Classification of birds and drones by exploiting periodical motions in Doppler spectrum series

With the rapidly growing abuse of drones, monitoring and classification of birds and drones have become a crucial safety issue. With similar low radar cross sections(RCSs), velocities, and heights, drones are usually difficult to be distinguished from birds in radar measurements. In this paper, we propose to exploit different periodical motions of birds and drones from highresolution Doppler spectrum sequences(DSSs) for classification.This paper presents an elaborate feature vector representing the periodic fluctuations of RCS and micro kinematics. Fed by the Doppler spectrum and feature sequence, the long to short-time memory(LSTM) is used to solve the time series classification.Different classification schemes to exploit the Doppler spectrum series are validated and compared by extensive real-data experiments, which confirms the effectiveness and superiorities of the proposed algorithm.

Tolerance analysis of multiple-element linear retrodirective cross-eye jamming

Tolerance sensitivity limits the practical application of the cross-eye jammer.Previous literature has demonstrated that retrodirective cross-eye jamming with multiple antenna elements possesses the advantage of loose tolerance requirements compared to traditional cross-eye jamming.However,the previous analysis was limited,because there are still some factors affecting the parameter tolerance of the multiple-element retrodirective cross-eye jamming(MRCJ)system and they have not been investigated completely,such as the loop difference,the baseline ratio and the jammer-to-signal ratio.This paper performs a comprehensive tolerance analysis of the MRCJ system with a nonuniformspacing linear array.Simulation results demonstrate the tolerance effects of the above influence factors and give reasonable advice for easing tolerance sensitivity.

A Novel Kind of Transverse Micro-Stack High-Power Diode Bars

新奇横向的微栈的半导体激光工具条被提出改进半导体激光的光力量在低注射电流锁的输出。更重要地，新奇激光酒吧有 a coupled 大光洞，它能克服灾难的光损坏的问题并且改进轻横梁质量线索到协调地联合了沿着横向的方向射出。微栈的隧道新生 tri 活跃的区域激光结构被金属种器官的化学蒸汽免职。为一微弱地联合了非常 ated 设备，光力量在 50 一开车电流下面超过 60 W，斜坡效率到达 1.55 W/A。进一步的实验证明 23 度的垂直分叉从横向的强烈联合的设备被完成。

Radio frequency compatibility analysis for BDSBAS and GPS/WAAS

Radio frequency signal compatibility is the basis of interoperability of the Satellite eBased Augmentation System(SBAS).SBAS should abide by relative international radio regulations of International Telecommunication Union(ITU) and meet the compatibility requirements of radio frequency signal between the Global Navigation Satellite System(GNSS)/SBAS,in order to avoid negative mutual interference.According to ITU Proposal and related reference and assumptions,the paper made simulation of signal receiving maximum power in the Bei Dou Satellite-Based Augmentation System(BDSBAS)global signal coverage.And then,interference of BDSBAS to Global Positioning System(GPS)/WAAS(Wide Area Augmentation System) on L1/L5 bands were calculated and analyzed,with equivalent carrierto-noise ratio as the evaluation parameter.The result shows that the carrier-to-noise ratio decrease of GPS/WAAS caused by BDSBAS B1 C and B2 a signals are extremely lower than inter-system interference of GPS/WAAS,and thus can be ignored in practical applications.Therefore,BDSBAS will not cause the service performance degradation of GPS and WAAS.

Design of 1.5 bit quantization correlator in satellite navigation software receiver

Currently, 1 bit or 2 bit signal quantization is widely used in satellite navigation software receivers. The bit-wise parallel algorithm has been proposed for 1 bit and 2 bit signal quantization, which performs correlation with high efficiency. In order to improve the performance of the correlator, this paper proposes a new 1.5 bit quantization method. Theoretical analyses are made from the aspects of complexity and quantization loss, and performance comparison between 1.5 bit quantization correlator and traditional correlators is discussed. The results show that the 1.5 bit quantization algorithm can save about 30 percent complexity under similar quantization loss, reduce more than 0.5 dB signal noise ratio（SNR） loss under similar complexity. It shows great performance improvement for correlators of satellite navigation software receivers.

