全文获取类型
收费全文 | 855篇 |
免费 | 201篇 |
国内免费 | 294篇 |
专业分类
航空 | 631篇 |
航天技术 | 270篇 |
综合类 | 114篇 |
航天 | 335篇 |
出版年
2024年 | 9篇 |
2023年 | 21篇 |
2022年 | 40篇 |
2021年 | 38篇 |
2020年 | 54篇 |
2019年 | 58篇 |
2018年 | 77篇 |
2017年 | 38篇 |
2016年 | 51篇 |
2015年 | 48篇 |
2014年 | 64篇 |
2013年 | 57篇 |
2012年 | 94篇 |
2011年 | 89篇 |
2010年 | 77篇 |
2009年 | 75篇 |
2008年 | 67篇 |
2007年 | 48篇 |
2006年 | 57篇 |
2005年 | 36篇 |
2004年 | 31篇 |
2003年 | 32篇 |
2002年 | 18篇 |
2001年 | 19篇 |
2000年 | 10篇 |
1999年 | 13篇 |
1998年 | 24篇 |
1997年 | 12篇 |
1996年 | 4篇 |
1995年 | 12篇 |
1994年 | 8篇 |
1993年 | 12篇 |
1992年 | 13篇 |
1991年 | 20篇 |
1990年 | 7篇 |
1989年 | 5篇 |
1988年 | 8篇 |
1987年 | 4篇 |
排序方式: 共有1350条查询结果,搜索用时 46 毫秒
241.
In this paper, we consider an amplify-and-forward (AF) cooperative communication system when the channel state information (CSI) used in relay selection differs from that during data transmission, i.e., the CSI used in relay selection is outdated. The selected relay may not be actually the best for data transmission and the outage performance of the cooperative system will deteriorate. To improve its performance, we propose a relay selection strategy based on maximum a posteriori (MAP) estimation, where relay is selected based on predicted signal-to-noise ratio (SNR). To reduce the computation complexity, we approximate the a posteriori probability density of SNR and obtain a closed-form predicted SNR, and a relay selection strategy based on the approximate MAP estimation (RS-AMAP) is proposed. The simulation results show that this approximation leads to trivial performance loss from the perspective of outage probability. Compared with relay selection strategies given in the literature, the outage probability is reduced largely through RS-AMAP for medium-to-large transmitting powers and medium-to-high channel correlation coefficients. 相似文献
242.
Associating environmental stresses (ESs) with built-in test (BIT) output is an important means to help diagnose intermittent faults (IFs). Aiming at low efficiency in association of traditional time stress measurement device (TSMD), an association model is built. Thereafter, a novel approach is given to evaluate the integrated environmental stress (IES) level. Firstly, the selection principle and approach of main environmental stresses (MESs) and key characteristic parameters (KCPs) are presented based on fault mode, mechanism, and ESs analysis (FMMEA). Secondly, reference stress events (RSEs) are constructed by dividing IES into three stress levels according to its impact on faults; and then the association model between integrated environmental stress event (IESE) and BIT output is built. Thirdly, an interval grey association approach to evaluate IES level is proposed due to the interval number of IES value. Consequently, the association output can be obtained as well. Finally, a case study is presented to demonstrate the proposed approach. Results show the proposed model and approach are effective and feasible. This approach can be used to guide ESs measure, record, and association. It is well suited for on-line assistant diagnosis of faults, especially IFs. 相似文献
243.
In Bayesian multi-target fltering,knowledge of measurement noise variance is very important.Signifcant mismatches in noise parameters will result in biased estimates.In this paper,a new particle flter for a probability hypothesis density(PHD)flter handling unknown measurement noise variances is proposed.The approach is based on marginalizing the unknown parameters out of the posterior distribution by using variational Bayesian(VB)methods.Moreover,the sequential Monte Carlo method is used to approximate the posterior intensity considering non-linear and non-Gaussian conditions.Unlike other particle flters for this challenging class of PHD flters,the proposed method can adaptively learn the unknown and time-varying noise variances while fltering.Simulation results show that the proposed method improves estimation accuracy in terms of both the number of targets and their states. 相似文献
244.
信道估计是无线通信系统的关键技术之一。文章提出一种基于压缩感知(Compressed Sensing)的信道估计方法,这种方法通过分析信道时延和多普勒域的稀疏度,可以有效降低导频的数量,从而提高系统的频谱利用率。通过与最小均方(Least Square,LS)信道估计方法的对比,可以明显看到导频数量的降低。 相似文献
245.
文章针对众多性能优良的超分辨DOA(Direction—Of-Arrival)估计算法大都是以预知信源数为前提、信源数估计不准可能会导致DOA估计失败这一问题,提出了一种基于协方差矩阵对角加载的超分辨DOA估计算法。该算法不需要预判信源个数和进行特征值分解,且通过对协方差矩阵进行对角加载,可以平滑小快拍数时噪声特征值分散程度,因此,该算法更适用于快拍数较少的情况。理论分析表明:该算法的统计估计性能接近于MUSIC(Multiple Signal Classification)算法。计算机仿真结果验证了该算法的鲁棒性和可行性。 相似文献
246.
247.
多操纵面飞翼构型飞机舵面故障在线诊断方法 总被引:2,自引:0,他引:2
舵面故障会影响飞机的操纵性,为实现飞机的控制重构需在线诊断出舵面的故障信息.针对多操纵面飞翼构型飞机,提出了一种在线诊断其舵面故障的方法.飞翼构型飞机舵面故障在线诊断需辨识的操纵导数个数较多,直接使用最小二乘法难以精确辨识得到各操纵导数.基于限定记忆最小二乘法,采用分组辨识法对飞机的操纵导数进行辨识,减少了每次辨识操纵... 相似文献
248.
信噪比(SNR,Signalto Noise Ratio)是衡量通信系统性能的重要指标。针对传统信噪比估计算法要求利用训练序列或一帧数据范周内信道系数恒定等问题,提出了一种8PSK信号在信道系数服从高斯分布的衰落信道下肯信噪比估计算法。该算法取接收信号实部或虚部数值运算构成中间观测量,理论分析8PSK信号经过衰落信道后... 相似文献
249.
Chikako Takahashi Makoto Suzuki Chihiro Mitsuda Satoshi Ochiai Naohiro Manago Hiroo Hayashi Yoshitaka Iwata Koji Imai Takuki Sano Masahiro Takayanagi Masato Shiotani 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2011
We estimate the capability of ozone (O3) retrieval with the Superconducting Submillimeter-Wave Limb-Emission Sounder (SMILES) instrument attached to the Exposed Facility of the Japanese Experiment Module (JEM) on the International Space Station (ISS). SMILES carries a 4-K mechanical refrigerator to cool superconducting devices in space. Since SMILES has high sensitivity thanks to the superconducting receiver, it is expected that SMILES has ability to retrieve O3 profiles more precisely than the previous millimeter–submillimeter limb measurements from satellites. 相似文献
250.