perturbative force in near-earth space formation satellites configuration design, then used the 2-norm of maximum bounded range to improve the classical LQR method and improved the robustness of classical LQR controller in controlling formation satellites configuration. In order to evaluate the effectiveness of improved method, a quantitative criterion for judging the robustness was given. The simulation results show that the improved method can greatly improve the robustness of classical LQR method and improve the resistance ability of formation satellites control methods for all kinds of uncertainty.

Abstract:Si-O-C material was prepared by a polymer-derived method using copolymer of phenyl-substituted polysiloxane and divinylbenzene as raw materials. The porous Si/Si-O-C anode material was prepared with the chemical reduction of Si-O-C material by magnesium at high temperature under argon atmosphere. The composition, structure, morphology, and formation process of porous Si/Si-O-C anode material were investigated by X-ray diffraction, energy spectrum analysis, elemental analysis and field emission scanning electron microscope. The electrochemical properties of the material were characterized by using the electrochemical test instrument. The research results show that the magnesium will react with oxygen of Si-O-C material and turn into MgO and Mg₂SiO₄ during the reduction process. When washed with HCl, MgO and Mg₂SiO₄ react with HCl and turn into MgCl₂, which can dissolve in the solution. Then, the porous Si/Si-O-C anode materials are formed. The silicon distribute in the porous Si-O-C material, which can improve the cycle performance of silicon. It can confirms that the material prepared by using magnesium reduction method is a potential material for the lithium-ion battery.

Abstract:Graphene paper is a potential candidate for heat spreader because of its special thermal conductivity. As graphene paper is assembled by the graphene sheets, the size of graphene sheets has an important effect on assembly method, microstructure and thermal conductivity of the graphene paper. The influence of the size of graphene sheets on the microstructure and thermal property of graphene paper was studied. Firstly, the filtration-assembling method was used to fabricate graphene oxide paper. Then, the grapheme paper was obtained from the heat-reduced process of the graphene oxide paper under the Ar/H₂ atmosphere. The SEM(scanning electron microscope) characterization result shows that the graphene paper with the bigger graphene sheet size is of higher density. The XRD(X-ray diffraction) characterization result also shows that the crystalline of the graphene paper is increased when the graphene sheet size is bigger. When graphene sheet size increased from the range of 0.3 μm～5 μm to that of 50 μm～100 μm, the thermal conductivity is about 632.8 W/mK and 683.7 W/mK respectively，which is increased by 8%. 

Abstract:To study the multi-hit performance of steel-tube-confined concrete, experiments of thick steel-tube-confined concrete targets normally impacted by 12.7 mm armor piercing projectile were carried out, the damage modes and the depth of penetration were obtained, and the formula for multi-hit depth of penetration was established. The results show that: steel-tube-confined concrete targets of 300 mm in thickness can effectively resist armor piercing projectile for 3 times, and the penetration depth increment for the second and third impacts to their former impact are less than 20% and 10%, respectively; the formula for multi-hit penetration depth is in good agreement with that of experiment. Results are beneficial to the studies of anti-bullet structures and bursting layer structures of steeltube-confined concrete.

Abstract:In order to analyze the structure stress and the current density distribution on the pulsed reactor in the course of working, the test on the stress was designed. The hoop stress on both the edge and the center of copper belt was obtained by the resistance transducer. The stress result was validated by calculating the thick wall cylinder model with the inflation pressure. To calibrate the stress value, the copper belt coil with a ballonet inside was designed. The half-bridge circuit, electrical grounding of the bridge cases and shielding wires were designed and put in practice to reduce the pulsed electromagnetic interference. The disturbance generated by the parasitic resistance and the stray capacitance was reduced by electrical grounding of the bridge cases. The experiment results show that the hoop strain is larger than the axial strain, the strain on the edge is larger than the strain in the center. Reinforcement should be applied on the edge of the copper belt. And it can be speculated that the current density on the edge is about 1.2 times near the center.

Abstract:An algorithm for modulation recognition of multiple quadrature amplitude modulation signals based on constellation recovery was proposed. Firstly, parameters of the carrier frequency and signal to noise ratio were estimated from its spectrum. According to the baud rate and the symbol timing, the baud rate sampling was finished. Then a non-data-aided carrier frequency offset estimation method was used to mitigate the effect of frequency offset and phase offset in constellation recovery. Finally, the modulation type was recognized by the average likelihood ratio test method. Simulation results demonstrate that the proposed algorithm has a better recognition performance when comparing with the amplitudebased maximum likelihood algorithm. 

Abstract:The principle of static angle measurement based on mechanically dithered ring laser gyro was described. The expression of angle measurement random error with gyro′s angle random walk coefficient and measurement time was analyzed and verified by checking the indexing errors of turntable. The systematic error of static angle measurement and the indexing errors of turntable were separated by using the permutation intercomparison method. Test results and analysis show that the static angle measurement method is robust under practical environment, the standard deviation of random error is less than 0.26″ and the systematic error is less than 1″. The key techniques for further improvement in accuracy were analyzed.