An anomaly detection method for spacecraft solar arrays based on the ILS-SVM model

Solar arrays are important and indispensable parts of spacecraft and provide energy support for spacecraft to operate in orbit and complete on-orbit missions.When a spacecraft is in orbit,because the solar array is exposed to the harsh space environment,with increasing working time,the performance of its internal electronic components gradually degrade until abnormal damage occurs.This damage makes solar array power generation unable to fully meet the energy demand of a spacecraft.Therefore,timely and accurate detection of solar array anomalies is of great significance for the on-orbit operation and maintenance management of spacecraft.In this paper,we propose an anomaly detection method for spacecraft solar arrays based on the integrated least squares support vector machine(ILS-SVM)model:it selects correlated telemetry data from spacecraft solar arrays to form a training set and extracts n groups of training subsets from this set,then gets n corresponding least squares support vector machine(LS-SVM)submodels by training on these training subsets,respectively;after that,the ILS-SVM model is obtained by integrating these submodels through a weighting operation to increase the prediction accuracy and so on;finally,based on the obtained ILS-SVM model,a parameterfree and unsupervised anomaly determination method is proposed to detect the health status of solar arrays.We use the telemetry data set from a satellite in orbit to carry out experimental verification and find that the proposed method can diagnose solar array anomalies in time and can capture the signs before a solar array anomaly occurs,which reflects the applicability of the method.

Characteristics of penetration and distribution of a liquid jet in a divergent cavity-based combustor

The atomization process of a liquid jet in a divergent cavity-based combustor was investigated experimentally using high-speed photography and schlieren techniques under a Mach number 2.0 supersonic crossflow.Gas-liquid flow field was studied at different divergent angles and injection schemes.It is found that complex wave structures exist in the divergent cavity-based combustor.The spray field can be divided into three distinct zones:surface wave-dominated breakup zone,rapid atomization zone and cavity mixing zone.A dimensionless spray factor is defined to describe the concentration of spray inside the cavity qualitatively.As a result,it is revealed that for the large divergent angle cavity,the injection scheme near the upstream inlet has a higher penetration depth but a lower spray distribution,where the injection scheme near the cavity has a more spray distribution.For the small divergent angle cavity,the injection scheme near the upstream inlet also has a higher penetration depth and the injection scheme near the start point of the divergent section has a more sufficient spray distribution.The small divergent angle cavity-based combustor with the upstream wall transverse injection is an optimized injection scheme to improve both penetration and spray distribution inside the cavity.Finally,a penetration depth formula is proposed to explain the spray and distribution behaviors in the divergent cavity-based combustor.

Review of atomization mechanism and spray characteristics of a liquid jet in supersonic crossflow

The injection and atomization process of a liquid fuel jet is critical for an ignition start of a scramjet engine.Airwall-mounted crossflow injection strategy is widely used in scramjet combustors,avoiding high total pressure loss and allowing the liquid fuel to rapidly undergo atomization,mixing,and evaporation.In this review,research progress on a liquid jet in supersonic crossflow was evaluated from aspects of atomization mechanism and spray characteristics.When a liquid jet is injected into a supersonic crossflow,primary and secondary breakups occur successively.The surface instability of liquid can significantly affect the breakup process.This review discusses the current understanding of the breakup process and spray characteristics of a liquid jet in supersonic crossflow including the mechanism of atomization and the characteristics of distribution and atomization.The development of windward Rayleigh-Taylor(R-T)unstable waves is the main factor in column breakup.The development of Kelvin-Helmholtz(K-H)unstable waves along the circumferential direction of the jet or droplets is the main factor of surface and droplet breakups.The liquid-gas momentum ratio is the most important factor affecting the penetration depth.The span width of the liquid jet is affected by the windward area.Breakup and coalescence lead to a transformation of the size distribution of droplets from S-or C-shaped to I-shaped,and the velocity distribution of the droplets on the central symmetry plane has a mirrored S-shape.The droplet distribution on the spanwise cross-section retains a structure similar to an“Ω”shape.At last,some promising recommendations have been proposed,namely a theoretical predictive model which can describe the breakup mechanism of a liquid jet,the distribution characteristics and droplets size distribution of a liquid jet under a cavity combustion chamber,especially for enthalpy flows with complex wave structures.

