全文获取类型
收费全文 | 5692篇 |
免费 | 29篇 |
国内免费 | 14篇 |
专业分类
航空 | 2750篇 |
航天技术 | 2117篇 |
综合类 | 26篇 |
航天 | 842篇 |
出版年
2021年 | 48篇 |
2019年 | 49篇 |
2018年 | 101篇 |
2017年 | 64篇 |
2016年 | 69篇 |
2015年 | 32篇 |
2014年 | 113篇 |
2013年 | 141篇 |
2012年 | 135篇 |
2011年 | 187篇 |
2010年 | 140篇 |
2009年 | 227篇 |
2008年 | 317篇 |
2007年 | 155篇 |
2006年 | 128篇 |
2005年 | 143篇 |
2004年 | 137篇 |
2003年 | 189篇 |
2002年 | 119篇 |
2001年 | 192篇 |
2000年 | 113篇 |
1999年 | 131篇 |
1998年 | 164篇 |
1997年 | 105篇 |
1996年 | 146篇 |
1995年 | 195篇 |
1994年 | 168篇 |
1993年 | 103篇 |
1992年 | 144篇 |
1991年 | 67篇 |
1990年 | 67篇 |
1989年 | 142篇 |
1988年 | 62篇 |
1987年 | 63篇 |
1986年 | 58篇 |
1985年 | 179篇 |
1984年 | 150篇 |
1983年 | 139篇 |
1982年 | 144篇 |
1981年 | 166篇 |
1980年 | 53篇 |
1979年 | 62篇 |
1978年 | 50篇 |
1977年 | 41篇 |
1976年 | 28篇 |
1975年 | 45篇 |
1974年 | 37篇 |
1973年 | 31篇 |
1972年 | 46篇 |
1970年 | 27篇 |
排序方式: 共有5735条查询结果,搜索用时 31 毫秒
991.
Adaptive control and stabilization of elastic spacecraft 总被引:1,自引:0,他引:1
This work treats the question of large angle rotational maneuver and stabilization of an elastic spacecraft (spacecraft-beam-tip body configuration). It is assumed that the parameters of the system are completely unknown. An adaptive control law is derived for the rotational maneuver of the spacecraft. Using the adaptive controller, asymptotically decoupled control of the pitch angle of the space vehicle is accomplished, however this maneuver causes elastic deformation of the beam connecting the orbiter and tip body. For the stabilization of the zero dynamics (flexible dynamics), a stabilizer is designed using elastic mode velocity feedback. In the closed-loop system including the adaptive controller and the stabilizer, reference pitch angle trajectory tracking and vibration suppression are accomplished. Simulation results are presented to show the maneuver capability of the control system 相似文献
992.
A theoretical analysis of the sequential lobing technique for target angle tracking is presented. The signal power received by each antenna beam is assumed to pass through a logarithmic amplifier. A rigorous statistical approach is adopted in the analysis for both non-fluctuating and fluctuating targets. Closed form expressions are derived for the normalized mean error and rms error of the angle estimate for Swerling 0, I, II, and III targets. Results are compared with those obtained using a simplified approach for the non-fluctuating target 相似文献
993.
Structures for radar detection in compound Gaussian clutter 总被引:1,自引:0,他引:1
Sangston K.J. Gini F. Greco M.V. Farina A. 《IEEE transactions on aerospace and electronic systems》1999,35(2):445-458
The problem of coherent radar target detection in a background of non-Gaussian clutter modeled by a compound Gaussian distribution is studied here. We show how the likelihood ratio may be recast into an estimator-correlator form that shows that an essential feature of the optimal detector is to compute an optimum estimate of the reciprocal of the unknown random local power level. We then proceed to show that the optimal detector may be recast into yet another form, namely a matched filter compared with a data-dependent threshold. With these reformulations of the optimal detector, the problem of obtaining suboptimal detectors may be systematically studied by either approximating the likelihood ratio directly, utilizing a suboptimal estimate in the estimator-correlator structure or utilizing a suboptimal function to model the data-dependent threshold in the matched filter interpretation. Each of these approaches is studied to obtain suboptimal detectors. The results indicate that for processing small numbers of pulses, a suboptimal detector that utilizes information about the nature of the non-Gaussian clutter can be implemented to obtain quasi-optimal performance. As the number of pulses to be processed increases, a suboptimal detector that does not require information about the specific nature of the non-Gaussian clutter may be implemented to obtain quasi-optimal performance 相似文献
994.
