全文获取类型
收费全文 | 1468篇 |
免费 | 321篇 |
国内免费 | 433篇 |
专业分类
航空 | 1069篇 |
航天技术 | 513篇 |
综合类 | 213篇 |
航天 | 427篇 |
出版年
2024年 | 13篇 |
2023年 | 33篇 |
2022年 | 57篇 |
2021年 | 66篇 |
2020年 | 85篇 |
2019年 | 88篇 |
2018年 | 125篇 |
2017年 | 73篇 |
2016年 | 85篇 |
2015年 | 79篇 |
2014年 | 112篇 |
2013年 | 98篇 |
2012年 | 147篇 |
2011年 | 146篇 |
2010年 | 111篇 |
2009年 | 132篇 |
2008年 | 108篇 |
2007年 | 71篇 |
2006年 | 97篇 |
2005年 | 74篇 |
2004年 | 48篇 |
2003年 | 52篇 |
2002年 | 34篇 |
2001年 | 37篇 |
2000年 | 22篇 |
1999年 | 24篇 |
1998年 | 31篇 |
1997年 | 19篇 |
1996年 | 14篇 |
1995年 | 18篇 |
1994年 | 18篇 |
1993年 | 21篇 |
1992年 | 22篇 |
1991年 | 27篇 |
1990年 | 11篇 |
1989年 | 7篇 |
1988年 | 10篇 |
1987年 | 5篇 |
1984年 | 2篇 |
排序方式: 共有2222条查询结果,搜索用时 265 毫秒
951.
952.
953.
954.
955.
956.
针对传统多智能体轨迹估计算法信息交换量大,计算量随群规模指数增长,可扩展性差等诸多不足,提出了一种基于超松弛迭代(SOR)的分布式多智能体轨迹估计算法,通过将最大似然(ML)准则下的轨迹估计转化为两级线性优化问题,并综合利用分布式超松弛迭代(Distributed SOR)和标记初始化方法,加快求解速度并简化信息交换流程,最终实现了多智能体位姿轨迹优化和协作定位。实验表明,所提的分布式方法能达到集中式算法的精度水平,在49个智能体规模条件下,位置估计误差小于0.15 m,姿态估计误差小于0.03°,且数据交换量仅到现有主流分布式方法DDF-SAM的0.06%,能很好用于大规模集群的场景。 相似文献
957.
T. Bullett A. Malagnini M. Pezzopane C. Scotto 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2010
In November 2008, the ionosonde station at Boulder, Colorado, USA (40.0°N; 105.3°W) became the host of a new ionosonde (VIPIR, Vertical Incidence Pulsed Ionospheric Radar) developed and built by Scion Associates. 相似文献
958.
K.G. Ratovsky B.G. ShpynevA.V. Oinats A.V. Medvedev 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2010
The topside ionosphere parameters are studied based on the long-duration Irkutsk incoherent scatter radar (52.9N, 103.3E) measurements conducted in September 2005, June and December 2007. As a topside ionosphere parameter we chose the vertical scale height (VSH) related to the gradient of the electron density logarithm above the peak height. For morphological studies we used median electron density profiles. Besides the median behavior we also studied VSH disturbances (deviations from median values) during the magnetic storm of September 11th 2005. We compared the Irkutsk incoherent scatter radar data with the Millstone Hill and Arecibo incoherent scatter radar observations, the IRI-2007 prediction (using the two topside options) and VSH derived from the Irkutsk DPS-4 Digisonde bottomside measurements. 相似文献
959.
《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2020,65(7):1772-1780
Doppler frequency estimation with high accuracy is very important for precise orbit determination and scientific studies in deep space exploration. In this paper, we propose a new method which implements cross correlation of only received signals from one single station to estimate Doppler frequency. The algorithm is relatively simple and it can achieve high accuracy even when signal to noise ratio (SNR) is very low. Moreover, the accuracy can be further improved by implementing frequency compensation, especially when the frequency dynamic range is high. Simulations were performed and results proved the validity of this method. X-band observations on MEX (Mars Express) and Juno were performed by a 13 m telescope of Wuhan University, which was equipped by two back ends, i.e. CDAS (Chinese Data Acquisition System) and RSR (Radio Science Receiver). Raw data recorded by the CDAS were processed by using the proposed method, and the estimated frequencies were in good agreement with the real time frequency estimation values by the RSR. What’s more, the accuracy by using cross correlation was higher than the one of RSR. 相似文献
960.
《中国航空学报》2020,33(5):1375-1391
The performance of compact, aggressive ducts in advanced propulsion systems is limited by the internal flow separation coupled with the formation of secondary counter-rotating vortices that give rise to intensive flow distortions at the duct exit. An experimental investigation was conducted to study the flow field and passive suppression of flow separation and Aerodynamic Interface Plane (AIP) distortion in a serpentine air inlet duct. Tests were performed by a turbofan engine at several Engine Operating Points (EOPs) from 56% (idle) to 100% (max). A large total pressure deficit region arose at the upper part of the AIP, which was associated with the upper surface flow separation. Using the new mechanical S-type vortex generators in two longitudinal positions (VG1 and VG2), separation and loss were diminished at the upper part of the duct and AIP. The VG2 arrangement attained the maximum reductions in distortion coefficients which were 73.72%, 60.7% and 37.8% in DC(90°), DC(60°) and ΔPC/P metrics, respectively. In the next step of the study, some unsteady aspects of the flow field were analyzed inside the duct. The separation onset and reattachment points were determined by the standard deviation of static pressure on the upper surface. The AIP spectral distribution showed that the boundary of low pressure and high pressure recovery regions was dominated by the maximum fluctuations. Furthermore, the PSD diagram of several probes at AIP revealed the vortex shedding frequency and its higher harmonics at separation region. The energy content of distinct unsteady spectral features in the bare configuration was significantly reduced using VGs, which showed the improvement of flow at the duct exit. 相似文献