全文获取类型
收费全文 | 2290篇 |
免费 | 351篇 |
国内免费 | 287篇 |
专业分类
航空 | 516篇 |
航天技术 | 929篇 |
综合类 | 66篇 |
航天 | 1417篇 |
出版年
2024年 | 28篇 |
2023年 | 87篇 |
2022年 | 81篇 |
2021年 | 110篇 |
2020年 | 90篇 |
2019年 | 87篇 |
2018年 | 140篇 |
2017年 | 62篇 |
2016年 | 86篇 |
2015年 | 121篇 |
2014年 | 199篇 |
2013年 | 127篇 |
2012年 | 190篇 |
2011年 | 202篇 |
2010年 | 144篇 |
2009年 | 152篇 |
2008年 | 129篇 |
2007年 | 139篇 |
2006年 | 100篇 |
2005年 | 88篇 |
2004年 | 63篇 |
2003年 | 61篇 |
2002年 | 47篇 |
2001年 | 71篇 |
2000年 | 41篇 |
1999年 | 27篇 |
1998年 | 34篇 |
1997年 | 26篇 |
1996年 | 24篇 |
1995年 | 24篇 |
1994年 | 19篇 |
1993年 | 22篇 |
1992年 | 19篇 |
1991年 | 21篇 |
1990年 | 29篇 |
1989年 | 9篇 |
1988年 | 14篇 |
1987年 | 8篇 |
1986年 | 2篇 |
1985年 | 4篇 |
1984年 | 1篇 |
排序方式: 共有2928条查询结果,搜索用时 468 毫秒
351.
对民航卫星通信网进行可视通信图像压缩编码方法进行了研究。通过在多媒体计算机上进行模拟实验表明,将视频信息及语音信息压缩为64kbps通过TES卫星网传输方案是可行的。 相似文献
352.
353.
Massimo Materassi Lucilla Alfonsi Giorgiana De Franceschi Vincenzo Romano Cathryn Mitchell Paolo Spalla 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2009
The study of amplitude scintillation on GPS radio links is usually done after detrending the time series of the transmitted power so to define scintillations as the chaotic fluctuation around a unitary value. In a sense, the choice of how to detrend the time series is part of the definition of scintillation. 相似文献
354.
GuiFeng Wei DanLing Tang Sufen Wang 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2008,41(1):12-19
Monitoring of spatial and temporal distribution of chlorophyll (Chl-a) concentrations in the aquatic milieu is always challenging and often interesting. However, the recent advancements in satellite digital data play a significant role in providing outstanding results for the marine environmental investigations. The present paper is aimed to review ‘remote sensing research in Chinese seas’ within the period of 24 years from 1978 to 2002. Owing to generalized distributional pattern, the Chl-a concentrations are recognized high towards northern Chinese seas than the southern. Moreover, the coastal waters, estuaries, and upwelling zones always exhibit relatively high Chl-a concentrations compared with offshore waters. On the basis of marine Chl-a estimates obtained from satellite and other field measured environmental parameters, we have further discussed on the applications of satellite remote sensing in the fields of harmful algal blooms (HABs), primary production and physical oceanographic currents of the regional seas. Concerned with studies of HABs, satellite remote sensing proved more advantageous than any other conventional methods for large-scale applications. Probably, it may be the only source of authentic information responsible for the evaluation of new research methodologies to detect HABs. At present, studies using remote sensing methods are mostly confined to observe algal bloom occurrences, hence, it is essential to coordinate the mechanism of marine ecological and oceanographic dynamic processes of HABs using satellite remote sensing data with in situ measurements of marine environmental parameters. The satellite remote sensing on marine environment and HABs is believed to have a great improvement with popular application of technology. 相似文献
355.
SPOT-5卫星系统性能概述 总被引:4,自引:0,他引:4
概要介绍SPOT - 5卫星的主要成像传感器 ,精确定位技术及数据格式。介绍分析卫星数据的应用前景 相似文献
356.
