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排序方式: 共有718条查询结果,搜索用时 906 毫秒
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近年来,卫星导航技术发展迅速.卫星导航系统以精密时间测量技术为基础,实现了伪距测量,进而实现定位.同时,卫星导航系统还提供了高精度授时功能.综述了卫星导航系统的授时和时间频率传递技术、基于通信卫星的授时技术以及双向卫星时间频率传递(TWSTFT)技术等.随着我国北斗卫星导航系统(BDS)的建成和提供服务,BDS授时应用研究正在快速发展.基于BDS/GNSS多系统的精密单点定位(PPP)时间传递技术已成为重点研究方向,未来将会应用于国际时间比对.同时,随着卫星通信技术尤其是低轨通信卫星技术的快速发展,低轨通信卫星授时会成为一个有潜力的研究方向. 相似文献
714.
《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2023,71(5):2416-2436
Several global gravity models (GGMs) are freely available in the public domain, which can be utilised to study the earth's gravity field in almost every part of the globe. The present study compared the free-air gravity anomalies calculated from the five GGMs EGM2008, EIGEN6C4, GECO, XGM2019e_2159, and SGG-UGM-2 archived by the International Centre for Global Earth Models (ICGEM) with respect to shipborne gravity in the Bay of Bengal. The average correlation coefficient and covariance are ~ 96 % and ~ 450mGal2. The mean difference between the shipborne and the modelled gravity is ? 5 mGal. Relatively higher amplitude gravity differences observed at the continental-oceanic transition, the 85°E and Ninetyeast ridges, and the western basin are possibly due to high gradient, dominant density contrasts, and rugged topography. The average standard deviation and root-mean-square-error (RMSE) of the differences are ~ 6.5 mGal and ~ 7.5 mGal. A significantly lower standard deviation and RMSE found for the models generated at higher degree/order compared to lower degree/order is due to diminishing omission error of the GGMs with increasing degrees of truncation. The spectral analysis and coherence estimation of the modelled gravity demonstrate excellent correspondence for anomalies wider than ~ 25 km. The agreement between anomaly amplitudes and shapes and calculated statistics indicates that the accuracy and resolution of the modelled gravity data are certainly good enough for regional-scale studies aiming to map Moho topography and mantle structures. However, the delineation of shorter wavelength features originating from the shallow-level basement/sedimentary might be uncertain and requires further validations. The present study confirms that all five models show promising results in terms of their accuracy and resolution. Moreover, the SGG-UGM-2 and XGM2019e_2159 models compare favourably with respect to the GECO, EIGEN6C4 and EGM2008 models in the Bay of Bengal. 相似文献
715.
飞翼布局无人机具有独特的气动特性,研究飞翼布局无人机着舰飞行动力学特性对设计无人机着舰控制律具有重要意义。针对飞翼布局无人机着舰下滑飞行过程,建立六自由度飞行动力学模型,并通过对着舰飞行轨迹稳定性的分析,根据飞行品质对飞行轨迹稳定性的约束,计算达到一级飞行品质要求的着舰飞行速度。通过配平计算和小扰动线性化处理,得到无人机着舰下滑运动线性模型,并分析无人机纵向和横航向的固有模态特性。结果表明,飞翼无人机着舰下滑过程中,纵向的长、短周期模态及横航向的滚转和螺旋模态收敛但收敛慢,荷兰滚模态发散。 相似文献
716.
《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2023,71(3):1486-1509
Sea-surface solar radiation (abbreviated as photosynthetically available radiation, PAR) in the visible wavelength (400–700 nm) is an essential parameter to estimate marine primary productivity and understanding phytoplankton dynamics, upper ocean physics and biogeochemical processes. Although many remote-sensing models were developed to estimate daily PAR (DPAR) from ocean colour data, these models often produce biases in the DPAR products under cloudy-sky and complex atmospheric conditions due to the lack of parameterization to deal with the cloud cover conditions and insufficient in-situ DPAR data. This study presents an Extended Sea-surface Solar Irradiance Model (ESSIM) for estimating DPAR over the global ocean. The ESSIM uses the direct and diffuse components from the Simple sea-surface Solar Irradiance Model (SSIM) along with a new parameter to handle cloudy conditions. The ESSIM produced DPAR products with greater accuracy under both clear and cloudy conditions. Its performance was tested on the time-series MODIS-Aqua images and compared with the concurrent in-situ data and the results from two global models. Results showed that the DPAR values produced by ESSIM agree with in-situ data better than the global models for all-sky conditions (with a mean relative error of 11.267 %; a root mean square error of 5.563 Em?2day?1; and a mean net bias of 2.917 Em?2day?1). The ESSIM performed slightly better than the SSIM for clear conditions and the Frouin's Operational Algorithm (FOA) for all-sky conditions. As the new parameterization accounts for cloudy conditions, the ESSIM produced more accurate results for cloud cover conditions across latitudes (up to 60°). The time-series Level-3 binned MODIS-Aqua data (global gridded) also demonstrated that the ESSIM improved the accuracy of DPAR products and produced spatially and temporally consistent DPAR products over the global ocean regardless of the seasons and sky conditions. 相似文献
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718.
GNSS导航星座主要由GPS、GLONASS、Galileo、BDS四大系统卫星组成,到2020年,用户有望同时观测到40多颗导航卫星,大幅提升导航性能的同时,也扩展了其应用领域。针对靶船、渔船对航向测量的需求,基于GNSS接收机应用现状,设计了低成本GNSS测向系统,使用2块Ublox LEA-M8T模块和STM32F407处理器搭建原理样机硬件平台,开展基于Kalman滤波器的载波相位差分姿态测量算法研究和基于Kalman滤波器的伪距差分姿态测量算法研究。静态实验表明,在8.5m基线的情况下,航向角测量标准差为0.0396°,俯仰角测量标准差为0.0889°。使用伪距差分算法,纬度方向上测量误差标准差为0.5923m,经度方向上测量误差标准差为0.4609m,高度方向上测量误差标准差为1.0766m。 相似文献