全文获取类型
收费全文 | 7749篇 |
免费 | 13篇 |
国内免费 | 33篇 |
专业分类
航空 | 3397篇 |
航天技术 | 2774篇 |
综合类 | 32篇 |
航天 | 1592篇 |
出版年
2021年 | 77篇 |
2019年 | 47篇 |
2018年 | 202篇 |
2017年 | 146篇 |
2016年 | 142篇 |
2015年 | 55篇 |
2014年 | 200篇 |
2013年 | 246篇 |
2012年 | 228篇 |
2011年 | 327篇 |
2010年 | 235篇 |
2009年 | 395篇 |
2008年 | 429篇 |
2007年 | 247篇 |
2006年 | 183篇 |
2005年 | 232篇 |
2004年 | 226篇 |
2003年 | 262篇 |
2002年 | 176篇 |
2001年 | 260篇 |
2000年 | 149篇 |
1999年 | 176篇 |
1998年 | 195篇 |
1997年 | 156篇 |
1996年 | 181篇 |
1995年 | 242篇 |
1994年 | 216篇 |
1993年 | 108篇 |
1992年 | 170篇 |
1991年 | 58篇 |
1990年 | 65篇 |
1989年 | 154篇 |
1988年 | 49篇 |
1987年 | 51篇 |
1986年 | 68篇 |
1985年 | 216篇 |
1984年 | 171篇 |
1983年 | 134篇 |
1982年 | 156篇 |
1981年 | 214篇 |
1980年 | 62篇 |
1979年 | 44篇 |
1978年 | 47篇 |
1977年 | 45篇 |
1976年 | 39篇 |
1975年 | 37篇 |
1974年 | 42篇 |
1972年 | 33篇 |
1971年 | 30篇 |
1970年 | 32篇 |
排序方式: 共有7795条查询结果,搜索用时 203 毫秒
771.
T. Kohno K. Munakata T. Imai M. Matsuoka 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1989,9(12):75-78
A computer simulation was carried out to evaluate the basic characteristics of a Δ E×E cosmic ray telescope consisting of 23 solid state detectors including 3 position sensitive detectors with large effective area. Based on the simulation, the geometric factor of the telescope is deduced to be as large as 22.5 cm2sr, almost independent of charge and energy concerned. The energy ranges to be covered by the telescope are, for example, 18–98 MeV/n for Li and 56–339 MeV/n for Fe. By analyzing simulated data, the mass resolution for iron in the overall energy range covered by the telescope is estimated as about 0.22 amu in standard deviation. The expected counting rates and mass-histograms are simulated for Galactic cosmic rays and solar energetic particles. 相似文献
772.
E M Nedukha 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1989,9(11):83-86
The pyroantimonate method was used to study the localization of free and weakly bound calcium in cells of moss protonema of Funaria hygrometrica Hedw. cultivated on a clinostat (2rev/min). Electroncytochemical study of control cells cultivated at 1 g revealed that granular precipitate marked chloroplasts, mitochondria, Golgi apparatus, lipid drops, nucleoplasma, nucleolus, nucleus membranes, cell walls and endoplasmic reticulum. In mitochondria the precipitate was revealed in stroma, in chloroplast it was found on thylakoids and envelope membranes. The cultivation of protonema on clinostat led to the intensification in cytochemical reaction product deposit. A considerable intensification of the reaction was noted in endomembranes, vacuoles, periplasmic space and cell walls. At the same time analysis of pectinase localization was made using the electroncytochemical method. A high reaction intensity in walls in comparison to that in control was found out to be a distinctive peculiarity of the cells cultivated on clinostat. It testifies to the fact that increasing of free calcium concentrations under conditions of clinostation is connected with pectinic substances hydrolysis and breaking of methoxy groups of pectins. Data obtained are discussed in relation to problems of possible mechanisms of disturbance in calcium balance of plant cells and the role of cell walls in gomeostasis of cell grown under conditions of simulated weightlessness. 相似文献
773.
V.E. Zuev S.I. Kavkyanov G.M. Krekov 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1985,5(6):81-85
A single-frequency lidar, using aerosol scattering as an informative component, is the simplest and reliable facility for remote sensing of the atmosphere. The information on vertical distribution of atmospheric aerosol which can be obtained using such a lidar is necessary for investigating the physics of atmospheric processes and forecast of optical state of the atmosphere. At the same time, the interpretation of data on single-frequency sounding is associated with some difficulties of fundamental character, mainly due to insufficient software of the experiment. Under such conditions the problems of optimal processing of lidar returns aiming at extracting the useful information on aerosol are of great importance, especially if one takes into account the hindering effects of atmospheric background and optical noises. This paper presents a statistical approach to this problem, and the possibilities of single-frequency sounding from space are analyzed. 相似文献
774.
VXI has matured into a flexible bus architecture with which to develop various types of instrumentation systems. This paper details the multi-computer approach used in the VXI-based system that CACI developed for the Air Force. The system, the Engine Test/Trim Automated System II (ETTAS II) is designed to test: all Air Force jet engines. The paper discusses how to integrate multiple computers in a VXI-based system, including discussions on: setting up the computers; selecting register-based versus message-based computers; setting up and using shared-memory; defining and separating tasks for each computer. The shared memory discussion talks about different ways to structure the shared memory, including setting up a system-level “Current-Value Table” (CVT) for all instruments, as well as how other devices, including another computer can access the shared memory space. The paper shows how Commercial-Off-the-Shelf (COTS) software products NI-VXI, LabVIEW, and NI-VISA (National Instruments) can be used to satisfy all these requirements. The paper shows how the multi-computer approach can be cost-effective in many cases 相似文献
775.
