全文获取类型
收费全文 | 4468篇 |
免费 | 13篇 |
国内免费 | 7篇 |
专业分类
航空 | 2086篇 |
航天技术 | 1605篇 |
综合类 | 16篇 |
航天 | 781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 | 130篇 |
2013年 | 138篇 |
2012年 | 143篇 |
2011年 | 194篇 |
2010年 | 127篇 |
2009年 | 217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4488条查询结果,搜索用时 0 毫秒
451.
452.
Servidia P.A. Sanchez Pena R.S. 《IEEE transactions on aerospace and electronic systems》2005,41(2):584-598
The attitude stabilization of a spacecraft using thrusters is considered from a practical point of view, i.e., when actuator constraints, uncertainties and failures, measurement noise, fuel consumption, and inertia matrix uncertainty are considered. A variable structure controller based on sliding modes is obtained first, which guarantees global exponential stability under actuator constraints, uncertainties, and failures. The design is based on a continuous average-proportional torque selection function and a set of feasible sliding surfaces, which may be adapted to improve performance. Exponential convergence towards ultimate bounds is guaranteed when all other practical issues arise. 相似文献
453.
Track-before-detect procedures for early detection of moving target from airborne radars 总被引:2,自引:0,他引:2
Buzzi S. Lops M. Venturino L. 《IEEE transactions on aerospace and electronic systems》2005,41(3):937-954
In this paper we present a family of track-before-detect (TBD) procedures for early detection of moving targets from airborne radars. Upon a sectorization of the coverage area, the received echoes are jointly processed in the azimuth-range-Doppler domain and in the time domain through a Viterbi-like algorithm that exploits the physically admissible target transitions between successive illuminations, in order to collect all of the energy back-scattered during the time on target (TOT). A reduced-complexity implementation is derived assuming, at the design stage, that the target does not change resolution cell during the TOT in each scan. The constant false alarm rate (CFAR) constraint is also englobed in the proposed procedures as well as the possibility of working with quantized data. Simulation results show that the proposed algorithms have good detection and tracking capabilities even for high target velocities and low quantization rates. 相似文献
454.
The ionosphere of Mars has been explored mostly with the radio occultation experiment onboard Mariners 6, 7, 9; Mars 2, 3, 4, 6; Viking 1, 2, and more recently on Mars Global Surveyor (MGS) and Mars Express (MEX). In addition to the radio occultation experiment, MEX also carried Mars Advanced Radar for the Subsurface and Ionosphere Sounding (MARSIS) experiment which provided electron density profiles well above the main ionospheric peak. The atmosphere of Mars was measured directly by the neutral mass spectrometer onboard Viking 1 and 2 Landers. Later, an accelerometer and radio occultation experiment on MGS provided large data sets of atmospheric density at various locations in the upper and lower atmospheres of Mars, respectively. In this paper we review results of these upper and lower atmospheric/ionospheric measurements. Results of these measurements have been compared with theoretical models by several workers; therefore, we also review various atmospheric and ionospheric models of Mars. 相似文献
455.
Beers T.C. Rossi S. Norris J.E. Ryan S.G. Molaro P. Rebolo R. 《Space Science Reviews》1998,84(1-2):139-144
Ongoing spectroscopy and photometry of stars selected in the HK objective-prism/interference-filter survey of Beers and colleagues has resulted in the identification of many hundreds of additional stars in the halo (and possibly the thick disk) of the Galaxy with abundances [Fe/H] -2.0. A new calibration of the technique for estimation of metal abundance based on a CaII K index as a function of broadband B - V color is applied to obtain metallicities for stars observed with the SSO 2.3m and INT 2.5m telescopes. This new data is combined with other samples of extremely metal-deficient stars (Ryan and Norris, 1991a; Beers et al., 1992; Carney et al., 1994) to form a large database of objects of low metallicity. The combined sample is examined and compared with expectations derived from a Simple Model of Galactic chemical evolution. There appears to be a statistically-significant deficit of stars more metal-weak than [Fe/H] = -3.0. An abundance of [Fe/H] -4.0 can be taken as the low-metallicity limit for presently-observable stars in the Galaxy. 相似文献
456.
