Abstract:Radio frequency is a non-renewable resource, and frequency sharing has become the consensus of the International Telecommunication Union. Multi-service integration has been the development trend of the world radio satellite system. S band 2483.5 MHz ~ 2500 MHz has been allocated into the sharing band of satellite multi-service including radio determinations satellite service, radio navigation satellite service and mobile satellite service by International Telecommunication Union, and is expected to provide four functions: satellite navigation, route tracking, emergency rescue and information relay. The development of S band space based system, the frequency characteristics of navigation communication and the prospect of S band multi-service were introduced, the future application and social and economic benefits of the S band navigation in China were predicted. 

Abstract:For the demand of structural enhancement and backwards compatibility of modern BDS navigation signal which is represented by the pilot channel, BDS has proposed several new constant envelope modulations recently, including QMBOC and ADualQPSK modulations applied to B1 band, TD-AltBOC and ACE-BOC modulations applied to B2 band, DualQPSK modulation applied to B3 band, GCE-BOC modulation that multiplexes arbitrary signals at two different carrier frequencies. All the new modulations were systematically analyzed and a unified analytical expression of constant envelope modulation and multiplexing was summarized on the basis of the GCE-BOC modulation, which can provide an important reference for the comprehensive understanding of the signal design of the BDS global navigation satellite system and the development of new type of constant envelope modulation and multiplexing technology on this basis.

Abstract:The atmospheric PWV (precipitable water vapor) was derived from GPS observations of 5 stations over Hong Kong during four years from 2013 to 2016 and the accuracy of PWV(GPS) was assessed with PWV(ECMWF). Based on harmonic analysis, the PWV(GPS) was then compared with rainfall. Taking PWV(ECMWF) monthly means as reference, results show that the RMS(root mean square) of PWV(GPS) monthly means is between 1.02 mm and 3.07 mm and the bias is between -2.14 mm and 2.52 mm. Both PWV and rainfall have similar periodic characteristics. The peaks of PWV occur in July with amplitudes varying between 14.65mm and 16.79mm. And the peaks of rainfall also occur in July and the amplitudes are between 147.64 mm and 211.01 mm. The annual increase rate of PWV is positive at each GPS station, which indicates that the PWV over Hong Kong has an increasing trend during the past 4 years. While the annual increase rate of the actual rainfall is negative except in HKOH station, which means a decreasing trend during the 4 years.

Abstract:In order to overcome the faulty channels’ influence on the receiver and improve the robustness of the vector tracking loops, a faulty channel detection algorithm, which employs the innovative sequences of the navigation filter as the input vector of a support vector machine, was proposed. A trained support vector machine was used to classify the channel as faulty or not faulty. If the channel is classified as faulty, it will not be included in the navigation filter to update the navigation results. In this way, the algorithm can eliminate the bad influence of the faulty channels. The simulation results show that this method can accurately detect faulty channels, and improve the performance and robustness of the vector tracking loops. 

Abstract:The atomic clock′s frequency stability estimated by Allan variance will significantly deviate from its true value if anomalies occur. An uncorrelated differential robust estimation method based on Huber weight function was proposed. Firstly, a difference estimation model of Allan variance was established, and the computation complexity of noise identification and whitening introduced by correlated difference was analyzed on the basis of the statistical property of the clock difference sequence. In order to solve this problem, a construction model of uncorrelated difference sequence was established to avoid complex noise identification and whitening computation effectively. Based on this, the uncorrelated difference robust estimation method of Allan variance was derived, followed by a counteracted technique of error accumulation introduced by robust estimation. Finally, the integrated robust estimation solution was given, and its effectiveness was verified by experiment. The results show that the method has significance robustness for phase outliers and phase jumps, and the relative deviation introduced by anomalies can be reduced from almost 200% to less than 10%.

