全文获取类型
收费全文 | 5991篇 |
免费 | 51篇 |
国内免费 | 34篇 |
专业分类
航空 | 2803篇 |
航天技术 | 2262篇 |
综合类 | 82篇 |
航天 | 929篇 |
出版年
2021年 | 56篇 |
2019年 | 37篇 |
2018年 | 96篇 |
2017年 | 80篇 |
2016年 | 63篇 |
2015年 | 45篇 |
2014年 | 126篇 |
2013年 | 158篇 |
2012年 | 146篇 |
2011年 | 232篇 |
2010年 | 166篇 |
2009年 | 235篇 |
2008年 | 350篇 |
2007年 | 175篇 |
2006年 | 165篇 |
2005年 | 167篇 |
2004年 | 139篇 |
2003年 | 190篇 |
2002年 | 140篇 |
2001年 | 174篇 |
2000年 | 135篇 |
1999年 | 153篇 |
1998年 | 165篇 |
1997年 | 132篇 |
1996年 | 133篇 |
1995年 | 176篇 |
1994年 | 179篇 |
1993年 | 110篇 |
1992年 | 133篇 |
1991年 | 70篇 |
1990年 | 71篇 |
1989年 | 133篇 |
1988年 | 60篇 |
1987年 | 59篇 |
1986年 | 57篇 |
1985年 | 195篇 |
1984年 | 144篇 |
1983年 | 130篇 |
1982年 | 127篇 |
1981年 | 176篇 |
1980年 | 50篇 |
1979年 | 64篇 |
1978年 | 53篇 |
1977年 | 39篇 |
1976年 | 34篇 |
1975年 | 48篇 |
1974年 | 34篇 |
1972年 | 45篇 |
1971年 | 34篇 |
1969年 | 29篇 |
排序方式: 共有6076条查询结果,搜索用时 343 毫秒
571.
W. Menn O. Adriani G.C. Barbarino G.A. Bazilevskaya R. Bellotti M. Boezio E.A. Bogomolov L. Bonechi M. Bongi V. Bonvicini S. Borisov S. Bottai A. Bruno F. Cafagna D. Campana R. Carbone P. Carlson M. Casolino G. Castellini L. Consiglio M.P. De Pascale C. De Santis N. De Simone V. Di Felice A.M. Galper W. Gillard L. Grishantseva G. Jerse A.V. Karelin S.V. Koldashov S.Y. Krutkov A.N. Kvashnin A. Leonov V. Malakhov V. Malvezzi L. Marcelli A.G. Mayorov V.V. Mikhailov E. Mocchiutti A. Monaco N. Mori N. Nikonov G. Osteria F. Palma P. Papini M. Pearce P. Picozza C. Pizzolotto M. Ricci S.B. Ricciarini 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2013
572.
Security applications of computer vision 总被引:1,自引:0,他引:1
In an age which bears witness to a proliferation of Closed Circuit Television (CCTV) cameras for security and surveillance monitoring, the use of image processing and computer vision techniques which were provided as top end bespoke solutions can now be realised using desktop PC processing. Commercial Video Motion Detection (VMD) and Intelligent Scene Monitoring (ISM) systems are becoming increasingly sophisticated, aided, in no small way, by a technology transfer from previously exclusively military research sectors. Image processing is traditionally concerned with pre-processing operations such as Fourier filtering, edge detection and morphological operations. Computer vision extends the image processing paradigm to include understanding of scene content, tracking and object classification. Examples of computer vision applications include Automatic Number Plate Recognition (ANPR), people and vehicle tracking, crowd analysis and model based vision. Often image processing and computer vision techniques are developed with highly specific applications in mind and the goal of a more global understanding computer vision system remains, at least for now, outside the bounds of present technology. This paper will review some of the most recent developments in computer vision and image processing for challenging outdoor perimeter security applications. It also describes the efforts of development teams to integrate some of these advanced ideas into coherent prototype development systems 相似文献
573.
Alvin L. Morris 《Space Science Reviews》1972,13(2):243-257
The services available for scientific ballooning are described with special emphasis on the facilities of the NCAR Balloon Flight Station at Palestine, Texas. The preparations for a typical launching, the operations during the launch, the flight and the recovery are described for illustrative purposes. Considerations on the future development of scientific ballooning are given; technological problems for extended flights are considered to be less serious than the political problems. 相似文献
574.
K Nitta K Otsubo A Ashida 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2000,26(2):335-338
CEEF (Closed Ecology Experiment Facilities) were installed at Rokkasho village in northern Japan, for the purpose of clarifying life-support mechanisms in a completely closed space, such as a Lunar or Mars base. An integration test using the Closed Plantation Experiment Facility and Closed Animal Breeding & Habitation Experiment Facility is needed before conducting an entire closed experiment including plants, animals and humans. These integration tests are planned to be conducted step by step from fiscal 2001 to 2008. 相似文献
575.
