Probability Density of the Click Duration in an Ideal FM Discriminator

The expression of the probability density function PClick(T) of the duration r of the clicks is determined as a function of 1) the expected number of clicks per second, Felick 2) the expected number of times per second that the noise component x(t) in phase with the carrier has instantaneous amplitude larger than that of the carrier itself 3) the probability density function peross(T) of the duration of the time intervals in which x(t) < -A 4) the probability that the values of the noise component in quadrature with the carrier will be of opposite sign at the extremes of time intervals of duration v. Using for Pcross(T) an approximate formula that holds fairly well in the range of the values of the signal-to-noise power ratio ? usually encountered, PIick(T) has been calculated for several values of a and for two shapes of the noise spectrum, Gaussian and rectangular. Finally, an expression is given for the mean value of T.     

元件强度可靠性的模糊概率计算模型

首先建立了元件强度可靠性分析时基本变量只具有随机性而元件的安全失效状态含有模糊性时,元件的模糊失效概率Ｐｇｆ计算模型,也称广义概率计算模型,在此模型中给出了两种方法,即基本随机变量法和综合随机变量法;然后给出了基本随机变量具有随机模糊性时元件的Ｐｇｆ的计算模型,此模型中不论元件的安全失效状态是否具有模糊性,Ｐｇｆ均可以表示成条件概率     

A method of obtaining signal components of residual carrier signalwith their power content and computer simulation

An algorithm for obtaining all signal components of a residual carrier signal with any number of channels is presented. The phase modulation type may be NRZ-L or split phase (Manchester). The algorithm also provides an easy way of obtaining the power contents of the signal components. Steps to recognize the signal components that influence the carrier tracking loop and the data tracking loop at the receiver are given. A computer program for numerical computation is also given     

Noncoherent Detection of Split-Phase FSK by Multiple Predetection Filtering

When the cumulative drift in the center frequency of a binary split-phase FSK signal exceeds the peak deviation of the signal, a conventional noncoherent receiver (i.e., one provided with only two IF filters) may be unable to achieve the probability of error per bit which the designer desires. This limitation may be overcome if the receiver is provided with a bank of more than two contiguous filters (each followed by an envelope detector) tospan the total IF band the instantaneous IF signal might occupy. It is shown that the probability of error per bit for such a receiver is a function of 1) the ratio F of peak frequency deviation to peak frequency drift, 2) the number M of IF filter/detectors, and 3) the signal-to-noise ratio ? in the output of the filter containing the signal. It is further shown thatfor a given value of F an increase in M reduces the amount of transmitter power the communication system designer must provide to yield a given probability of error per bit.     

Optimal Aircraft Go?Around and Flare Maneuvers

This paper analyzes in detail two of the critical aircraft maneuvers associated with approach and landing: the go-around maneuver and the flare maneuver. Optimal solutions that include state and control variable constraints are obtained for both problems. Two algorithms are given for computation of the minimum and maximum altitude loss associated with the pilot-controlled go-around maneuver. A matrix operator is obtained that can be used for in-flight computation of the altitude loss on a small general-purpose digital computer. The flare optimization presented is for a cost functional that includes both the longitudinal touchdown dispersion and the normal acceleration. A closed-loop mechanization is given that approximates the optimal trajectory. A second matrix operator which can be used for prediction of the longitudinal touchdown point is obtained. Uncertainties are also obtained for the purpose of establishing a prediction confidence level. It is proposed that these prediction techniques should be incorporated into a decision making performance monitor. This monitor could provide the pilot with a continuous assessment of the approach and could generate a preflare decision on whether or not to commit the aircraft to the flare maneuver.     

High-Resolution, Radar Coherent Linear FM Microwave Source

The microwave CLFM study was directed to generating 14 ?s S-band pulses of 1000 MHz bandwidth and an rms phase error of a few degrees. Over 972 MHz bandwidth, the sampled phase error relative to the reference was 7 degrees rms and 17 degrees peak, with a maximum Fourier component of 4 degrees. The FM pulse train is generated by a gated BWO driven by a stable linearizing waveform. Phase coherency during each pulse is obtained by a sampling technique, where the phase is corrected at intervals of 1/6 ?s, the RF phase having changed an integral number of cycles in each interval. Multiplication of the BWO signal by the sampling pulse train results in band-limited phase error pulses which are applied in a feedback loop. Pulse-to-pulse coherency is obtained by phase lock of the BWO starting frequency to the crystal reference. Feedback leveling holds the output constant to 0.3 dB. The basic MITRE technique was originally demonstrated at 10 MHz in 1964. Range results measured with the X-band model radar using the CLFM generator are given and confirm the phase errors of the CLFM.     

