Thermal Noise Errors in DF Monopulse Systems Having Log Amplifiers

An analysis of the effects of thermal noise on the accuracy of direction finding (DF) monopulse systems which employ logarithmic amplifiers is presented. Numerical results are presented which indicate that a simple model may be used to compute rms thermal errors for a wide range of signal levels. Analytical results obtained in the limit of large signal-to-noise ratios are compared with standard results used to analyze monopulse systems employing automatic gain control (AGC) circuitry instead of log amplifiers.     

10. The surface and interior of venus

Present ideas about the surface and interior of Venus are based on data obtained from (1) Earth-based radio and radar: temperature, rotation, shape, and topography; (2) fly-by and orbiting spacecraft: gravity and magnetic fields; and (3) landers: winds, local structure, gamma radiation. Surface features, including large basins, crater-like depressions, and a linear valley, have been recognized from recent ground-based radar images. Pictures of the surface acquired by the USSR's Venera 9 and 10 show abundant boulders and apparent wind erosion.On the Pioneer Venus 1978 Orbiter mission, the radar mapper experiment will determine surface heights, dielectric constant values and small-scale slope values along the sub-orbital track between 50°S and 75°N. This experiment will also estimate the global shape and provide coarse radar images (40–80 km identification resolution) of part of the surface. Gravity data will be obtained by radio tracking. Maps combining radar altimetry with spacecraft and ground-based images will be made. A fluxgate magnetometer will measure the magnetic fields around Venus.The radar and gravity data will provide clues to the level of crustal differentiation and tectonic activity. The magnetometer will determine the field variations accurately. Data from the combined experiments may constrain the dynamo mechanism; if so, a deeper understanding of both Venus and Earth will be gained.     

Wavelet de-noising for IMU alignment

Inertial navigation system (INS) is presently used in several applications related to aerospace systems and land vehicle navigation. An INS determines the position, velocity, and attitude of a moving platform by processing the accelerations and angular velocity measurements of an inertial measurement unit (IMU). Accurate estimation of the initial attitude angles of an IMU is essential to ensure precise determination of the position and attitude of the moving platform. These initial attitude angles are usually estimated using alignment techniques. Due to the relatively low signal-to-noise ratio of the sensor measurement (especially for the gyroscopes), the initial attitude angles may not be computed accurately enough. In addition, the estimated initial attitude angles may have relatively large uncertainties that may affect the accuracy of other navigation parameters. This article suggests processing the gyro and accelerometer measurements with multiple levels of wavelet decomposition to remove the high frequency noise components. The proposed wavelet de-noising method was applied on a navigational grade inertial measurement unit (LTN90-100). The results showed that accurate alignment procedure and fast convergence of the estimation algorithm, in addition to reducing the estimation covariance of the three attitude angles, could be obtained.     

A global solution for robust parameter design of aeronautical electrical apparatus based on interactions analysis and polynomial fitting

Robust Parameter Design (RPD) has been widely applied for improving quality and reliability of products. One of the key drawbacks of applying RPD using Taguchi method is that the stable factors may not be independent of the adjustment factors, resulting in unsatisfactory design. Moreover, the Taguchi method cannot guarantee global optimality since the levels set in the experiment are usually discrete to ensure orthogonal design. In this paper, robust solutions of the stable factors are obtained via a nonlinear model based on polynomial fitting; while the adjustment factors are obtained via interactions analysis so that they are independent of the stable factors. In particular, the values of the adjustment factors are determined by output offset compensation so as to achieve robustness of the design scheme. An example on the design of an aeronautical electrical apparatus is presented to illustrate the procedure. The results show that the proposed method can take full advantage of the nonlinearity in the response and achieve the desired outcome.     

粉末冶金镍基高温合金FGH96高温疲劳寿命分散性特征

通过宽载荷水平大子样试验研究了缺陷对粉末冶金镍基高温合金FGH96的疲劳寿命分散性的影响,获得FGH96在宽载荷水平下的疲劳寿命分布特征.通过扫描电镜对疲劳失效断口进行统计分析,揭示缺陷在不同载荷条件下的作用.结果表明:①FGH96中导致疲劳失效的缺陷主要为非金属夹杂;②在高应力水平下（1200,1100MPa）下,导致表面萌生裂纹的夹杂是最差疲劳寿命的主导因素,使得疲劳寿命分散性较大;③在中间应力水平（1000MPa）下,在材料内部萌生裂纹的夹杂并不影响疲劳寿命的分散性;④在低应力水平（900MPa）下,疲劳破坏均萌生于内部,在材料内部夹杂处萌生的裂纹并不影响疲劳寿命的分散性.因此,在高应力水平下的寿命预测需要考虑缺陷信息.      

