全文获取类型
收费全文 | 8994篇 |
免费 | 16篇 |
国内免费 | 38篇 |
专业分类
航空 | 4317篇 |
航天技术 | 3090篇 |
综合类 | 200篇 |
航天 | 1441篇 |
出版年
2021年 | 78篇 |
2019年 | 54篇 |
2018年 | 158篇 |
2017年 | 96篇 |
2016年 | 87篇 |
2014年 | 168篇 |
2013年 | 222篇 |
2012年 | 222篇 |
2011年 | 326篇 |
2010年 | 213篇 |
2009年 | 372篇 |
2008年 | 403篇 |
2007年 | 235篇 |
2006年 | 193篇 |
2005年 | 236篇 |
2004年 | 236篇 |
2003年 | 292篇 |
2002年 | 277篇 |
2001年 | 331篇 |
2000年 | 183篇 |
1999年 | 223篇 |
1998年 | 264篇 |
1997年 | 188篇 |
1996年 | 247篇 |
1995年 | 302篇 |
1994年 | 267篇 |
1993年 | 148篇 |
1992年 | 225篇 |
1991年 | 97篇 |
1990年 | 98篇 |
1989年 | 223篇 |
1988年 | 89篇 |
1987年 | 90篇 |
1986年 | 102篇 |
1985年 | 272篇 |
1984年 | 209篇 |
1983年 | 185篇 |
1982年 | 217篇 |
1981年 | 259篇 |
1980年 | 91篇 |
1979年 | 66篇 |
1978年 | 79篇 |
1977年 | 61篇 |
1976年 | 63篇 |
1975年 | 70篇 |
1974年 | 68篇 |
1972年 | 65篇 |
1971年 | 60篇 |
1970年 | 54篇 |
1969年 | 54篇 |
排序方式: 共有9048条查询结果,搜索用时 0 毫秒
221.
We have performed a joint survey of anisotropic ≳40 keV electron events from August 1997 to September 2000 using the matched
detectors on the Ulysses (ULS)/HI-SCALE and the ACE/EPAM instruments. A computer algorithm selected events with strong, statistically significant
pitch-angle anisotropies. Electron pitch-angle distributions at ACE (∼1 AU) are often ‘beams’ that are strongly collimated
along the local interplanetary magnetic field (IMF). These flare-associated impulsive injections can display rapid rise times
(∼15 min) and slower decays, or more irregular intensity histories. At ULS, the electron intensities are lower and the time
histories smoother, but strong anisotropies are still observable, indicating direct, nearly field-aligned propagation outward
from the Sun. We focus on four event periods, selected from the survey, during times when the angle between the footpoints
of the IMF lines intersecting ACE and ULS is small. These events span three full years and cover a wide range of distances
and heliographic latitudes. We found one reasonably good association between impulsive electron events at ACE and ULS, and
two events with small field-aligned gradients.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
222.
Detecting moving targets in SAR imagery by focusing 总被引:1,自引:0,他引:1
A new method for detecting moving targets in a synthetic aperture radar (SAR) image is presented. It involves segmenting a complex-valued SAR image into patches, focusing each patch separately, and measuring the sharpness increase in the focused patch. The algorithm is sensitive to azimuth velocities and is exquisitely sensitive to radial accelerations of the target, allowing it to detect motion in any direction. It is complementary to conventional Doppler-sensing moving target indicators, which can sense only the radial velocity of rapidly moving targets. 相似文献
223.
Drumheller D.M. Glasser M.L. 《IEEE transactions on aerospace and electronic systems》1997,33(3):784-794
In a recent paper, general expressions were derived for the density and cumulative probability functions of the amplitude of a linear matched-filter output given a nonfluctuating target in a clutter-limited environment. These expressions were based on the clutter amplitude density function. The results are extended to calculate the cumulative probability function of the output of a linear matched filter used to detect a chi-square fluctuating target in a clutter-limited environment. The resulting method is applied to a common radar clutter model, and experimental sonar data. 相似文献
224.
We present the results from a study of the variations of the cosmic-ray intensity with time, heliographic latitude, and longitude, and for varying interplanetary conditions, using our three-dimensional, time-dependent computer code for cosmic-ray transport in the heliosphere. Our code also produces a solar-wind and interplanetary magnetic field (IMF) configuration which is compared with observations. Because of the fully threedimensional nature of the model calculations, we are able to model time variations which would be expected to be observed along Ulysses's trajectory as it moves to high latitudes. In particular we can model the approximately 13-and 26-day solar-rotation induced variations in cosmic rays, solar wind and IMF, as a function of increasing heliographic latitude, as one moves poleward of the interplanetary current sheet. Our preliminary model results seem to be in general form quite similar to published data, but depend on the physical parameters used such as cosmic-ray diffusion coefficients, boundary conditions, and the nature of the solar wind and IMF and current sheet. 相似文献
225.
This paper considers the problem of guiding N interceptors so that they intercept or rendezvous with N targets at the same time T. It is assumed that the interceptors and targets are described by linear time-varying differential equations (deterministic or stochastic). Under certain assumptions both the optimal (static or dynamic) interceptor-target allocation and the guidance of each interceptor for the optimal allocation can be obtained with modest real-time and storage computer requirements. 相似文献
226.
