全文获取类型
收费全文 | 24572篇 |
免费 | 1172篇 |
国内免费 | 1382篇 |
专业分类
航空 | 13961篇 |
航天技术 | 7006篇 |
综合类 | 1334篇 |
航天 | 4825篇 |
出版年
2022年 | 149篇 |
2021年 | 302篇 |
2020年 | 200篇 |
2019年 | 212篇 |
2018年 | 335篇 |
2017年 | 208篇 |
2016年 | 289篇 |
2015年 | 316篇 |
2014年 | 786篇 |
2013年 | 885篇 |
2012年 | 888篇 |
2011年 | 1094篇 |
2010年 | 966篇 |
2009年 | 1317篇 |
2008年 | 1309篇 |
2007年 | 851篇 |
2006年 | 824篇 |
2005年 | 722篇 |
2004年 | 679篇 |
2003年 | 751篇 |
2002年 | 714篇 |
2001年 | 790篇 |
2000年 | 611篇 |
1999年 | 745篇 |
1998年 | 697篇 |
1997年 | 547篇 |
1996年 | 546篇 |
1995年 | 591篇 |
1994年 | 593篇 |
1993年 | 495篇 |
1992年 | 451篇 |
1991年 | 370篇 |
1990年 | 323篇 |
1989年 | 478篇 |
1988年 | 266篇 |
1987年 | 287篇 |
1986年 | 251篇 |
1985年 | 641篇 |
1984年 | 521篇 |
1983年 | 407篇 |
1982年 | 492篇 |
1981年 | 614篇 |
1980年 | 246篇 |
1979年 | 185篇 |
1978年 | 189篇 |
1976年 | 155篇 |
1975年 | 186篇 |
1974年 | 180篇 |
1973年 | 161篇 |
1972年 | 188篇 |
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
991.
A relative navigation system for formation flight 总被引:1,自引:0,他引:1
A relative navigation system based on both the Inertial Navigation System (INS) and the Global Positioning System (GPS) is developed to support situational awareness during formation flight. The architecture of the system requires an INS/GPS integration across two aircraft via a data link. A fault-tolerant federated filter is used to estimate the relative INS errors based on relative GPS measurements and a range measurement obtained from the data link. The filter is constructed based on a reduced-order model of the relative INS error process. A method for analyzing the filter performance is presented. A case involving two helicopters in formation flight is studied under three different night trajectories to account for the effect of vehicle motion on the INS state transition matrix. The results of the covariance analysis are compared with actual night results over an instrumented test range. 相似文献
992.
993.
994.
G. Sigl 《Space Science Reviews》1996,75(1-2):375-385
In this paper we review the hypothesis that a substantial part of the cosmic ray flux observed above about 1019 eV may be produced by decaying or annihilating topological defects left over from phase transitions in the early universe at grand unification energy scales ( 1016 GeV). Possible signatures of cosmic ray producing defect models are discussed which could be tested experimentally in the near future. We thereby focus on model independent universal spectral properties of the predicted particle fluxes. 相似文献
995.
J. Scudder F. Hunsacker G. Miller J. Lobell T. Zawistowski K. Ogilvie J. Keller D. Chornay F. Herrero R. Fitzenreiter D. Fairfield J. Needell D. Bodet J. Googins C. Kletzing R. Torbert J. Vandiver R. Bentley W. Fillius C. McIlwain E. Whipple A. Korth 《Space Science Reviews》1995,71(1-4):459-495
HYDRA is an experimental hot plasma investigation for the POLAR spacecraft of the GGS program. A consortium of institutions has designed a suite of particle analyzers that sample the velocity space of electron and ions between 2 keV/q – 35 keV/q in three dimensions, with a routine time resolution of 0.5 s. Routine coverage of velocity space will be accomplished with an angular homogeneity assumption of 16°, appropriate for subsonic plasmas, but with special 1.5° resolution for electrons with energies between 100 eV and 10 keV along and opposed to the local magnetic field. This instrument produces 4.9 kilobits s–1 to the telemetry, consumes on average 14 W and requires 18.7 kg for deployment including its internal shielding. The scientific objectives for the polar magnetosphere fall into four broad categories: (1) those to define the ambient kinetic regimes of ions and electrons; (2) those to elucidate the magnetohydrodynamic responses in these regimes; (3) those to assess the particle populations with high time resolution; and (4) those to determine the global topology of the magnetic field. In thefirst group are issues of identifying the origins of particles at high magnetic latitudes, their energization, the altitude dependence of the forces, including parallel electric fields they have traversed. In thesecond group are the physics of the fluid flows, regimes of current, and plasma depletion zones during quiescent and disturbed magnetic conditions. In thethird group is the exploration of the processes that accompany the rapid time variations known to occur in the auroral zone, cusp and entry layers as they affect the flow of mass, momentum and energy in the auroral region. In thefourth class of objectives are studies in conjunction with the SWE measurements of the Strahl in the solar wind that exploit the small gyroradius of thermal electrons to detect those magnetic field lines that penetrate the auroral region that are directly open to interplanetary space where, for example, the Polar Rain is observed. 相似文献
996.
