Arrival Times of Satellite-Broadened Laser Pulses

A method for measuring the time of arrival of very narrow laser pulses which have been reflected and randomly broadened by a target is examined. It is known that these return pulses from the target have very small rise times. A threshold detection algorithm that detects the rising edge of the pulse is used for obtaining the pulse arrival times. The errors of the scheme are evaluated numerically for different pulse shapes, and a loose bound on the errors of detecting a typical pulse is obtained. Finally, a gamma-density model is used to characterize the random gain processes of the optical receiver, and the effect of such random gains on the errors of threshold detection is analyzed.     

钛合金钣金件脉冲电流辅助热压成形精度控制

针对钛合金钣金件冷加工成形精度低、热成形机成形成本高的问题,提出一种采用脉冲电流辅助热压成形工艺。以TC1钛合金U型件为对象,研究了脉冲电流辅助热压成形工艺对钛合金钣金件的缺陷、尺寸精度及力学性能的影响。结果表明,脉冲电流加热可提高成形效率,当脉冲电流密度为9.2A/mm^2,合模速度为15mm/s时,补偿了成形过程的热量损失,使钛合金钣金件保持在良好的成形温度区间,增加其成形极限,得到零件表面质量良好、无裂纹。在成形过程中,随着成形压力、保压时间、模具预热温度的升高,零件的尺寸精度逐渐提高,当成形压力10t、保压时间10s、模具不预热时,零件的尺寸精度即可达到±0.2mm。脉冲电流辅助热压成形后零件的晶粒尺寸比热成形机成形的细小,且力学性能也较好。     

Next generation GPS-aided space navigation systems

The next generation of low cost Global Positioning System (GPS) receivers for space navigation and attitude determination are positioned to take full advantage of the improvements made in the commercial GPS receivers used for terrestrial applications. There have been recent improvements made to the GPS receivers that include the addition of extra GPS satellite channels that can be tracked simultaneously. The older style GPS receivers were only able to handle five channels at a time. In order for proper determination of three-dimensional position, a minimum of four channels was required and the fifth channel of the receiver was reserved to perform search functions for finding the next satellite. This included searching for satellites that could be used to replace exiting satellites moving out of the Field of View (FOV). The search function also enables the GPS receiver to search for the best constellation for maximum performance accuracy. The fifth roaming channel also provided a best next-satellite selection capability in case the field of view to one of the satellites was blocked or shaded.     

Mismatched Filtering of Sonar Signals

A replica correlator (matched filter) is an optimum processor for a receiver employing a pulse of continuous wave (CW) signal in a white Gaussian noise background. In an active sonar, however, when the target of interest has low Doppler shift and is embedded in a high reverberation background, this is not so. High sidelobes of the correlator frequency response pass a significant portion of the signal contained in the mainlobe of the reverberation spectrum. In order to reduce the sidelobes of the correlator output spectrum and at the same time keep the increase in its 3 dB bandwidth to a small amount, we propose lengthening of the replica of the transmitted signal and weighting it by a Kaiser window. It is demonstrated that by extending the weighted replica by 50 percent compared with the transmitted signal, it is possible to reduce the sidelobe levels to at least 40 dB below the mainlobe peak, with the concomitant increase of the 3 dB band-width by less than 5 percent. The degradation in signal-to-noise ratio (SNR) performance for such a ?mismatched? filter receiver with respect to the matched filter is less than 1.5 dB.     

The Detection of Moderately-Fluctuating Rayleigh Targets

Pulse train detection of fluctuating targets whose coherence time is long compared to the time between pulses and short compared to the pulse train duration is compared for two systems: (1) a recursive digital optimal receiver operating in conjunction with a coherent pulse train transmitter, and (2) a good (but not optimal) receiver operating with a block coherent frequency-hopping transmitter. The equivalence of this type of problem to noise-in-noise detection problems is demonstrated, the performance relations for both systems are derived and comparisons are made.     

Geometrical and electrical optimization of NS-SDBD streamwise plasma heat knife for aircraft anti-icing

Ice accretion on aircraft encountering supercooled water droplets in clouds poses great risks to flight performance and safety. With the aim of optimizing the newly developed streamwise plasma heat knife method for anti-icing, a parametric investigation is carried out in this work. The influence of the detailed voltage profile on the heating effects of a Surface Dielectric Barrier Discharge driven by Nanosecond Pulses (NS-SDBD) is investigated, and a comparison of the anti-icing performance among different configurations of streamwise plasma heat knife is made. The results show that columnar high-temperature regions produced by a multi-streamer discharge appear at small pulse rise time, but become diffuse as the pulse rise time increases. An optimal pulse rise time exists to provide a wide range and high value of temperature, which is found to be 150 ns for the setup in the present study. The influence of the pulse fall time is much weaker than that of the rise time. The range and value of the temperature decrease with increasing pulse fall time. A greater pulse width is found to improve the heating effect by increasing the discharge power. When a spanwise electrode is placed connecting the streamwise electrodes of the streamwise plasma heat knife at the airfoil leading edge, the anti-icing performance becomes poorer, whereas good performance is achieved when the spanwise electrode is at the edge of the streamwise electrodes. Based on this, a three-level configuration of the plasma heat knife is proposed, and its anti-icing performance is found to be much better than that of the original configuration.     