Understanding commutations in switching converters. II. Analysis and synthesis of DC-DC regulators

For pt.I see ibid., vol.39, no.1, p. 282-297 (2003). This paper illustrates a general method to formulate the equations for the analysis of commutations and of the effects caused by large step-wise perturbations, large change of parameters, and circuit failures in switching converters including transformers, feedback circuitry, and soft-switching cells. The matrix equations obtained by means of the modified nodal analysis-based method proposed in this work are useful for theoretical inspection of switching converters as well as for numerical simulations. The examples of applications concern the analysis and the synthesis of switching converters and show that the Compensation Theorem (CT)-based model may help in selecting circuit topologies valid for the circuit design.     

An experimental method to obtain the hard alpha anomaly distribution for titanium alloy aeroengine disks

A probability-based damage tolerance methodology has been proposed to improve the recognition of material anomalies, especially hard alpha (TiN) anomalies for aeroengine rotor disks. A key input to this method is hard alpha anomaly distribution, which reflects the occurrence rate and size of anomalies present in the finished part material of titanium rotors. Since anomalies rarely occur naturally, an experimental method is proposed to obtain the anomaly distribution for titanium alloy aeroengine disks to reflect and equivalently replace the manufacturing development in titanium industry. In general, the anomaly distribution information can be divided into two parts: the Probability of Detection (POD) curve and the detected anomaly distribution, which contains the size and frequency data of the detected anomalies. The distribution can be established based on several appropriate assumptions and derivation steps with different confidence levels of POD curves and detected anomaly distributions. In this case, the distribution can be obtained in a relatively short time as a key input to the probability-based damage tolerance methodology. Then the Probability of Failure (POF) can be calculated, and the value is found to vary with different confidence levels. On this basis, the conservative estimated POF can be obtained in conjunction with confidence levels.     

Robust guidance for electro-optical missiles

A robust guidance law is presented which renders zero miss distance (ZMD) against deterministically or randomly maneuvering targets for all missile parametric uncertainties. Since the resulting guidance controller is a phase-lead network, it is mainly suitable for systems characterized by moderate glint levels such as electro-optical missiles. The structured uncertainties in missile dynamics are modeled by interval transfer functions. It is first shown that for the nominal case, when the total missile transfer function is positive real, ZMD can be obtained. When uncertainties are considered, the problem becomes design of a guidance controller which renders a family of transfer functions positive real. A new algorithm for the design of such controllers is proposed. An example illustrating a typical design procedure for a nonlinear real-life missile model is given, showing the simplicity and effectiveness of the proposed robust guidance. The main conclusion of this work is that the newly developed guidance law performs well against highly maneuvering targets and may be a suitable alternative to optimal guidance laws in low-glint systems.     

固体火箭喷管中的烧蚀控制机制

对碳基材料,以热化学烧蚀三方程模型为基础,在考虑粒子侵蚀,烧蚀与传热耦合的情况下,进行了全喷管烧蚀控制机制的研究,喷管中的烧蚀控制机制有化学动力学控制,扩散控制和双控制三种机制。通过研究,得到如下结论:1)对于由扩散控制和化学动力学控制确定的烧蚀率相差约20倍以上时,可以简化为按烧蚀率低的一种控制机制来计算;否则,应当按既考虑扩散控制,又考虑化学动力学控制的双控制机制来计算。2)在固体火箭喷管中,大体上喉部和扩张段的烧蚀是化学动力学控制的,而收敛段的烧蚀是由扩散控制的。3)由于在收敛段由两种机制控制的烧蚀率相差较小,因此,在收敛段的烧蚀率应当按双控制机制来计算。喉部和扩张段的烧蚀可简化为动力学控制机制。     

Noise subspace techniques in non-gaussian noise using cumulants

We consider noise subspace methods for narrowband direction-of-arrival or harmonic retrieval in colored linear non-gaussian noise of unknown covariance and unknown distribution. The non-gaussian noise covariance is estimated via higher order cumulants and combined with correlation information to solve a generalized eigenvalue problem. The estimated eigenvectors are used in a variety of noise subspace methods such as multiple signal classification (MUSIC), MVDR and eigenvector. The noise covariance estimates are obtained in the presence of the harmonic signals, obviating the need for noise-only training records. The covariance estimates may be obtained nonparametrically via cumulant projections, or parametrically using autoregressive moving average (ARMA) models. An information theoretic criterion using higher order cumulants is presented which may be used to simultaneously estimate the ARMA model order and parameters. Third- and fourth-order cumulants are employed for asymmetric and symmetric probability density function (pdf) cases, respectively. Simulation results show considerable improvement over conventional methods with no prewhitening. The effects of prewhitening are particularly evident in the dominant eigenvalues, as revealed by singular value decomposition (SVD) analysis     

