全文获取类型
收费全文 | 23746篇 |
免费 | 984篇 |
国内免费 | 1360篇 |
专业分类
航空 | 13304篇 |
航天技术 | 6936篇 |
综合类 | 1381篇 |
航天 | 4469篇 |
出版年
2021年 | 216篇 |
2019年 | 161篇 |
2018年 | 262篇 |
2017年 | 145篇 |
2016年 | 217篇 |
2015年 | 239篇 |
2014年 | 767篇 |
2013年 | 868篇 |
2012年 | 784篇 |
2011年 | 972篇 |
2010年 | 903篇 |
2009年 | 1270篇 |
2008年 | 1235篇 |
2007年 | 793篇 |
2006年 | 792篇 |
2005年 | 671篇 |
2004年 | 624篇 |
2003年 | 663篇 |
2002年 | 606篇 |
2001年 | 762篇 |
2000年 | 593篇 |
1999年 | 792篇 |
1998年 | 735篇 |
1997年 | 564篇 |
1996年 | 631篇 |
1995年 | 628篇 |
1994年 | 604篇 |
1993年 | 546篇 |
1992年 | 464篇 |
1991年 | 380篇 |
1990年 | 354篇 |
1989年 | 488篇 |
1988年 | 273篇 |
1987年 | 305篇 |
1986年 | 260篇 |
1985年 | 656篇 |
1984年 | 518篇 |
1983年 | 409篇 |
1982年 | 491篇 |
1981年 | 610篇 |
1980年 | 245篇 |
1979年 | 182篇 |
1978年 | 189篇 |
1976年 | 155篇 |
1975年 | 183篇 |
1974年 | 180篇 |
1973年 | 161篇 |
1972年 | 188篇 |
1971年 | 148篇 |
1969年 | 147篇 |
排序方式: 共有10000条查询结果,搜索用时 156 毫秒
71.
通过对影响万能工具显微镜滑板移动直线度的分析 ,提出有针对性的修理方法 ,并分别论述了产生线值偏差和角度偏差的主要原因。 相似文献
72.
73.
The robustness of a moving-bank multiple model adaptive estimator/controller to order reduction in the controller design model is examined. It is shown that the adaptive mechanism and bank-moving logic are not confounded by the effects of unmodeled higher order modes of a large flexible spacestructure. Control characteristics are achieved that are essentially equivalent to those of an artificially informed benchmark controller 相似文献
74.
Design for validation 总被引:2,自引:0,他引:2
An approach is outlined for the development of ultrareliable avionics for civil air transports using a design-for-validation philosophy that includes rigorous application of formal methods. The basic concept of the methodology is introduced, and the role of formal methods is explored. The impact of the design-for-validation philosophy on the system design process is then demonstrated by two simple examples. More details about the design-for-validation methodology are then given 相似文献
75.
This paper discusses and evaluates two innovative SETI education programs conducted at the University of Western Sydney, viz: the SETI Pathways Program and the Life in the Universe Curriculum Project. 相似文献
76.
The problem of optimal data fusion in multiple detection systems is studied in the case where training examples are available, but no a priori information is available about the probability distributions of errors committed by the individual detectors. Earlier solutions to this problem require some knowledge of the error distributions of the detectors, for example, either in a parametric form or in a closed analytical form. Here we show that, given a sufficiently large training sample, an optimal fusion rule can be implemented with an arbitrary level of confidence. We first consider the classical cases of Bayesian rule and Neyman-Pearson test for a system of independent detectors. Then we show a general result that any test function with a suitable Lipschitz property can be implemented with arbitrary precision, based on a training sample whose size is a function of the Lipschitz constant, number of parameters, and empirical measures. The general case subsumes the cases of nonindependent and correlated detectors. 相似文献
77.
An experimental investigation of three eigen DF techniques 总被引:1,自引:0,他引:1
The comparative direction-finding (DF) performance of multiple signal classification (MUSIC), ROOT-MUSIC, and estimation of signal parameters via rotational invariance techniques (ESPRIT) is considered. Data were collected from two target transmitters operating simultaneously. The objective of the experiment was to evaluate multipath resolution capability using measurement precision equivalent to that found in modern radio direction-finding systems 相似文献
78.
Bucci N.J. Nespor J.D. Urkowitz H. Mokry D. Brown R. Baldygo W. 《Aerospace and Electronic Systems Magazine, IEEE》1995,10(8):37-43
A major technology barrier to the application of pulse compression for the meteorological functions required by a next generation ATC radar is range/time sidelobes which mask and corrupt observations of weak phenomena occurring near areas of strong extended meteorological scatterers. Techniques for suppressing range sidelobes are well known but without prior knowledge of the scattering medium's velocity distribution their performance degrades rapidly in the presence of Doppler. Recent investigations have presented a “doppler tolerant” range sidelobe suppression technique. The thrust of the work described herein is the extension of previous simulations to actual transmitted dispersed/coded waveforms using the S-band surveillance radar located at Rome Laboratory Surveillance Facility. The objectives of the experiment are: 1) to extend the verification of the simulation of the Doppler tolerant technique; and 2) to demonstrate that the radar transmitter, waveform generator, and receiver imperfections do not significantly degrade resolution, performance or reliability of meteorological spectral moment estimates 相似文献
79.
并发演算CC(Concurrent Calculus)是高阶并发通信系统的数学模型,它把λ-演算作为子理论并包含一阶通信系统演算CCS、活动进程演算CMP、和高阶通信系统演算CHOCS的主要特征。在CC中,通信端口可为任意表达式并且进程和通信端口都可以作为在通信中传递的一等对象(First-class Objects)。从而CC不仅可以描述一阶通信行为而且可以刻划通信网络的动态自修改行为。另外,由于CC把λ-演算和进程演算纳入同一形式系统,故CC可以作为并发函数式程序设计的核心语言和理论模型。本文首先给出CC的语法、语义和例子,然后研究CC的高阶双向模拟等价关系以及CC的代数定律。最后简单讨论了相关的工作和今后的研究方向。 相似文献
80.
Liu H. Ghafoor A. Stockmann P.H. 《IEEE transactions on aerospace and electronic systems》1992,28(2):324-334
The Gram-Schmidt orthogonalization (GSO) algorithm has excellent numerical performance and is readily applicable to systolic implementations such as in a field of adaptive cancellation systems. A modified GSO algorithm for a fully adaptive array is proposed and computer simulations show that the proposed algorithm gives superior performance. A systolic implementation of the proposed GSO algorithm for fully adaptive array is presented. A feedback mode GSO algorithm for use with analog weights is also presented and has been shown to have excellent performance in the presence of weight errors 相似文献