航空装备计量中脉冲上升时间测量不确定度分析和评定

脉冲信号是航空装备计量保障中非常重要的基础参数,示波器校准仪检定装置是测量脉冲信号上升时间的重要标准。介绍示波器校准仪检定装置的组成,对示波器校准仪检定装置测量高速脉冲上升时间进行分析,对测量结果的不确定度进行评定。结果表明:可通过设置较小的时基、提高脉冲的输入电平等手段减小测量不确定度。研究结果可为航空装备计量技术机构编制示波器校准仪检定装置的建标报告和评定脉冲上升时间的不确定度提供参考。     

脉冲干扰对软件接收机的影响分析

卫星信号经过长距离传播,信号能量损耗严重,到达地面的功率很弱,容易受到各种干扰的影响。脉冲干扰为常见的干扰类型,所以针对不同功率、不同周期,以及不同占空比的脉冲干扰信号,通过接收前端采集受脉冲干扰的GPS L1信号,利用软件接收机及多相关器生成技术,详细分析了脉冲干扰对接收机信号捕获与跟踪性能的影响。分析结果表明,周期为1ms的脉冲干扰信号,能对接收机产生强烈的干扰效果,捕获图中的噪声明显增大;跟踪过程中,载噪比和相关值突发性减小,造成跟踪数据异常。而长周期的脉冲信号仅在脉冲到达时影响接收机的捕获和跟踪,但由于信号跟踪不能连续进行,导致伪距观测量的不连续与导航数据不能正常解码,从而干扰接收机。     

Calculation of Detection Probability for Frequency Compressive Receivers

A numerical method is described for predicting the detection probability of a frequency compressive pulse receiver. The approach is general; i.e., input pulse envelope shapes, pulse offset frequencies, and filter magnitude responses are arbitrary. The performance of the frequency compressive receiver is analyzed and compared to that of a crystal video receiver for a specific case.     

Delay?Lock Repeater Tracking System Utilizing Superregenerative Interrogator

A unique delay?lock tracking system is described. The system includes an interrogator and a repeater operating on the same radio frequency, with a pulse repetition rate which is related to the distance. Single radio frequency operation allows utilization of a superregenerative radio frequency stage, which serves as both the receiver and the transmitter of the interrogator unit.     

空中加油调度的研究

研究了执行大规模空中加油任务的加油机群和受油机群的匹配方案，以完成总耗油量最少（或总耗时最短）为目标函数，有杉球面上航线约束的方法优化确定每次加油的最优加要用克尼格效能矩阵优化确定空中加油的调度方案。方案中了各种加油机基地和受油机基地的相对位置，受油机的安全可加油区域，最大可用加油机数目和最大需用加油机数目等约束条件。本研究可为实际应用提供理论依据。     

Clutter Suppression Properties of Weighted Pulse Trains

A common but troublesome requirement on radar sensors is the detection of a target in the interference from undesired scatterers, or clutter. Systems with coherent processing of pulse trains are uniquely suited for the purpose because, with pulse trains, it is possible to concentrate the receiver output for particular values of Doppler and thus suppress the clutter by Doppler filtering. This paper discusses to what degree the effectiveness of the method can be enhanced by tapering, or weighting, of the pulse amplitudes. The general results are illustrated by computer-plotted response functions for weighted pulse trains. The clutter suppression efficiency of weighting is calculated both for unilateral weighting in the receiver and for bilateral weighting in both receiver and transmitter. The significance of additional phase weighting is discussed and the results for pure amplitude weighting are compared with publishedwork on phase and amplitude weighting.     

适用于矢量接收机的模糊自适应CKF方法研究

卫星导航接收机矢量跟踪环路的核心就是用一个Kalman滤波器将标量接收机的信号跟踪和导航解算一起完成,优点是能够形成通道之间的相互辅助,缺点是也会相互影响。尤其在部分卫星信号被遮挡或者部分通道信号质量较差的环境下,问题通道会影响其他通道,甚至导致矢量跟踪环路滤波器发散,常规的方法是检测故障通道然后将故障通道剔除,这样需要对导航滤波器进行变维操作。针对此问题,提出了一种新的消除问题通道对其他通道影响的方法,同时不需要对导航滤波器进行变维。首先给出了一种标度因子,用来判断通道是否存在故障通道,然后给出一种利用模糊控制的导航滤波器自适应调整方法。仿真表明,在通道卫星信号被频繁遮挡的极端情况,矢量接收机依旧能保持正常的导航精度,并没有明显受到误差通道的影响,同时避免了对导航滤波器进行变维操作。     

High Dynamic GPS Receiver Using Maximum Likelihood Estimationand Frequency Tracking

A new high dynamic global positioning system (GPS) receiver ispresented and its performance characterized by analysis,simulation, and demonstration. The demonstration receiver is abreadboard model capable of tracking a single simulated satellitesignal in pseudorange and range rate. Pseudorange and range rateestimates are made once every 20 ms, using a maximum likelihoodestimator, and are tracked by means of a third-order fadingmemory filter in a feedback configuration. The receiver trackspseudorange with rms errors of under 1 m when subjected tosimulated 50 g, 40 g/s circular trajectories. The tracking thresholdis approximately 28 dB·Hz, which provides 12 dB margin relativethe the minimum specified signal strength, assuming 3.5 dB systemnoise figure and 0 dB antenna gain.     