The magnetotail and substorms 总被引:5,自引:0,他引:5
The tail plays a very active and important role in substorms. Magnetic flux eroded from the dayside magnetosphere is stored here. As more and more flux is transported to the magnetotail and stored, the boundary of the tail flares more, the field strength in the tail increases, and the currents strengthen and move closer to the Earth. Further, the plasma sheet thins and the magnetic flux crossing the neutral sheet lessens. At the onset of the expansion phase, the stored magnetic flux is returned from the tail and energy is deposited in the magnetosphere and ionosphere. During the expansion phase of isolated substorms, the flaring angle and the lobe field strength decrease, the plasma sheet thickens and more magnetic flux crosses the neutral sheet.In this review, we discuss the experimental evidence for these processes and present a phenomenological or qualitative model of the substorm sequence. In this model, the flux transport is driven by the merging of the magnetospheric and interplanetary magnetic fields. During the growth phase of substorms the merging rate on the dayside magnetosphere exceeds the reconnection rate in the neutral sheet. In order to remove the oversupply of magnetic flux in the tail, a neutral point forms in the near earth portion of the tail. If the new reconnection rate exceeds the dayside merging rate, then an isolated substorm results. However, a situation can occur in which dayside merging and tail reconnection are in equilibrium. The observed polar cap electric field and its correlation with the interplanetary magnetic field is found to be in accord with open magnetospheric models. 相似文献
227.
Haines K. Hipkin R. Beggan C. Bingley R. Hernandez F. Holt J. Baker T. Bingham R.J. 《Space Science Reviews》2003,108(1-2):205-216
Accurate local geoids derived from in situ gravity data will be valuable in the validation of GOCE results. In addition it will be a challenge to use GOCE data in an
optimal way, in combination with in situ gravity, to produce better local geoid solutions. This paper discusses the derivation of a new geoid over the NW European
shelf, and its comparison with both tide gauge and altimetric sea level data, and with data from ocean models. It is hoped
that over the next few years local geoid methods such as these can be extended to cover larger areas and to incorporate both
in situ and satellite measured gravity data.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
228.
Present-Day Sea Level Change: Observations and Causes 总被引:3,自引:0,他引:3
Cazenave A. Cabanes C. Dominh K. Gennero M.C. Le Provost C. 《Space Science Reviews》2003,108(1-2):131-144
We investigate climate-related processes causing variations of the global mean sea level on interannual to decadal time scale.
We focus on thermal expansion of the oceans and continental water mass balance. We show that during the 1990s where global
mean sea level change has been measured by Topex/Poseidon satellite altimetry, thermal expansion is the dominant contribution
to the observed 2.5 mm/yr sea level rise. For the past decades, exchange of water between continental reservoirs and oceans
had a small, but not totally negligible contribution (about 0.2 mm/yr) to sea level rise. For the last four decades, thermal
contribution is estimated to about 0.5 mm/yr, with a possible accelerated rate of thermosteric rise during the 1990s. Topex/Poseidon
shows an increase in mean sea level of 2.5 mm/yr over the last decade, a value about two times larger than reported by historical
tide gauges. This would suggest that there has been significant acceleration of sea level rise in the recent past, possibly
related to ocean warming.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
229.
Jurewicz A.J.G. Burnett D.S. Wiens R.C. Friedmann T.A. Hays C.C. Hohlfelder R.J. Nishiizumi K. Stone J.A. Woolum D.S. Becker R. Butterworth A.L. Campbell A.J. Ebihara M. Franchi I.A. Heber V. Hohenberg C.M. Humayun M. McKeegan K.D. McNamara K. Meshik A. Pepin R.O. Schlutter D. Wieler R. 《Space Science Reviews》2003,105(3-4):535-560
Genesis (NASA Discovery Mission #5) is a sample return mission. Collectors comprised of ultra-high purity materials will be
exposed to the solar wind and then returned to Earth for laboratory analysis. There is a suite of fifteen types of ultra-pure
materials distributed among several locations. Most of the materials are mounted on deployable panels (‘collector arrays’),
with some as targets in the focal spot of an electrostatic mirror (the ‘concentrator’). Other materials are strategically
placed on the spacecraft as additional targets of opportunity to maximize the area for solar-wind collection.
Most of the collection area consists of hexagonal collectors in the arrays; approximately half are silicon, the rest are for
solar-wind components not retained and/or not easily measured in silicon. There are a variety of materials both in collector
arrays and elsewhere targeted for the analyses of specific solar-wind components.
Engineering and science factors drove the selection process. Engineering required testing of physical properties such as the
ability to withstand shaking on launch and thermal cycling during deployment. Science constraints included bulk purity, surface
and interface cleanliness, retentiveness with respect to individual solar-wind components, and availability.
A detailed report of material parameters planned as a resource for choosing materials for study will be published on a Genesis
website, and will be updated as additional information is obtained. Some material is already linked to the Genesis plasma
data website (genesis.lanl.gov). Genesis should provide a reservoir of materials for allocation to the scientific community
throughout the 21st Century.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
230.
Spectral-domain covariance estimation with a priori knowledge 总被引:2,自引:0,他引:2
Gurram P.R. Goodman N.A. 《IEEE transactions on aerospace and electronic systems》2006,42(3):1010-1020
A knowledge-aided spectral-domain approach to estimating the interference covariance matrix used in space-time adaptive processing (STAP) is proposed. Prior knowledge of the range-Doppler clutter scene is used to identify geographic regions with homogeneous scattering statistics. Then, minimum-variance spectral estimation is used to arrive at a spectral-domain clutter estimate. Finally, space-time steering vectors are used to transform the spectral-domain estimate into a data-domain estimate of the clutter covariance matrix. The proposed technique is compared with ideal performance and to the fast maximum likelihood technique using simulated results. An investigation of the performance degradation that can occur due to various inaccurate knowledge assumptions is also presented 相似文献