全文获取类型
收费全文 | 17867篇 |
免费 | 49篇 |
国内免费 | 131篇 |
专业分类
航空 | 9898篇 |
航天技术 | 5228篇 |
综合类 | 239篇 |
航天 | 2682篇 |
出版年
2021年 | 157篇 |
2018年 | 203篇 |
2016年 | 153篇 |
2014年 | 434篇 |
2013年 | 513篇 |
2012年 | 418篇 |
2011年 | 582篇 |
2010年 | 407篇 |
2009年 | 758篇 |
2008年 | 794篇 |
2007年 | 368篇 |
2006年 | 422篇 |
2005年 | 377篇 |
2004年 | 426篇 |
2003年 | 502篇 |
2002年 | 472篇 |
2001年 | 543篇 |
2000年 | 359篇 |
1999年 | 446篇 |
1998年 | 419篇 |
1997年 | 310篇 |
1996年 | 366篇 |
1995年 | 430篇 |
1994年 | 409篇 |
1993年 | 353篇 |
1992年 | 308篇 |
1991年 | 250篇 |
1990年 | 241篇 |
1989年 | 395篇 |
1988年 | 206篇 |
1987年 | 234篇 |
1986年 | 233篇 |
1985年 | 639篇 |
1984年 | 519篇 |
1983年 | 406篇 |
1982年 | 488篇 |
1981年 | 611篇 |
1980年 | 244篇 |
1979年 | 184篇 |
1978年 | 189篇 |
1977年 | 146篇 |
1976年 | 155篇 |
1975年 | 187篇 |
1974年 | 180篇 |
1973年 | 161篇 |
1972年 | 188篇 |
1971年 | 148篇 |
1970年 | 143篇 |
1969年 | 147篇 |
1967年 | 142篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
231.
This paper describes a method of applying digital techniques to the control of a 1-kVA three-phase dc-ac inverter to generate a sinusoidal 400-Hz output, using high-frequency bridge-chopper techniques. The model which was constructed used predominantly off-theshelf digital microcircuits and resulted in a device with an overall efficiency of 85 percent, in a 0.52 cubic-foot package which weighed 19.5 lbs. Sinusoidal output with less than 2 percent harmonic distortion at 115 volts line to neutral was obtained with 28 volts dc input. 相似文献
232.
233.
A general performance index is developed for evaluating aircraft landing trajectories. The primary term in the index is the effect of noise on people residing near the air terminal. Other terms included are passenger comfort, fuel consumed, and the time spent in the near-terminal area. Models are developed for aircraft engine noise, passenger comfort, the population distribution about a specific airport, and the aircraft flight behavior. While this performance index may be used in computing optimal trajectories, it is also useful for comparing nonoptimal trajectories which, for one reason or another, may be worthy of consideration. Some examples of such comparisons are included through simulations of landing. The aircraft considered is a Boeing 737. 相似文献
234.
The Electric Antennas for the STEREO/WAVES Experiment 总被引:1,自引:0,他引:1
S. D. Bale R. Ullrich K. Goetz N. Alster B. Cecconi M. Dekkali N. R. Lingner W. Macher R. E. Manning J. McCauley S. J. Monson T. H. Oswald M. Pulupa 《Space Science Reviews》2008,136(1-4):529-547
The STEREO/WAVES experiment is designed to measure the electric component of radio emission from interplanetary radio bursts and in situ plasma waves and fluctuations in the solar wind. Interplanetary radio bursts are generated from electron beams at interplanetary shocks and solar flares and are observed from near the Sun to 1 AU, corresponding to frequencies of approximately 16 MHz to 10 kHz. In situ plasma waves occur in a range of wavelengths larger than the Debye length in the solar wind plasma λ D ≈10 m and appear Doppler-shifted into the frequency regime down to a fraction of a Hertz. These phenomena are measured by STEREO/WAVES with a set of three orthogonal electric monopole antennas. This paper describes the electrical and mechanical design of the antenna system and discusses efforts to model the antenna pattern and response and methods for in-flight calibration. 相似文献
235.
