Percolation transition in temporal airport network

The air transportation system has a critical impact on the global economy. While the system reliability is essential for the operational management of air traffic, it remains challenging to understand the network reliability of the air transportation system. This paper focuses on how the global air traffic is integrated from local scale along with operational time. The integration process of air traffic into a temporally connected network is viewed as percolation process by increasing the integration time constantly. The critical integration time T_P which is found during the integration process can measure the global reliability of air traffic. The critical links at T_P are also identified, the delay of which will influence the global integration of the airport network. These findings may provide insights on the reliability management for the temporal airport network.     

结构弹性碰撞分析三种隐式时间积分法结果的比较

 采用与Newmark法和Wilsonθ法相同的基本假设,即设在每一时间步长内加速度为线性变化,然后经过等加权处理而得到一种条件稳定的隐式时间积分格式。作为积分法特性的标志,其周期延长率和幅值衰减率均以封闭的解析形式给出。分析表明,此法的周期延长率远小于Newmark法和Wilsonθ法,而幅值衰减也远低于Wilsonθ法。最后以质点-弹性杆碰撞分析为例,比较了上述3种隐式时间积分法所得的结果。     

Hybrid integration method for highly maneuvering radar target detection based on a Markov motion model

To detect highly maneuvering radar targets in low signal-to-noise ratio conditions, a hybrid long-time integration method is proposed, which combines Radon-Fourier Transform (RFT), Dynamic Programming (DP), and Binary Integration (BI), named RFT-DP-BI. A Markov model with unified range-velocity quantification is formulated to describe the maneuvering target’s motion. Based on this model, long-time hybrid integration is performed. Firstly, the whole integration time is divided into multiple time segments and coherent integration is performed in each segment via RFT. Secondly, non-coherent integration is performed in all segments via DP. Thirdly, 2/4 binary integration is performed to further improve the detection performance. Finally, the detection results are exported together with target range and velocity trajectories. The proposed method can perform the long-time integration of highly maneuvering targets with arbitrary forms of motion. Additionally, it has a low computational cost that is linear to the integration time. Both simulated and real radar data demonstrate that it offers good detection and estimation performances.     

Gravity Field Recovery from GRACE: Unique Aspects of the High Precision Inter-Satellite Data and Analysis Methods

The very high accuracy of the Doppler and range measurements between the two low-flying and co-orbiting spacecraft of the GRACE mission, which will be at the μm/sec and ≈10 μm levels respectively, requires that special procedures be applied in the processing of these data. Parts of the existing orbit determination and gravity field parameters retrieval methods and software must be modified in order to fully benefit from the capabilities of this mission. This is being done in the following areas: (i) numerical integration of the equations of motion (summed form, accuracy of the predictor-corrector loop, Encke's formulation): (ii) special inter-satellite dynamical parameterization for very short arcs; (iii) accurate solution of large least-squares problems (normal equations vs. orthogonal decomposition of observation equations); (iv) handling the observation equations with high accuracy. Theoretical concepts and first tests of some of the newly implemented algorithms are presented. This revised version was published online in August 2006 with corrections to the Cover Date.     

北斗导航接收机高增益相干积分算法

近年来,针对弱信号的高灵敏度接收机已逐渐成为国内外的研究热点。加长相干积分时间可以提高信噪比,从而跟踪到更弱的信号。但是,北斗导航接收机跟踪环路并不可以无限加长相干积分时间,相干积分时间的长短和功效还受到卫星导航电文比特跳变的限制。为了消除导航电文比特跳变对相干积分的影响,提出了一种改进的基于最大似然估计的北斗信号位同步方法,完成位同步后再利用先猜后检的方法便可以实现长相干积分。利用软件接收机进行编程设计,仿真结果表明：该长相干积分算法能够稳定可靠地实现对弱信号的跟踪,20ms相干积分环路信噪比约提升12dB,40ms相干积分环路信噪比约提升15dB,80ms相干积分环路信噪比约提升17dB,提高了北斗导航接收机的灵敏度。     

