全文获取类型
收费全文 | 6011篇 |
免费 | 28篇 |
国内免费 | 17篇 |
专业分类
航空 | 2790篇 |
航天技术 | 2119篇 |
综合类 | 185篇 |
航天 | 962篇 |
出版年
2021年 | 43篇 |
2019年 | 39篇 |
2018年 | 99篇 |
2017年 | 72篇 |
2016年 | 67篇 |
2015年 | 37篇 |
2014年 | 132篇 |
2013年 | 161篇 |
2012年 | 145篇 |
2011年 | 215篇 |
2010年 | 135篇 |
2009年 | 245篇 |
2008年 | 282篇 |
2007年 | 167篇 |
2006年 | 136篇 |
2005年 | 163篇 |
2004年 | 156篇 |
2003年 | 181篇 |
2002年 | 217篇 |
2001年 | 256篇 |
2000年 | 107篇 |
1999年 | 142篇 |
1998年 | 163篇 |
1997年 | 124篇 |
1996年 | 178篇 |
1995年 | 198篇 |
1994年 | 172篇 |
1993年 | 85篇 |
1992年 | 129篇 |
1991年 | 61篇 |
1990年 | 58篇 |
1989年 | 132篇 |
1988年 | 52篇 |
1987年 | 46篇 |
1986年 | 60篇 |
1985年 | 185篇 |
1984年 | 175篇 |
1983年 | 113篇 |
1982年 | 134篇 |
1981年 | 187篇 |
1980年 | 42篇 |
1979年 | 40篇 |
1978年 | 48篇 |
1977年 | 45篇 |
1975年 | 47篇 |
1974年 | 48篇 |
1972年 | 39篇 |
1971年 | 37篇 |
1970年 | 38篇 |
1969年 | 42篇 |
排序方式: 共有6056条查询结果,搜索用时 656 毫秒
641.
642.
This presents recent progress of the state-of-the-art of space-borne radar technology in case of either earth-orbiting or planetary-orbiting satellites and space probes, respectively. In addition to that, recent progress is also discussed concerning specific inversion techniques to evaluate radar measurements; i.e., the art of deriving the relevant physical quantities to be determined such as terrain and depth profiles for planetary surface and/or subsurface structures ((3D)-profiles) from radar data measured depending on signal frequency, aspect angle, polarization, etc., as a function of time. 相似文献
643.
This paper describes some new concepts in dealing with the circuitry for Loran-C automatic timing systems. The conventional analog techniques associated with phase-adjusting networks have been replaced by an incremental digital phase-shifting device. The Loran-C period generator includes facilities for delay compensation by means of an epoch monitor producing a 1-Hz output coincident with the master station TOC (time of coincidence). The required initial time information has to be accurate within ± 20 ms. The automatic format identification and decoding equipment together form a system which takes into account the information of every Loran-C pulse. Owing to the use of digital signal treatment, the synchronization accuracy is limited only by the resolution of the incremental phase shift. The automatic cycle selection device is based on sampling techniques where the derivative of the envelope is calculated. The time of coincidence has to be precalculated and fed into the thumbwheel memory of the epoch monitor, which is automatically initiated when the synchronizing operations are concluded. For VLBI purposes and transcontinental use, the accuracy of this system will be better than 1 ?s when post corrections, supplied by the U.S. Naval Observatory, are taken into account. 相似文献
644.
Conte E. Lops M. Ricci G. 《IEEE transactions on aerospace and electronic systems》1998,34(4):1058-1069
Radar detection of coherent pulse trains embedded in compound-Gaussian disturbance with partially known statistics is discussed. We first give a thorough derivation of two recently proposed adaptive detection structures. Next, we derive a different detection scheme exploiting the assumption that the clutter is wide-sense stationary. Resorting to the theory of circulant matrices, in fact, we demonstrate that the estimation of the structure of the clutter covariance matrix can be reduced to the estimation of its eigenvalues, which in turn can be (efficiently) done via fast Fourier transform codes. After a thorough performance assessment, mostly carried on via computer simulations, the results show that the newly proposed detector achieves better performance than the two previously introduced adaptive detectors. Moreover, a sensitivity analysis shows that, even though this detector does not strictly guarantee the constant false alarm rate property with respect to the clutter covariance matrix, it is robust, in the sense that its performance is only slightly affected by variations in the clutter temporal correlation 相似文献
645.
Much of the mass of a battery is comprised of nonreactive materials. In an NiH2 battery, this includes the pressure vessel and 50% of the positive electrode. PowerCore reconfigures the battery materials to serve as a structural sandwich panel. The effective specific energy of the new device can exceed 100 Wh/kg. PowerCore is intended to handle power demands of low Earth orbiting communications satellites such as IRIDIUM. This paper describes the concept and development progress 相似文献
646.