SWE,a comprehensive plasma instrument for the WIND spacecraft 总被引:1,自引:0,他引:1
K. W. Ogilvie D. J. Chornay R. J. Fritzenreiter F. Hunsaker J. Keller J. Lobell G. Miller J. D. Scudder E. C. Sittler Jr. R. B. Torbert D. Bodet G. Needell A. J. Lazarus J. T. Steinberg J. H. Tappan A. Mavretic E. Gergin 《Space Science Reviews》1995,71(1-4):55-77
The Solar Wind Experiment (SWE) on the WIND spacecraft is a comprehensive, integrated set of sensors which is designed to investigate outstanding problems in solar wind physics. It consists of two Faraday cup (FC) sensors; a vector electron and ion spectrometer (VEIS); a strahl sensor, which is especially configured to study the electron strahl close to the magnetic field direction; and an on-board calibration system. The energy/charge range of the Faraday cups is 150 V to 8 kV, and that of the VEIS is 7 V to 24.8 kV. The time resolution depends on the operational mode used, but can be of the order of a few seconds for 3-D measurements. Key parameters which broadly characterize the solar wind positive ion velocity distribution function will be made available rapidly from the GGS Central Data Handling Facility. 相似文献
997.
R. Bodmer P. Bochsler J. Geiss R. von Steiger G. Gloeckler 《Space Science Reviews》1995,72(1-2):61-64
This is the first study of the isotopic composition of solar wind helium with the SWICS time-of flight mass spectrometer. Although the design of SWICS is not optimized to measure3He abundances precisely,4He/3He flux ratios can be deduced from the data. The long term ratio is 2290±200, which agrees with the results obtained with the ICI magnetic mass spectrometer on ISEE-3 and with the Apollo SWC foil experiments.The ULYSSES spacecraft follows a trajectory which is ideal for the study of different solar wind types. During one year, from mid-1992 to mid-1993, it was in a range of heliographic latitudes where a recurrent fast stream from the southern polar coronal hole was observed every solar rotation. Solar wind bulk velocities ranged from 350 km/s to 950 km/s which would, in principle allow us to identify velocity-correlated compositional variations. Our investigation of solar wind helium, however, shows an isotopic ratio which does not depend on the solar wind speed. 相似文献
998.
ANALYSISANDMEASUREMENTONRESIDUALSTRESSOFARAMIDALUMINIUMLAMINATELiHongyun,HuHongiun,ZhengRulqi(BeijinginstituteofAeronauticalM... 相似文献
999.
In the case of a single sinusoid or multiple well-separated sinusoids, a coarse estimator consisting of a windowed Fourier transform followed by a fine estimator which is an interpolator is a good approximation to an optimal frequency acquisition and measurement algorithm. The design tradeoffs are described. It is shown that for the fine-frequency estimator a good method is to fit a Gaussian function to the fast-Fourier-transform (FFT) peak and its two neighbors. This method achieves a frequency standard deviation and a bias in the order of only a few percent of a bin. In the case of short-time stationarity, for a moderate number of averages and for an adaptive threshold detector, only between 0.5 and 1 dB is lost when averaging is traded off for FFT length, in contrast to the asymptotic result of 1.5 dB. The COSPAS-SARSAT satellite system for emergency detection and localization is used to illustrate the concepts. The algorithm is analyzed theoretically, and good agreement is found with test results 相似文献
1000.
Multiposition alignment of strapdown inertial navigation system 总被引:3,自引:0,他引:3
Lee J.G. Park C.G. Park H.W. 《IEEE transactions on aerospace and electronic systems》1993,29(4):1323-1328
The authors demonstrate that the stationary alignment of strapdown inertial navigation system (SDINS) can be improved by employing the multiposition/technique. Using an observability analysis, it is shown that an optimal two-position alignment not only satisfies complete observability conditions but also minimizes alignment errors. This is done by analytic rank testing of the stripped observability matrix and numerical calculation of the error covariance. It is also shown that an optimal three-position alignment accelerates the convergence of the alignment error compared with two-position alignment 相似文献