Abstract:In order to reduce the hardware realization difficulty, the GNSS(global navigation satellite system) sole antenna anti-jamming correlation receivers were studied in partialband interferences circumvent. Their frequency responses, SINR (signal-to-interference-noise ratio) losses and correlation output functions were analyzed. It is shown that the SINR losses of anti-jamming optimal receivers and correlation receivers will increase with the bandwidth and power increase of interferences. The SINR losses will decrease with the bandwidth increase of noise and the frequency difference will increase between interferences and signals. Moreover, the match and correlation output main lobes will be depressed and the side lobes will be raised by jammers so as to affect the acquisition, track and multipath mitigation performance of anti-jamming receivers. Analysis results will provide the design basis for the GNSS anti-jamming receivers.

Abstract:Since carrier tracking in presence of ionospheric scintillation is non-stationary, nonlinear and non-Gaussian, the conventional discriminator or Kalman filter-based methods used to enhance the robustness of tracking loops are less effective. A carrier tracking algorithm for scintillation mitigation was proposed based on online wavelet denoising, which can characterize the nonstationary scintillation signal much better. By designing a sliding windowbased online wavelet de-noising filer after the integration of I and Q channels, the non-Gaussian noise caused by scintillation can be filtered effectively. As a result, the effect of ionospheric scintillation on loop discriminator is mitigated, and the accuracy and robustness of the tracking loop is improved. Experimental results prove the validity of the analysis and the proposed carrier tracking algorithm.

Abstract:Aiming at the problem that fixed measurement environments cannot establish a fixed base station and thus cannot carry out precise positioning, a differential positioning algorithm for mobile base station based on BeiDou navigation system was proposed, which means achieving high-precise differential positioning while the base station and the rover both moving. Based on the carrier phase measurement, under the condition of dynamic short baseline, the datum were inter-station and inter-satellite double-difference processed, eliminating receiver clock error and other common errors. Based on the linear combination of multifrequency measurement, the observation of the super-wide lane, the wide lane, the middle lane and the narrow lane was obtained, the window-moving average filter was carried out and the ambiguities were fixed by the stepwise ambiguity determination method, that is, along the order from the super-wide lane to the narrow lane to solve the ambiguity. In order to verify the effectiveness of the algorithm, a verification experiment of the external geometric parameter detector based on the BeiDou navigation system was designed to realize the static relative positioning accuracy of millimeter and the RTK relative positioning accuracy of centimeter.

Abstract:For solving the real-time problem of the software correlator, a parallel receiving model of spread spectrum signal based on vector dot product was proposed. The calculation speed of the signal correlation was improved by parallel computing correlation value of each channel with a large number of ALU in GPU(graphics processing unit). Simulation results show that, 25 channels with 150 correlation computation consume 967 μs in all for 1 ms signal of 25 MHz sampling rate by using the vector dot product parallel correlation algorithms based on GPU. Compared with the implementation on CPU based on MKL(math kernel library), the proposed method speeds up to 61.4 times, and the real-time correlation of the broadband spread spectrum signals in software can be realized.

Abstract:BeiDou satellites concentrates near the equator, which results in a low positioning accuracy within the homeland in north-south direction. Because the visible satellites are positive elevation, the vertical positioning accuracy is also low. To solve the above problems, a method to improve BeiDou satellites positioning accuracy was proposed, which uses pseudolites to improve the user observation geometry. The least square positioning algorithm based on the combination of pseudolites and satellites was derived. Dilution of positioning in north-south and vertical direction were also proposed to evaluate the positioning precision. Simulation results show that the dilution of positioning in north-south and vertical direction can be reduced effectively by lying pseudolites in the north area of users, thus the positioning accuracy can be effectively improved.

Abstract:Based on nonlocal Euler-Bernoulli beam theory, the dynamic characteristics analysis model for nanobeams resting on viscoelastic foundation and subjected to a magnetic field was built. The Kelvin viscoelastic foundation model and the Lorentz magnetic force were introduced to derive the governing equations of the system. The new general analytical expressions for the complex natural frequencies of the nanobeams were obtained on the basis of the Kelvin-Voigt model and some typical special cases were also discussed. Then the governing equations of motion were solved by using the transfer function method to obtain the natural frequencies and corresponding mode shapes in closed form for the nanobeams with arbitrary boundary condition. Considering a single-walled carbon nanotube as a numerical example, the first three natural frequencies under various boundary conditions were obtained, and a detailed parametric study was conducted to examine the effect of nonlocal parameter, the strength of the magnetic field, the aspect ratio, the damping parameter and the boundary conditions on the vibration characteristics of nanobeams. The results demonstrate the efficiency of the proposed model for dynamic characteristics analysis of the nanobeams resting on viscoelastic foundation under a magnetic field.