R.C Miller S.G Martin W.R Hanson S.A Marino E.J Hall 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1998,22(12):1719-1723
The oncogenic potential of high-energy 56Fe particles (1 GeV/nucleon) accelerated with the Alternating Gradient Synchrotron at the Brookhaven National Laboratory was examined utilizing the mouse C3H 10T1/2 cell model. The dose-averaged LET for high-energy 56Fe is estimated to be 143 keV/μm with the exposure conditions used in this study. For 56Fe ions, the maximum relative biological effectiveness (RBEmax) values for cell survival and oncogenic transformation were 7.71 and 16.5 respectively. Compared to 150 keV/μm 4He nuclei, high-energy 56Fe nuclei were significantly less effective in cell killing and oncogenic induction. The prostaglandin E1 analog misoprostol, an effective oncoprotector of C3H 10T1/2 cells exposed to X rays, was evaluated for its potential as a radioprotector of oncogenic transformation with high-energy 56Fe. Exposure of cells to misoprostol did not alter 56Fe cytotoxicity or the rate of 56Fe-induced oncogenic transformation. 相似文献
647.
Clinorotation reduces number, but not size, of cartilaginous nodules formed in micromass cultures of mouse limbbud cells. 总被引:1,自引:0,他引:1
P J Duke D Montufar-Solis T Hamazaki A Sato 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1998,21(8-9):1065-1072
In previous studies we used a ground based model to investigate the cellular responses to microgravity by exposing micromass cultures of embryonic limb cells to simulated weightlessness on a clinostat. Cultures set up in T-flasks and rotated at 30 rpm showed that clinostatted cultures had less chondrocyte differentiation than stationary or rotation controls, as assessed by number of nodules/culture stained with cartilage specific Alcian blue. In the current study, nodule size and shape of these nodules was assessed by interactive measurement of area, perimeter, circularity, and equivalent diameters, using the Optimas imaging software. Results show no significant difference in any of the measurements, indicating that clinorotation has no effect on expansion of the nodules either by differentiation of cells within the nodule, or by recruitment of cells into the nodule. The reduction in number of nodules without an alteration in size and shape indicates that the effect of simulated microgravity is to reduce the cell interactions required for the initial condensation of cells into a nodule, probably by interference with cell adhesion molecules. 相似文献
648.
H.J. Völk E.G. Berezhko L.T. Ksenofontov 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2008,41(3):473-476
For the case of Tycho’s supernova remnant (SNR), we present the relation between the blast wave and contact discontinuity radii calculated within the nonlinear kinetic theory of cosmic ray (CR) acceleration in SNRs. It is demonstrated that these radii are confirmed by recently published Chandra measurements which show that the observed contact discontinuity radius is very close to the shock radius. Therefore a consistent explanation of these observations can be given in terms of efficient CR acceleration which makes the medium more compressible. 相似文献
649.
E J Ainsworth S M Afzal D A Crouse W R Hanson R J Fry 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1989,9(10):299-313
Early and late murine tissue responses to single or fractionated low doses of heavy charged particles, fission-spectrum neutrons or gamma rays are considered. Damage to the hematopoietic system is emphasized, but results on acute lethality, host response to challenge with transplanted leukemia cells and life-shortening are presented. Low dose rates per fraction were used in some neutron experiments. Split-dose lethality studies (LD 50/30) with fission neutrons indicated greater accumulation of injury during a 9 fraction course (over 17 days) than was the case for gamma-radiation. When total doses of 96 or 247 cGy of neutrons or gamma rays were given as a single dose or in 9 fractions, a significant sparing effect on femur CFU-S depression was observed for both radiation qualities during the first 11 days, but there was not an earlier return to normal with dose fractionation. During the 9 fraction sequence, a significant sparing effect of low dose rate on CFU-S depression was observed in both neutron and gamma-irradiated mice. CFU-S content at the end of the fractionation sequence did not correlate with measured LD 50/30. Sustained depression of femur and spleen CFU-S and a significant thrombocytopenia were observed when a total neutron dose of 240 cGy was given in 72 fractions over 24 weeks at low dose rates. The temporal aspects of CFU-S repopulation were different after a single versus fractionated neutron doses. The sustained reduction in the size of the CFU-S population was accompanied by an increase in the fraction in DNA synthesis. The proliferation characteristics and effects of age were different for radial CFU-S population closely associated with bone, compared with the axial population that can be readily aspirated from the femur. In aged irradiated animals, the CFU-S proliferation/redistribution response to typhoid vaccine showed both an age and radiation effect. After high single doses of neutrons or gamma rays, a significant age- and radiation-related deficiency in host defense mechanisms was detected by a shorter mean survival time following challenge with transplantable leukemia cells. Comparison of dose-response curves for life shortening after irradiation with fission-spectrum neutrons or high energy silicon particles indicated high initial slopes for both radiation qualities at low doses, but for higher doses of silicon, the effect per Gy decreased to a value similar to that for gamma rays. The two component life-shortening curve for silicon particles has implications for the potential efficacy of radioprotectants. Recent studies on protection against early and late effects by aminothiols, prostaglandins, and other compounds are discussed. 相似文献
650.
The Theory and experimental results of a simple optical correlator are given. The optical components consist only of an incoherent source, condensing, collimating, and transforming lenses, and a photodetector. For real-time operation, a reference signal on a photographic transparency is correlated with an unknown signal on an ultrasonic light modulator. The output is a function of time proportional to the correlation function's amplitude rather than the squared amplitude generated by other optical correlators. When used for correlation in radar signal processing, the optical processor may be used for both pulse coding and decoding (pulse expansion and compression) and the decoded signal is shown to be described by Woodward's ambiguity function. The experimental results demonstrated the validity of the theory in both real and scaled time. However, developmental work is still required to improve the operation. 相似文献