全文获取类型
收费全文 | 1496篇 |
免费 | 343篇 |
国内免费 | 272篇 |
专业分类
航空 | 1120篇 |
航天技术 | 313篇 |
综合类 | 175篇 |
航天 | 503篇 |
出版年
2024年 | 8篇 |
2023年 | 16篇 |
2022年 | 67篇 |
2021年 | 76篇 |
2020年 | 86篇 |
2019年 | 69篇 |
2018年 | 66篇 |
2017年 | 78篇 |
2016年 | 71篇 |
2015年 | 63篇 |
2014年 | 85篇 |
2013年 | 101篇 |
2012年 | 99篇 |
2011年 | 104篇 |
2010年 | 104篇 |
2009年 | 106篇 |
2008年 | 93篇 |
2007年 | 107篇 |
2006年 | 111篇 |
2005年 | 74篇 |
2004年 | 55篇 |
2003年 | 63篇 |
2002年 | 73篇 |
2001年 | 64篇 |
2000年 | 38篇 |
1999年 | 50篇 |
1998年 | 28篇 |
1997年 | 24篇 |
1996年 | 13篇 |
1995年 | 21篇 |
1994年 | 17篇 |
1993年 | 10篇 |
1992年 | 9篇 |
1991年 | 10篇 |
1990年 | 9篇 |
1989年 | 12篇 |
1988年 | 11篇 |
1987年 | 4篇 |
1986年 | 2篇 |
1985年 | 2篇 |
1984年 | 2篇 |
1982年 | 1篇 |
1981年 | 2篇 |
1980年 | 1篇 |
1978年 | 2篇 |
1977年 | 1篇 |
1970年 | 1篇 |
1969年 | 1篇 |
1900年 | 1篇 |
排序方式: 共有2111条查询结果,搜索用时 625 毫秒
821.
Virus protein assembly in microgravity. 总被引:2,自引:0,他引:2
D Chang A Paulsen T C Johnson R A Consigli 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1993,13(7):251-257
The coat of polyomavirus is composed of three proteins that can self-assemble to form an icosahedral capsid. VP1 represents 75% of the virus capsid protein and the VP1 capsomere subunits are capable of self assembly to form a capsid-like structure. Ground-based and orbiter studies were conducted with VP1 protein cloned in an expression vector and purified to provide ample quantities for capsomere-capsid assembly. Flight studies were conducted on STS-37 on April 5-9, 1991. Assembly initiated when a VP1 protein solution was interfaced with a Ca+2 buffer solution (pH 5.0). After four days a second alignment terminated the assembly process and allowed for glutaraldehyde fixation. Flight and ground-based samples were analyzed by electron microscopy. Ground-based experiments revealed the assembly of VP1 into capsid-like structures and a heterogenous size array of capsomere subunits. Samples reacted in microgravity, however, showed capsomeres of a homogenous size, but lack of capsid-like assembly. 相似文献
822.
Chang K.C. Tian Zhi Saha R.K. 《IEEE transactions on aerospace and electronic systems》2002,38(2):455-466
In a multisensor environment, each sensor detects multiple targets and creates corresponding tracks. Fusion of tracks from these, possibly dissimilar, sensors yields more accurate kinematic and attribute information regarding the target. Two methodologies have been employed for such purpose, which are: measurement fusion and state vector fusion. It is well known that the measurement fusion approach is optimal but computationally inefficient and the state vector fusion algorithms are more efficient but suboptimal, in general. This is so because the state vector estimates to be fused obtained from two sensors, are not conditionally independent in general due to the common process noise from the target being tracked. It is to be noted that there are three approaches to state vector fusion, which are: weighted covariance, information matrix, and pseudomeasurement. This research is restricted solely to performance evaluation of the information matrix form of state vector fusion. Closed-form analytical solution of steady state fused covariance has been derived as a measure of performance using this approach. Note that the results are derived under the assumptions that the two sensors are synchronized and no misassociation or merged measurement is considered in the study. Results are compared with those using Monte Carlo simulation, which was used in the past to predict fusion system performance by various authors. These results provide additional insight into the mechanism of track fusion and greatly simplify evaluation of fusion performance. In addition, availability of such a solution facilitates the trade-off studies for designing fusion systems under various operating conditions 相似文献
823.
Binary parallel distributed-detection architectures employ a bank of local detectors to observe a common volume of surveillance, and form binary local decisions about the existence or nonexistence of a target in that volume. The local decisions are transmitted to a central detector, the data fusion center (DEC), which integrates them to a global target or no target decision. Most studies of distributed-detection systems assume that the local detectors are synchronized. In practice local decisions are made asynchronously and the DFC has to update its global decision continually. In this study the number of local decisions observed by the central detector within any observation period is Poisson distributed. An optimal fusion rule is developed and the sufficient statistic is shown to be a weighted sum of the local decisions collected by the DFC within the observation interval. The weights are functions of the individual local detector performance probabilities (i.e., probabilities of false alarm and detection). In this respect the decision rule is similar to the one developed by Chair and Varshney for the synchronized system. Unlike the Chair-Varshney rule, however, the DFC's decision threshold in the asynchronous system is time varying. Exact expressions and asymptotic approximations are developed for the detection performance with the optimal rule. These expressions allow performance prediction and assessment of tradeoffs in realistic decision fusion architectures which operate over modern communication networks 相似文献
824.
就近年来战斗机敏捷性研究中的某些问题,如横向敏捷性尺度,敏捷性与飞行品质之间关系,大迎角下俯卸载尺度以及敏捷性战斗机的飞行控制系统一体化设计,进行了综合分析和讨论,并得出了结论:(1)横向敏捷性以空战效益的影响比俯仰和轴向敏捷性的影响大;(2)空战中下俯敏捷性与上仰敏捷性同样重要;(3)敏捷性与飞行品质之间存在着明显的相关性;(4)敏捷性战斗机在初始设计阶段,必须考虑飞机控制系统一体化设计。 相似文献
825.
826.
Liuchen Chang 《Aerospace and Electronic Systems Magazine, IEEE》1996,11(8):24-28
High power IGBTs have been widely used for medium power inverters up to 200 kVA. Their driver circuits often require multiple-output, isolated power supplies. This paper presents a simple flyback power supply especially designed for the driver boards of three-phase IGBT inverters. The detailed circuit diagrams and typical waveforms are reported. Certain performance data are given. The power supplies have been used in 100 kVA inverters for an electric vehicle drive and for a wind turbine inverter 相似文献
827.
Thompson M.W. Ho-Shin Chang 《IEEE transactions on aerospace and electronic systems》1994,30(2):452-461
The discrete-time detection of a time-varying, additive signal in independent Laplace noise is considered. Previous efforts in this area have been restricted to the constant signal, and identically distributed noise case. Theoretical (closed form) expressions for the false alarm and detection probabilities are developed for both the Neyman-Pearson optimal detector and the classical matched filter detector. Comparisons between the two detectors are made which illustrate the effects of signal-to-noise power ratio and sample size for certain false alarm and detection probability constraints. In view of the fact that the optimal Laplace detector is not UMP, we also investigate the effect of signal amplitude mismatch 相似文献
828.
829.
本文由前轮定位参数的作用出发,详细分析了影响其作用的诸多因素,为合理选择汽车的前轮定位参数提供依据。 相似文献
830.