全文获取类型
收费全文 | 24911篇 |
免费 | 1508篇 |
国内免费 | 1566篇 |
专业分类
航空 | 14644篇 |
航天技术 | 6955篇 |
综合类 | 1186篇 |
航天 | 5200篇 |
出版年
2022年 | 246篇 |
2021年 | 412篇 |
2020年 | 285篇 |
2019年 | 312篇 |
2018年 | 407篇 |
2017年 | 272篇 |
2016年 | 376篇 |
2015年 | 371篇 |
2014年 | 794篇 |
2013年 | 939篇 |
2012年 | 895篇 |
2011年 | 1080篇 |
2010年 | 840篇 |
2009年 | 1197篇 |
2008年 | 1178篇 |
2007年 | 746篇 |
2006年 | 762篇 |
2005年 | 692篇 |
2004年 | 712篇 |
2003年 | 817篇 |
2002年 | 801篇 |
2001年 | 919篇 |
2000年 | 611篇 |
1999年 | 729篇 |
1998年 | 658篇 |
1997年 | 513篇 |
1996年 | 554篇 |
1995年 | 599篇 |
1994年 | 590篇 |
1993年 | 511篇 |
1992年 | 470篇 |
1991年 | 477篇 |
1990年 | 423篇 |
1989年 | 586篇 |
1988年 | 269篇 |
1987年 | 290篇 |
1986年 | 257篇 |
1985年 | 642篇 |
1984年 | 522篇 |
1983年 | 415篇 |
1982年 | 490篇 |
1981年 | 612篇 |
1980年 | 247篇 |
1979年 | 193篇 |
1978年 | 189篇 |
1976年 | 155篇 |
1975年 | 189篇 |
1974年 | 180篇 |
1973年 | 161篇 |
1972年 | 189篇 |
排序方式: 共有10000条查询结果,搜索用时 570 毫秒
581.
582.
复熵及其应用——Bayes-E决策分析法 总被引:1,自引:0,他引:1
本文将熵的概念推广到复数域,并讨论了它的性质,进行了后验预分析,从而克服了Bayes风险决策方法对信息分析的不足,得到一个Bayes-E风险决策方法的新测度——传递熵。 相似文献
583.
584.
一个扩压叶栅角区流向涡的物理模式 总被引:1,自引:0,他引:1
本文采用烟显示和三维流场测量的方法,研究扩压叶栅角涡的物理模式。试验结果揭示了角涡发生、发展程中的流动形态和结构特点,以及角涡与叶面附面层相互作用的物理过程。在这个物理模式的基础上讨论了角涡引起高流动损失的物理机制。 相似文献
585.
PHD filters of higher order in target number 总被引:14,自引:0,他引:14
The multitarget recursive Bayes nonlinear filter is the theoretically optimal approach to multisensor-multitarget detection, tracking, and identification. For applications in which this filter is appropriate, it is likely to be tractable for only a small number of targets. In earlier papers we derived closed-form equations for an approximation of this filter based on propagation of a first-order multitarget moment called the probability hypothesis density (PHD). In a recent paper, Erdinc, Willett, and Bar-Shalom argued for the need for a PHD-type filter which remains first-order in the states of individual targets, but which is higher-order in target number. In this paper we show that this is indeed possible. We derive a closed-form cardinalized PHD (CPHD) filter, which propagates not only the PHD but also the entire probability distribution on target number. 相似文献
586.
587.
复杂空间四连杆传动系统的一种算法 总被引:1,自引:0,他引:1
1.引言 空间四连杆机构的运动关系是一个空间几何关系。确定四连杆相互位置时,存在着一球面与圆弧相交或两圆弧相交等情况,它们的运动与几何关系都以一些二次联立方程式表达。求解与根的判定都比较复杂,有时还要用迭代方法。本文所介绍的方法采用空间坐标变换,将空间四连杆机构的运动点转换到特定的平面内,利用平面三角形的几何关系求解。所用的方法都是典型的,简便易行,特别适用于计算机计算。 相似文献
588.
A New Mathematical Formulation for Strapdown Inertial Navigation 总被引:12,自引:0,他引:12
A differential equation is developed for the orientation vector relating the body frame to a chosen reference frame. The time derivative of this vector is the sum of the inertially measurable angular velocity vector and of the inertially nonmeasurable noncommutativity rate vector. It is precisely this noncommutativity rate vector that causes the computational problems when numerically integrating the direction cosine matrix. The orientation vector formulation allows the noncommutativity contribution to be isolated and, therefore, treated separately and advantageously. An orientation vector mechanization is presented for a strap down inertial system. Further, an example is given of the applica tion of this formulation to a typical rigid body rotation problem. 相似文献
589.
The application of existing estimation theory to the problem of specification and performance of passive sonar spectral estimators is considered. The classification function is addressed, so that the signal is assumed to be present, and so that the energy arrival angle is known. The spatial filter considered is a line array of M equally spaced omnidirectional hydrophones. Signal and ambient noise are both zero-mean, wide-sense, stationary Gaussian random processes that differ in their spatial correlation across the face of the array. The signal is a plane wave that can be made totally spacially corrected between array elements by inserting delays between sensors to invert the signal propagation delay. The noise correlation is a function of frequency, bandwidth, element separation, and the relative time delay between sensors. Under these assumptions, the Cramer-Rao lower bound is derived for the class of unbiased estimates of signal power in a narrow frequency band at the hydrophone in the presence of correlated ambient noise of known power. The bound is examined numerically, resulting in a threshold phenomenon with M that constitutes a new design consideration. In addition, there is a striking insensitivity to realistic values of ambient noise correlation, and there are ranges in signal-to-noise ratio for which one gains more by increasing M than by increasing the bandwidth-time product. Specific processors, including a new unbiased estimator when noise power is unknown, are developed. 相似文献
590.
The complex angle (CA) method for resolving a low angle target from its multipath signal is evaluated in the presence of system noise. It is shown that standard deviation improvements of around 3-to-1 can be achieved at a 20-dB signal-to-noise power ratio relative to a normal monopulse system without the CA. It is also shown that the CA method is unbiased, giving bias improvements of as much as 100 times relative to normal monopulse. Evaluation of the assumptions in the technique shows very little sensitivity to knowledge of the reflecting surface's conductivity or dielectric constant. However, the method is somewhat sensitive to knowledge of surface roughness. 相似文献