全文获取类型
收费全文 | 4466篇 |
免费 | 11篇 |
国内免费 | 7篇 |
专业分类
航空 | 2086篇 |
航天技术 | 1602篇 |
综合类 | 16篇 |
航天 | 780篇 |
出版年
2021年 | 44篇 |
2019年 | 25篇 |
2018年 | 104篇 |
2017年 | 73篇 |
2016年 | 76篇 |
2015年 | 39篇 |
2014年 | 129篇 |
2013年 | 137篇 |
2012年 | 143篇 |
2011年 | 193篇 |
2010年 | 127篇 |
2009年 | 216篇 |
2008年 | 293篇 |
2007年 | 128篇 |
2006年 | 107篇 |
2005年 | 138篇 |
2004年 | 117篇 |
2003年 | 153篇 |
2002年 | 89篇 |
2001年 | 153篇 |
2000年 | 74篇 |
1999年 | 99篇 |
1998年 | 120篇 |
1997年 | 87篇 |
1996年 | 90篇 |
1995年 | 124篇 |
1994年 | 121篇 |
1993年 | 64篇 |
1992年 | 95篇 |
1991年 | 53篇 |
1990年 | 36篇 |
1989年 | 88篇 |
1988年 | 40篇 |
1987年 | 42篇 |
1986年 | 36篇 |
1985年 | 96篇 |
1984年 | 86篇 |
1983年 | 75篇 |
1982年 | 86篇 |
1981年 | 118篇 |
1980年 | 41篇 |
1979年 | 36篇 |
1978年 | 32篇 |
1977年 | 25篇 |
1976年 | 28篇 |
1975年 | 22篇 |
1974年 | 22篇 |
1973年 | 23篇 |
1972年 | 23篇 |
1971年 | 21篇 |
排序方式: 共有4484条查询结果,搜索用时 814 毫秒
701.
C E Swenberg L S Myers J H Miller 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1994,14(10):181-201
The relation between the site of energy deposition and the site of its biological action is an important question in radiobiology. Even at 77 degrees K, evidence is clear that these two sites must be separated since energy deposition is random but specific products are formed. Several processes that may contribute to this separation are: 1) hole migration and stabilization through deprotonation to give neutral oxidation product radicals; 2) electron trapping and transfer to form specific radical anions, possibly followed by protonation to give neutral reduction product radicals; and 3) recombination of spatially separated charges or radicals. These microscopic processes will be reviewed critically in an analysis using electron paramagnetic resonance spectroscopy (EPR) evidence for and against long-range transfer of energy and/or charge in frozen, hydrated DNA. 相似文献
702.
A method for estimating parameters of K-distributed clutter 总被引:1,自引:0,他引:1
A method for estimating the parameters of K -distributed clutter when the available sample size of the data is limited is proposed. In this method, the arithmetic mean and geometric mean of the given data are used to estimate the model parameters. Expressions characterizing the performance of the proposed estimator are presented, along with some simulation results. For spiky clutter, simulations show that parameter estimates obtained from the arithmetic and geometric mean are approximately equal to the numerically evaluated maximum-likelihood (ML) estimates. The method is also used to estimate the parameter of the Weibull density 相似文献
703.
We review here observations and models related to the chemical and thermal structures, airglow and auroral emissions and dynamics of the Venus thermosphere, and compare empirical models of the neutral densities based in large part on in situ measurements obtained by the Pioneer Venus spacecraft. Observations of the intensities of emissions are important as a diagnostic tool for understanding the chemical and physical processes taking place in the Venus thermosphere. Measurements, ground-based and from rockets, satellites, and spacecraft, and model predictions of atomic, molecular and ionic emissions, are presented and the most important sources are elucidated. Coronas of hot hydrogen and hot oxygen have been observed to surround the terrestrial planets. We discuss the observations of and production mechanisms for the extended exospheres and models for the escape of lighter species from the atmosphere. Over the last decade and a half, models have attempted to explain the unexpectedly cold temperatures in the Venus thermosphere; recently considerable progress has been made, although some controversies remain. We review the history of these models and discuss the heating and cooling mechanisms that are presently considered to be the most important in determining the thermal structure. Finally, we discuss major aspects of the circulation and dynamics of the thermosphere: the sub-solar to anti-solar circulation, superrotation, and turbulent processes. 相似文献
704.
