The Effect of Hard Limiting in the Presence of Large Out-of-Band Interfering Signals

The effect of hard limiting on the phase of a test signal was investigated when large out-of-band interfering signals are present at the input to the limiter. First a single and then two discrete interfering signals were considered. The interfering signals and the test signal are restricted to narrow but separated bands. The test signal is recovered after limiting by narrowband filtering. The analysis of the single interfering signal is essentially the same as that performed by Cahn.1 In this correspondence experimental evidence is presented to verify the analysis and, in addition, the case of two interfering signals is considered.     

Costas编码信号的循环谱特征分析与参数估计

研究了准确反映Costas编码信号特点的关键参数集合。基于循环谱相关密度估计方法,分析了Costas编码的循环谱特征及其与辐射源侍号参数的对应关系,提出了一种没有先验知识条件下Costas编码截获信号特征提取方法。通过对典型的Costas编码信号参数提取归一化均方根误差的计算,验证了该算法的有效性。     

Multiple Signal Analysis for the Delay-Lock Discriminator

This correspondence deals with the effect of multiple signals on the control voltage of the delay-lock discriminator (tracker). Given n returned signals with known delay and power, a formula for the tracker estimate is derived. The analysis has specific application to low-altitude altimeters where the leak signal from the transmitter to the receiver may have delay comparable to the delay of the true signal.     

Loss from Approximations to Square-Law Detectors in Quadrature Systems with Postdetection Integration

The trend toward digital processing of radar and sonar signals has ledto the use of ?baseband? or bipolar video signals. The processingof both the in-phase (I) and quadrature (Q) channels generally allows an exact representation of the signals, and no loss of sensitivity need occur. However, for reasons of economy or weight only one channel is often implemented. This correspondence shows that the sensitivity loss for this case often deviates considerably from the 3-dB rule of thumb.     

准定常强激波马赫反射波形的数值研究

应用DCD频散控制激波捕捉格式,求解二维、多组分、带有化学反应的Euler方程组,数值模拟了准定常强激波的马赫反射问题.研究结果表明:与经典马赫反射理论相比,在强激波条件下,激波诱导的气体分子振动激发和化学反应使马赫反射的三波点轨迹角变小、马赫杆高度变低、楔顶附体激波倾角变小;马赫杆的相对突出量随入射激波马赫数和楔角的增大而增大,而气体分子的振动、离解等真实气体效应能进一步加剧马赫杆的向前突出.     

An Experimental Determination of Distortion and Delay in the FM-FM Telemetry System of the Saturn Space Vehicle

The distortion of the FM-FM Saturn telemetry system for dc input (static error) has been the subject of many studies. The purpose of this correspondence is to measure the additional distortion (dynamic error) that results when the input signal is time-varying. A digital method is developed for measuring both the system delay and the system mean square error for various input signals.     

The Effects of Notch Filters on the Correlation Properties of a PN Signal

With wideband pseudo-noise (PN) communications systems, it is sometimes desirable to supplement the inherent interference rejection capabilities by adding notch filters to attenuate relatively narrowband interference. This correspondence presents an investigation of the effects of notch filters on the performance of PN correlation receivers. A theoretical analysis of the correlation drop due to filter distortion has been conducted and confirmed by experimentation. Additional measurements and analysis have established the trade-off between correlation drop and interference suppression as a function of interference bandwidth. A typical result is that by incurring a penalty of a 1-dB drop in correlation peak, interfering signals having bandwidths of 2 to 3 percent of the PN chip rate can be attenuated by 25 dB.     

基于单缝射流的二元推力矢量喷管设计及数值模拟

喷管是发动机产生推力的主要部件,其气动性能对发动机的性能具有决定性的影响。本文利用简化特征线法设计二元收敛-扩张（2DCD）推力矢量喷管模型;采用RNGk-ε湍流模型和非平衡壁面函数对单缝二次流喷射后的喷管流场进行数值模拟,分析了射流位置、主流落压比（NPR）、二次流与主流总压比（SPR）等参数对矢量喷管气动性能的影响...     