On the Performance of Some Sequential Multiple-Decision Procedures

The performance of several sequential procedures for the following multiple-decision problem is investigated. Samples from k random processes (or populations) are available, k at a time (one from each process), to a receiver or data processor. One process contains a signal; the other k - 1 are statistically identical noise. The receiver is to select the odd process (locate the signal), with prescribed probability of error. The optimal receiver makes the selection in minimum average time. Analytical and Monte Carlo computations were performed under the hypothesis that the processes sampled are Rayleigh; however, a method for extrapolating results to other cases is given. The parameter k is allowed to vary from 2 to 1000.     

A One-bit Memory Receiver for Channels with Memory

A receiver is investigated with one-bit memory for communication channels with memory. The memory of the channel is characterized by the statistical correlation of the received bits due to fading. An optimal solution to the problem is considered but is found to be quite complex to implement. Therefore, a suboptimal receiver having a decision-feedback structure is examined. The performance of the receiver is shown to be better than a receiver without memory. Binary on-off keying system is considered as an example and numerical results are obtained.     

Self-calibrating SSR monopulse receiver

An extension of the monopulse technique for estimating the target azimuth in a secondary surveillance radar (SSR) is considered. The idea is to associate in pairs monopulse measurements coming from the amplitude processor (AP) at the dwell time processing level. This allows the automatic compensation of the bias errors due to the misalignments in the receiver channels, thus eliminating the necessity for periodic system calibration. This dual-pulse technique also allows for the practical use of the dot product receiver as a modification of the AP receiver. This, in turn, implies that the variance of each dual-pulse estimate is uniformly maintained at the monopulse maximum-likelihood level over the whole off-boresight angle (OBA) range     

Present-Day Sea Level Change: Observations and Causes

We investigate climate-related processes causing variations of the global mean sea level on interannual to decadal time scale. We focus on thermal expansion of the oceans and continental water mass balance. We show that during the 1990s where global mean sea level change has been measured by Topex/Poseidon satellite altimetry, thermal expansion is the dominant contribution to the observed 2.5 mm/yr sea level rise. For the past decades, exchange of water between continental reservoirs and oceans had a small, but not totally negligible contribution (about 0.2 mm/yr) to sea level rise. For the last four decades, thermal contribution is estimated to about 0.5 mm/yr, with a possible accelerated rate of thermosteric rise during the 1990s. Topex/Poseidon shows an increase in mean sea level of 2.5 mm/yr over the last decade, a value about two times larger than reported by historical tide gauges. This would suggest that there has been significant acceleration of sea level rise in the recent past, possibly related to ocean warming. This revised version was published online in August 2006 with corrections to the Cover Date.     

一种基于半符号的PSWFs调制信号PAPR抑制方法

针对椭圆球面波(Prolate Spheroidal Wave Functions,PSWFs)调制信号峰均功率比(Peak-to-Average Power Ratio,PAPR)较高,易受信道非线性特性影响这一问题,文章引入半符号的思想,针对仅传输半个周期的奇对称、偶对称信号,降低并行传输信号路数,提出了一种基于半符号 PSWFs调制信号 PAPR抑制方法。该方法在发射端仅传输部分波形,用以降低调制信号 PAPR;在接收端,利用函数良好的奇偶对称性,对信号进行恢复。理论与仿真分析结果表明,所提方法在未明显降低系统误码性能的前提下,能够有效降低椭圆球面波调制信号峰均功率比。     

Time-Dependent Nuclear Decay Parameters: New Evidence for New Forces?

This paper presents an overview of recent research dealing with the question of whether nuclear decay rates (or half-lives) are time-independent constants of nature, as opposed to being parameters which can be altered by an external perturbation. If the latter is the case, this may imply the existence of some new interaction(s) which would be responsible for any observed time variation. Interest in this question has been renewed recently by evidence for a correlation between nuclear decay rates and Earth–Sun distance, and by the observation of a dip in the decay rate for ⁵⁴Mn coincident in time with the solar flare of 2006 December 13. We discuss these observations in detail, along with other hints in the literature for time-varying decay parameters, in the framework of a general phenomenology that we develop. One consequence of this phenomenology is that it is possible for different experimental groups to infer discrepant (yet technically correct) results for a half-life depending on where and how their data were taken and analyzed. A considerable amount of attention is devoted to possible mechanisms which might give rise to the reported effects, including fluctuations in the flux of solar neutrinos, and possible variations in the magnitudes of fundamental parameters, such as the fine structure constant and the electron-to-proton mass ratio. We also discuss ongoing and future experiments, along with some implications of our work for cancer treatments, ¹⁴C dating, and for the possibility of detecting the relic neutrino background.