We study the directional stability of rigid and deformable spinning satellites in terms of two attitude angles. The linearized attitude motion of a free system about an assumed uniform-spin reference solution leads to a generic MGK system when the satellite is rigid or deformable. In terms of Lyapunov’s stability theory, we investigate the stability with respect to a subset of the variables. For a rigid body, the MGK system is 6-dimensional, i.e., 3 rotational and 3 translational variables. When flexible parts are present the system can have any arbitrary dimension. The 2×2 McIntyre–Myiagi stability matrix gives sufficient conditions for the attitude stability. A further development of this method has led to the Equivalent Rigid Body method. We propose an alternative practical method to establish sufficiency conditions for directional stability by using the Frobenius–Schur reduction formula. As practical applications we discuss a spinning satellite augmented with a spring–mass system and a rigid body appended with two cables and tip masses. In practice, the attitude stability must also be investigated when the spinning satellite is subject to a constant axial thrust. The generic format becomes MGKN as the thrust is a follower force. For a perfectly aligned thrust along the spin axis, Lyapunov’s indirect method remains valid also when deformable parts are present. We illustrate this case with an apogee motor burn in the presence of slag. When the thrust is not on the spin axis or not pointing parallel to the spin axis, the uniform-spin reference motion does not exist and none of the previous methods is applicable. In this case, the linearization may be performed about the initial state. Even when the linearized system has bounded solutions, the non-linear system can be unstable in general. We illustrate this situation by an instability that actually happened in-flight during a station-keeping maneuver of ESA’s GEOS-I satellite in 1979. 相似文献
576.
A rigorous theory is developed for the course scalloping amplitudes observed in the bearing indications of an airborne standard VOR receiver in the presence of multipath signals. The VOR transmitting antenna pattern characteristics, the scattering properties of the multipath source, and the effects of ground reflection are all built into the theory so that their combined effects on the scalloping may be quantitatively investigated. The analysis assumes that the VOR receiver characteristics are ideal. 相似文献
577.
The problem of estimating the state of a turbopump rotating assembly and identifying unknown products of inertia and unknown bearing parameters is considered. A linearized, extended Kalman filtering approach has been used and found to be successful for both state estimation and parameter identification in an inherently nonlinear problem. The input data was simulated Bently test data which was obtained from a verified 12-dimensional state-space model and was compared with real Bently test data obtained from NASA. The results prove the feasibility of using this model and this type of test data to obtain the hidden parameters of a typical turbopump rotating assembly. This method should lead to improved performance by allowing improved balancing of the rotor and giving improved knowledge of the bearing reactions. 相似文献
578.
T Takano K Inada J Takanashi 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1987,7(4):149-152
Rockwool, as an inert medium covered or bagged with polyethylene film, can be effectively used for plant culture in space station. The most important machine is the pump adjusting the dripping rate in the feeding system. Hydro-aeroponics may be adaptable to a space laboratory. The shortening of the light-dark cycles inhibits plant growth and induces an abnormal morphogenesis. A photoperiod of 12-hr-dark may be needed for plant growth. 相似文献
579.
Compact antennas for UWB applications 总被引:1,自引:0,他引:1
Taeyoung Yang Seong-Youp Suh Nealy R. Davis W.A. Stutzman W.L. 《Aerospace and Electronic Systems Magazine, IEEE》2004,19(5):16-20
A report on an investigation of spherical, disc, and half-disc antennas in the frequency and time domains with the objective of developing small planar versions of the antennas. These antennas have an omni-directional impulse response in azimuth and pulse duration of 0.5-0.65 nanoseconds. In addition, the measured data show a reasonable peak received signal in a pulse communication link using two identical antennas. 相似文献
580.
J.L. Hall D. Fairbrother T. Frederickson V.V. Kerzhanovich M. Said C. Sandy J. Ware C. Willey A.H. Yavrouian 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2008
This paper describes the design, fabrication and testing of a full scale prototype balloon intended for long duration flight in the upper atmosphere of Venus. The balloon is 5.5 m in diameter and is designed to carry a 45 kg payload at an altitude of 55 km. The balloon material is a 180 g/m2 multi-component laminate comprised of the following layers bonded together from outside to inside: aluminized Teflon film, aluminized Mylar film, Vectran fabric and a polyurethane coating. This construction provides the required balloon functional characteristics of low gas permeability, sulfuric acid resistance and high strength for superpressure operation. The design burst superpressure is 39,200 Pa which is predicted to be 3.3 times the worst case value expected during flight at the highest solar irradiance in the mission profile. The prototype is constructed from 16 gores with bi-taped seams employing a sulfuric acid resistant adhesive on the outside. Material coupon tests were performed to evaluate the optical and mechanical characteristics of the laminate. These were followed by full prototype tests for inflation, leakage and sulfuric acid tolerance. The results confirmed the suitability of this balloon design for use at Venus in a long duration mission. The various data are presented and the implications for mission design and operation are discussed. 相似文献