全文获取类型
收费全文 | 915篇 |
免费 | 230篇 |
国内免费 | 245篇 |
专业分类
航空 | 825篇 |
航天技术 | 228篇 |
综合类 | 110篇 |
航天 | 227篇 |
出版年
2024年 | 10篇 |
2023年 | 31篇 |
2022年 | 48篇 |
2021年 | 65篇 |
2020年 | 58篇 |
2019年 | 78篇 |
2018年 | 64篇 |
2017年 | 61篇 |
2016年 | 66篇 |
2015年 | 56篇 |
2014年 | 83篇 |
2013年 | 71篇 |
2012年 | 57篇 |
2011年 | 77篇 |
2010年 | 46篇 |
2009年 | 48篇 |
2008年 | 45篇 |
2007年 | 49篇 |
2006年 | 37篇 |
2005年 | 34篇 |
2004年 | 31篇 |
2003年 | 28篇 |
2002年 | 27篇 |
2001年 | 21篇 |
2000年 | 21篇 |
1999年 | 24篇 |
1998年 | 15篇 |
1997年 | 17篇 |
1996年 | 28篇 |
1995年 | 15篇 |
1994年 | 20篇 |
1993年 | 18篇 |
1992年 | 14篇 |
1991年 | 4篇 |
1990年 | 8篇 |
1989年 | 9篇 |
1988年 | 2篇 |
1987年 | 4篇 |
排序方式: 共有1390条查询结果,搜索用时 640 毫秒
631.
介绍了纤维缠绕金属内衬压力容器的工艺特点、性能优势和爆破前先泄漏(LBB)的安全失效模式,以及一般设计原则、构形、材料和工艺要求。分析了金属内衬的厚度、过渡区、焊接区和纤维缠绕层。以及输入输出极孔的设计技术。阐明了用于静力分析的解析法和数值法,以及用于循环寿命预测的断裂力学分析法。最后概述了一些新技术、新材料的应用情况,并指出了压力容器设计的发展趋势。 相似文献
632.
针对目前应用于SINS/GPS组合导航系统中的扩展卡尔曼滤波(Extended Kalman
Filter, EKF)存在精度低、实时性差的缺点,提出一种基于模型误差预测(Model Error
Prediction, MEP)的Unscented 卡尔曼滤波(Unscented Kalman Filter,
UKF)。MEP-UKF滤波算法将惯性器件测量误差作为模型误差使用MEP进行实时预测的同时,采 用UKF估计载体的姿态、速度及位置等误差信息,并反馈给SINS系统来校正导航参数。MEP-U KF不仅克服了UKF必须假设惯性器件误差为高斯白噪声的局限性,而且降低了SINS/GPS组合 导航系统状态变量的维数,大大缩短了导航解算的时间。仿真结果表明,MEP-UKF的收敛速 度和滤波精度均明显优于EKF,更好地满足了工程应用中对导航精度和实时性的要求。 相似文献
Filter, EKF)存在精度低、实时性差的缺点,提出一种基于模型误差预测(Model Error
Prediction, MEP)的Unscented 卡尔曼滤波(Unscented Kalman Filter,
UKF)。MEP-UKF滤波算法将惯性器件测量误差作为模型误差使用MEP进行实时预测的同时,采 用UKF估计载体的姿态、速度及位置等误差信息,并反馈给SINS系统来校正导航参数。MEP-U KF不仅克服了UKF必须假设惯性器件误差为高斯白噪声的局限性,而且降低了SINS/GPS组合 导航系统状态变量的维数,大大缩短了导航解算的时间。仿真结果表明,MEP-UKF的收敛速 度和滤波精度均明显优于EKF,更好地满足了工程应用中对导航精度和实时性的要求。 相似文献
633.
634.
635.
636.
基于均匀加权天线模型,推导星载ScanSAR天线指向不稳定引起的多普勒中心频率估计误差,建立多普勒中心频率估计误差与图像辐射校正精度之间的数学模型,进而提出星载ScanSAR辐射校正对天线指向稳定度的要求。最后,通过计算机仿真,验证了理论分析的正确性。 相似文献
637.
Guanwen Huang Bobin Cui Qin Zhang Pingli Li Wei Xie 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2019,63(5):1681-1696
The information of the satellite clock switching and performance variations on-orbit of Chinese BeiDou-2 Navigation System (BDS) is not available for the public. In order to detect the BDS satellite clock switching and performances variation, we analyzed the precise clock offset products with a total duration of 5?years every BDS satellite equipped four atomic clocks from four different manufactures from January 2013 to October 2017. Three important contributions are concluded as follows. (1) It is found that the average time of on-orbit operation for BDS satellite clocks is about 1–2?years. There have been 22 times of clock switching for BDS satellites, of which the C05 and C08 satellites have been switched to the fourth (last) atomic clock. (2) There are frequent phase adjustments for BDS on-orbit satellite clocks, and the frequency series is relatively stable. Furthermore, there are semi-annual sinusoid cycles in the frequency drift series of C06 and C09 satellites. (3) The performances of MEO satellite clocks perform better than the IGSO and GEO satellite clocks. The average ten-thousand frequency stability of BDS satellite clocks is about 1E-13, which is worse than that of GPS and Galileo but better than that of GLONASS. 相似文献
638.
In this paper, a modified unscented Kalman filter (UKF) for nonlinear stochastic systems is proposed, and it is applied to autonomous orbit determination for Earth satellites. Based on some standard results about the boundedness of stochastic processes and a new formulation of the unscented transformation (UT), it is demonstrated that the design of the noise covariance matrix plays an important role in enhancing the filter stability. Furthermore, a particular design of the noise covariance matrix is proposed as a modification of the UKF. The modified UKF is less sensitive to the initial error than the usual one. High performance of the modified UKF is illustrated in comparison with the usual one by using the real data obtained from an Earth sensor. 相似文献
639.
The present paper continues the discussion in Part-I. Model and Formulation. Based on the theory proposed in Part-I, the formulae predicting stiffness moduli of the composites in some typical cases of whisker orientations and loading conditions are derived and compared with theoretical representatives in literatures, experimental measurement and commonly-used empirical formulae. It seems that (1) with whisker reinforcing and matrix-hardening considered, the present prediction is in well agreement with the experimental measurement; (2) the present theory can predict accurate moduli with the proper pre-calculated parameters; (3) the upper-bound and lower-bound of the present theory are just the predictions of equal strain theory and equal stress theory; (4) the present theory provides a physical explanation and theoretical base for the present commonly-used empirical formulae. Compared with the microscopic mechanical theories, the present theory is competent for modulus prediction of practical engineering composite in accuracy and simplicity. 相似文献
640.
结合工程中应用最为广泛的Paris公式对多裂纹板的疲劳寿命进行了预计,计算结果与现有试验结果比较吻合,表明了该方法的可行性。由于力学响应的计算采用有限元方法,所以该方法适用于裂纹任意分布的多部位损伤板,可直接用于工程实际结构。 相似文献