首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   6277篇
  免费   41篇
  国内免费   35篇
航空   3250篇
航天技术   2269篇
综合类   56篇
航天   778篇
  2021年   43篇
  2019年   39篇
  2018年   64篇
  2017年   36篇
  2014年   102篇
  2013年   150篇
  2012年   133篇
  2011年   184篇
  2010年   135篇
  2009年   218篇
  2008年   309篇
  2007年   154篇
  2006年   171篇
  2005年   181篇
  2004年   148篇
  2003年   194篇
  2002年   120篇
  2001年   218篇
  2000年   127篇
  1999年   176篇
  1998年   194篇
  1997年   140篇
  1996年   196篇
  1995年   239篇
  1994年   206篇
  1993年   138篇
  1992年   157篇
  1991年   87篇
  1990年   76篇
  1989年   161篇
  1988年   78篇
  1987年   71篇
  1986年   72篇
  1985年   211篇
  1984年   173篇
  1983年   146篇
  1982年   149篇
  1981年   193篇
  1980年   64篇
  1979年   45篇
  1978年   55篇
  1977年   55篇
  1976年   41篇
  1975年   64篇
  1974年   40篇
  1973年   44篇
  1972年   52篇
  1971年   43篇
  1970年   47篇
  1969年   40篇
排序方式: 共有6353条查询结果,搜索用时 0 毫秒
691.
Studies evaluating the transport coefficients for energetic particles in interplanetary space are described in relation to particle data.In position space, the main mode of propagation is along field lines but perpendicular diffusion and drift motion is also possible. Diffusion coefficients based on interplanetary magnetic field data are either derived from quasi-linear, adiabatic theory or this theory corrected for finite scattering near 90° pitch angle or by numerical techniques. Relevant particle data includes solar proton event time profile and anisotropy measurements. In general, when Fokker-Planck transport equation solutions are fitted to particle data, the parallel diffusion coefficients obtained still appear rather larger than those given by theoretical estimates. Perpendicular diffusion is shown to be due to field line wandering and random drift motion effects. The importance of drift motion in cosmic ray modulation theory is mentioned.Although much emphasis is currently placed upon shock acceleration in CIR's, statistical acceleration in interplanetary space must be considered. Energetic particles may gain energy from longitudinal waves and cyclotron resonance interactions. Analytical and numerical estimates of the energy space diffusion coefficients are considered. Some reveal a surprising importance to this statistical acceleration and can explain a variety of data.Presented at the Fifth International Symposium on Solar-Terrestrial Physics, held at Ottawa, Canada, May 1982.  相似文献   
692.
本文研究了人力资源管理与服务质量的关系。透过问卷调查及数据分析,确立人力资源的重点、结构与相互关系,促进服务质量。研究选择了零售服务为试点,给管理人员提供了一个更直接、更清晰的指引,发挥人力资源本身的素质。研究结果把希士杰等质量大师在美国哈佛商报中提出的“服务利润连接(Serviceprofitchain)”具体化,使管理人员可以直接在企业内运用。  相似文献   
693.
Neugebauer  M.  Steinberg  J.T.  Tokar  R.L.  Barraclough  B.L.  Dors  E.E.  Wiens  R.C.  Gingerich  D.E.  Luckey  D.  Whiteaker  D.B. 《Space Science Reviews》2003,105(3-4):661-679
Some of the objectives of the Genesis mission require the separate collection of solar wind originating in different types of solar sources. Measurements of the solar wind protons, alpha particles, and electrons are used on-board the spacecraft to determine whether the solar-wind source is most likely a coronal hole, interstream flow, or a coronal mass ejection. A simple fuzzy logic scheme operating on measurements of the proton temperature, the alpha-particle abundance, and the presence of bidirectional streaming of suprathermal electrons was developed for this purpose. Additional requirements on the algorithm include the ability to identify the passage of forward shocks, reasonable levels of hysteresis and persistence, and the ability to modify the algorithm by changes in stored constants rather than changes in the software. After a few minor adjustments, the algorithm performed well during the initial portion of the mission. This revised version was published online in August 2006 with corrections to the Cover Date.  相似文献   
694.
An Exact Trajectory Solution from Doppler Shift Measurements   总被引:2,自引:0,他引:2  
Three radar Doppler shift measurements are sufficient to determine the range and velocity, and the angle between them, of a target in uniform rectilinear motion. A solution using derivative approximations has recently appeared. An exact solution in terms of the original measurements is presented here.  相似文献   
695.
A detector which is designed to operate in a correlated Gaussian-plus-impulsive-noise environment is presented. The detector whitens the data robustly and then uses a two-sided threshold test to determine the presence of impulsive samples. The impulsive samples are discarded, and the remaining samples are used to detect the presence or absence of a signal using a matched filter. An approximate analysis is presented, and simulations are used to demonstrate the effectiveness of this approach  相似文献   
696.
The nonstationarity of the low-frequency radar echo envelope observed from an aircraft is shown and a simple model of the echo is suggested. This model, devised specifically for simulation studies using computers, reproduces the characteristics of the observed envelope, demonstrating the correlation between the observed nonstationarity and aircraft motion relative to the receiver.  相似文献   
697.
To determine the orbital parameters of a satellite using star occultations, it is necessary to measure the intensity of stars as they occult. The accuracy of the intensity measurements is reduced by interfering sources of light and internal noise, and is expressed in terms of the magnitude of various sources of interference. Subsequently, a specific system is analyzed in detail. Moonlit cloud tops introduce the largest systematic error. Photon noise from the star radiation, airglow, and moonlit cloud tops are the largest sources of random errors.  相似文献   
698.
699.
This paper presents a target identification method based on an estimation of the natural frequencies of oscillation in transient radar signatures. The emphasis is placed upon signal modeling and estimation ation strategy rather than relating resonance locations to physical structures. Salient features of this identification method are: 1) target aspect angle is not needed, 2) multiple targets of the same type can be illuminated simultaneously, and 3) bandpass interrogation ion pulses can be used. The latter feature is compatible with existing radar facilities. The method is applied to some simulated transfer functions, and factors affecting estimate accuracy are discussed.  相似文献   
700.
A direction-finding technique is presented that is capable of simultaneously estimating the arrival angles of multiple signals. Pulsed as well as continuous signals can be handled with the signal form only approximately specified. An adaptive antenna array is used as a processing device in the estimation technique. The effect of input signal and feedback loop parameters upon estimate bias is discussed.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号