Modeling of Ground Deformation and Shallow Surface Waves Generated by Martian Dust Devils and Perspectives for Near-Surface Structure Inversion

We investigated the possible seismic signatures of dust devils on Mars, both at long and short period, based on the analysis of Earth data and on forward modeling for Mars. Seismic and meteorological data collected in the Mojave Desert, California, recorded the signals generated by dust devils. In the 10–100 s band, the quasi-static surface deformation triggered by pressure fluctuations resulted in detectable ground-tilt effects: these are in good agreement with our modeling based on Sorrells’ theory. In addition, high-frequency records also exhibit a significant excitation in correspondence to dust devil episodes. Besides wind noise, this signal includes shallow surface waves due to the atmosphere-surface coupling and is used for a preliminary inversion of the near-surface S-wave profile down to 50 m depth. In the case of Mars, we modeled the long-period signals generated by the pressure field resulting from turbulence-resolving Large-Eddy Simulations. For typical dust-devil-like vortices with pressure drops of a couple Pascals, the corresponding horizontal acceleration is of a few nm/s² for rocky subsurface models and reaches 10–20 nm/s² for weak regolith models. In both cases, this signal can be detected by the Very-Broad Band seismometers of the InSight/SEIS experiment up to a distance of a few hundred meters from the vortex, the amplitude of the signal decreasing as the inverse of the distance. Atmospheric vortices are thus expected to be detected at the InSight landing site; the analysis of their seismic and atmospheric signals could lead to additional constraints on the near-surface structure, more precisely on the ground compliance and possibly on the seismic velocities.     

A Semi-Empirical Approach to the PLL Threshold

This correspondence considers the response of the PLL near threshold to an input consisting of a modulated carrier and white, Gaussian noise. For high input signal-to-noise power ratios ?, the output noise power is Gaussian with a parabolic spectrum. As ? is decreased, the PLL tends to lose lock which gives rise to impulses or ?spikes? in the output with a resulting white power spectrum. The additional output noise due to these ?spikes? causes a threshold in the output signal-to-noise ratio. Unfortunately the loss of lock rate in the PLL depends on the modulation as well as the noise power. A semiempirical approximate expression for the loss of lock rate as a function of the noise and sinusoidal frequency modulation is presented and is used to determine the optimum design procedure for PLL's to demodulate FM signals of varying modulation indexes, ?.     

二次流喷射位置对流体推力矢量喷管气动性能影响的数值模拟

数值模拟了二次流喷射位置对基于激波控制的二维收-扩(2DCD)喷管的流体推力矢量气动性能,结果表明喷射位置对矢量角有较大影响,在喷管落压比NPR=4.5时,二次流喷射位置相对位于喷管发散段中部时,矢量角最大;在NPR≥7.2时,二次流喷射位置越靠近出口,矢量角越大.喷射位置对推力系数及流量系数的影响不大.      

基于激波控制的流体推力矢量喷管试验

以二元收扩喷管为对象,开展了基于二次流喷射的流体推力矢量技术研究。基于试验研究,得到了不同喷管落压比、不同的二次流总压比和不同的二次流喷射角度多种工况下的喷管上下壁面中心线压力分布规律以及喷管壁面油流分布图。通过对不同工况下参数变化规律分析,给出了基于二次流喷射的流体推力矢量喷管的主次流气动参数及几何参数对流体推力矢量喷管流场结构和性能影响的关联关系。从试验和分析结果可以看出,喷管落压比、二次流总压比和二次流喷射角度等喷管的主次流气动几何参数对基于流体推力矢量喷管参数变化有明显的影响。     

二股流喷射控制推力矢量数值计算研究

利用求解雷诺平均的N-S方程的数值模拟方法,对一种新型的推力矢量喷管———基于次流喷射控制的二维收缩-扩张喷管(2DCD)———的推力矢量性能进行了研究。根据数值模拟结果分析了外流马赫数、喷管压强比和次流总压与主流总压之比对矢量偏角的影响。     