Sampling Bandpass Signals

In a recent paper [1], Brown examined the sampling of a real finiteenergy bandpass signal having an (angular) bandwidth ? (in radians per second) at the theoretically minimum (average) rate of ?/? samples per second. Following Grace and Pitt's [2] quadrature sampling, a particular case of Kohlenberg's second-order sampling [3, 4], Brown has proved the feasibility of a separate interpolation of the in-phase and quadrature components of the signal when ?o = k?/2 (Brown's condition), where ?o is the center (angular) frequency of the signal and k is an arbitrary positive integer. Here the problem is reconsidered from a general point of view, introducing, under Brown's condition, an interpolation formula which includes that of Grace and Pitt and which extends a theorem of Populis [5-7]. We indicate the necessary and sufficient conditions to obtain a separate interpolation, offering closed formulas to obtain the interpolation functions. We also discuss the minimum oversampling rate needed whenBrown's condition is not verified.     

Calculation of Output Noise Variances for Discrete Time-Invariant Filters

Certain calculations to minimize output noise variance are introduced. Many applied problems in sampled data systems require that data be smoothed in the presence of noise for the prediction of future positions, velocities, or accelerations. Smoothing coefficients in discrete time-invariant filters are computed to minimize the output noise variance, but under the constraints that the function and derivatives be predicted ahead. The output noise variance is seen to be a function of the input noise, the number of input signals (N+1) that the filter has to smooth, and the prediction time ?T. Four examples are given in the derivation of smoothing coefficients for step and ramp inputs subjected to either almost white noise or Gaussian-Markoff noise. The examples illustrate the number of constraint relations that the filter smoothing coefficients must satisfy for function and/or derivative convergence under noise-free conditions. The smoothing coefficients are also a function of the type of noise input into the system or the discrete filter. From the examples, it can be observed that as N becomes larger, the output noise variance becomes smaller, but the computation time is increased.     

A reduced-order model for integrated GPS/INS

The dominant factor in determining the computation time of the Kalman filter is the dimension n of the model state vector. The number of computations per iteration is on the order of n³. Any reduction in the number of states will benefit directly in terms of increased computation time. In this paper, a high order model in integrated GPS/INS is described first, then a reduced-order model based on the high-order model, is developed. Finally, a faster tracking approach for Kalman filters is discussed. A typical aircraft trajectory is designed for a complex high-dynamic aircraft flight experiment. A Monte Carlo analysis shows that the reduced order model presented in this paper provides satisfactory accuracy for aircraft navigation     

二阶离散系统LQ问题加权阵的选择

本文针对单输入条件下二阶定常线性离散系统,讨论了线性二次型(LQ)问题中加权阵的选择问题。证明并给出了当要求闭环系统是渐近稳定时,加权阵与系统开、闭环特征多项式之间以及系统开、闭环极点之间的一组解析关系式。对实例的计算结果表明,利用这组关系,可以很方便地得到加权阵。     

移动代理(MA)综述

随着网络规模的日益扩大，如何保证网络管理系统的可扩展性和高效率性已成为网络管理人员迫切需要解决的问题。移动代理技术是新型的分布计算技术，具有很好的应用前景。移动代理系统是实现移动代理应用的基础框架。文章简述了移动代理(mobile agent)的定义和特点、代理的分类及移动代理的实现语言、以及移动的分类和移动机制；还提出了移动代理的特有技术优势，并对其发展前景进行了展望。文章在论述移动代理系统基本组成的同时，对IBM的移动代理平台aglet系统进行了介绍。     

应用TVD格式分离求解不可压N-S流动

提出一种新的将连续方程与动量方程分离求解的TVD引用方法。对动量方程中的对流项采用三阶精度、迎风TVD格式离散, 粘性项和压力项采用中心差分格式离散。压力采用压力替代法求解。计算所用的网格是具有交错网格主要特点的修正非交错网格, 并采用一种压力修正法和G-S中心对称迭代法加速收敛。给出了二维管流和二维空穴流动在不同状态下的计算结果, 并与其它解法的计算结果或精确解进行比较。结果表明, TVD格式不仅能用于不可压流数值计算, 而且还能得到比非TVD差分法精度更高的结果。      

钝体绕流流场分离涡的数值模拟

 采用近似因式分解和特殊Jacobin系数矩阵分裂法,求解可压缩Navier-Stokes方程,数值模拟矩形柱体绕流流场中分离涡的形成和发展。给出了Ma_∞=0.3,0.8;Re=10~4,10~6的计算结果。从结果中可清楚地看到柱体非对称涡形成的非定常过程。     

Statistical properties of fatigue damage of Gaussian random loadings

Firstly, the fatigue damages associated with the random loadings were always deemed as high-cycle or very-high-cycle fatigue problems, and based on Chebyshev theorem, the number of rainflow cycles in a given time interval could be recognized as a constant by neglecting its randomness. Secondly, the randomness of fatigue damage induced by the distribution of rainflow cycles was analyzed. According to central limit theorem, the fatigue damage could be assumed to follow Gaussian distribution, and the statistical parameters: mean and variance, were derived from Dirlik's solution. Finally, the proposed method was used to a simulate Gaussian random loading and the measured random loading from an aircraft. Comparisons with observed results were carried out extensively. In the first example, the relative errors of the proposed method are 2.29%, 3.52% and 1.16% for the mean, standard deviation and variation coefficient of fatigue damage, respectively. In the second example, these relative errors are 11.70%, 173.32% and 18.20%, and the larger errors are attributable to non-stationary state of the measured loading to some extent.      