基于模糊分数阶PI~λD~μ的柔性结构振动主动控制

为防止翅翼结构因干扰而产生振颤,以柔性悬臂梁结构为被控对象,提出一种参数自整定的分数阶比例积分微分控制器,在利用遗传算法取得优化参数后,通过模糊规则第2次对控制器参数进行在线整定,以达到实时最优控制的目的.在Simulink软件环境下进行了数值仿真,结果表明:设计的分数阶比例积分微分控制器具有记忆性,动态表征能力强.与常规比例积分微分控制器相比,冲击激励下的系统响应稳态时间缩短0.9s;简谐激励下的系统响应衰减幅度从88%提升到96%;随机激励下系统响应抖动减小,提高了稳定性.     

Time-implicit simulation of particle-fluid systems

This paper presents one-dimensional particle-fluid hybrid simulations in which the strongly collisional components of the plasma (e.g., ions and thermal electrons with ν_cfΔt > 1) are treated as fluids and the weakly collisional components (e.g., energetic electrons with ν_cpΔt ≪ 1) are treated as particles. Here ν_cf denotes the fluid ion and electron collision frequencies, ν_cp is the energetic particle collision frequency and Δt is the time step. Collisions between particle and fluid components are treated by a Monte-Carlo method and mass transfers between the particle and fluid electron components are governed by collision frequency thresholds. The field is computed implicitly to allow time steps with Ω_p Δt > 1 (Ω_p: plasma frequency).     

Rapid development of tightly-coupled GPS/INS systems

This paper addresses the question: “Why aren't tightly-coupled OPS/INS systems everywhere, on aircraft, ships and land vehicles?” Two barriers to the widespread use are cited. One is the high cost of the INS, and the other is the cost and complexity of tightly-coupled OPS/INS integration. One of those two barriers has recently been diminished drastically with the development of a standardized software package for tightly-coupled integration. In the past, only the largest corporations have been able to pay the initial development cost for tightly-coupled OPS/INS integration, usually with funding from a large defense program. Using the new software package, integration and van test can be accomplished in a matter of days, and this has been demonstrated with field trials. The package is intended primarily for small companies that otherwise would not be able to build tightly-coupled OPS/INS systems at all. What would have been a prohibitive 3- or 4-man year development effort is reduced to a few man weeks. To accomplish an integration, the system integrator has to find a way, through serial interfaces or by some other means, to get the INS measurements of acceleration (accumulated velocity change ΔV) and attitude rate (accumulated angle change Δ&thetas;) into a processor, along with the raw data of a GPS receiver. He also has to find a way to time tag the INS ΔV, Δ&thetas; with GPS time. The rest of tightly-coupled OPS/INS integration is predominately accomplished in the standardized software package. That leaves the cost of the INS as the only remaining barrier to the very widespread use of OPS/INS, and invites new development of low cost inertial sensors. The focus of this paper is on the software package, and how it achieves standardization and ease of use while retaining the flexibility to produce optimal results with a variety of INS and GPS receiver types     

Advanced Technologies Demonstrated by the Miniature Integrated Camera and Spectrometer (MICAS) Aboard Deep Space 1

