全文获取类型
收费全文 | 451篇 |
免费 | 99篇 |
国内免费 | 91篇 |
专业分类
航空 | 258篇 |
航天技术 | 200篇 |
综合类 | 28篇 |
航天 | 155篇 |
出版年
2024年 | 5篇 |
2023年 | 13篇 |
2022年 | 14篇 |
2021年 | 36篇 |
2020年 | 24篇 |
2019年 | 38篇 |
2018年 | 28篇 |
2017年 | 30篇 |
2016年 | 31篇 |
2015年 | 27篇 |
2014年 | 67篇 |
2013年 | 33篇 |
2012年 | 31篇 |
2011年 | 27篇 |
2010年 | 30篇 |
2009年 | 29篇 |
2008年 | 26篇 |
2007年 | 25篇 |
2006年 | 19篇 |
2005年 | 22篇 |
2004年 | 15篇 |
2003年 | 15篇 |
2002年 | 15篇 |
2001年 | 5篇 |
2000年 | 6篇 |
1999年 | 5篇 |
1998年 | 5篇 |
1997年 | 2篇 |
1996年 | 2篇 |
1995年 | 1篇 |
1994年 | 3篇 |
1993年 | 2篇 |
1992年 | 3篇 |
1990年 | 3篇 |
1989年 | 2篇 |
1988年 | 1篇 |
1981年 | 1篇 |
排序方式: 共有641条查询结果,搜索用时 15 毫秒
231.
针对月球探测中软着陆与采样返回段弹道计算问题,提出用数值逼近弹道确定方法。通过B样条对探测器状态进行建模,进而综合全弧段数据进行统计定轨的方法。由于样条法良好的数值逼近性能,使得该方法对探测器弹道异常复杂情况下的状态确定较为有效。对嫦娥三号探测器动力软着陆弧段进行了仿真与实测数据处理。分析了采样返回段的基本动力学与控制特征,为后续的嫦娥五号探测器的软着陆及其采样返回提供初步的可行弹道计算方法。在嫦娥三号探测器动力落月段实测数据处理中,通过评估,该段弹道确定精度优于100 m,其弹道末点与NASA的月球勘测轨道器(LRO)给出的结果差异优于50 m,证实了文章提出的软着陆弹道确定方法的有效性。 相似文献
232.
月地返回轨道存在各种摄动误差,终端约束复杂,有必要对其进行中途修正研究。显式制导法通过二体轨道与精确轨道之间的差别进行多次迭代求解给定时刻所需的修正速度。文章利用显式制导法,采用月球段及地球段分段进行中途修正的策略,给出了基于分段落点预报显式制导的月地返回轨道中途修正方案。该方案无需计算雅克比矩阵,算法简单、计算快速、实用性强,能满足再入点参数要求。算例仿真与蒙特卡洛仿真验证了该方案的适用性。 相似文献
233.
234.
军用运输机运输效能指标 总被引:1,自引:0,他引:1
给出了运输任务类型和运输效能定义,从效果与耗费两方面着手建立了运输效能指标体系,将运输效能归纳为安全性、质量或体积、距离、准确性、时间、场地、人员、设备及费用等9种因素,在确立各个因素运输效能单项指标的基础上提出了一种运输效能综合指标,以四种典型运输任务为例,建立了常用的运输效能指标。 相似文献
235.
S. Takechi K. Nogami T. Miyachi M. Fujii N. Hasebe T. Iwai S. Sasaki H. Ohashi H. Shibata E. Grün R. Srama N. Okada 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2009
A cosmic dust monitor for use onboard a spacecraft is currently being developed using a piezoelectric lead zirconate titanate element (PZT). Its characteristics of the PZT sensor is studied by ground-based laboratory impact experiments using hypervelocity particles supplied by a Van de Graaff accelerator. The output signals obtained from the sensor just after the impact appeared to have a waveform that was explicitly related to the particle’s impact velocity. For velocities less than ∼6 km/s, the signal showed an oscillation pattern and the amplitude was proportional to the momentum of the impacting particle. For higher velocities, the signal gradually changed to a single waveform. The rise time of this single waveform was proportional to the particle’s velocity for velocities above ∼6 km/s. The present paper reports on results for the low velocity case and especially discusses the effect of an outer coating of the sensor with a paint, which is used to reduce heating by solar radiation. 相似文献
236.
