全文获取类型
收费全文 | 3256篇 |
免费 | 687篇 |
国内免费 | 590篇 |
专业分类
航空 | 2169篇 |
航天技术 | 849篇 |
综合类 | 563篇 |
航天 | 952篇 |
出版年
2024年 | 20篇 |
2023年 | 39篇 |
2022年 | 99篇 |
2021年 | 140篇 |
2020年 | 135篇 |
2019年 | 98篇 |
2018年 | 99篇 |
2017年 | 97篇 |
2016年 | 103篇 |
2015年 | 127篇 |
2014年 | 162篇 |
2013年 | 169篇 |
2012年 | 204篇 |
2011年 | 222篇 |
2010年 | 239篇 |
2009年 | 198篇 |
2008年 | 251篇 |
2007年 | 203篇 |
2006年 | 154篇 |
2005年 | 149篇 |
2004年 | 103篇 |
2003年 | 110篇 |
2002年 | 100篇 |
2001年 | 117篇 |
2000年 | 105篇 |
1999年 | 143篇 |
1998年 | 147篇 |
1997年 | 113篇 |
1996年 | 107篇 |
1995年 | 78篇 |
1994年 | 87篇 |
1993年 | 91篇 |
1992年 | 71篇 |
1991年 | 65篇 |
1990年 | 52篇 |
1989年 | 34篇 |
1988年 | 20篇 |
1987年 | 33篇 |
1986年 | 7篇 |
1985年 | 9篇 |
1984年 | 9篇 |
1983年 | 5篇 |
1982年 | 13篇 |
1981年 | 3篇 |
1980年 | 1篇 |
1965年 | 2篇 |
排序方式: 共有4533条查询结果,搜索用时 296 毫秒
981.
982.
983.
984.
Due to the particularity of its location algorithm,there are some unique difficulties and features regarding the test of target motion states of multilateration(MLAT)system for airport surface surveillance.This paper proposed a test method applicable for the airport surface surveillance MLAT system,which can effectively determine whether the target is static or moving at a certain speed.Via a normalized test statistic designed in the sliding data window,the proposed method not only eliminates the impact of geometry Dilution of precision(GDOP)effectively,but also transforms the test of different motion states into the test of different probability density functions.Meanwhile,by adjusting the size of the sliding window,it can fulfill different test performance requirements.The method was developed through strict theoretical extrapolation and performance analysis,and simulations results verified its correctness and effectiveness. 相似文献
985.
Hao Peng Xiaoli Bai 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2018,61(10):2628-2646
Due to the lack of information such as the space environment condition and resident space objects’ (RSOs’) body characteristics, current orbit predictions that are solely grounded on physics-based models may fail to achieve required accuracy for collision avoidance and have led to satellite collisions already. This paper presents a methodology to predict RSOs’ trajectories with higher accuracy than that of the current methods. Inspired by the machine learning (ML) theory through which the models are learned based on large amounts of observed data and the prediction is conducted without explicitly modeling space objects and space environment, the proposed ML approach integrates physics-based orbit prediction algorithms with a learning-based process that focuses on reducing the prediction errors. Using a simulation-based space catalog environment as the test bed, the paper demonstrates three types of generalization capability for the proposed ML approach: (1) the ML model can be used to improve the same RSO’s orbit information that is not available during the learning process but shares the same time interval as the training data; (2) the ML model can be used to improve predictions of the same RSO at future epochs; and (3) the ML model based on a RSO can be applied to other RSOs that share some common features. 相似文献
986.
Fast coating analysis and modeling for RCS reduction of aircraft 总被引:1,自引:0,他引:1
In order to fast analyze the aircraft Radar Cross Section(RCS) and accurately reduce it with Radar Absorbing Materials(RAM), a comprehensive analysis method based on Higher-Order Method of Moments(HOMOM), termed Locally Coating Method(LCM), is proposed in this paper. There are two steps to fast analyze coatings for RCS reduction in this method: analyze the RCS of various parts before coating the aircraft; model a coating over the aircraft and analyze the wave absorbing effect of it. The aircraft RCS is calculated as a whole but analyzed in various parts by LCM, and thus the RCS contribution of different parts can be compared without disturbing the current continuity. A model expansion algorithm is also presented in LCM to model absorption coatings on specified aircraft parts for later stage RCS calculation of the coated aircraft. 相似文献
987.
邬磊 《中国民航飞行学院学报》2013,24(3)
在自主学习中心大部分学生选择观看原声电影作为主要的学习手段,但学生在电影赏析时缺乏明确的学习目标和可行的学习方法.本文在介绍实验操作的具体步骤后,通过对比分析实验对象在实验前后的考试数据,证明了教师合理科学地干预学生原声电影赏析活动,可以让学生收获更好的语言学习效果. 相似文献
988.
Zongjie Hao Dunhai Li Yanhui Li Zhicong Wang Yuan Xiao Gaohong Wang Yongding Liu Chunxiang Hu Qifang Liu 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2011
Some phytoplankton can be regarded as possible candidates in the establishment of Controlled Ecological Life Support System (CELSS) for some intrinsic characteristics, the first characteristic is that they should grow rapidly, secondly, they should be able to endure some stress factors and develop some corresponding adaptive strategies; also it is very important that they could provide food rich in nutritious protein and vitamins for the crew; the last but not the least is they can also fulfill the other main functions of CELSS, including supplying oxygen, removing carbon dioxide and recycling the metabolic waste. According to these characteristics, Nostoc sphaeroides, a potential healthy food in China, was selected as the potential producer in CELSS. It was found that the oxygen average evolution rate of this algae is about 150 μmol O2 mg−1 h−1, and the size of them are ranged from 2 to 20 mm. Also it can be cultured with high population density, which indicated that the potential productivity of Nostoc sphaeroides is higher than other algae in limited volume. We measured the nutrient contents of the cyanobacterium and concluded it was a good food for the crew. Based on above advantages, Nostoc sphaeroides was assumed to a suitable phytoplankton for the establishment of Controlled Ecological Life Support System. We plan to develop suitable bioreactor with the cyanobacterium for supplying oxygen and food in future space missions. 相似文献
989.
990.