Robust reinforcement learning with UUB guarantee for safe motion control of autonomous robots

This paper addresses the issue of safety in reinforcement learning(RL)with disturbances and its application in the safety-constrained motion control of autonomous robots.To tackle this problem,a robust Lyapunov value function(rLVF)is proposed.The rLVF is obtained by introducing a data-based LVF under the worst-case disturbance of the observed state.Using the rLVF,a uniformly ultimate boundedness criterion is established.This criterion is desired to ensure that the cost function,which serves as a safety criterion,ultimately converges to a range via the policy to be designed.Moreover,to mitigate the drastic variation of the rLVF caused by differences in states,a smoothing regularization of the rLVF is introduced.To train policies with safety guarantees under the worst disturbances of the observed states,an off-policy robust RL algorithm is proposed.The proposed algorithm is applied to motion control tasks of an autonomous vehicle and a cartpole,which involve external disturbances and variations of the model parameters,respectively.The experimental results demonstrate the effectiveness of the theoretical findings and the advantages of the proposed algorithm in terms of robustness and safety.

Principle and performance of BDSBAS and PPP-B2b of BDS-3

Within the framework of diferential augmentation,this paper introduces the basic technical framework and performance of the BeiDou Global Navigation Satellite System(BDS-3)Satellite-Based Augmentation System(BDSBAS),including orbit products,satellite clock ofset products,ionosphere and its integrity performance.The basic principle of BDS-3 Precise Point Positioning(PPP-B2b)is expounded,the similarities and diferences between the PPP service provided by BDS-3 and International Global Navigation Satellite System(GNSS)Service(IGS)are discussed,and the limitations of PPP-B2b are analyzed.Since both the BDSBAS and PPP-B2b utilize a ground monitoring station network to determine the satellite orbits and clock ofset corrections,and broadcast diferential corrections through the three Geostationary Orbit(GEO)satellites of BDS-3,the feasibility of the co-construction of BDSBAS and PPP-B2b is analyzed,strategies for the infrastructure sharing and correction broadcasting are presented,and the infuences of BDSBAS correction broadcasting strategy adjustment are evaluated.In addition,it assesses the possibility of broadcasting diferential corrections through the Inclined Geosynchronous Orbit(IGSO)satellites of BDS-3,and the feasibility of augmenting satellite navigation with Low Earth Orbit(LEO)satellites.

Propagation and self-healing properties of Lommel-Gaussian beam through atmospheric turbulence

The superposition of basic non-diffracting beams triggered new research hotspots lately,laying opportunities for long-distance wireless optical communication.The Lommel-Gaussian(LMG)beam formed by the superposition of Bessel-Gaussian light not only possesses non-diffraction feature,but also has tunable symmetry.With the help of Poynting vector analysis,we observed a smaller radial energy flow component during the propagation of the high order symmetrical LMG beam,which allows it to maintain the original beam profile over long distance.Thanks to the energy oscillation of the mainlobe and sidelobes,the mainlobe blocked by the symmetrical LMG beam can be restored.Also,the random phase screen with angular spectrum method is used to describe the beam behaviors in turbulence.The results show that the symmetry LMG is preferred in free space optical communication,and the asymmetric LMG performs poorly due to asymmetric energy transfer.