Optical guidance for autonomous landing of spacecraft 总被引:7,自引:0,他引:7
Miso T. Hashimoto T. Ninomiya K. 《IEEE transactions on aerospace and electronic systems》1999,35(2):459-473
An autonomous rendezvous guidance scheme for spacecraft to descend to small celestial bodies by using optical information is presented. First, a new guidance, navigation, and control (GNC) method based on fixation-point (FP) inheritance is proposed. A spacecraft can safely descend toward the target point on the celestial body by tracking and autonomously renewing the FPs on the surface. Next, we deal with the method of extracting the FPs. A spatial band-pass filter (BPF) is applied to pictures taken to enhance features having comparable size with the tracking window. Local variance of the filtered image is used as a criterion of the extraction. Then, the relative information between the spacecraft and the celestial body (position, velocity, attitude, etc.) is calculated from the image coordinates and the range measurements of the FPs from the spacecraft. To suppress observation noise and improve navigation accuracy, an application of the extended Kalman filter is also presented. Finally, simulations are conducted to verify the guidance precision and the fuel consumption of the proposed guidance scheme 相似文献
995.
W. H. Matthaeus G. P. Zank R. J. Leamon C. W. Smith D. J. Mullan S. Oughton 《Space Science Reviews》1999,87(1-2):269-275
Mechanisms for the deposition of heat in the lower coronal plasma are discussed, emphasizing recent attempts to reconcile
the fluid and kinetic perspectives. Structures at the MHD scales are believed to act as reservoirs for fluctuation energy,
which in turn drive a nonlinear cascade process. Kinetic processes act at smaller spatial scales and more rapid time scales.
Cascade-driven processes are contrasted with direct cyclotron absorption, and this distinction is echoed in the contrast between
frequency and wavenumber spectra of the fluctuations. Observational constraints are also discussed, along with estimates of
the relative efficiency of cascade and cyclotron processes.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
996.
Interplanetary origin of geomagnetic storms 总被引:8,自引:0,他引:8
Gonzalez Walter D. Tsurutani Bruce T. Clúa de Gonzalez Alicia L. 《Space Science Reviews》1999,88(3-4):529-562
Around solar maximum, the dominant interplanetary phenomena causing intense magnetic storms (Dst<−100 nT) are the interplanetary
manifestations of fast coronal mass ejections (CMEs). Two interplanetary structures are important for the development of storms,
involving intense southward IMFs: the sheath region just behind the forward shock, and the CME ejecta itself. Whereas the
initial phase of a storm is caused by the increase in plasma ram pressure associated with the increase in density and speed
at and behind the shock (accompanied by a sudden impulse [SI] at Earth), the storm main phase is due to southward IMFs. If
the fields are southward in both of the sheath and solar ejecta, two-step main phase storms can result and the storm intensity
can be higher. The storm recovery phase begins when the IMF turns less southward, with delays of ≈1–2 hours, and has typically
a decay time of 10 hours. For CMEs involving clouds the intensity of the core magnetic field and the amplitude of the speed
of the cloud seems to be related, with a tendency that clouds which move at higher speeds also posses higher core magnetic
field strengths, thus both contributing to the development of intense storms since those two parameters are important factors
in genering the solar wind-magnetosphere coupling via the reconnection process.
During solar minimum, high speed streams from coronal holes dominate the interplanetary medium activity. The high-density,
low-speed streams associated with the heliospheric current sheet (HCS) plasma impinging upon the Earth's magnetosphere cause
positive Dst values (storm initial phases if followed by main phases). In the absence of shocks, SIs are infrequent during
this phase of the solar cycle. High-field regions called Corotating Interaction Regions (CIRs) are mainly created by the fast
stream (emanating from a coronal hole) interaction with the HCS plasma sheet. However, because the Bz component is typically highly fluctuating within the CIRs, the main phases of the resultant magnetic storms typically have
highly irregular profiles and are weaker. Storm recovery phases during this phase of the solar cycle are also quite different
in that they can last from many days to weeks. The southward magnetic field (Bs) component of Alfvén waves in the high speed stream proper cause intermittent reconnection, intermittent substorm activity,
and sporadic injections of plasma sheet energy into the outer portion of the ring current, prolonging its final decay to quiet
day values. This continuous auroral activity is called High Intensity Long Duration Continuous AE Activity (HILDCAAs).
Possible interplanetary mechanisms for the creation of very intense magnetic storms are discussed. We examine the effects
of a combination of a long-duration southward sheath magnetic field, followed by a magnetic cloud Bs event. We also consider the effects of interplanetary shock events on the sheath plasma. Examination of profiles of very
intense storms from 1957 to the present indicate that double, and sometimes triple, IMF Bs events are important causes of such events. We also discuss evidence that magnetic clouds with very intense core magnetic
fields tend to have large velocities, thus implying large amplitude interplanetary electric fields that can drive very intense
storms. Finally, we argue that a combination of complex interplanetary structures, involving in rare occasions the interplanetary
manifestations of subsequent CMEs, can lead to extremely intense storms.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
997.