Abstract:In order to inhibit the influence of the output signals of three-axis simulator exhibited the phase delays and noise on the performance of missile’s hardware-in-loop simulation system, a modified tracking differentiator which was used to filter and phase advance was analyzed and designed. Based on the principle of traditional tracking differentiator, the basic structure of tracking the output signals of threeaxis simulator was designed. Due to the differential signal obtained from the tracking differentiator trembles existed noise, the vibration phenomenon of the output signals was analyzed. On this basis, a design method that the differential signal cascaded another tracking differentiator was proposed to filter the signal pollution and output signals noise. Finally, as the examples of one missile’s hardware-in-loop simulation system which were the output signals of three-axis simulator exhibited the phase delays and noise, the performance of the system was verified by comparing with the lead correction method. Simulation results show that the attitude angular velocity can be extracted by the modified phase advancer efficiently, and the modified phase advancer possesses excellent filter and phase advanced performance, which prove the correctness and validity of the modified phase advancer in the hardware-in-loop simulation system for missile.

Abstract:According to the loading mode and failure mechanism of a certain anti-detaching spring, a testing device for creep of anti-detaching spring was designed and manufactured. The creep accelerated aging test was done, and the effect of the temperature and loading on the creep behavior of antidetaching spring was investigated with image measuring technology. Furthermore, the creep kinetic equation of anti-detaching spring subjected to loading and temperature based on Arrhenius equation was derived to predict the service life of the anti-detaching spring. The results show that the displacement of the spring due to creep is a monotonic non-decreasing function of time. Increasing the temperature or initial preload will result in accelerating its creep displacement. The method and conclusion may be helpful in creep anti-detaching spring design and life prediction.

Abstract:Aiming at the servo mechanism fault of launch vehicle, a FDD (fault detection and diagnosis）based on extended multiple model adaptive estimation was proposed. Attitude dynamic model of launch vehicle considering servo mechanism fault was established; the fault angle was used as the state variable to obtain the augmented state space model; the nonlinear estimation of state vectors and fault parameters was carried out by using the extended Kalman filter, and based on the sensor measurement data, the occurrence probability of faults was calculated by the hypothesis testing algorithm; the fault detection and diagnosis procedure based on the extended multiple model adaptive estimation was presented. Simulation results shows that, not only the health monitoring of the servo mechanism can be carried out by the algorithm without fault, but also under single servo mechanism fault, the core servo mechanism whose fault appeared can be timely and exactly detected through the algorithm, and the angle of nozzle under servo mechanism fault can be estimated accurately.

Abstract:In order to improve the operational effectiveness and operational indicators of the UAV (unmanned aerial vehicle), and to enhance the accuracy and reliability of the UAV relative navigation system, a novel relative navigation method was proposed. Under the background of relative navigation system, the cubature information filter based on the cubature Kalman filter and information filter was studied. Moreover, an INS/GPS/VisNav relative navigation filter was designed by making use of the multi-sensor information fusion theory and distributed information fusion structure to fuse the information from INS, VisNav and GPS, and then the relative position, velocity and attitude were obtained. By making use of this algorithm, the accuracy, reliability and stability of the reliability navigation system were all improved. In addition, the accuracy decrease and numerical instability which often occur to traditional filter were avoided by cubature information filter. Mathematical simulation results indicate that the method can improve the accuracy and reliability of the UAV relative navigation system, and the proposed algorithm is verified.