二元收-扩喷管与球形收敛调节片喷管的性能对比研究

为了对比二元收-扩喷管(2DCD)与球形收敛调节片喷管(SCFN)的性能,基于CFD数值计算软件对2种喷管在相同工作点下进行了内外流场计算与分析。数值模拟结果表明:在相同高度及飞行马赫数条件下,SCFN的流量系数相较于2DCD呈现出较大不同,推力系数则略差于2DCD;在低落压比条件下,2种喷管内流对后体阻力特性影响不大;在高落压比条件下,2种喷管内流对后体阻力特性影响较大。     

Comments on "Optimum Bandwidth of a Low-Pass Filter for Detection of a Pulse in Nonstationary Noise"

In a recent correspondence1 a calculation of the optimum bandwidth of a low-pass RC filter for the detection of a pulse signal in nonstationary noise was presented. The purpose of this correspondence is: 1) to point out additional references to the work which has been conducted in the stationary noise case, and 2) to present an interesting alternate derivation of the expected output noise power for the nonstationary noise case.     

高等函授教育学生信息管理系统的设计与实现

结合高等函授教育的特点，对函授生信息管理系统进行了规划，阐述了研究生信息管理系统的功能、开发模型设计、数据库设计和应用程序开发。     

Comments on "A Novel Feedforward Compensation Canceling Input Filter-Regulator Interaction"

This correspondence clarifies the conditions under which the canceling of the input filter dynamics of a buck regulator by partial state feedback can be performed.     

Counterweapon Aiming for Defense of a Moving Target

Previous correspondence [1, 2] considered the problem of defending moving target by means of a counterweapon launched from the target to intercept the attacker, deriving relations between attacker-target ranges at counterweapon launch and at intercept of the attacker, and the speed capabilities of the counterweapon. The present correspondence derives the location of the intercept point in target-centered coordinates (the counterweapon aimpoint), analyzes the complicated behavior of range and bearing settings with attack geometry, and formulates the relations for determining whether any given attack can be countered under restrictions on speed or angle at which the counterweapon may be fired. Exhaustive analysis is not attempted. A simplified treatment portrays kinematic conditions which must exist if the target is to counter the attack before the target is impacted by the attacking vehicle, through the use of examples.     

Clock Synchronization from Satellite Tracking

The tracking of synchronous satellites to provide propagation delays for the synchronization of clocks is described. The tracking is accomplished by range measurements to the satellite from three stations using signals transponded by the satellite. These same signals also functioned as the timing signals for the synchronization of other stations' clocks. Although the range measurements were of low resolution by usual standards, approximately 3000 meters, they provided the delays necessary to synchronize clocks to 40 microseconds or better. These results were obtained over a 4-month period using two satellites with measurements from five stations in the United States and South America.     

基于小波神经网络的齿轮系统故障诊断

通过对齿轮系统在不同的运转状态下不同的故障类型进行试验测试分析,获取了有关的测试信号,对振动特征信号进行了小波阈值去噪,采用离散小波变换(DWT)对去噪后的信号进行8层分解处理,对各层的小波系数进行了小波重构,得到8层细节信号和1层近似信号,并计算了各层信号的能量,得到了信号的能量分布特征.在此基础上把各层信号特征作为神经网络的输入,进行了网络的研究、分析处理和故障分类,并对小波神经网络方法与单独采用神经网络方法的故障诊断结果进行了比较评价.研究表明,去噪处理后的效果比没有去噪的信号特征更加明显,而采用小波神经网络诊断方法,对于齿轮无故障、齿根裂纹故障、分度圆裂纹故障和齿面磨损故障能够进行很好地区分与诊断,其诊断成功率均在95%以上,可对实际工程工作的齿轮系统进行故障诊断.