全文获取类型
收费全文 | 958篇 |
免费 | 145篇 |
国内免费 | 100篇 |
专业分类
航空 | 512篇 |
航天技术 | 310篇 |
综合类 | 130篇 |
航天 | 251篇 |
出版年
2023年 | 9篇 |
2022年 | 26篇 |
2021年 | 27篇 |
2020年 | 19篇 |
2019年 | 25篇 |
2018年 | 29篇 |
2017年 | 26篇 |
2016年 | 35篇 |
2015年 | 40篇 |
2014年 | 43篇 |
2013年 | 64篇 |
2012年 | 51篇 |
2011年 | 61篇 |
2010年 | 55篇 |
2009年 | 62篇 |
2008年 | 60篇 |
2007年 | 37篇 |
2006年 | 29篇 |
2005年 | 44篇 |
2004年 | 26篇 |
2003年 | 26篇 |
2002年 | 20篇 |
2001年 | 28篇 |
2000年 | 15篇 |
1999年 | 31篇 |
1998年 | 30篇 |
1997年 | 20篇 |
1996年 | 32篇 |
1995年 | 23篇 |
1994年 | 23篇 |
1993年 | 16篇 |
1992年 | 19篇 |
1991年 | 17篇 |
1990年 | 15篇 |
1989年 | 18篇 |
1988年 | 9篇 |
1987年 | 12篇 |
1986年 | 7篇 |
1985年 | 8篇 |
1984年 | 13篇 |
1983年 | 9篇 |
1982年 | 11篇 |
1981年 | 13篇 |
1980年 | 2篇 |
1972年 | 2篇 |
1971年 | 4篇 |
1970年 | 2篇 |
1969年 | 2篇 |
1967年 | 1篇 |
1965年 | 2篇 |
排序方式: 共有1203条查询结果,搜索用时 140 毫秒
141.
卫星能源约束检查模型对卫星任务规划有重要参考作用,可防止因能源不足而导致卫星故障或任务不能顺利完成。现有的能源约束检查模型存在部分问题:人为设定计算初始值,导致计算不准确;能源约束检查不通过时未给出符合能源约束的任务调整建议,导致任务不能快速调整;未对计算误差进行校正,导致误差积累。对此,提出了一种改进的能源约束检查模型,在原有模型中加入能源数据库对卫星运行的能源数据进行记录,从而准确获取初始值,并在模型中加入任务调整和遥测校正功能。该模型可实现能源约束计算自动管理及任务自动调整功能,缩短任务规划周期,提升紧急任务的执行效率。对改进的模型进行仿真,结果表明:模型预测结果与预期值相符,任务调整和能源校正功能实现良好,能实际应用到运控系统中进行规划任务能源约束检查和能源管理。 相似文献
142.
飞机装配中铆接部位的孔垂直度直接影响铆钉的连接质量和疲劳寿命,进而影响飞机的力学性能。为了保证飞机铆接垂直度要求,必须分析自动钻铆系统法向误差的影响因素。首先,确定了倾角误差、数学模型的线性误差与法向误差的关系。其次,给出了激光位移传感器安装参数的选取规律,并研究了其对位姿标定的影响。最后,实验验证了安装参数优化后的实际效果。研究结果可为法向测量装置的设计与分析提供理论指导,对于提升自动钻铆系统的法向精度,进而保证孔垂直度要求具有重要意义。 相似文献
143.
144.
145.
146.
J Z Kiss R E Edelmann 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1999,24(6):755-762
In order to help resolve some of the controversy associated with ground-based research that has supported the starch-statolith theory of gravity perception in plants, we performed spaceflight experiments with Arabidopsis in Biorack during the January 1997 and May 1997 missions of the Space Shuttle. Seedlings of wild-type (WT) Arabidopsis, two reduced-starch strains, and a starchless mutant were grown in microgravity and then were given either a 30, 60, or 90 minute gravity stimulus on a centrifuge. By the 90 min 1-g stimulus, the WT exhibited the greatest magnitude of curvature and the starchless mutant exhibited the smallest curvature while the two reduced starch mutants had an intermediate magnitude of curvature. In addition, space-grown plants had two structural features that distinguished them from the controls: a greater number of root hairs and an anomalous hypocotyl hook structure. However, the morphological changes observed in the flight seedlings are likely to be due to the effects of ethylene present in the spacecraft. (Additional ground-based studies demonstrated that this level of ethylene did not significantly affect gravitropism nor did it affect the relative gravitropic sensitivity among the four strains.) Nevertheless, this experiment on gravitropism was performed the "right way" in that brief gravitational stimuli were provided, and the seedlings were allowed to express the response without further gravity stimuli. Our spaceflight results support previous ground-based studies of these and other mutants since increasing amounts of starch correlated positively with increasing sensitivity to gravity. 相似文献
147.
The two-target technique proposed by the author in an earlier paper [1] for reducing radar multipath angle tracking errors has been simulated on a digital computer assuming an actual closed-loop system. When tracking with noise, the technique provides angle error performance which compares quite favorably with the expected performance given in [1] Furthermore, the large bias errors usually encountered in normal monopulse systems at low elevation angles are removed. Results of typical tracks are given, both for the method of [1], and for a modified version of the method which applies primarily to shipboard radar systems. Some results on loss of lock are also presented. 相似文献
148.
R.Z. Sagdeev G.G. Managadze A.A. Martinson Yu.A. Romanovsky R.I. Moisya W.K. Riedler M.F. Friedrich T.G. Adeishvily S.B. Lyakhov L.S. Novikov N.A. Leonov T.I. Gagua I.I. Slyusarenko 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1981,1(2):129-140
This paper describes two rocket experiments “Aelita” with high power lithium plasma injection. The results of onboard magnetometer, massspectrometer, photometer, plasma, corpuscular and ground radar measurements are given. Dynamics and structure of plasma formation are discussed. 相似文献
149.
Xi Lin Xiaohua Deng Zhigang Yuan Meng Zhou Shiyou Li Ye Pang 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2011
We study the structure and kinetic properties of slow-mode shocks near the plasma sheet boundary layer (PSBL) associated with magnetic reconnection by Cluster observation. The presence of slow-mode shocks is confirmed by traditional Rankine–Hugoniot (RH) analysis and Monte-Carlo shock fitting method. The Walén analysis, applied to the tailward flow associated with slow-mode shocks, also supports that plasma was accelerated across a Petschek-type slow-mode shock connected to the diffusion region. Back-streaming ions were observed on the shock layer, and cold ions were accelerated and heated by slow-mode shocks. In addition, whistler and electrostatic solitary waves were observed around the slow-mode shocks. These waves might be excited by the observed field-aligned electron beams near the shocks. 相似文献
150.
Peebles Peyton Z. Berkowitz R. S. 《IEEE transactions on aerospace and electronic systems》1968,(6):845-854
The problem of determining target parameters of a known number of radar targets falling in the same range-Doppler-angle-angle resolution cell is examined for the noise-free case. The required minimum number of radar beams is determined, based upon approximating the beam patterns by a Taylor series expansion, both for the general problem and for factorable beams. Signal processors for target position estimation are developed for the two-target case and equations are presented for the general case. 相似文献