G.J. Nagy D.N. Severov V.A. Pshennikov M. De los Santos J.J. Lagomarsino K. Sans E.G. Morozov 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2008,41(11):1876-1881
This paper deals with the application of SeaWIFS images to characterize spatial and temporal variability of fronts in the Rio de la Plata estuarine system over the period 2000–2003. We aim to depict the relationship between river outflow and variability of fronts’ loci on monthly to ENSO-related timescales and the influence of the winds along Rio de la Plata (axial winds) on the abrupt changes in frontal dynamics over synoptic timescales. During the studied period both La Niña (July 1999–June 2000) and El Niño (April 2002–May 2003) events induced significant displacements of fronts. Three distinct fronts were analyzed between river, estuarine, coastal and marine waters of the Rio de la Plata: Main Turbidity Front, Main Marine Front, and Secondary Marine Front. Their number, location and separation seem to be mainly related to river outflow and second, to fresh (>8 m/s) axial winds. During low discharge periods (i.e. summer time and/or La Niña events) these winds induce abrupt changes in the location of fronts (100–200 km) and greater separation between them over synoptic timescale, whereas during high river discharge or ENSO years some of the variability of fronts location is explained by the river’s outflow fluctuations, especially by the high variability of the River Uruguay discharge. 相似文献
776.
The Hubble space-based telescope is a great tribute to our progress in space. The ability to place an optical telescope at a significant distance from the Earth's surface, away from the interference of the planet's unsteady atmosphere, have already paid off by producing magnificent records of astronomical activities in the depths of outer space. In the past the problems with the alignment of the Hubble's optics were blamed on the manufacturers of it's optical components. The hastily set investigation concluded that the problem is a spherical aberration of the primary mirror (the primary mirror is said to be 2 microns too flat at the edges). It is suggested that the real culprit is the Parker Effect. Since the time of Galileo Galilei, all telescopes were built, aligned, and used on the Earth's surface. Hubble is the first telescope to be built and aligned on Earth for use in space. Because of this we have to consider the fundamental differences between the alignment of surface-based and space-based telescopes. For those who missed our article “The Parker Effect and Navigation in Space” published in the January issue. The Parker Effect describes the result of interaction between inertial bodies (anything that has mass) and non-inertial media (light or other E/M fields) 相似文献
777.
Estimates of drag characteristics of the space vehicles with orbit heights of 450–540 and 700–900 km before and after strong
(with a magnitude M ≥ 6.5) crust earthquakes of 2000–2006 are presented. The method of estimation of seismic orbital effects is presented using
as an example the small Mozhaets-4 spacecraft. Two weeks prior to earthquakes, variations in the drag of low-orbital spacecraft increase. 3–6 days prior to
strong crust earthquakes with epicenters on the land, the drag of low-orbit spacecraft in the upper atmosphere increases.
The effect of increased viscosity of the neutral component of the atmosphere at spacecraft heights 3–6 days prior to strong
crust earthquakes is consistent with the results of studies of disturbances in the ionization density variations in the ionospheric
F region prior to earthquakes. No anomalies are found in the day of the earthquake. In the future, it is proposed to use elements
of space debris for diagnostics of seismic orbital effects and disturbances of the upper atmosphere. 相似文献
778.
M.B. Moldwin D. Fiello E. Harter G. Holman N. Nagumo A. Pryharski C. Takunaga 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2008
An elementary science education professional development partnership between Culver City Unified School District teachers and UCLA has been formed. The project was designed to assist teachers to comfortably present introductory space science concepts, to support them in their efforts, and to aid them in encouraging their students to develop inquiry skills related to space sciences. The project encourages teacher use of observational science techniques in their classrooms, the use of NASA solar mission images and enhanced use of astronomical observation to facilitate discovery learning. The integrated approach of the project has fostered collegial learning activities among the participating teachers and offered them opportunities for continued renewal and professional development of teacher competencies in astronomy and space science. The activities used in the classroom were developed by others, classroom tested, and specifically address National Science Education and California Science Content Standards. These activities have been sustained through on-going collaboration between the scientist and the teachers, a summer Research Experience for Teachers program, and on-going, grade-specific, district-sponsored workshops. Assessment of the value of the program is done by the school district and is used to continuously improve each workshop and program component. Culver City (California) Unified School District is a small urban school district located on the Westside of Los Angeles. This paper describes the program and the plans for incorporating IHY-themed science into the classroom. 相似文献
779.
Algorithms for determining fixed-pitch propeller parameters and some flight characteristics of a superlight aircraft at the early stage of designing are described. 相似文献
780.
Rice M. Oliphant T. Haddadin O. McIntire W. 《IEEE transactions on aerospace and electronic systems》2007,43(4):1484-1495
This paper describes data-aided signal level and noise variance estimators for Gaussian minimum shift keying (GMSK) when the observations are limited to the output of a filter matched to the first pulse-amplitude modulation (PAM) pulse in the equivalent PAM representation. The estimators are based on the maximum likelihood (ML) principle and assume burst-mode transmission with known timing and a block of L0 known bits. While it is well known that ML estimators are asymptotically unbiased and efficient, the analysis quantifies the rate at which the estimators approach these asymptotic properties. It is shown that the carrier phase, amplitude, and noise variance estimators are unbiased and can achieve their corresponding Cramer-Rao bounds with modest combinations of signal-to-noise ratio and observation length. The estimates are used to estimate the signal-to-noise ratio. It is shown that the mean squared error performance of the ratio increases with signal-to-noise ratio while the mean squared error performance of the ratio in decibels decreases with signal-to-noise ratio. Simulation results are provided to confirm the accuracy of the analytic results. 相似文献