全文获取类型
收费全文 | 2274篇 |
免费 | 10篇 |
国内免费 | 10篇 |
专业分类
航空 | 1111篇 |
航天技术 | 815篇 |
综合类 | 10篇 |
航天 | 358篇 |
出版年
2021年 | 22篇 |
2018年 | 38篇 |
2017年 | 21篇 |
2016年 | 26篇 |
2014年 | 48篇 |
2013年 | 57篇 |
2012年 | 49篇 |
2011年 | 91篇 |
2010年 | 64篇 |
2009年 | 91篇 |
2008年 | 98篇 |
2007年 | 53篇 |
2006年 | 47篇 |
2005年 | 59篇 |
2004年 | 70篇 |
2003年 | 73篇 |
2002年 | 38篇 |
2001年 | 58篇 |
2000年 | 39篇 |
1999年 | 47篇 |
1998年 | 67篇 |
1997年 | 42篇 |
1996年 | 67篇 |
1995年 | 79篇 |
1994年 | 56篇 |
1993年 | 49篇 |
1992年 | 64篇 |
1991年 | 31篇 |
1990年 | 20篇 |
1989年 | 49篇 |
1988年 | 24篇 |
1987年 | 20篇 |
1986年 | 22篇 |
1985年 | 83篇 |
1984年 | 53篇 |
1983年 | 54篇 |
1982年 | 60篇 |
1981年 | 72篇 |
1980年 | 21篇 |
1979年 | 27篇 |
1978年 | 27篇 |
1977年 | 27篇 |
1976年 | 20篇 |
1975年 | 20篇 |
1974年 | 20篇 |
1973年 | 11篇 |
1972年 | 14篇 |
1970年 | 18篇 |
1969年 | 19篇 |
1967年 | 10篇 |
排序方式: 共有2294条查询结果,搜索用时 156 毫秒
461.
Green J.L. Benson R.F. Fung S.F. Taylor W.W.L. Boardsen S.A. Reinisch B.W. Haines D.M. Bibl K. Cheney G. Galkin I.A. Huang X. Myers S.H. Sales G.S. Bougeret J.-L. Manning R. Meyer-Vernet N. Moncuquet M. Carpenter D.L. Gallagher D.L. Reiff P.H. 《Space Science Reviews》2000,91(1-2):361-389
The Radio Plasma Imager (RPI) will be the first-of-its kind instrument designed to use radio wave sounding techniques to perform repetitive remote sensing measurements of electron number density (N
e) structures and the dynamics of the magnetosphere and plasmasphere. RPI will fly on the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) mission to be launched early in the year 2000. The design of the RPI is based on recent advances in radio transmitter and receiver design and modern digital processing techniques perfected for ground-based ionospheric sounding over the last two decades. Free-space electromagnetic waves transmitted by the RPI located in the low-density magnetospheric cavity will be reflected at distant plasma cutoffs. The location and characteristics of the plasma at those remote reflection points can then be derived from measurements of the echo amplitude, phase, delay time, frequency, polarization, Doppler shift, and echo direction. The 500 m tip-to-tip X and Y (spin plane) antennas and 20 m Z axis antenna on RPI will be used to measures echoes coming from distances of several R
E. RPI will operate at frequencies between 3 kHz to 3 MHz and will provide quantitative N
e values from 10–1 to 105 cm–3. Ray tracing calculations, combined with specific radio imager instrument characteristics, enables simulations of RPI measurements. These simulations have been performed throughout an IMAGE orbit and under different model magnetospheric conditions. They dramatically show that radio sounding can be used quite successfully to measure a wealth of magnetospheric phenomena such as magnetopause boundary motions and plasmapause dynamics. The radio imaging technique will provide a truly exciting opportunity to study global magnetospheric dynamics in a way that was never before possible. 相似文献
462.
The Radio Plasma Imager investigation on the IMAGE spacecraft 总被引:1,自引:0,他引:1
Reinisch B.W. Haines D.M. Bibl K. Cheney G. Galkin I.A. Huang X. Myers S.H. Sales G.S. Benson R.F. Fung S.F. Green J.L. Boardsen S. Taylor W.W.L. Bougeret J.-L. Manning R. Meyer-Vernet N. Moncuquet M. Carpenter D.L. Gallagher D.L. Reiff P. 《Space Science Reviews》2000,91(1-2):319-359
Radio plasma imaging uses total reflection of electromagnetic waves from plasmas whose plasma frequencies equal the radio sounding frequency and whose electron density gradients are parallel to the wave normals. The Radio Plasma Imager (RPI) has two orthogonal 500-m long dipole antennas in the spin plane for near omni-directional transmission. The third antenna is a 20-m dipole along the spin axis. Echoes from the magnetopause, plasmasphere and cusp will be received with the three orthogonal antennas, allowing the determination of their angle-of-arrival. Thus it will be possible to create image fragments of the reflecting density structures. The instrument can execute a large variety of programmable measuring options at frequencies between 3 kHz and 3 MHz. Tuning of the transmit antennas provides optimum power transfer from the 10 W transmitter to the antennas. The instrument can operate in three active sounding modes: (1) remote sounding to probe magnetospheric boundaries, (2) local (relaxation) sounding to probe the local plasma frequency and scalar magnetic field, and (3) whistler stimulation sounding. In addition, there is a passive mode to record natural emissions, and to determine the local electron density, the scalar magnetic field, and temperature by using a thermal noise spectroscopy technique. 相似文献
463.