Abstract:In fixed-receiver bistatic SAR (synthetic aperture radar), the spaceborne SAR was used as the illuminator. The direct-path signal and the backscattering echo were sampled, synchronized and focused by the receiver. The geometry of fixed receiver bistatic SAR was introduced. After the direct-path signal was used as reference signal for range match filtering, the spatial variation of chirp rate, range cell migration and two-dimensional spectrum were analyzed. Based on azimuth preprocessing, direct-path signal compensation and nonlinear Chirp Scaling imaging algorithm, a new focusing algorithm was presented. Simulation results of point targets were presented to validate the feasibility and efficiency of the proposed method. This algorithm was also validated by the measured data of the HITCHHIKER system.

Abstract:Under the multi-path channel condition, the preamble-based channel estimation methods have very large pilots overhead in OQAM/OFDM (offset quadrature amplitude modulation/orthogonal frequency division multiplexing). To solve this problem, a scattered pilots aided channel estimation method based on the compressed sensing for OQAM/OFDM system was proposed by utilizing the sparsity of wireless channel. The principle of OQAM/OFDM channel estimation method based on compressed sensing was established and a scattered pilots pattern, which deploys pilots symbols on few subcarriers randomly, was also designed. The proposed method can reduce the pilots overhead significantly and realize the highly accurate channel estimation. The simulation results validate the efficacy and the superior performance of the proposed method in both slow and fast time-varying wireless channel.

Abstract:A particle filtered matching pursuit for compressive sensing of blind sparsity signal polluted by non-Gaussion noise was proposed, while the conventional detectors(e.g. least-squares estimates) were known to be sensitive to the non-Gaussion nature of noise. The proposed algorithm which combined the Huber cost(loss) function with an l₁-norm did not need the sparse prior while it eliminated the interference of measuring noise by particle filter estimation. Meanwhile, sparsity adaptive matching pursuit was used to sift the effective support set so as to inverse the original states. Simulation results indicate that the proposed algorithm outperforms the existing greedy iterative inversions in the same condition, especially in the non-Gaussion noise situation.

Abstract:Multipath is the key factor which affects communication, navigation system performance. It not only affects the signal power, but also causes time delay, frequency shift and polarization parameters changing, resulting in a decrease in the communication and navigation system. To solve this problem, the fast reverse ray tracing algorithm based on virtual source tree, partition, backward detection, was proposed to predict multipath signal loss and delay distribution accurately. Besides, the urban scenes were taken as examples to show the algorithm of signal multipath transmission, and the diffraction algorithm was studied to improve the accuracy of the prediction. Taking the signal propagation through simplified street models for example, the consistency of reflection path and power distribution can be proved by comparing with the MATLAB program based on the proposed algorithm and the simulation from using Wireless Insite software, in which way, the validity of the algorithm was also verified.

Abstract:In order to improve the ranging accuracy between satellites and increase the available bandwidth of systems in the simulation and tests, a design algorithm for high-accuracy fractional delay filter based on the boundary fitting Remez algorithm was proposed. The proposed algorithm used the polynomial approximation thought of Farrow structure to fit the impulse response boundary coefficients of the filter designed by Remez algorithm, and achieved fractional delay filters through the polyphase decomposition. The proposed algorithm achieved continuous impulse response boundary when high-order filters was used, while the group delay error was reduced and the accuracy was improved. The simulation results show that the proposed algorithm improves the accuracy of fractional delay filters a little, doubles the available bandwidth and reduces the number of multipliers need to be used.

Abstract:The rapid development of the Internet also causes more and more network threats. How to detect the network threats in a real-time and accurate manner becomes one of the key technique issues. The alert-correlation-based network threat detection technique is becoming the research hotspot, which couples with the widely used security products and fully exploits the relation between abnormal events to reconstruct the attack scenario. Starting from the features of network threats and security environment, the requirements and classification of network threat detection were introduced. Then the basic concepts and system model of alert-correlation-based network threat detection technique were illustrated in detail. The key module of the model, aler correlation method, and the fundamentals and features of different kinds of typical algorithm were studied in detail, including causal-relation-based method, case-based method, similarity-based method and data-mining-based method. Furthermore, three kinds of representative detection system architectures were discussed with practical instances, namely centralized architecture, hierarchical architecture and distributed architecture. Finally, based on the analysis of recent research work, the future work is discussed and outlined.

