全文获取类型
收费全文 | 3269篇 |
免费 | 23篇 |
国内免费 | 13篇 |
专业分类
航空 | 1580篇 |
航天技术 | 1147篇 |
综合类 | 4篇 |
航天 | 574篇 |
出版年
2021年 | 26篇 |
2019年 | 23篇 |
2018年 | 54篇 |
2017年 | 51篇 |
2016年 | 45篇 |
2015年 | 26篇 |
2014年 | 72篇 |
2013年 | 92篇 |
2012年 | 83篇 |
2011年 | 121篇 |
2010年 | 80篇 |
2009年 | 137篇 |
2008年 | 156篇 |
2007年 | 100篇 |
2006年 | 72篇 |
2005年 | 92篇 |
2004年 | 90篇 |
2003年 | 100篇 |
2002年 | 72篇 |
2001年 | 115篇 |
2000年 | 65篇 |
1999年 | 68篇 |
1998年 | 87篇 |
1997年 | 59篇 |
1996年 | 94篇 |
1995年 | 112篇 |
1994年 | 111篇 |
1993年 | 50篇 |
1992年 | 71篇 |
1991年 | 34篇 |
1990年 | 32篇 |
1989年 | 76篇 |
1988年 | 28篇 |
1987年 | 25篇 |
1986年 | 33篇 |
1985年 | 90篇 |
1984年 | 90篇 |
1983年 | 65篇 |
1982年 | 66篇 |
1981年 | 99篇 |
1980年 | 19篇 |
1979年 | 25篇 |
1978年 | 29篇 |
1977年 | 27篇 |
1975年 | 29篇 |
1974年 | 23篇 |
1973年 | 21篇 |
1972年 | 20篇 |
1969年 | 20篇 |
1968年 | 18篇 |
排序方式: 共有3305条查询结果,搜索用时 31 毫秒
501.
We review the X- and gamma-ray observations of Cygnus X-1 and their theoretical interpretations, with emphasis on new developments since the mid-1970's. The overall data base at present is most consistent with the inverse Compton model by hot thermal electrons of T
e 109 K, for the hard X-ray luminosity (10–200 keV). However, the origin of the soft X-rays ( 10 keV) in high states and gamma rays (> 200 keV) remain unsettled.Operated under DOE Contract W-7405-Eng-48.Partially supported by NASA Grant NGR 05-020-668.NRC/NRL Research Associate. 相似文献
502.
E. Costa M. Salvati G. Agnetta R. Di Raffaele G. Gerardi B. Sacco L. Scarsi E. Massaro B. Agrinier J.C. Christi J.L. Masnou A. Bui-Van J.P. Chabaud P. Frabel P. Mandrou J. Narbonne M. Niel G. Rouaix G. Vedrenne 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1983,3(4):113-116
We present a large area, balloon borne, NaI(Tl) detector for low-energy gamma rays with temporal signature : FIGARO.The main detector is a mosaic of 12 NaI(Tl) tiles 22.5 × 15 × 5 cm, for a total geometric area of 4050 cm2.In the energy band 140 keV - 6 MeV, the expected background counting rate at float altitude is in the range of two to three thousands counts per second.For pulsar analysis the expected 3δ sensitivity for 5 hours exposition time is 2.5 10?4 ph/cm2.s.MeV (150–500 keV) 1.5 10?4 ph/cm2.s.MeV (1–6 MeV). This performance, together with the large effective area and the relatively short duration of a balloon flight, make FIGARO particularly suitable for the identification of sources by means of temporal analysis.For objectives in the Northern sky, including the Crab pulsar, a transmediterranean flight is planned for the summer of 1982 ; a Southern mission is scheduled in Brazil for the fall of 1983 (Vela, PSR 1822-09). 相似文献
503.
E E Maksimova L Y Popova 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2001,27(9):1581-1586
Modeling of transgenic microorganism introduction into small man-made ecosystems can help forecast changes in expression of cloned genes under different conditions of existence. Introduction of the E. coli Z905/pPHL7 strain containing a plasmid with luminescent system genes of luminous bacteria led to changes in cell and colony morphology, reduction in metabolic activity of cells, and, as a result, a lower level of expression of cloned gene. A low concentration of nutrients has been shown to favor greatly the phenotypic change of cells of the recombinant strain. Expression of cloned genes changed due to: a lower concentration of plasmid DNA, a change in regulation of cloned genes, and a change in cells of biosynthesis of substrates needed for expression of luminescent genes. The conducted investigations can provide a basis for the use of marker transgenic microorganisms in closed ecosystems of different types. Grant numbers: 99-04-96017, 00-07-9011. 相似文献
504.