航空发动机可靠性评估中故障样本时效性分析 总被引:2,自引:0,他引:2
通过故障数据建立可靠性模型反映系统可靠性状态的变化具有一定的滞后性,随着发动机可靠性水平的不断提高,对航空发动机进行可靠性评估需要将早期故障样本进行时效性处理.提出衰减因子 α 用于表示故障强度的衰弱程度,对Gompertz可靠性增长模型改进得到故障的衰减函数,来描述故障强度的衰弱过程;用Bayes方法合理利用发动机全部故障样本进行数据融合,可有效解决样本的时效性问题.对在研的某涡扇发动机进行故障统计分析,研究了典型故障模式的时效性变化,结果表明:考虑样本的时效性后进行可靠性评估对发动机当前的可靠性水平能给予更加科学的评价. 相似文献
237.
238.
This paper deals with the relative navigation of a formation of two spacecrafts separated by hundreds of kilometers based on processing dual-frequency differential carrier-phase GPS measurements. Specific requirements of the considered application are high relative positioning accuracy and real-time on board implementation. These can be conflicting requirements. Indeed, if on one hand high accuracy can be achieved by exploiting the integer nature of double-difference carrier-phase ambiguities, on the other hand the presence of large ephemeris errors and differential ionospheric delays makes the integer ambiguities determination challenging. Closed-loop schemes, which update the relative position estimates of a dynamic filter with feedback from integer ambiguities fixing algorithms, are customarily employed in these cases. This paper further elaborates such approaches, proposing novel closed loop techniques aimed at overcoming some of the limitations of traditional algorithms. They extend techniques developed for spaceborne long baseline relative positioning by making use of an on-the-fly ambiguity resolution technique especially developed for the applications of interest. Such techniques blend together ionospheric delay compensation techniques, nonlinear models of relative spacecraft dynamics, and partial integer validation techniques. The approaches are validated using flight data from the Gravity Recovery and Climate Experiment (GRACE) mission. Performance is compared to that of the traditional closed-loop scheme analyzing the capability of each scheme to maximize the percentage of correctly fixed integer ambiguities as well as the relative positioning accuracy. Results show that the proposed approach substantially improves performance of the traditional approaches. More specifically, centimeter-level root-mean square relative positioning is feasible for spacecraft separations of more than 260 km, and an integer ambiguity fixing performance as high as 98% is achieved in a 1-day long dataset. Results also show that approaches exploiting ionospheric delay models are more robust and precise of approaches relying on ionospheric-delay removal techniques. 相似文献
239.
240.
Maria S. Pulinets Elizaveta E. Antonova Maria O. Riazantseva Svetlana S. Znatkova Igor P. Kirpichev 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2014
Crossings of the magnetopause near the subsolar point are analyzed using data of THEMIS mission. Variations of the magnetic field near magnetopause measured by one of THEMIS satellites are studied and compared with simultaneous measurements in the solar wind by another THEMIS satellite. The time delay of the solar wind arrival at the subsolar point of the magnetopause is taken into account. 30 and 90 s averaging of the magnetic field in the magnetosheath is produced. The results of averaging are compared with the results of measurements in the solar wind before the bow shock and foreshock. It is shown, that Bx component of the magnetic field near magnetopause is near to zero, which supports the possibility to consider the magnetopause as the tangential discontinuity. Comparatively good correlation of By component in the solar wind and near the magnetopause is observed. The correlation of Bz component near the magnetopause and IMF is practically absent, the sign of the Bz near the subsolar point does not coincide with the sign of IMF Bz in ∼30% cases. 相似文献