MICAS is an integrated multi-channel instrument that includes an ultraviolet imaging spectrometer (80–185 nm), two high-resolution visible imagers (10–20 μrad/pixel, 400–900 nm), and a short-wavelength infrared imaging spectrometer (1250–2600 nm). The wavelength ranges were chosen to maximize the science data that could be collected using existing semiconductor technologies and avoiding the need for multi-octave spectrometers. It was flown on DS1 to validate technologies derived from the development of PICS (Planetary Imaging Camera Spectrometer). These technologies provided a novel systems approach enabling the miniaturization and integration of four instruments into one entity, spanning a wavelength range from the UV to IR, and from ambient to cryogenic temperatures with optical performance at a fraction of a wavelength. The specific technologies incorporated were: a built-in fly-by sequence; lightweight and ultra-stable, monolithic silicon-carbide construction, which enabled room-temperature alignment for cryogenic (85–140 K) performance, and provided superb optical performance and immunity to thermal distortion; diffraction-limited, shared optics operating from 80 to 2600 nm; advanced detector technologies for the UV, visible and short-wavelength IR; high-performance thermal radiators coupled directly to the short-wave infrared (SWIR) detector optical bench, providing an instrument with a mass less than 10 kg, instrument power less than 10 W, and total instrument cost of less than ten million dollars. The design allows the wavelength range to be extended by at least an octave at the short wavelength end and to ∼50 microns at the long wavelength end. Testing of the completed instrument demonstrated excellent optical performance down to 77 K, which would enable a greatly reduced background for longer wavelength detectors. During the Deep Space 1 Mission, MICAS successfully collected images and spectra for asteroid 9969 Braille, Mars, and comet 19/P Borrelly. The Borrelly encounter was a scientific hallmark providing the first clear, high resolution images and excellent, short-wavelength infrared spectra of the surface of an active comet’s nucleus.     

Pull-In Behavior of Third-Order Generalized Phase-Locked Loops

This paper analyzes third-order phase-locked loops with an arbitrary phase-detector characteristic and imperfect integration by the quasi-stationary approach with a suitable linearization of the non-linear differential equation deseribing the loop behavior; ideal integration is also considered. The main feature of this technique is the use of an easy mathematical approach which allows a direct physical insight into acquisition behavior and the development of simple expressions for the time and pull-in range.     

基于LEO辅助的北斗B1C高灵敏快速捕获算法

随着低轨(LEO)卫星数量的不断增加,利用LEO星座辅助增强GNSS导航性能已经成为一种新的趋势.针对低信噪比环境下B1C信号难以捕获的问题,提出了一种基于LEO辅助的B1C信号高灵敏快速捕获算法.首先对提升接收机捕获灵敏度进行了分析,对比了相干积分与非相干积分对于信号处理增益的影响,得出在低轨导航增强信号的辅助下采用增加相干积分时间的捕获算法对低信噪比条件下B1C信号的捕获更有效.然后提出了一种基于LEO辅助的B1C高灵敏快速捕获算法,从理论分析和实验仿真两方面,对比验证了在LEO辅助下可以显著提高B1C信号的捕获灵敏度,缩短捕获时间,提高捕获效率.     

Planetary Accumulation with a Continuous Supply of Planetesimals

Previous calculations of the accumulation of small (∼10 km) planetesimals at ∼1 AU to form Mars-sized bodies assumed that the initial assemblage of planetesimals were all present at the outset. This is an obviously reasonable assumption in systems in which the time scale for growth time of ∼10²⁶ g planetary bodies is long compared to estimates of the evolutionary time scale of a protosolar disk, as was the case in the pioneering work of Safronov (1969). It is now found that as a result of the preplanetary assemblage being unstable with respect to the runaway growth of the largest bodies, this is unlikely to be the case. The more realistic alternative of adding the initial planetesimals on a ∼10⁵ year time scale is considered here, as well as the consequences of the initial planetesimals being considerably smaller than those assumed previously. It is found that although the time scale for runaway growth is now actually controlled by the availability of planetesimals, for planetesimal production time scales of ∼10⁵ yrs, the final consequences are very similar. These calculations do show, however, that as a consequence of continuous infall during the runaway growth process, the late initial planetesimals are likely to be catastrophically disrupted by mutual collisions. For this reason, a more detailed treatment of the growth of planetesimals into planetary embryos will require a better understanding of the difficult problem of formation of the initial planetesimals themselves. This revised version was published online in June 2006 with corrections to the Cover Date.     

