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911.
M.R. Saradjian M. Hosseini 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2011
In the past studies, different soil moisture estimation models were developed for bare soil areas by using remotely-sensed data. However, there are few models that can be used to estimate soil moisture in vegetated areas. Water Cloud Model (WCM) model is a widely used soil moisture estimation model has been developed for vegetated areas. In this study, the WCM model was extended to take soil roughness parameter into consideration. The modeling and its accuracy assessment were done by using multi-polarization Airborne Synthetic Aperture Radar (AIRSAR) images and ground data collected during field Soil Moisture Experiments. 相似文献
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Zhengshi Yu Pingyuan Cui Shengying Zhu 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2014
A thorough observability analysis of the Mars entry navigation using radiometric measurements from ground based beacons is performed. This analysis involves the evaluation of the Fisher information matrix which is derived from the maximum likelihood estimation. A series of navigation cases with multiple beacons are investigated, and both range and range-rate measurements are considered. The determinant of Fisher information matrix is used to quantify the observability of navigation system, while the trace of Fisher information matrix is used to determine the lower-bound of estimation errors. For one and two beacon cases, the navigation system is unobservable. However, the eigenvectors of Fisher information matrix give the observable and unobservable component. When three or more beacon measurements are employed, the states of entry vehicle become observable. Some valuable analytic conclusions on the relationship between the geometric configuration of beacons and observability are obtained consequently. Finally, simulation results from two navigation examples indicate that our effort is useful for understanding and assessing the observability of the Mars entry navigation using radiometric measurements. 相似文献
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918.
Attitude control techniques for the pointing and stabilization of very large, inherently flexible spacecraft systems are investigated. The attitude dynamics and control of a long, homogeneous flexible beam whose center of mass is assumed to follow a circular orbit is analyzed. In this study, first order effects of gravity-gradient are included, whereas external perturbations and related orbital station keeping maneuvers are neglected. A mathematical model which describes the system deflections within the orbital plane has been developed by treating the beam as having a maximum of three discretized mass particles connected by massless, elastic structural elements. The uncontrolled dynamics of this system are simulated and, in addition, the effects of the control devices are considered. The concept of distributed modal control, which provides a means for controlling a system mode independently of all other modes, is examined. The effect of varying the number of modes in the model as well as the number and location of the control devices are also considered. 相似文献
919.
以航天运输为背景,研究了磁悬浮助推发射的磁悬浮、直线电机、空气动力影响和运载器分离发射等关键技术问题.通过磁悬浮系统比较,确定高温超导体EDS(Electrodynamic Suspension)系统是适合的磁悬浮技术方案,在单元静、动态实验测试基础上,研究其基本静态悬浮导向性能和刚度、阻尼特性,并建立缩比研究试验平台以评估磁悬浮系统的综合性能.通过分析2种直线电机类型及优缺点,确定双边感应直线电机是适合磁悬浮发射系统的一种直线电机加速方式,并确定脉冲磁流体发电的能量供给方案.通过磁悬浮发射空气动力影响分析,优化设计磁悬浮发射装置气动外形和气动布局,得出其基本气动特性,并提出利于实现系统俯仰力矩平衡的气动翼控制方案.研究了运载器同时滑离导轨式定向器分离发射方案,并通过动静法计算分析得出运载器从定向器滑轨上成功分离所需具备的基本动力学参数. 相似文献
920.
利用涡喷发动机试车台和新研制的小型涡喷发动机完成了一系列静电监测实验,获取了涡喷发动机尾气静电信号特性,并通过分析静电信号特征证实了尾气静电监测技术对发动机气路部件机械故障的监测.研究发现发动机尾气静电信号的细微变化与发动机工况相关,通过分析第102试车阶段的静电信号发现活动率水平出现了与发动机工况同步变化的情况,验证了活动率水平是一种可作为表征尾气中微小颗粒的带电情况的特征参数;在102~108试车阶段中,发动机出现了燃烧室内壁积炭、燃烧室内壁轻微烧蚀和气路润滑油泄漏故障,根据静电信号的统计分析以及大时间尺度的趋势分析,发现活动率水平、正(负)事件率可作为预警参数来反映因这些故障而导致尾气静电整体水平的变化. 相似文献