From magnetic fields and coronal heating observed in flares, active regions, quiet regions, and coronal holes, we propose
that exploding sheared core magnetic fields are the drivers of most of the dynamics and heating of the solar atmosphere, ranging
from the largest and most powerful coronal mass ejections and flares, to the vigorous microflaring and coronal heating in
active regions, to a multitude of fine-scale explosive events in the magnetic network, driving microflares, spicules, global
coronal heating, and, consequently, the solar wind.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
998.
K. Miyamoto M. Oka R. Yamamoto Y. Masuda T. Hoson S. Kamisaka J. Ueda 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1999,23(12):2033-2036
Activity of auxin polar transport in inflorescence axes of Arabidopsis thaliana grown under simulated microgravity conditions was studied in relation to the growth and development. Seeds were germinated and allowed to grow on an agar medium in test tubes on a horizontal clinostat. Horizontal clinostat rotation substantially reduced the growth of inflorescence axes and the productivity of seeds of Arabidopsis thaliana (ecotypes Landsberg erecta and Columbia), although it little affected seed germination, development of rosette leaves and flowering. The activity of auxin polar transport in inflorescence axes decreased when Arabidopsis plants were grown on a horizontal clinostat from germination stage, being ca. 60% of 1 g control. On the other hand, the auxin polar transport in inflorescence axes of Arabidopsis grown in 1 g conditions was not affected when the segments were exposed to various gravistimuli, including 3-dimensional clinorotation, during transport experiments. Pin-formed mutant of Arabidopsis, having a unique structure of the inflorescence axis with no flower and extremely low levels of the activity of auxin polar transport in inflorescence axes and endogenous auxin, did not continue its vegetative growth under clinostat rotation. These facts suggest that the development of the system of auxin polar transport in Arabidopsis is affected by microgravity, resulting in the inhibition of growth and development, especially during reproductive growth. 相似文献
999.
Stone E.C. Cohen C.M.S. Cook W.R. Cummings A.C. Gauld B. Kecman B. Leske R.A. Mewaldt R.A. Thayer M.R. Dougherty B.L. Grumm R.L. Milliken B.D. Radocinski R.G. Wiedenbeck M.E. Christian E.R. Shuman S. von Rosenvinge T.T. 《Space Science Reviews》1998,86(1-4):357-408
The Solar Isotope Spectrometer (SIS), one of nine instruments on the Advanced Composition Explorer (ACE), is designed to provide
high- resolution measurements of the isotopic composition of energetic nuclei from He to Zn (Z=2 to 30) over the energy range
from ∼10 to ∼100 MeV nucl−1. During large solar events SIS will measure the isotopic abundances of solar energetic particles
to determine directly the composition of the solar corona and to study particle acceleration processes. During solar quiet
times SIS will measure the isotopes of low-energy cosmic rays from the Galaxy and isotopes of the anomalous cosmic-ray component,
which originates in the nearby interstellar medium. SIS has two telescopes composed of silicon solid-state detectors that
provide measurements of the nuclear charge, mass, and kinetic energy of incident nuclei. Within each telescope, particle trajectories
are measured with a pair of two-dimensional silicon-strip detectors instrumented with custom, very large-scale integrated
(VLSI) electronics to provide both position and energy-loss measurements. SIS was especially designed to achieve excellent
mass resolution under the extreme, high flux conditions encountered in large solar particle events. It provides a geometry
factor of ∼40 cm2 sr, significantly greater than earlier solar particle isotope spectrometers. A microprocessor controls the
instrument operation, sorts events into prioritized buffers on the basis of their charge, range, angle of incidence, and quality
of trajectory determination, and formats data for readout by the spacecraft. This paper describes the design and operation
of SIS and the scientific objectives that the instrument will address.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
1000.
Wong K.T. Zoltowski M.D. 《IEEE transactions on aerospace and electronic systems》1998,34(4):1320-1336
A novel sparse array geometry embedding two sizes of spatial invariances is presented for use with a new ESPRIT-based (estimation of signal parameters via rotational invariance techniques) algorithm for aperture extension. The half-wavelength invariance yields unambiguous but high-variance direction cosine estimates to disambiguate low-variance but cyclically ambiguous estimates from the larger invariance. With larger invariance at 60 half-wavelengths, resolution threshold for two closely spaced emitters is reduced by 50 dB and estimation error by 100-fold. Array design formulas are also presented 相似文献