结构动力学方程的显式与隐式数值计算

从理论上详细推导了动力学方程的显式隐式时间积分数值计算方法,针对经典中心差分格式在求解阻尼结构计算效率降低的问题,研究提出了一种新的显式积分格式,理论证明该数值积分格式能够处理各种复杂阻尼状态而不降低计算效率。对于不同的时间步长通过设定调节加速度参数,能得到比经典显式计算格式和NEWMARK逐步时间积分更高的精度,最后通过数值算例对三种计算格式的理论分析进行了验证。     

Geometric aspects of long-term noncoherent integration

Long-term integration is defined as integration, perhaps interrupted, over time periods long enough for targets to move through volumes in space resolvable by the radar. Because the motion of the target is unknown prior to detection, long-term integration must be performed along multiple paths representing plausible target paths. The geometry of such a set of integration paths affects detection performance in several ways. The simplest implementation of long-term integration, using constant radial velocity paths, is investigated. The effects of path geometry on detection is quantified and optimized for a target whose motion is nearly radial but otherwise unknown     

燃气轮机系统仿真集成概念及其分布对象实现

本文提出燃气轮机系统仿真集成的概念, 分析了关键实现技术, 然后从跨平台性入手, 给出 1个分布异构计算机平台上的面向对象实现方案。算例表明, 这一方案成功实现了系统仿真集成的主要功能, 从而为发动机系统的多学科开发、研制和实验提供了有效的仿真软、硬件平台, 并使发动机的远程协同设计成为可能。      

一类特殊的一阶常微分方程的初等积分法

微分方程是与微积分同时诞生的，从诞生的时候起，人们就试图用初等积分法求微分方程的解。经过一百多年的努力，人们掌握了一些可以用初等积分法求解的微分方程类型，也发现有许多微分方程无法用初等积分法求解。但还有很多微分方程可用初等积分法求解，只是人们还没有发现它们。所以，用初等积分法求解微分方程仍是微分方程领域的一个基本研究内容。在这篇文章里讨论了一类类似黎卡提方程的一阶微分方程的初等积分法。     

Architectures and GPS/INS integration: impact on missionaccomplishment

The GPS (Global Positioning System) signal contains information which, when properly combined with information from INS and other sensors, provides exceptionally high accuracy position, velocity, attitude, and time measurements. These ten elements of information (three each in position, velocity, and attitude, and one in time) are common, in various combinations, to most avionics functions. When viewed from a system perspective, this high-precision information can be thought of as an integration basis, or a reference set, which offers opportunities for reconfiguration of the offensive, defensive, communication, navigation, and other sensors. Various integration architectures for fusion of these sensors can inherently enhance, enable, or severely limit these potential mission capabilities. The choice of integration architecture, can directly and profoundly affect performance, cost of integration, cost of ownership, and exploitation of much greater mission capability. This is illustrated by an example     

结构动力响应精细积分级数解的并行计算

讨论了精细积分法的存储需求问题,指出级数解更适合计算大型结构的动力响应分析.研究了精细积分法级数解的并行计算,精细积分法的时程积分公式包含两项,采用第一项串行、第二项并行的并行算法对精细积分级数解实施并行计算,给出了相应的实现流程.讨论了并行算法的负载分配策略,为减少处理器等待初值的时间,对时间步数实施非平均分配.算例表明,并行算法具有好的加速比.     

Temporal and Spatial Variations of Heliospheric x-ray Emissions Associated with Charge Transfer of the Solar Wind with Interstellar Neutrals

A simple model has been developed that demonstrates that heliospheric X-ray emission can account for about 25%–50% of observed soft X-ray background intensities. Similar to cometary soft X-ray emission, these X-rays are thought to be produced in the heliosphere due to charge transfer collisions between heavy solar wind ions and interstellar neutrals. A more complex model has now been developed to take into account temporal and spatial variations of the solar wind and interstellar neutrals. Measured time histories of the solar wind proton flux are used in the model and the results are compared with the ‘long-term enhancements’ in the soft X-ray background measured by ROSAT for the same time period. This revised version was published online in August 2006 with corrections to the Cover Date.