The objective of the advanced launch system (ALS) program is to develop a family of launch vehicles which provides a substantial improvement in reliability, operability, and economy over existing systems. This requires that autopilot design and verification procedures be of minimum sensitivity to recurring costs while providing adequate, but not necessarily optimal, vehicle performance. One approach to accomplishing this task, namely, the utilization of advanced control design and analysis techniques, is considered. It is shown that the techniques, which represent a cross-section of developments in control theory, attempt to solve the ALS control problem in one of two ways: given increased knowledge of the launch vehicle and its environment, through greater model fidelity and additional sensor data; and autopilot design in the presence of quantified model/disturbance uncertainties and less stringent sensing requirements. It is concluded that the methods are promising and cost effective 相似文献
705.
Oli P.V.S. Nagarajan N. Rayan H.R. 《IEEE transactions on aerospace and electronic systems》1993,29(3):696-705
The concept of meridian orbits is briefly reviewed. It is shown that, if a satellite in the meridian orbit makes an odd number (>1) of revolutions per day, then the satellite passes over the same set of meridians twice a day. Satellites in such orbits pass over the same portion of the sky twice a day and every day. This enables a user to adopt a programmed mode of tracking, thereby avoiding a computational facility for orbit prediction, look angle generation, and auto tracking. A constellation of 38 or more satellites placed in a 1200-km altitude circular orbit is favorable for global communications due to various factors. It is shown that appropriate phasing in right ascension of the ascending node and mean anomaly results in a constellation wherein each satellite appears over the user's horizon one satellite after another. Visibility and coverage plots are provided to verify the continuous coverage 相似文献
706.
Chistopher H.A. Gilman S. Hamlen R.P. 《Aerospace and Electronic Systems Magazine, IEEE》1993,8(5):7-10
The current status and thrust of the US Army Research Laboratory's battery and fuel cell R&D programs that support emerging electronic battlefield equipment applications are reviewed. Major technical barriers are identified along with the approaches proposed to solve these anticipated problems 相似文献
707.
Zeitz F.H. III Maybeck P.S. 《IEEE transactions on aerospace and electronic systems》1993,29(4):1123-1136
The discrete-time Kalman filter is an optimal estimator for the states of a linear, stochastic system. It assumes that measurements are linear combinations of the states, and all disturbances are Gaussian. The influence diagram, a decision analysis tool that provides an algorithm for discrete-time filtering equivalent to the Kalman filter when the influence diagram represents Gaussian random variables, is discussed. The influence diagram algorithm is a factored form of the Kalman filter, similar to other factored forms such as the U-D filter. Compared with the Kalman filter, it offers improved numerical properties. Compared with other factored forms, it offers a reduced computational load 相似文献
708.
Dimogianopoulos D.G. Hios J.D. Fassois S.D. 《IEEE transactions on aerospace and electronic systems》2007,43(4):1385-1396
A statistical method for the on-board detection and compensation of adverse oscillations in pilot-aircraft systems is presented. A recursively updated linear model representing the pilot-aircraft system is used to continuously check for stability loss. The instability-related oscillations are detected early, using a statistical hypothesis test. Furthermore, a specially designed stability augmentation system assists the pilot during the instability periods. The method's effectiveness is demonstrated via data obtained from a flight simulator and a detailed simulation model. 相似文献
709.
Smart M.C. Ratnakumar B.V. Whitcanack L. Chin K. Rodriguez M. Surampudi S. 《Aerospace and Electronic Systems Magazine, IEEE》2002,17(12):16-20
The low temperature charge and discharge characteristics of experimental MCMB-Li/sub x/Ni/sub y/Co/sub 1-y/O/sub 2/ cells containing different electrolytes were investigated. The use of low ethylene carbonate (EC)-content electrolyte formulations has resulted in good discharge performance to temperatures as low as -40/spl deg/C. The effect of charge voltage and charge current upon the individual electrode potentials at low temperature was investigated using the three electrode cells (containing lithium reference electrodes). In some cases, lithium plating was observed to occur upon low temperature charge, and found to be facilitated by high charge voltages, high charge currents, and poor anode kinetics. Electrochemical characterization of the cells has helped to establish the conditions under which lithium plating can occur by providing information regarding the polarization effects present at each electrode. 相似文献
710.
The numerical investigation of the impact of time-dependent accelerations (vibrations) on the flow and heat and mass transfer in the melt is carried out for the case of modeling the crystal growth by the floating zone method under conditions of microgravity that exist onboard spacecraft. The effects of the Archimedean buoyancy force and of vibrations of the free surface of fluid are considered separately. When solving the problem of the effect of the vibrations of the free surface of fluid, the previously obtained data were used. It is shown that vibrations of the free surface have a much stronger effect on the processes under consideration than the buoyancy. Some problems that are related to the newly discovered effects are discussed. The use of vibroprotected systems and a rotating magnetic field can help solve these problems. We plan to continue our investigations in future spacecraft experiments, in particular, at the International Space Station, which is under construction at the moment. 相似文献