Abstract:A new type of cold atom interferometry gravimeter based on Bragg diffraction was presented, which is able to increase the gravity measurement sensitivity and stability of common Raman atom gravimeters significantly. By comparing with Raman transition, the principles and advantages of Bragg diffraction-based atom gravimeters were introduced. The theoretical model for a time-domain Bragg atom gravimeter with atomic incident direction parallel to the wave vector of Bragg lasers was constructed. Some key experimental requirements for an nth-order Bragg diffraction-based atom gravimeter were deduced, including the temperature of atom sources, the diameter, curvature radius, frequency, intensity, and timing sequence of Bragg pulses, etc. The analysis results were verified by the existing experimental data in discussion. The presented theoretical model and conclusions provide a meaningful reference for the understanding and construction of a Bragg diffraction-based cold atom gravimeter.

Abstract:Under the condition of satellite platform′s jitter, the push-broom high-resolution optical imaging often produces distortion, which affects the accuracy of the image positioning and information extraction. According to the critical compensation issue for distorted images, a steady-state imaging model with the linear accelerometer directly measuring platform jitter was proposed to achieve reconstruction of high-resolution distorted images. The effectiveness was validated by using the distorted panchromatic image of a high-resolution satellite affected by platform jitter. The analysis results show that the linear accelerometer can effectively detect and measure the platform jitter, the designed on-ground reconstruction model of jitter image can effectively improve the distorted imaging of push-broom optical image caused by platform jitter, and the jitter image′s distortion phenomenon can be obviously eliminated.

Abstract:The intuitionistic fuzzy matter-element model with intuitionistic fuzzy decision information was proposed for the traditional matter-element theory which cannot be applied in intuitionstic fuzzy decision-making. It presented relative similarity to replace the traditional correlation function and then integrally described intuitionistic fuzzy matter-element multi-attribute decision-making method and its steps. An example was used to show the effectiveness of this method, especially it analyzed the multi-attribute group decision-making problem with unknown weight by using the projection method and intuitionistic fuzzy entropy to prove the practicability of this method.It is proved that the method presented here can offer a new approach to solve intuitionistic fuzzy multi-attribute group decision-making problems.

Abstract:The experimental and finite element simulation methods were used to investigate the mode conversion, the reflection and the transmission phenomenon at the elbow in small diameter tubes. Excitation signals of different frequency were used to inspect tubes involve elbows of different bending radius. The results show that the mode conversion phenomenon occurs at the elbow and the L(0,1) guided wave converts to F(1,1) guided wave. The orientation of F(1,1) is identical to the extradosintrados direction of the elbow. With the increase of the inspection frequency and bending radius, the reflected F(1,1) mode decreases monotonically while the transmitted F(1,1) mode changes non-monotonically, the reflected L(0,1) mode decreases while the transmitted L(0,1) mode increases. When the inspection frequency and bending radius increase to some extent, the change of inspection frequency or bending radius has little influence on the reflected F(1,1) mode, reflected L(0,1) mode and transmitted L(0,1) mode. The experimental results agree with the simulation results well which validates the truth of the simulation results. The conclusion will provide theoretical guide for the inspection of small diameter tubes involve elbows.

Abstract:The reliability analysis approach of PMS (phased-mission system) based on Markov process encounters the state-explosion problem for large systems. To avoid the state-explosion problem, the PMS reliability model using hierarchical method was proposed. The hierarchical method uses a low-level continuous time Markov chain and an upper-level binary decision diagrams. The isomorphic nodes consolidation strategy and the redundant nodes removal strategy were given to construct reduced hierarchical model. The above nodes compressing strategies can improve the efficiency of model generation and storage. A recursive computing algorithm solving the hierarchical model was designed on the basis of the PMS components ordering, and its computational complexity was linear with the number of nodes in model. Finally, by a case study, the number of nodes in the original model was compared with the proposed model using nodes compressing strategy, also the mission reliability in the hierarchical model was compared with it in the Markov model. Results show the correctness and effectiveness of the reduced hierarchical model.