Initial analysis for characterizing and mitigating the pseudorange biases of BeiDou navigation satellite system

Pseudorange bias has become a practical obstacle in the field of high-precision global navigation satellite system(GNSS)applications,which greatly restricts the further development of high-precision applications.Unfortunately,no studies have been conducted on the pseudorange biases of the BeiDou navigation satellite system(BDS).To mitigate the effects of pseudorange biases on the BDS performance to the greatest extent possible,the origin of such BDS pseudorange biases are first thoroughly illustrated,based upon which the dependency of the biases on the receiver configurations are studied in detail.Owing to the limitations regarding the parameter re-settings for hardware receivers,software receiver technology was used to achieve the ergodicity of the receiver parameters,such as the correlator spacing and front-end bandwidth,using high-fidelity signal observations collected by a 40-m-high gain dish antenna at Haoping Observatory.Based on this,the pseudorange biases of the BDS B1I and B3I signals and their dependency on different correlator spacings and front-end bandwidths were adequately provided.Finally,herein,the suggested settings of the correlator spacing and front-end bandwidth for BDS receivers are in detail proposed for the first time.As a result,the pseudorange biases of the BDS signals will be less than 20 cm,reaching even under 10 cm,under this condition.This study will provide special attention to GNSS pseudorange biases,and will significantly promote a clear definition of the appropriate receiver parameter settings in the interface control documents of BDS and other individual satellite systems.

Fast BSC-based algorithm for near-field signal localization via uniform circular array

In this paper,we propose a beam space coversion(BSC)-based approach to achieve a single near-field signal local-ization under uniform circular array(UCA).By employing the centro-symmetric geometry of UCA,we apply BSC to extract the two-dimensional(2-D)angles of near-field signal in the Van-dermonde form,which allows for azimuth and elevation angle estimation by utilizing the improved estimation of signal para-meters via rotational invariance techniques(ESPRIT)algorithm.By substituting the calculated 2-D angles into the direction vec-tor of near-field signal,the range parameter can be conse-quently obtained by the 1-D multiple signal classification(MU-SIC)method.Simulations demonstrate that the proposed al-gorithm can achieve a single near-field signal localization,which can provide satisfactory performance and reduce computational complexity.

Two-order Hermite vector-interpolating subdivision schemes

A family of two-order Hermite vector-interpolating subdivision schemes is proposed and its convergence and con- tinuity are analyzed. The iterative level can be estimated for given error. The sufficient conditions of C2 continuity are proved. Geometric features of subdivision curves, such as line segments, cusps and inflection points, are obtained by appending some conditions to initial vectorial Hermite sequence. An algorithm is presented for generating geometric features. For an initial se- quence of two-order Hermite elements from unit circle, the numerical error of the 4th subdivided level is O(10?4).

Multiplexing modulation design optimization and quality evaluation of BDS-3 PPP service signal

The Precise Point Positioning(PPP)service of BeiDou-3 Navigation Satellite System(BDS-3)is implemented on its Geostationary Earth Orbit(GEO)satellites.However,its signal design is limited by the actual power of satellite and other conditions.Furthermore,the design needs to fully consider the compatibility of diferent service phases.Starting from the actual state of the BDS-3 GEO satellite,this paper studies the multiplexing modulation of the BDS PPP service signal that is based on the Asymmetric Constant Envelope Binary Ofset Carrier(ACE-BOC)technique and proposes several feasible schemes for this signal.Comparison and optimization of these techniques are made from the aspects of transmission efciency,multiplexing efciency,and service forward compatibility.Based on the Type-III ACE-BOC multiplexing modulation technique,phase rotation and intermodulation reconstruction techniques are proposed to suppress the intermodulation interference issue.Finally,a signal based on improved ACE-BOC multiplexing is designed.The quality of the proposed signal was continuously monitored and tested using large-diameter antennas.The evaluation results show that the power spectrum deviation of the signal is 0.228 dB,the correlation loss is 0.110 dB,the S-curve slope deviation is 1.558%on average,the average length diference between the positive/negative chip and the ideal chip is only 0.0006 ns,and the coherence between the carrier and the pseudo code is 0.082°.All quality indicators are satisfactory,indicating that the proposed signal multiplexing modulation technique is an ideal solution that meets all the requirements of the design constraints,and can achieve efcient information broadcasting and forward compatibility of the BDS PPP service.