André Balogh Réjean Grard Sean C. Solomon Rita Schulz Yves Langevin Yasumasa Kasaba Masaki Fujimoto 《Space Science Reviews》2007,132(2-4):611-645
Mercury is a very difficult planet to observe from the Earth, and space missions that target Mercury are essential for a comprehensive
understanding of the planet. At the same time, it is also difficult to orbit because it is deep inside the Sun’s gravitational
well. Only one mission has visited Mercury; that was Mariner 10 in the 1970s. This paper provides a brief history of Mariner
10 and the numerous imaginative but unsuccessful mission proposals since the 1970s for another Mercury mission. In the late
1990s, two missions—MESSENGER and BepiColombo—received the go-ahead; MESSENGER is on its way to its first encounter with Mercury
in January 2008. The history, scientific objectives, mission designs, and payloads of both these missions are described in
detail. 相似文献
236.
Johannsen K. Tustioson G. Egli S. 《IEEE transactions on aerospace and electronic systems》1980,(5):683-711
In many domestic satellite communication systems single-channel per-carrier FM (SCPC-FM) has become the accepted transmission method for thin route communication. The SCPC-FM modem operates over a wide range of carrier frequencies and is designed to use satellite power and bandwidth efficiently. It incorporates elements which make the signal transfer complex, so the system behavior is not immediately obvious. This article investigates the signal processes and analyzes the performance of a companded and preemphasized SCPC-FM system and it furnishes insight into the choice of crucial system parameters. 相似文献
237.
An analysis is given of the tracking accuracy of a conopulse radar that angle-tracks on target echo pulses. The analysis includes effects of noises in both sum and difference channels, the effect of the tracking range gate's duration, and applies to any shape of radio frequency (RF) pulse envelope. For large single-pulse signalto-noise ratio and any shape of RF pulse envelope, accuracy approaches the theoretical accuracy of a conopulse system when a short-duration range gate is used. The inportant case of rectangular RF pulses is solved in detail. 相似文献
238.
Centralized/distributed recursive algorithms for temporal-spatial information integration using the Dempster-Shafer technique are developed. Compared with the Bayesian approach, the Dempster-Shafer technique has the strong capability of handling information uncertainties, which are particularly desirable in many applications. In the centralized integration algorithm, all information is pooled into the central processor and integrated. In contrast, the distributed integration algorithm shares the computational burden among the local processors, which increases the computational efficiency. The developed algorithms are effectively applied to a target identification problem with three sensors: identification of friend-foe-neutral (IFFN), electronic support measurement (ESM), and infrared search and track (IRST) 相似文献
239.
藜蒿中绿原酸溶剂提取方法研究 总被引:1,自引:0,他引:1
应用醇溶剂回流法提取藜蒿中绿原酸,通过正交试验,对藜蒿中绿原酸进行溶剂提取条件的优化研究,并分剐讨论了各因素对绿原酸提取率的影响。实验表明,乙醇浓度70%、12倍的提取剂用量、温度85℃、提取时间4h、提取两次为乙醇溶剂提取藜蒿中绿原酸的最佳条件,总绿原酸提取率为1.24%。 相似文献
240.
Metallic Power has demonstrated a regenerative zinc air fuel cell for applications in industrial and specialty vehicles. The fuel cell uses zinc pellets and atmospheric oxygen to generate electric current; the reaction product is zinc oxide, which is collected in a tank. In its present stage of development the 36 V fuel cell will deliver approximately 6 kWh, with a maximum power of 4 kW. The device is refueled at a zinc recycling/refueling station where zinc pellets are pumped into each cell; ZnO is pumped from the tank and replaced with KOH electrolyte. The recycling/refueling unit uses an electrolytic process to convert zinc oxide powder into zinc pellets 相似文献