Abstract:Aimed at the high velocity and unpredictable trajectory characteristics of the hypersonic targets in near space, the optimal midcourse trajectory design and trajectory cluster generation methods were proposed for the effective coverage of the target’s predicted impact area. Firstly, by analyzing the challenges that the hypersonic targets have formed to the existing air defense systems, the necessity of the trajectory cluster design and generation in midcourse phase was illustrated. Secondly, the optimal midcourse trajectory design was treated as the problem of searching for optimal solutions under numerous constrains, and the nominal optimal trajectory was generated with the optimal control theory. Thirdly, based on the NOC (neighboring optimal control) theory, the neighboring optimal trajectory cluster generation algorithm was developed to meet the adjusted terminal constraints. Finally, the simulation results testify the effectiveness of the proposed method.

Abstract:Affected by the influence of the attitude variation and vibration of the satellite platform in the process of tracking and pointing, free space optical communication terminal will therefore reduce the tracking precision of the system. In order to improve the system′s ability for the purpose of resisting disturbance, on the basis of coarse tracking system, the cascade linear active disturbance rejection controller was put forward. The coarse tracking system used direct current brushless torque motor driving two-dimensional turntable as the actuator, and was controlled by the coarse tracking controller. Analysis of the system disturbance is required, for which the tracking differentiator is applied to extract. With azimuth axis of the two-dimensional turntable as the object, the cascade linear active disturbance rejection controller was applied to position loop and speed loop for the sack of active disturbance compensation through linear extended state observer. Therefore, double isolation of internal and external loop disturbance can be achieved, and the tracking precision shall be improved. The experimental analysis, and a comparison with the proportion integral derivation controller were also included. The research results show that there is a 50% decrease of standard deviation referred to the tracking error，when inputting 2 Hz, 5 Hz and 〖JP2〗8 Hz sinusoidal signals with the 〖JP2〗same disturbance signal simulation. 〖JP3〗At the same time, the observer′s〖JP〗 disturbance compensation effect in the circumstance of linear expansion was verified. A high tracking accuracy is still ensured when active disturbance rejection control is not sensitive to the frequency variation of inputted sinusoidal signal with changing frequency.

Abstract:Accelerometer is one of the fundamental measurement units of inertial navigation system. It is difficult to meet the precision requirement for low-cost accelerometer due to the manufacturing process and all kinds of sensor errors. Calibration for accelerometer is essential before being used. Therefore, an accelerometer self-calibration algorithm based on maximum likelihood estimation was proposed. The sensor errors model was established by taking comprehensive consideration of zero bias, scale errors, non-orthogonal errors, installation errors and measurement noise of the accelerometer, based on which the calibration problem of accelerometer was transformed into maximum likelihood estimation problem of calibration parameters. The self-calibration algorithm based on maximum likelihood estimation was tested by both numerical simulation and real data experiment. The result shows the maximum likelihood estimation algorithm has a high precision of parameters estimation and can calibrate the errors caused by factors mentioned above effectively.

Abstract:In order to reduce the disturbance torque during the assembly of the seeker’s servomechanism, a drive torque measurement system of servo-mechanism was built on the basis of electrical measurement. In the process of precise assembly and adjustment, the drive torque was measured in real time and the assembly parameters were adjusted to reduce the disturbance torque. The characteristics of the mass unbalance torque and the moving cable torque in the driving torque were experimentally verified, and the balancing and routing methods were determined. The relationship between the preload of the bearing and the friction torque of the servo mechanism was also experimentally verified. It is of great significance to solve the problem of automatic running process to reduce the fluctuation amplitude of the friction torque. The method of measuring the rotation error of the rotating shaft is determined, which can be used for determining optimum preload of bearing. Results show that the traditional assembly process of servo mechanism is optimized, and the qualitative method based on the experience of technician′s experience is optimized as a quantitative method.