全文获取类型
收费全文 | 8005篇 |
免费 | 9篇 |
国内免费 | 24篇 |
专业分类
航空 | 3924篇 |
航天技术 | 2843篇 |
综合类 | 31篇 |
航天 | 1240篇 |
出版年
2021年 | 52篇 |
2018年 | 106篇 |
2017年 | 68篇 |
2016年 | 63篇 |
2014年 | 127篇 |
2013年 | 202篇 |
2012年 | 176篇 |
2011年 | 260篇 |
2010年 | 190篇 |
2009年 | 281篇 |
2008年 | 366篇 |
2007年 | 209篇 |
2006年 | 182篇 |
2005年 | 224篇 |
2004年 | 200篇 |
2003年 | 251篇 |
2002年 | 151篇 |
2001年 | 253篇 |
2000年 | 151篇 |
1999年 | 192篇 |
1998年 | 228篇 |
1997年 | 172篇 |
1996年 | 207篇 |
1995年 | 269篇 |
1994年 | 276篇 |
1993年 | 155篇 |
1992年 | 179篇 |
1991年 | 102篇 |
1990年 | 92篇 |
1989年 | 186篇 |
1988年 | 85篇 |
1987年 | 82篇 |
1986年 | 92篇 |
1985年 | 267篇 |
1984年 | 225篇 |
1983年 | 196篇 |
1982年 | 184篇 |
1981年 | 285篇 |
1980年 | 83篇 |
1979年 | 66篇 |
1978年 | 73篇 |
1977年 | 69篇 |
1976年 | 61篇 |
1975年 | 85篇 |
1974年 | 62篇 |
1973年 | 67篇 |
1972年 | 85篇 |
1971年 | 58篇 |
1970年 | 64篇 |
1969年 | 52篇 |
排序方式: 共有8038条查询结果,搜索用时 46 毫秒
251.
A two-pole filter is proposed as a detector for a scanning radar. The optimum values of the filter coefficients are found and are approximated by a simple expression. The optimum two-pole filter requires a 0.15-dB increase in signal-to-noise ratio in order to provide the same detection capability as the optimum detector, and yields azimuth estimates whose standard deviation are within 15 percent of the Cramér-Rao lower bound. The estimator is simple to implement, avoiding the storage requirements of the moving window detector and the bias complications of the feedback integrator. 相似文献
252.
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
253.
A ground testing method has been devised to evaluate the dynamic errors of an inertial navigation system. A trimmed stationary inertial system in the navigation mode can be subjected to programmed platform orm drift rates, and generate position outputs which are compared with those of a perfect navigator. The linearized error equations for this testing mode are derived and the resulting position error propagation is analyzed using Kalman filtering in order to identify the error sources. A simulated platform with a fixed set of error sources is analyzed to evaluate this testing concept. An example is presented to show that the gyro and accelerometer scale factor and misalignment error coefficients can be estimated. 相似文献
254.
255.
Moyer L.R. Morgan C.J. Rugger D.A. 《IEEE transactions on aerospace and electronic systems》1989,25(4):584-587
An exact expression for the bistatic resolution-cell area (A B) is developed for the special case in which either the transmit or receive antenna has a broad azimuth beamwidth or is omnidirectional. Quantitative examples are presented to illustrate the variation of A B with location. A comparison is made with the performance of a commonly used approximation formula 相似文献
256.
257.
Parameters of expanding magnetic loops and arches and of mass flows generated by them in the corona have been computed in a 1D two-fluid approximation. Two possible trigger mechanisms of the coronal transients have been considered: (i) sudden increase of the background magnetic field strength, and (ii) heating and compression plasma inside these magnetic structures. We discuss the formation of shock waves and their dependence on dynamics and geometry of the magnetic structures. 相似文献
258.
The nonlinear modeling and analysis of dc-dc converters has been done by averaging and discrete-sampling techniques. The averaging technique is simple, but inaccurate, as the modulation frequencies approach the theoretical limit of one-half the switching frequency. The discrete technique is accurate even at high frequencies, but is very complex and cumbersome. An improved model is developed by combining the aforementioned techniques. This new model is easy to implement in circuit and state variable forms and is accurate to the theoretical limit. 相似文献
259.
E. J. Smith 《Space Science Reviews》1983,34(1):101-110
Interplanetary shock observations since the prior Solar Terrestrial Physics Symposium in 1978 are reviewed. Since the interval coincides with the recent solar maximum, emphasis is placed on shocks associated with transient solar phenomena, including coronal transients and eruptive prominences as well as flares. A good correlation between shocks and Storm Sudden Commencements has persisted into the recent maximum. Shocks have been identified that are associated with disappearing filaments and coronal transients rather than with flares. Significant progress has been made in the indirect observation of shocks near the Sun as a result of radio wave measurements in interplanetary space and measurement of the scintillation and spectral broadening of spacecraft radio transmissions. Preliminary results regarding the thickness of interplanetary shocks have appeared. Several quasi-parallel shocks propagating more nearly along, rather than across, the magnetic field have been identified. The plasma drivers accompanying interplanetary shocks have received increased attention and distinctive features have been found in electron, ion and magnetic field data. 相似文献
260.
J. P. Cox J. C. Wheeler C. J. Hansen D. S. King A. N. Cox S. W. Hodson 《Space Science Reviews》1980,27(3-4):529-535
The radial pulsations of very luminous, low-mass models (L/M 104, solar units), which are possible representatives of the R CrB stars, have been examined. These pulsations are extremely nonadiabatic. We find that there are in some cases at least one extra (strange) mode which makes interpretation difficult. The blue instability edges are also peculiar, in that there is an abrupt excursion of the blue edge to the blue for L/M sufficiently large. The range of periods of the model encompasses observed periods of the Cepheid-like pulsations of actual R CrB stars. 相似文献