Abstract:A new bearing fault detection method based on the signal sparse decomposition theory was developed. An over-complete dictionary on which the bearing vibration signals in normal state can be represented sparsely was trained by the dictionary learning method. According to the fact that this dictionary just can sparsely represent the signals in normal state, the bearing vibration signal in unknown state was decomposed on this dictionary. The bearing state was determined by comparing the representation error of the signal on the dictionary with the given error threshold, and then the bearing fault detection was achieved. Experimental tests validate the effectiveness of the proposed method in bearing fault detection when setting an appropriate error threshold.

Abstract:MC-CDMA (multicarriercode division multiple access) is an important multiple access technique for mobile communications. In conventional MC-CDMA, construction of spreading sequences and multiuser detection are considered separately, which means that the transmitter and the receiver are not designed jointly. Inspired by LDPC (low density parity check) codes, the transceiver models of MC-CDMA based on graph theory were designed, and the low density Tanner graph was used to describe the relationship between spreading chips and data symbols. In addition, message passing algorithm, which was suitable for the Tanner graph, was utilized to perform iterative multiuser detection. Flooding and serial schedules for the message passing were deduced and analyzed. Simulation results show that the conventional MC-CDMA cannot perform well under the overloading conditions, while the proposed low density Tanner graph for MC-CDMA achieves satisfactory performance, which provides the useful reference for the design of next generation mobile networks.

Abstract:In view of the problems that sole guide system cannot meet the modern battlefield demand and data update of multi-sensor is not synchronized, a new MIMU/MMW/IR (miniature inertial measurement unit/ millimeterwave/infrared) composite guidance system was built and the time synchronization and space match problems of multi-sensor asynchronous information fusion in this system were studied. A novel adaptive UKF (unscented Kalman filter) algorithm was presented. The statistics of status model error was built by prediction error in this algorithm; contribution of status model information to status parameter estimation was adjusted by adaptive factor. So the influence of status model noise is effectively regulated. The proposed algorithm was applied to the MIMU/MMW/IR compound guidance system and the algorithm performance was tested. The simulation results show that the adaptive UKF is better than the standard UKF and extended Kalman filter, and it can improve the positioning precision effectively.

Abstract:Based on Zwart-Gerber-Belamri cavitation model, a numerical calculation method of cavity flow of water-exit missile with complex configuration was proposed, in which the technology of dynamic mesh and composite mesh was used. Cavitation formation, evolvement and influence of shoulder, rudder and protuberance of water-exit missile were analyzed. The simulation results acclaim that in the beginning of water-exit course, body-attached bubble quickly grows due to reduction of cavitation number. After that, the effect of bubble break-off and oscillation becomes more obvious. The cavitation of shoulder firstly appears at the water-exit speed of 15 m/s. And it grows quickly with water-exit speed. The dissymmetry of shoulder cavitation becomes obvious in the attack angle of 5°～10°. The cavitation of rudder is hardly influenced by waterexit speed. The proportion of cavitation area of rudder is less than 10% when the attack angle is less than 5°, and rapidly increases to 50% when the attack angle beyond 5°. Reduction of protuberance size is beneficial to cavitation avoidance. The influence of the attack angle on protuberance cavitation is unapparent. Protuberance can induce the cavitation on nearby missile body at high speed.

Abstract:The backbone network is the main part of the Internet traffic transfer system, and the router-level topology of backbone network is of great significance for the research on network survivability. Due to various reasons, it is difficult to get real Internet backbone router level topology. By analyzing the forming driving factors of backbone network, the geographical constraints, the exchanging strength of nodes, infrastructure cost and robustness, etc. were combined; the router-level topology generation method for Internet backbone networks under multiple constraints was proposed. The method can generate router level topology for network which cannot be inferred from publicly accessible measurement data, and can generate topology set that consists of realistic alternatives for a backbone network. Finally, the effectiveness of the method is verified by two real Internet backbone networks. 

Abstract:A study about how to effectively achieve semi-structured spatio-temporal data storage and query in HBase was carried out. The formal description of the problem was issued; the HBase semi-structured spatio-temporal storage model was proposed by using a semi-structured approach TwigStack. On this basis, semi-structured spatio-temporal range query and kNN queries were carried out. An experiment was made in terms of real data and a comparison was made with MongoDB which needs higher hardware configuration, the results show that the performance of semistructured spatiotemporal query algorithm is similar to MongoDB in the machine with general configuration, so that it has the advantage in the practical application.

Abstract:For the estimation problem of pose and attitude of fixed-wing unmanned aerial vehicles in the terminal landing stage, a vision-based landing method using structured line features was proposed. One forward looking camera equipped in the fixed-wing unmanned aerial vehicles was used to capture multiple pictures of the structured line features and these features were extracted automatically. 6 degrees of freedom pose and attitude parameters (pitch angle, yaw angle, roll angle, longitudinal position, lateral position and altitude) were calculated by using geometric constraint of full configuration structured line features in the earlier stage of the landing, the key parameters (pitch angle, yaw angle, lateral position and altitude) were calculated by using degenerate configuration structured line features (only the runway edges) in the final stage of the landing. In the 3D stimulation experiment, the accuracy of distance is less than 0.5 m, the accuracy of angle is less than 0.1°when the fixed-wing unmanned aerial vehicle is 200 m distant from the airport. 