全文获取类型
收费全文 | 19545篇 |
免费 | 45篇 |
国内免费 | 129篇 |
专业分类
航空 | 10284篇 |
航天技术 | 5899篇 |
综合类 | 250篇 |
航天 | 3286篇 |
出版年
2021年 | 160篇 |
2018年 | 287篇 |
2017年 | 171篇 |
2016年 | 178篇 |
2014年 | 435篇 |
2013年 | 518篇 |
2012年 | 446篇 |
2011年 | 673篇 |
2010年 | 494篇 |
2009年 | 843篇 |
2008年 | 868篇 |
2007年 | 466篇 |
2006年 | 438篇 |
2005年 | 445篇 |
2004年 | 496篇 |
2003年 | 572篇 |
2002年 | 505篇 |
2001年 | 637篇 |
2000年 | 382篇 |
1999年 | 475篇 |
1998年 | 486篇 |
1997年 | 350篇 |
1996年 | 438篇 |
1995年 | 506篇 |
1994年 | 509篇 |
1993年 | 369篇 |
1992年 | 385篇 |
1991年 | 251篇 |
1990年 | 248篇 |
1989年 | 430篇 |
1988年 | 216篇 |
1987年 | 248篇 |
1986年 | 249篇 |
1985年 | 648篇 |
1984年 | 530篇 |
1983年 | 422篇 |
1982年 | 493篇 |
1981年 | 623篇 |
1980年 | 247篇 |
1979年 | 189篇 |
1978年 | 189篇 |
1977年 | 147篇 |
1976年 | 156篇 |
1975年 | 192篇 |
1974年 | 181篇 |
1973年 | 161篇 |
1972年 | 188篇 |
1971年 | 148篇 |
1970年 | 143篇 |
1969年 | 147篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
971.
The benefits of using transmitting phased array antennas for radar systems are examined. Accurate performance prediction for the transmitting phased array antennas requires theories describing both the antenna system and the power generation devices. These theories were created and applied to the design and performance evaluation of the Russian 3-D mobile solid-state surveillance radar 67N6E (GAMMA-DE), a PAA designed for long-range air defense. 相似文献
972.
T. R. Robinson F. Honary A. Stocker T. B. Jones 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1995,15(12):41-44
Results are presented from recent ionospheric modification experiments in which the EISCAT UHF radar measured the E-region temperature and density response to high power RF heating above Tromsø. A variety of electrojet conditions were encountered during these experiments. In particular, the electron drift velocity varied considerably allowing the heating efficiency of the RF heater to be investigated as a function of electron flow velocity. These observations constitute the first direct investigation of electrojet temperature modifications by high power radio waves and provide a test of a recent theoretical model in which the combined effects of RF heating and of natural plasma turbulence associated with the Farley-Buneman instability have been considered. 相似文献
973.
The fundamental TPBVP usually underlying true “optimal sensor selection strategy” is revisited to obtain practical real-time mechanizations as a solution to an exclusively initial value problem 相似文献
974.
975.
T H Sweetser C A Halsell R J Cesarone 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1995,15(3):257-260
Planetary protection has been an important consideration during the process of designing the Mars Observer mission. It affected trajectory design of both the interplanetary transfer and the orbits at Mars; these in turn affected the observation strategies developed for the mission. The Project relied mainly on the strategy of collision avoidance to prevent contamination of Mars. Conservative estimates of spacecraft reliability and Martian atmosphere density were used to evaluate decisions concerning the interplanetary trajectory, the orbit insertion phase at Mars, and operations in orbit at Mars and afterwards. Changes in the trajectory design, especially in the orbit insertion phase, required a refinement of those estimates. 相似文献
976.
C. J. Gramling A. C. Long 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1995,16(12):77-80
The National Aeronautics and Space Administration (NASA) Goddard Space Flight Center (GSFC) is currently developing a Tracking and Data Relay Satellite (TDRS) System (TDRSS) Onboard Navigation System (TONS) to provide onboard knowledge of highly accurate navigation products autonomously to users of TDRSS. The TONS I operational system processes Doppler data derived from scheduled forward-link S-band services to provide onboard orbit and frequency determination and time maintenance. TONS I is implemented using a stable onboard frequency reference, a Doppler measurement capability in the user transponder, and onboard navigation/signal acquisition software. The success of the TONS experiment (1992/1993), which was implemented on the Explorer Platform/Extreme Ultraviolet Explorer (EP/EUVE) spacecraft, demonstrated the flight readiness of TONS I. TONS II-A provides the user with additional Doppler data derived from an S-band beacon signal, available approximately 80 percent of the time. This paper describes the TONS operational capabilities, configurations, and expected performance. 相似文献
977.
The paper describes a recent effort to design, build, and test a 77 GHz radar to be used for “intelligent” cruise control of automobiles. An analysis of requirements is given, design trade-offs are accounted for, and main features of prototypes in manufacturing are detailed. The radar has been thoroughly tested and evaluated both under “controlled” conditions and on roads, also in dense traffic. Results so far are very promising 相似文献
978.
The authors present the communication system architecture for air traffic management and weather information dissemination as viewed within the context of the overall National Airspace System (NAS) and the services it provides. This presents the architecture in the 2015 time frame when the final phases of transition take place from analog voice to digital data exchange of common data via integrated networks. The results of this transition are an integrated collection of systems and procedures that efficiently use the capacity of the NAS while balancing access to all user classes and maintaining the highest levels of safety. Efficient collaboration among users is built on a foundation of common data. This information base provides common situational awareness to all participants. All users are accommodated and will receive benefits commensurate with equipage level. This architecture development provides a technical framework for decision-making, research, and analysis of communication technologies 相似文献
979.
A discussion of the current and emerging core technologies and philosophies that will enable Air Force personnel to quickly, accurately and intuitively diagnose faults in increasingly complex systems 相似文献
980.
Earp S.L. Hughes E.S. Elkins T.J. Vickers R. 《Aerospace and Electronic Systems Magazine, IEEE》1996,11(9):30-39
The suitability of ultra-wideband ground-penetrating radar as a tool for the detection of buried metallic mines is explored in this paper. The analysis centers around a 200-800 MHz, dual-polarized ground penetrating radar (GPR) designed and built by SRI International. The analysis consisted of fusing the images from the dual polarizations into a single image to enhance the target objects and suppress clutter. Results are shown for several variations of a Mahalanobis-based fusion technique, and “soft decision” minefield detection results based upon Monte Carlo statistical techniques are also presented. Although relatively few scenes were analyzed, these results show that the dual-polarized GPR is potentially very effective at finding buried mines and minefields 相似文献