Liu S. Singer C.H. Dougal R.A. 《IEEE transactions on aerospace and electronic systems》2006,42(2):612-624
Electric power anomalies or disturbances can disrupt the normal operation of equipment, accelerate aging, or even cause outright failures thus resulting in increased costs of maintenance and reduced system reliability. Past research on the effects caused by power anomalies has been mostly focused on industrial, commercial, or residential systems, or on power distribution equipment. A literature survey reveals that there is no comprehensive review related to low-voltage (LV) power systems and utilization equipment applicable to military combat vehicles, such as aircraft and ships. This paper summarizes the results of a new literature survey that focused on the causes, effects, and mitigation methods for power anomalies typical of LV mobile power systems. Electric power anomaly cost data collected from the literature are also presented, from which the costs of anomalies to the national defense are estimated using some simple rationales. 相似文献
457.
Capturability of the pure proportional navigation (PPN) guidance law against a target executing bounded piecewise continuous time-varying manoeuvres is investigated. A qualitative analysis is carried out to obtain a set of sufficient conditions for capture defined on the engagement parameters and initial conditions. These conditions are significantly less restrictive than the ones obtained previously by others using the Lyapunov method. It is shown that the actual capture region for time-varying target manoeuvres, obtained by using the conditions derived, is much larger than that obtained from the Lyapunov technique. We also show that though a bounded tine varying target manoeuvre does change the constant target manoeuvre capture region to some extent, it does not reduce it drastically. Further, we show that the worst case capture region is obtained when the target executes a constant manoeuvre equal to the bound on the manoeuvre level. Some bounds on the missile lateral acceleration are also obtained for certain regions in the engagement plane. These results are generalizations and extensions of existing results on the capturability of the PPN guidance law against targets executing constant or time-varying target manoeuvres 相似文献
458.
Messenger S. Stadermann F.J. Floss C. Nittler L.R. Mukhopadhyay S. 《Space Science Reviews》2003,106(1-4):155-172
Interplanetary dust particles collected in the stratosphere frequently exhibit enrichments in deuterium (D) and 15N relative to terrestrial materials. These effects are most likely due to the preservation of presolar interstellar materials.
While the elevated D/H ratios probably resulted from mass fractionation during chemical reactions at very low < 100 K temperatures,
the origin of the N isotopic anomalies remains unresolved. The bulk of the N-bearing material may have obtained its isotopic
signatures from low temperature chemistry, but a nucleosynthetic origin is also possible.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
459.
460.
A Class of TVD Type Combined Numerical Scheme for MHD Equations With a Survey About Numerical Methods in Solar Wind Simulations 总被引:3,自引:0,他引:3
It has been believed that three-dimensional, numerical, magnetohydrodynamic (MHD) modelling must play a crucial role in a
seamless forecasting system. This system refers to space weather originating on the sun; propagation of disturbances through
the solar wind and interplanetary magnetic field (IMF), and thence, transmission into the magnetosphere, ionosphere, and thermosphere.
This role comes as no surprise to numerical modelers that participate in the numerical modelling of atmospheric environments
as well as the meteorological conditions at Earth. Space scientists have paid great attention to operational numerical space
weather prediction models. To this purpose practical progress has been made in the past years. Here first is reviewed the
progress of the numerical methods in solar wind modelling. Then, based on our discussion, a new numerical scheme of total
variation diminishing (TVD) type for magnetohydrodynamic equations in spherical coordinates is proposed by taking into account
convergence, stability and resolution. This new MHD model is established by solving the fluid equations of MHD system with
a modified Lax-Friedrichs scheme and the magnetic induction equations with MacCormack II scheme for the purpose of developing
a combined scheme of quick convergence as well as of TVD property. To verify the validation of the scheme, the propagation
of one-dimensional MHD fast and slow shock problem is discussed with the numerical results conforming to the existing results
obtained by the piece-wise parabolic method (PPM). Finally, some conclusions are made.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献