通过相似性分析,得到了大气污染建筑物影响风洞试验应满足的基本相似条件.由一个实例研究了利用常规风洞模拟装置和测试仪器进行大气污染建筑物影响风洞试验在技术上的可行性.结果显示:由于模拟装置和仪器条件的限制,在大多数风洞试验中存在大气边界层不完全模拟和仪器响应过慢等问题,但通过合理模拟来流的主要湍流特征和改进采样方法,可以在风洞中较真实地模拟建筑物对大气扩散的影响. 相似文献
464.
S.J. Bauer L.M. Brace H.A. Taylor T.K. Breus A.J. Kliore W.C. Knudsen A.F. Nagy C.T. Russell N.A. Savich 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1985,5(11):233-267
Physical properties of the Venus ionosphere obtained by experiments on the US Pioneer Venus and the Soviet Venera missions are presented in the form of models suitable for inclusion in the Venus International Reference Atmosphere. The models comprise electron density (from 120 km), electron and ion temperatures, and relative ion abundance in the altitude range from 150 km to 1000 km for solar zenith angles from 0° to 180°. In addition, information on ion transport velocities, ionopause altitudes, and magnetic field characteristics of the Venus ionosphere, are presented in tabular or graphical form. Also discussed is the solar control of the physical properties of the Venus ionosphere. 相似文献
465.
The interaction between the input filter and the regulator often causes serious degradation of performance. The reduction in loop gain due to input filter interaction can result in system instability. An exact stability analysis of the buck regulator system is presented. The input filter parameter values are varied and system instability is predicted for the case without feedforward. The eigenvalues of the system can be brought back into the unit circle and the system thus stabilized with the addition of the feedforward loop. Measurements made for the cases with and without feedforward confirm the analytical prediction. 相似文献
466.
467.
Gearing G.M. Cimino M.B. Terzuoli A.J. Fritts D.H. Leonard J.F. 《IEEE transactions on aerospace and electronic systems》1985,(5):644-653
This investigation consisted of several tests of specially fabricated nickel-cadmium batteries having circular disk-type electrodes. These batteries were evaluated as filter elements between a constant current power supply and a 5 Hz pulsed load demanding approximately twice the power supply current during the load on a portion of the cycle. Short tests lasting 104 cycles were conducted at up to a 21 C rate and an equivalent energy density of over 40 J/Ib. In addition, two batteries were subjected to 10h dischar cycles, one at a 6.5 C rate and the other at a 13 C rate. Assuming an electrode-to-battery weight ratio of 0.5, these tests represent an energy density of about 7 and 14 J/Ib, respectively. Energy density, efficiency, capacitance, average voltage, and available capacity were tracked during these tests. After 10y capacity degradation was negligible for one battery and about 20 percent for the other. Cadmium electrode failure may be the factor limiting lifetime at extremely low depth of discharge cycling. The output was examined and a simple equivalent circuit was proposed. 相似文献
468.
469.
W Schimmerling J W Wilson J E Nealy S A Thibeault F A Cucinotta J L Shinn M Kim R Kiefer 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1996,17(2):31-36
Ions of galactic origin are modified but not attenuated by the presence of shielding materials. Indeed, the number of particles and the absorbed energy behind most shield materials increases as a function of shield thickness. The modification of the galactic cosmic ray composition upon interaction with shielding is the only effective means of providing astronaut protection. This modification is intimately connected with the shield transport properties and is a strong function of shield composition. The systematic behavior of the shield properties in terms of microscopic energy absorption events will be discussed. The shield effectiveness is examined with respect to conventional protection practice and in terms of a biological endpoint: the efficiency for reduction of the probability of transformation of shielded C3H10T1/2 mouse cells. The relative advantage of developing new shielding technologies is discussed in terms of a shield performance as related to biological effect and the resulting uncertainty in estimating astronaut risk. 相似文献
470.
E F Wheeler J Kossowski E Goto R W Langhans G White L D Albright D Wilcox 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1996,18(1-2):233-236
A Linear Programming model has been constructed which aids in selecting appropriate crops for CELSS (Controlled Environment Life Support System) food production. A team of Controlled Environment Agriculture (CEA) faculty, staff, graduate students and invited experts representing more than a dozen disciplines, provided a wide range of expertise in developing the model and the crop production program. The model incorporates nutritional content and controlled-environment based production yields of carefully chosen crops into a framework where a crop mix can be constructed to suit the astronauts' needs. The crew's nutritional requirements can be adequately satisfied with only a few crops (assuming vitamin mineral supplements are provided) but this will not be satisfactory from a culinary standpoint. This model is flexible enough that taste and variety driven food choices can be built into the model. 相似文献