全文获取类型
收费全文 | 8174篇 |
免费 | 40篇 |
国内免费 | 29篇 |
专业分类
航空 | 3708篇 |
航天技术 | 2819篇 |
综合类 | 30篇 |
航天 | 1686篇 |
出版年
2021年 | 90篇 |
2019年 | 61篇 |
2018年 | 236篇 |
2017年 | 158篇 |
2016年 | 132篇 |
2015年 | 59篇 |
2014年 | 202篇 |
2013年 | 251篇 |
2012年 | 245篇 |
2011年 | 384篇 |
2010年 | 278篇 |
2009年 | 403篇 |
2008年 | 436篇 |
2007年 | 265篇 |
2006年 | 194篇 |
2005年 | 220篇 |
2004年 | 219篇 |
2003年 | 244篇 |
2002年 | 177篇 |
2001年 | 255篇 |
2000年 | 150篇 |
1999年 | 179篇 |
1998年 | 218篇 |
1997年 | 142篇 |
1996年 | 192篇 |
1995年 | 229篇 |
1994年 | 203篇 |
1993年 | 137篇 |
1992年 | 177篇 |
1991年 | 56篇 |
1990年 | 58篇 |
1989年 | 162篇 |
1988年 | 74篇 |
1987年 | 70篇 |
1986年 | 71篇 |
1985年 | 217篇 |
1984年 | 177篇 |
1983年 | 132篇 |
1982年 | 139篇 |
1981年 | 236篇 |
1980年 | 75篇 |
1979年 | 54篇 |
1978年 | 64篇 |
1977年 | 45篇 |
1975年 | 61篇 |
1974年 | 47篇 |
1973年 | 38篇 |
1972年 | 41篇 |
1971年 | 41篇 |
1970年 | 40篇 |
排序方式: 共有8243条查询结果,搜索用时 93 毫秒
411.
McGoey-Smith A.D. Vant M.R. 《IEEE transactions on aerospace and electronic systems》1992,28(3):666-674
When the basic step transform algorithm is used to compress synthetic-aperture radar (SAR) signals in azimuth, the linear FM rate and sampling rate must satisfy certain tight constraints. In practice, these constraints cannot be satisfied and errors are introduced into the compressed SAR image. A modification is described of the basic step transform which incorporates interpolation and resampling into the algorithm. These changes allow the removal of the constraints and make the step transform more useful for the compression of real data. An autofocusing capability is also included, without introducing much additional complexity 相似文献
412.
413.
414.
Frequency estimation techniques for high dynamic trajectories 总被引:7,自引:0,他引:7
Vilnrotter V.A. Hinedi S. Kumar R. 《IEEE transactions on aerospace and electronic systems》1989,25(4):559-577
A comparison is presented of four different estimation techniques applied to the problem of continuously estimating the rapidly varying parameters of a sinusoidal signal, observed in the presence of additive noise. Frequency estimates are emphasized, although phase and/or frequency rate are also estimated by some of the algorithms. These parameters are related to the velocity, position, and acceleration of the maneuvering receiver or transmitter. Estimated performance at low carrier-to-noise ratios and high dynamics is investigated for the purpose of determining the useful operating range of an approximate maximum likelihood estimator, an extended Kalman filter, a cross-product automatic frequency loop and a phase-locked loop. Numerical simulations are used to evaluate performance while tracking a common trajectory exhibiting high dynamics 相似文献
415.
416.
417.
Cascaded detector for multiple high-PRF pulse Doppler radars 总被引:1,自引:0,他引:1
A postdetection design methodology for a multiple high-pulse-repetition frequency (PRF) pulse Doppler radar has been developed. The postdetection processor consists of an M out of N detector where range and target ambiguities are resolved, followed by a square-law detector which enhances the minimum signal-to-noise (S/N) power-ratio per pulse burst performance. For given probabilities of false alarm and detection, formulas are derived from which the three thresholds associated with the cascaded detector can be found. Fundamental tradeoffs between the minimum S/N required, number of ghosts, and the number of operations (NOPs) that the cascaded detector must perform are identified. It is shown that the NOPs and the number of ghosts increase and the minimum S/N required decreases as the binary M out of N detector passes more detections to the square-law detector 相似文献
418.
B. Holback S. -E. Jansson L. Åhlén G. Lundgren L. Lyngdal S. Powell A. Meyer 《Space Science Reviews》1994,70(3-4):577-592
The Wave Experiment, F4, on the Swedish/German satelliteFreja, is designed to measure the electric wave fields up to 4 MHz, the magnetic wave fields up to 16 kHz and the plasma density and its relative variations up to 2 kHz. Six wave signals and four density probe signals can be measured simultaneously. The wave forms of all signals are transmitted to ground without any analysis onboard. The limited TM allocation does not allow continuous sampling of the wave signals, so normally the measurements are made in snapshots of various lengths dependent on sampling frequency, etc. Continuous sampling can be made for shorter time periods by using a 6 Mbyte memory as a buffer. 相似文献
419.
M. Pick A. Buttighoffer A. Kerdraon T. P. Armstrong E. C. Roelof S. Hoang L. J. Lanzerotti G. M. Simnett J. Lemen 《Space Science Reviews》1995,72(1-2):315-320
A remarkable streaming beam-like particle event of 60 keV-5 MeV ions and of 38–315 keV electrons has been reported previously. This event has been associated with the passage of a Coronal Mass Ejection (CME) over the Ulysses spacecraft on June 9–13, 1993. At this time, the spacecraft was located at 4.6 AU from the sun and at an heliolatitude of 32° south. It was proposed (Armstrong et al., 1994) that the particle injection source could have been of coronal origin. In this study, we analyse the solar activity during this period. We identify a region of solar radio noise storms in the corona and in particular, a flare on June 7 that presents all the required characteristics to produce the hot plasma beam observed in the interplanetary medium. 相似文献
420.
The Pioneer 10, Pioneer 11, and Voyager 2 spacecraft were launched in 1972, 1974, and 1977, respectively. While these three spacecraft are all at compartively low heliographic latitudes compared with Ulysses, their observation span almost two solar cycles, a range of heliocentric distances from 1 to 57 AU, and provide a unique insight into the long-term variability of the global structure of the solar wind. We examine the spatial and temporal variation of average solar wind parameters and fluxes. Our obsevations suggest that the global structure of the outer heliosphere during the declining phase of the solar cycle at heliographic latitudes up to 17.5°N was charaterized by two competing phenomena: 1) a large-scale increase of solar wind density, temperature, mass flux, dynamic pressure, kinetic energy flux, and thermal enery flux with heliographic latitude, similar to the large-scale latitudinal gradient of velocity seen in IPS observations, 2) a small-scale decrease in velocity and temperature, and increase in density near the heliospheric current sheet, which is associated with a band of low speed, low temperature, and high density solar wind similar to that observed in the inner heliosphere. 相似文献