Richard DalBello 《Space Policy》1985,1(3):289-297
This article examines the effect that private-sector remote sensing activities might have on the international community and on the future of international cooperation in space. It describes in detail The Land Remote Sensing Commercialization Act of 1984, recently passed by the US Congress and signed by President Reagan. The article concludes that it is possible to develop domestic regulations that balance the economic interests of the private sector with the legal and political concerns of the international community. 相似文献
357.
针对全球卫星导航星座网络建设初期或论证阶段所涉及的网络体系结构、协议体系及相关组网等技术问题,开展了基于导航星座星间链路构建空间信息网络的技术研究,分别提出了由子网、接入网、骨干网等节点及其相互之间星间、星地无线链路构成的分层网络系统结构,设计了兼容遥控、遥测、测量与网络交互支持等业务的基于IP over CCSDS(基于空间数据系统咨询委员会标准的空间链路承载互联网协议业务)的协议体系,给出了全系统基本通信业务运行模式等。与传统高轨卫星通信系统相比,该星座网络具有高覆盖、低时延、随遇接入等优点,可实现星座导航性能与中低轨及地面用户通信性能的全方位提升,相关结果对我国全球卫星导航星座网络技术研究具有一定的参考意义。 相似文献
358.
359.
基于TC35i和C8051F020的远程数据采集系统 总被引:1,自引:0,他引:1
本文提出了一种用混合信号系统级单片机C8051F020控制GSM网络引擎TC35i实现远程数据采集的方案。在描述了远程数据采集系统原理、构成和功能的基础上,对无线通讯模块及硬件和软件设计部分也进行了详细介绍。 相似文献
360.
Earth remote sensing (alongside communications) is one of the key application of Earth-orbiting satellites. Civilian satellites in the LANDSAT and SPOT series provide Earth images which have been used for a vast spectrum of applications in agriculture, meteorology, hydrology, urban planning and geology, to name but a few. In the defence sector, satellite remote sensing systems are a critical tool in strategic and tactical planning – for the countries which can afford them. To date, remote sensing satellites have fallen into one of these two categories: military missions driven by the requirement for very high resolution and orbital agility; and multipurpose civil satellites using general purpose sensors to serve a diverse community of end users. For military-style missions, the drive to high resolution sets the requirements for optics, attitude control and downlink data bandwidth. For civil missions, the requirement to satisfy multiple, diverse user applications forces compromises on spectral band and orbit selection. Although there are exceptions, many small satellite remote sensing missions carry on in this tradition, concentrating on ultra high resolution products for multiple user communities. This results in satellites costing on the order of US $100 M, not optimised for any particular application. This paper explores an alternative path to satellite remote sensing, aiming simultaneously to reduce cost and to optimise imaging products for specific applications. By decreasing the cost of the remote sensing satellite system to a critical point, it becomes appropriate to optimise the sensor's spectral and temporal characteristics to fit the requirements of a small, specialised user base. The critical engineering trade-off faced in a cost driven mission is how to reduce mission cost while still delivering a useful product to the selected user. At the Surrey Space Centre, we have pursued an engineering path using two dimensional CCD array sensors, commercial off-the-shelf lenses and gravity-gradient stabilised microsatellites. In spite of the inherent limitations of such systems, recent successes with the Thai Microsatellite Company's Thai-Phutt satellite show that a system costing in the region of US $3 million, can approach the spectral and spatial characteristics of LANDSAT. Surrey's UoSAT-12 minisatellite (to be launched April, 1999) will further develop this cost-driven approach to provide 10 m panchromatic resolution and 30 m multi-spectral resolution. This paper describes the Thai-Phutt and UoSAT-12 imaging systems, explaining the engineering methods and trade-offs. Although Surrey is presently the only centre presently pursuing such implementations, our paper shows that they deserve wider consideration. 相似文献