Distributive Demand-Assigned Packet Switching with Trailer Transmissions

In a packet satellite system using ALOHA type random access techniques, channels and on-board processing may be effectively used to provide significant gains in throughput, delay, and bandwidth efficiency. Motivated by this conjecture, a distributive demand-assigned packet broadcast system through a processing satellite with multiple request channels and user buffers is first presented. This scheme is shown to be able to achieve the above goals except, as any other demand type scheme, it also exhibits relatively poor delay performance at low traffic when compared with pure random access schemes. An extended scheme using trailer transmission to improve delay performance at low traffic is then introduced. Two techniques for handling trailer transmissions are studied and compared. Both are shown to provide lower packet delay at light trafric. lay at light traffic.     

材料非线性有限元分析的一种有效方法——预测割线刚度变刚法

 材料非线性有限元增量解法中如何减少加载次数,保证精度是两个重要的问题;而误差估计也是与之密切相关的问题,特别是对每次加载不进行迭代的增量变刚法。本文针对这些问题提出预测割线刚度变刚法。     

Neutral buoyancy and sleep-deprived serum factors alter expression of cytokines regulating osteogenesis

We examined expression of genes associated with cytokine production, and genes implicated in regulating bone metabolism, in bone stromal and osteoblast cells incubated under standard ground conditions and under conditions of neutral buoyancy, and in the presence/absence of serum from normal or sleep-deprived mice. We observed a clear interaction between these two conditions (exposure to neutral buoyancy and serum stimulation) in promoting enhanced osteoclastogenesis. Both conditions independently altered expression of a number of cytokines implicated in the regulation of bone metabolism. However, using stromal cells from IL-1 and TNFx cytokine(r) KO mice, we concluded that the increased bone loss under microgravity conditions was not primarily cytokine mediated.     

Orbit determination of BDS-3 satellite based on regional ground tracking station and inter-satellite link observations

The BeiDou global navigation satellite system (BDS-3) has established the Ka-band inter-satellite link (ISL) to realize a two-way ranging function between satellites, which provides a new observation technology for the orbit determination of BDS-3 satellites. Therefore, this study presents a BDS satellite orbit determination model based on ground tracking station (GTS) observations and ISL ranging observations firstly to analyze the impact of the ISL ranging observations on the orbit determination of BDS-3 satellites. Subsequently, considering the data fusion processing, the variance component estimation (VCE) algorithm is applied to the parameter estimation process of the satellite orbit determination. Finally, using the measured data from China’s regional GTS observations and BDS-3 ISL ranging observations, the effects of ISL ranging observations on the orbit determination accuracy of BDS-3 satellites are analyzed. Moreover, the impact of the VCE algorithm on the fusion data processing is evaluated from the aspects of orbit determination accuracy, Ka-band hardware delay parameter stability, and ISL ranging observation residuals. The results show that for China’s regional GTSs, the addition of BDS-3 ISL ranging observations can significantly improve the orbit determination accuracy of BDS-3 satellites. The observed orbit determination accuracy of satellite radial component is improved from 48 cm to 4.1 cm. In addition, when the initial weight ratio between GTS observations and ISL ranging observations is not appropriate, the various indicators which include orbit determination accuracy, ISL hardware delay, and ISL observation residuals were observed to have improved after the adjustment of the VCE algorithm. These results validate the effectiveness of the VCE algorithm for the fusion data processing of the GTS observations and ISL ranging observations.     

A method of estimating GPS instrumental biases with a convolution algorithm

This paper presents a method of deriving the instrumental differential code biases (DCBs) of GPS satellites and dual frequency receivers. Considering that the total electron content (TEC) varies smoothly over a small area, one ionospheric pierce point (IPP) and four more nearby IPPs were selected to build an equation with a convolution algorithm. In addition, unknown DCB parameters were arranged into a set of equations with GPS observations in a day unit by assuming that DCBs do not vary within a day. Then, the DCBs of satellites and receivers were determined by solving the equation set with the least-squares fitting technique. The performance of this method is examined by applying it to 361?days in 2014 using the observation data from 1311 GPS Earth Observation Network (GEONET) receivers. The result was crosswise-compared with the DCB estimated by the mesh method and the IONEX products from the Center for Orbit Determination in Europe (CODE). The DCB values derived by this method agree with those of the mesh method and the CODE products, with biases of 0.091?ns and 0.321?ns, respectively. The convolution method's accuracy and stability were quite good and showed improvements over the mesh method.     

双燃式冲压发动机中超燃燃烧室冷态流场数值模拟

采用Ｍａｃ Ｃｏｒｍ ａｃｋ 矢通分裂格式, 求解雷诺平均的Ｎ－Ｓ方程组, 采用Ｂａｌｄｗ ｉｎ－Ｌｏ－ｍ ａｘ 代数湍流模型和混合长度模型, 模拟超燃燃烧室内两股平行超声速来流的冷态流场, 取得了与试验相一致的结果, 计算表明回流区和射流流动参数对超燃燃烧室流场内波系的变化有着较大的影响。     

地下原油无缆电加热系统可靠性措施的计算机仿真与优化决策

本文根据给定的油井地质资料,用计算机对其进行拟合再现,着重讨论了井下原油无缆电加热系统的可靠性措施及其计算机仿真与优化决策方法。     

基于神经网络的机械加工信息融合

概述了基于神经网络的信息融合技术在机械加工中的刀具状态监控、加工精度预测、误差补偿等方面的应用,并展望了该技术的应用前景。     

Single-phase three-level rectifier and random PWM inverter drives

A novel ac/dc/ac converter topology with three-level pulsewidth modulated (PWM) scheme for the single-phase ac/dc rectifier and random PWM scheme for ac drives is proposed. In order to improve the power quality in the single-phase rectifier, a ROM-based (read-only memory) control scheme, based on hysteresis current comparator, region detector, and capacitor compensator, is used to achieve a sinusoidal line current with low current distortion. The control scheme of the adopted three-level rectifier is easy to implement. The blocking voltage of power switches is clamped to half of the dc bus voltage. To reduce the mechanical vibration from an induction motor, random pulse position PWM scheme is adopted to spread the harmonics in a wide frequency range which results in the reduction of torque pulsation in the ac motor drives. Simulation and experimental results based on the laboratory prototype circuit are presented to verify the proposed control scheme     

分析计算航天飞机气动系数的边界元局部方法

空间运载工具设计阶段为分析比较不同外形高超音速各流动领域的性能,广泛应用局部方法。在其原始形式下,形状函数计算繁复,并限于简单几何外形组成的轴对称体。用边界元局部方法进行形状函数计算,可免去原局部方法面积分的繁复和藉助表格的局限。对STS-1轨道飞行器外形进行计算得到丁形状函数,并根据实验数据确定领域系数。由此计算出飞行器的法向力、轴向力和俯仰力矩系数,计算结果与实验数据相符。