E.M. Nedukha 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1984,4(12):19-22
Changes in the ultrastructure of protonema cells of Funaria hygrometrica, cultivated during 20 days on a horizontal clinostat (2 rev/min), were determined by the electron microscopy method. About 20% of the cells were almost identical to those in the control, 20% were destructive cells, and in 60% ultrastructure changes were observed. The heterogeneity of the reaction demonstrated the evidence of sensitive cells on the clinostation process. Changes affected the ultrastructure of plastids, wall of the cell, and the form of the nucleus as well. Starch disappeared from chloroplasts practically completely, thylakoids swelled, granas frequently disappeared from plastids. Peroxisomes number in cells increased appreciably, width of cell walls decreased by almost half their size. Ca++-binding sites were revealed in cytoplasma of cells. Electronocytochemical exposure of ATPases activity with the presence of Mg++ and Ca++ ions showed that Mg2+-ATPase activity localization in clinosted cells was not too different from the control, while Ca2+-ATPase location presented differences in plasmalemma and Ca-sites. These changes are perhaps connected with the membranes permeability breaking and affect the plant cells adaptation to the influence of hypogravitation. 相似文献
505.
Attitude and Oribit Estimation Using Stars and Landmarks 总被引:2,自引:0,他引:2
White R.L. Adams M.B. Geisler E.G. Grant F.D. 《IEEE transactions on aerospace and electronic systems》1975,(2):195-203
An extended Kalman filter is used to process line-of-sight measurements to stars and known landmarks providing a statistical indication of performance in estimating spacecraft attitude, orbital ephemeris, and the bias drift of a set of three strapdown gyros. The landmark measurements were assumed to have been taken from the imagery of an Earth-observing multispectral scanner. It is shown that filtering of these noisy measurements results in highly accurate estimates of the above parameters. Results are given showing the sensitivity of performance to various system parameters such as star tracker accuracy, errors in the knowledge of landmark position, and number of stars and landmarks processed. 相似文献
506.
Global positioning by means of satellites requires simultaneous observation by at least four satellites. The problem is to determine the minimum number of satellites and the corresponding orbital geometry necessary to satisfy this requirement on a continuous basis. To model the problem, a fixed number of users are assumed uniformly distributed in a known manner over the surface of the earth, and the satellites are restricted to exist in either three or four orbital planes. However, the orbit radius and inclination angle are left as variables. Under these assumptions, and starting with a small number of satellites which will be increased afterwards, an algorithm is developed to determine the visibility of satellites at each surface location. In this way it is possible to specify the minimum number of satellites needed by any desired orbital geometry. It is found that the number of satellites required for three-dimensional continuous worldwide coverage decreases as the orbit radius is increased. There appears to be no general trend regarding the effect of the inclination angle on the minimum number of satellites. 相似文献
507.
E.E. Fenimore R.W. Klebesadel J.G. Laros 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1983,3(4):207-210
Spectral measurements by the Solar Maximum Mission have been used to confirm the cyclotron lines in gamma-ray bursts reported from the Konus experiment. We present ISEE-3 data for the same burst (GB800419) during the same period of time which show no line. We discuss various problems in the analysis of scintillator spectra and point out that unfolded spectra are not necessarily unique and that the position of a data point in a deconvolved spectrum may vary depending on the assumed overall shape of the spectrum. As a result, if the analysis assumes a soft spectrum (such as optically thin thermal bremsstrahlung) an absorption feature might appear, whereas a harder spectrum (such as a Comptonized blackbody) would not require the feature. Since the continuum shape probably changes during the duration of a typical burst, the nonuniqueness of the spectral unfolding, combined with the assumption that the continuum is optically thin thermal bremsstrahlung, could give rise to spurious “absorption” features which vary on a time scale of seconds. Despite these problems, there is still some evidence for spectral lines in the range 45 to 65 keV but not for the broad lines reported from the Konus experiment. Unfortunately, the range 45 to 65 keV is the most difficult spectral region to unfold. 相似文献
508.
Registration by integral-square error correlation of one-dimensional discrete-time waveforms which are treated as random processes with specified autocorrelation functions is considered. An important design parameter for this class of problems is the probability of anomaly (a false dip in the correlation function) because it gives an indication of system immunity to gross registration errors. Explicit expressions for this parameter are not possible, so bounds and appoximations must be derived. Two upper bounds and an approximation for the probability of anomaly are derived here. The use of these expressions is illustrated by an example. The relative utility of these performance indicators is shown for the example by comparison with actual values of the probability of anomaly obtained by computer simulation. 相似文献
509.
Many simple radars use fact-risetime pulses, but wide bandwidth does not translate into corresponding high resolution since the spectrum is far from flat. A scheme for enhancing down- and cross-range resolution of multiple targets through a two-step partial equalization of the spectrum is illustrated by detailed computer simulation, with emphasis on the tradeoffs between resolution and signal-to-noise ratios. Three examples are treated: two equal scattering centers, two unequal scattering centers, and compound target 相似文献
510.
Incremental planning to control time-constrained blackboard-basedproblem solver [vehicle monitoring]
A planner that improves the control decisions made by a blackboard-based problem solver is described. This planner interleaves planning and execution. It sketches out major plan steps but incrementally details actions for only the next step since earlier actions influence how (and whether) to pursue a plan. The planner also reasons about time constraints, revising plans to meet problem-solving deadlines and deciding when it has done enough work on a problem. Implementation of these mechanisms is outlined, and experiments are discussed showing how they reduce overall computation and improve problem solving 相似文献