A Sequential Logic for Improving Signal Detectability in Frequency-Agile Search Radars

Certain fading models have in the past become standard for taking into account the effects of fluctuations in target echoing area in the theoretical prediction of radar range performance. In the application of these models, fading has always been treated as an entirely random phenomenon. In this paper it is noted that this is not necessarily true for radars which have the transmission frequency as a controlled variable, since in that case the fading characteristics of the target may be modified by properly controlling the transmission frequency. A strategy for taking advantage of this possibility is developed and theoretical results obtained by Monte Carlo simulation for a specific logic indicate that gains in detectability of several decibels (relative to what is usually considered optimum) may be obtained when certain realistic assumptions are made for the target fading characteristics.     

Array shape estimation tracking using active sonar reverberation

This paper concerns the problem of array shape estimation and tracking for towed active sonar arrays, using received reverberation returns from a single transmitted CW pulse. Uniform linear arrays (ULAs) deviate from their nominal geometry while being towed due to ship maneuvers as well as ocean currents. In such scenarios, conventional beamforming performed under the assumption of a ULA can sometimes lead to unacceptably high spatial sidelobes. The reverberation leaking through the sidelobes can potentially mask weak targets in Doppler, especially when the target Doppler is close to that of the mainlobe reverberation and the reverberation-to-target ratio (RTR) is very high. Although heading sensors located along the array can be used to provide shape estimates, they may not be sufficiently available or accurate to provide the required sidelobe levels. We propose an array shape calibration algorithm using multipath reverberation returns from each ping as a distributed source of opportunity. More specifically, a maximum likelihood (ML) array shape calibration algorithm is developed, which exploits a deterministic relationship between the reverberation spatial and Doppler frequencies causing it to be low rank in the space-time vector space formed across a single coherent processing interval (CPI). In this application, a sequence of overlapped CPI length snapshots of duration less than the CW pulse is used. The ML estimates obtained for each snapshot are tracked using a Kalman filter with a state equation corresponding to the water pulley model for array dynamics. Simulations performed using real heading sensor data in conjunction with simulated reverberation suggest that 8-10 dB improvement in sidelobe level may be possible using the proposed array shape tracking algorithm versus an algorithm that uses only the available heading information.     

K. Koyama, S. Kitamoto and M. Itoh (eds.), The Hot Universe, IAU Symposium

This book concerns the publication of the proceedings of an IAU Symposium held in Tokio in the summer of 1997. As implied by the title, it provides an overall review of our knowledge on all aspects of high-energy phenomena occurring in the universe obtained via the observations in X- and gamma-rays with orbiting satellites. It contains 44 invited (4 pages each) and 132 (2 pages each) contributed papers covering: Sun, stars, supernovae and their remnants, galaxies and their clusters, white dwarfs and neutron stars, black hole binaries, active galactic nuclei, gamma-ray bursts, large scale structure and hot intergalactic medium, and a chapter on future space programs in X- and gamma-ray astronomy. Many of the contributions have since appeared in the astronomical literature. The invited reviews, although very concise, are generally valuable in presenting the most relevant points of the various subjects. The book is for professional astronomers and may serve as a quick and very useful reference to becoming acquainted with the main developments in the field of high-energy astrophysics beginning of 1998. This revised version was published online in August 2006 with corrections to the Cover Date.     

基于数控切齿的弧线齿面齿轮啮合特性研究

研究了弧线齿面齿轮的数控切齿及其啮合特性.基于假想产形齿轮的概念,同时考虑了刀具法向截面的修形,推导弧线齿面齿轮的齿面方程;以坐标变换为工具,建立其机床加工模型,确定各运动轴的多项式表达式;在此基础上,建立了考虑安装误差的弧线齿面齿轮副接触分析模型.计算结果表明:通过面齿轮的齿面修形,可以降低啮合转换点处的传动误差幅值,并获得较好的传动误差曲线.      

湍流燃烧模型在航空发动机喷雾燃烧中的应用

使用自主开发的计算软件,数值模拟航空发动机旋流杯环形燃烧室三维湍流煤油两相燃烧过程,采用多维经验分析法与多步反应火焰面模型预测旋流杯环形燃烧室污染物排放,对4种湍流燃烧模型预测喷雾燃烧流场与污染物排放能力进行评估.各模型所得的计算值与试验数据较为相符,表明这些模型都可用来预测旋流杯环形燃烧室两相燃烧流场和污染物排放能力,但是不同模型的计算精度不相同.其中多步反应火焰面模型所得的出口温度分布与污染物排放与试验结果最接近,其预测精度高于其他模型.因此合理选择湍流燃烧模型可以提高计算准确性和可靠性.      