The OH Maser Line Receiving System for the Urumqi 25m Radio Telescope

A maser spectral line system is newly implemented on the Urumqi 25m Radio Telescope. The system consists mainly of a cooling receiver and a 4096 channels digital correlation spectrometer. The frequency resolution of the spectrometer at the maximum signal bandwidth of 80MHz is 19.5kHz. After careful calibrations observation at the 1665 MHz OH maser emission was made towards a number of sources, including W49N and W75N. The observed results demonstrate that the digital correlation spectrometer is suitable for astronomical spectral line observations.

Precession–nutation correction for star tracker attitude measurement of STECE satellite

Space Technology Experiment and Climate Exploration （STECE） is a small satellite mis- sion of China for space technology experiment and climate exploration. A new test star tracker and one ASTRO 10 star tracker have been loaded on the STECE satellite to test the new star tracker＇s measurement performance. However, there is no autonomous precession nutation correction func- tion for the test star tracker, which causes an apparent periodic deflection in the inter-boresight angle between the two star trackers with respect to each other of up to - 500 arcsec, so the preces- sion and nutation effect needs to be considered while assessing the test star tracker. This paper researches on the precession-nutation correction for the test star traeker＇s attitude measurement and presents a precession-nutation correction method based on attitude quaternion data. The peri- odic deflection of the inter-boresight angle between the two star trackers has been greatly eliminated after the precession and nutation of the test star tracker＇s attitude data have been corrected by the proposed method and the validity of the proposed algorithm has been demonstrated. The in-flight accuracy of the test star tracker has been assessed like attitude noise and low-frequency errors after the precession-nutation correction.

Global capabilities of BeiDou Navigation Satellite System

The BeiDou Navigation Satellite System (BDS) has completed the constellation deployment and started to provide global services. After achieving the capabilities of global coverage, global first-class accuracy for Positioning, Navigation and Timing (PNT), global Inter-Satellite Links (ISL) networking, and global featured services, BDS will promote the construction of the comprehensive PNT infrastructure in the new era and play a more active role in international cooperation with other Global Navigation Satellite System (GNSS) providers to better serve humankind and the world.

Advances of SBAS authentication technologies

Satellite Based Augmentation System (SBAS) provides the corrections and integrity information to users, but as its signal format is opened to the public and Global Navigation Satellite System (GNSS) spoofing technology becomes more realistic, more feasible and cheaper. It’s foreseeable that there will be risks of spoofing threats against SBAS in the future. SBAS signal authentication technology provides a system-level solution to spoofing threats by adding special markers to SBAS signals so that receivers can verify whether the SBAS signals are from the on-orbit Geostationary Earth Orbit (GEO) satellites or whether the signal information has been forged and tampered with. First, this article introduces the existing anti-spoofing methods that can be applied to SBAS, especially the Elliptic Curve Digital Signature Algorithm (ECDSA) and Timed Efficient Stream Loss-Tolerant Authentication (TESLA) protocols. Then it discusses four possible solutions in a combination with the existing SBAS Interface Control Document (ICD). Two main Key Performance Indicators (KPIs), Time Between Authentication (TBA) and Authentication Latency (AL), obtained in the four main scenarios are compared. By analyzing the EGNOS Authentication Security Testbed (EAST) test simula-tion results of European Geostationary Navigation Overlay Service (EGNOS) in Europe, the impact of SBAS after joining the authentication service is obtained.

Two-maneuver indirect contingency return from a low lunar orbit

The problem of contingency return from the low lunar orbit is studied.A novel twomaneuver indirect return strategy is proposed.By effectively using the Earth’s gravity to change the orbital plane of the transfer orbit,the second maneuver in the well-known three-maneuver return strategy can be removed,so the total delta-v is reduced.Compared with the singlemaneuver direct return,our strategy has the advantage in that the re-entry epoch for the minimum delta-v cost can be advanced in time,with a minimum delta-v value similar to that of the direct return.The most obvious difference between our strategy and the traditional single-or multiplemaneuver strategies is that the complete transfer orbit is a patch between a two-body conic orbit and a three-body orbit instead of two conic orbits.Our strategy can serve as a useful option for contingency return from a low lunar orbit,especially when the delta-v constraint is stringent for a direct return and the contingency epoch is far away from the return window.