Fragment Identification and Statistics Method of Hypervelocity Impact SPH Simulation

A comprehensive treatment to the fragment identification and statistics for the smoothed particle hydrodynamics (SPH) simulation of hypervelocity impact is presented. Based on SPH method, combined with finite element method (FEM), the computation is performed. The fragments are identified by a new pre- and post-processing algorithm and then converted into a binary graph. The number of fragments and the attached SPH particles are determined by counting the quantity of connected domains on the binary graph. The size, velocity vector and mass of each fragment are calculated by the particles’ summation and weighted average. The dependence of this method on finite element edge length and simulation terminal time is discussed. An example of tungsten rods impacting steel plates is given for calibration. The computation results match experiments well and demonstrate the effectiveness of this method.     

大迎角体涡,翼涡干扰的Euler方程数值模拟

本文用Ｅｕｌｅｒ方程的简化区算法计算翼－身组合体绕流。当大迎角时，计算能自动捕捉到大后掠前缘的分离涡，机射的分离涡由强加Ｋｕｔｔａ条件生成，形成体涡与翼涡的相互干拢。本文对加长前机身的ＡＧＡＲＤ－Ｂ翼－射组合体模型，给出机射有分离和无分离时，横流截面的速度矢量分布、法向力和压力中心，分析体涡的影响。     

基于1553B总线星时精确产生硬件秒脉冲的方法

本设计在整星不能提供硬件秒脉冲的情况下,利用单片机对1553B总线协议芯片进行配置,使其只对星时数据产生中断.该中断触发外围电路自主产生硬件秒脉冲信号,进而实现校时.由于该方法为纯硬件的触发控制环节,在不考虑卫星平台发送星时数据随机误差的情况下,其误差仅取决于硬件的延时误差,通过标定可达到与整星提供秒脉冲相同的校时精度,而大大优于不采用校时的情况.此外,该成果通过对触发电路外围进行配置,可以实现任意脉宽及方向的秒脉冲信号输出,可满足不同型号对不同类型秒脉冲信号的需求,具有很大的灵活性及广泛的适用性.     

Chirp Waveform Generation Using Digital Samples

This paper describes two methods of generating an analog frequency-modulated waveform by the use of a small number of digital samples of the ?chirp? waveform. The number of digital samples required is a function of the time-bandwidth product. For certain values of time-bandwidth product, this type of signal generation becomes extremely efficient. Several proofs are offered which show how to select ?optimum? values of time-bandwidth products. Two hardware implementations are suggested. One is based on the use of modulo arithmetic and a small stored memory table. The second method utilizes the inherent signal symmetries available if ?optimum? time-bandwidth products are selected. The symmetrical signal patterns are stored in recirculating reversible shift registers which can be read out at high speeds.     

A closed-form solution for moving source localization using LBI changing rate of phase difference only

Due to the deficiencies in the conventional multiple-receiver localization systems based on direction of arrival(DOA) such as system complexity of interferometer or array and amplitude/phase unbalance between multiple receiving channels and constraint on antenna configuration,a new radiated source localization method using the changing rate of phase difference(CRPD)measured by a long baseline interferometer(LBI) only is studied. To solve the strictly nonlinear problem, a two-stage closed-form solution is proposed. In the first stage, the DOA and its changing rate are estimated from the CRPD of each observer by the pseudolinear least square(PLS) method,and then in the second stage, the source position and velocity are found by another PLS minimization. The bias of the algorithm caused by the correlation between the measurement matrix and the noise in the second stage is analyzed. To reduce this bias, an instrumental variable(IV) method is derived. A weighted IV estimator is given in order to reduce the estimation variance. The proposed method does not need any initial guess and the computation is small. The Cramer–Rao lower bound(CRLB) and mean square error(MSE) are also analyzed. Simulation results show that the proposed method can be close to the CRLB with moderate Gaussian measurement noise.     

高光谱观测卫星高精时间同步方案设计与应用

为了满足星上高光谱相机、全谱段光谱成像仪、星敏感器等时间精度敏感仪器的高精度时间同步需求,高光谱观测卫星上设计了可适应多种精度需求的时间同步方案。对于系统时间,综合通过星地时差集中校时、均匀校时、GPS总线校时、GPS硬件秒脉冲校时等校时方法进行时间修正;对于终端用户,分别给出直接采用广播的进行本地时间修正的时间同步方法、采用广播及内部秒脉冲的时间同步方法、采用广播授时和不同秒脉冲校时方案的地面时间重建算法。利用在轨数据,对卫星星敏感器、全谱段光谱成像仪及可见短波红外高光谱相机的曝光时刻时间重建方法进行了应用说明,并对不同方法的时统误差进行了分析。