Detection Performance of the Cell Averaging LOG/CFAR Receiver

The cell averaging LOG/CFAR receiver is a special implementation of a constant-false-alarm-rate (CFAR) receiver in which the noise level estimate is derived from a set of contiguous time samples of the output of a logarithmic (LOG) detector as obtained from a tapped delay line. This CFAR receiver is capable of operating over a larger dynamic range of noise levels than a conventional cell averaging CFAR receiver, but with somewhat poorer detectability. The performance in stationary Gaussian noise of the cell averaging LOG/CFAR receiver with no post-detection integration is determined in this paper. For a small number of reference noise samples, results were obtained by a Monte Carlo simulation using the technique of importance sampling. For a large number of reference noise samples, a second moment analysis gave the desired results. Both these results can be summarized in the following simple formula, NLOG = 1.65NLIN - 0.65, which relates the number of reference samples required by each of the two receivers for equivalent performance. Thus, for the cell averaging LOG/CFAR receiver to give the same detection performance as the conventional cell averaging CFAR receiver, the number of reference noise samples has to be increased by up to 65 percent.     

基于BP神经网络的多应力加速寿命试验预测方法

 针对多应力加速寿命试验(ALT)中传统的寿命预测方法存在建立加速模型及求解多元似然方程组困难的缺点,基于反向传播(BP)人工神经网络(ANN),利用BP神经网络良好的预测特性,建立了多应力恒定加速寿命试验寿命预测模型。首先,以加速寿命试验中的加速应力水平和通过经验分布得到的可靠度作为网络训练输入向量;以非线性最小二乘法对原始失效数据进行拟合并得到回归方程,利用回归方程生成大量的仿真数据作为训练目标向量;然后,建立3层BP神经网络并对网络进行训练。最后,把正常应力水平和设定的可靠度输入训练好的网络,得到预测的失效时间,进而给出可靠度函数的预测曲线。通过仿真算例对本方法进行验证,预测值和仿真值相比较表明,所建立的网络能反映应力水平、可靠度与寿命的关系,为多应力加速寿命试验的寿命预测提供了新的思路。     

双轴载荷下孔边疲劳裂纹的扩展

 在AISI316奥氏体不锈钢制成的十字形带中心孔（圆孔和椭圆孔）试件上进行的双轴载荷疲劳裂纹扩展试验表明:应力双轴性对疲劳裂纹扩展的影响,仅发生在靠近开孔的局部区域内。等双轴应力较单轴应力状态下的裂纹扩展速率减慢,而纯剪切状态下的裂纹扩展速率加快。在孔的影响区之外,所有的结果服从Paris规律,与应力双轴性无关。     

Reliability evaluation of avionics system with imperfect fault coverage and propagated failure mechanisms

Fault tolerance designs are essential techniques for systems that require high levels of reliability, such as aircraft or spacecraft control system. Imperfect Fault Coverage (IFC) may lead to the failure of a system or subsystem even with adequate redundancy. Previous studies of IFC mostly concentrated on evaluating Coverage Factor (CF), whereas the system failure behaviors with IFC have rarely been involved. Failures that occur in low-layer may be covered by high-layer. However, if the coverage is imperfect, uncovered failure will have functional and physical impact on the system behavior. In this thesis, the failure behavior and reliability of IFC of multi-layer systems are studied and a Binary Decision Diagram (BDD)-based modeling and simulation method are proposed to evaluate system reliability. As a case, the failure behavior of an aero engine electronic controller with IFC is studied. The results show that the IFC may impact system behavior without taking the IFC into account, the system maintenance intervals may reduce, and thus the maintenance costs will increase.     

The Use of a Spinning Dissipator for Attitude Stabilization of Earth-Orbiting Satellites

This paper considers a novel attitude stabilizing system which acts both as a passive nutation damper and a momentum source for an Earth-orbiting satellite. The system may be designed to use four identical wheels mounted coaxially with and at the ends of the arms of a cruciform structure. The structure may then be spun about the axis perpendicular to the arms to provide angular momentum along the spacecraft spin axis. The wheels experience torques produced by inertial forces. The necessary damping and restoring (spring) torques are provided by means of a torsional arrangement built around the center of each wheel. The performance characteristics of the proposed stabilizing unit are evaluated by considering spacecraft parameters used in one phase of development of the Communications Technology Satellite (CTS). The results obtained for a specific design of the stabilizer indicate that global stability of the desired attitude motion of the spacecraft can be guaranteed with associated damping time constant as low as one second.