全文获取类型
收费全文 | 7210篇 |
免费 | 25篇 |
国内免费 | 16篇 |
专业分类
航空 | 3690篇 |
航天技术 | 2358篇 |
综合类 | 193篇 |
航天 | 1010篇 |
出版年
2021年 | 55篇 |
2019年 | 45篇 |
2018年 | 114篇 |
2017年 | 65篇 |
2016年 | 68篇 |
2014年 | 121篇 |
2013年 | 148篇 |
2012年 | 162篇 |
2011年 | 241篇 |
2010年 | 164篇 |
2009年 | 271篇 |
2008年 | 310篇 |
2007年 | 171篇 |
2006年 | 140篇 |
2005年 | 156篇 |
2004年 | 170篇 |
2003年 | 215篇 |
2002年 | 229篇 |
2001年 | 287篇 |
2000年 | 146篇 |
1999年 | 179篇 |
1998年 | 221篇 |
1997年 | 152篇 |
1996年 | 203篇 |
1995年 | 243篇 |
1994年 | 219篇 |
1993年 | 133篇 |
1992年 | 174篇 |
1991年 | 85篇 |
1990年 | 90篇 |
1989年 | 182篇 |
1988年 | 90篇 |
1987年 | 84篇 |
1986年 | 77篇 |
1985年 | 219篇 |
1984年 | 186篇 |
1983年 | 162篇 |
1982年 | 175篇 |
1981年 | 206篇 |
1980年 | 79篇 |
1979年 | 68篇 |
1978年 | 73篇 |
1977年 | 56篇 |
1976年 | 53篇 |
1975年 | 75篇 |
1974年 | 61篇 |
1972年 | 77篇 |
1971年 | 59篇 |
1970年 | 41篇 |
1969年 | 45篇 |
排序方式: 共有7251条查询结果,搜索用时 31 毫秒
991.
Adaptive pulse compression via MMSE estimation 总被引:2,自引:0,他引:2
Radar pulse compression involves the extraction of an estimate of the range profile illuminated by a radar in the presence of noise. A problem inherent to pulse compression is the masking of small targets by large nearby targets due to the range sidelobes that result from standard matched filtering. This paper presents a new approach based upon a minimum mean-square error (MMSE) formulation in which the pulse compression filter for each individual range cell is adaptively estimated from the received signal in order to mitigate the masking interference resulting from matched filtering in the vicinity of large targets. The proposed method is compared with the standard matched filter and least-squares (LS) estimation and is shown to be superior over a variety of stressing scenarios. 相似文献
992.
N.M. Ashok R.K. Manchanda D.P.K. Banerjee S. Farrell R.K. Sood 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2006,38(12):2777-2778
We present near-infrared spectroscopy of the massive X-ray binary 2S 0114+650. These observations covering the spectral range 1.08–2.35 μm span the region where Paschen and Brackett series recombination lines of hydrogen are expected to be seen, namely, Paβ , Brγ and Br 10–17 lines. The absence of any of these lines in emission supports earlier inferences that the optical component in 2S 0114+650 is unlikely to be a Be star but rather a B type supergiant. Near-IR photometry gives J = 8.78, H = 8.53 and K = 7.96; these values show marginal variations from earlier reported measurements. 相似文献
993.
T. Muoz-Darias I.G. Martínez-Pais J. Casares T.R. Marsh R. Cornelisse D. Steeghs V.S. Dhillon P.A. Charles 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2006,38(12):2762-2764
We present preliminary results of a simultaneous X-ray/optical campaign of the prototypical LMXB Sco X-1 at 1–10 Hz time resolution. Lightcurves of the high excitation Bowen/HeII emission lines and a red continuum at λc 6000 Å were obtained through narrow interference filters with ULTRACAM, and these were cross-correlated with simultaneous RXTE X-ray lightcurves. We find evidence for correlated variability, in particular when Sco X-1 enters the Flaring branch. The Bowen/HeII lightcurves lag the X-ray lightcurves with a light travel time which is consistent with reprocessing in the companion star while the continuum lightcurves have shorter delays consistent with reprocessing in the accretion disc. 相似文献
994.
In developing radio-electronic devices (RED) of spacecraft operating in the fields of ionizing radiation in space, one of the most important problems is the correct estimation of their radiation tolerance. The “weakest link” in the element base of onboard microelectronic devices under radiation effect is the integrated microcircuits (IMC), especially of large scale (LSI) and very large scale (VLSI) degree of integration. The main characteristic of IMC, which is taken into account when making decisions on using some particular type of IMC in the onboard RED, is the probability of non-failure operation (NFO) at the end of the spacecraft’s lifetime. It should be noted that, until now, the NFO has been calculated only from the reliability characteristics, disregarding the radiation effect. This paper presents the so-called “reliability” approach to determination of radiation tolerance of IMC, which allows one to estimate the probability of non-failure operation of various types of IMC with due account of radiation-stimulated dose failures. The described technique is applied to RED onboard the Spektr-R spacecraft to be launched in 2007. 相似文献
995.
996.
N. F. Blagoveshchenskaya A. D. Andreev V. A. Kornienko 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1995,15(12):45-48
Effects of ionospheric modification produced by powerful high frequency radio waves are studied using the method of field-aligned scattering of diagnostic HF radio signals. Observations of scattered HF signals have been made by the Doppler spectrum method. Analysis of the experimental data shows the appearance of quasiperiodic variations in Doppler frequency shift fd, with periods 30–60 s during the heating cycles. Powerful HF waves are assumed to excite the Alfvén resonator generating oscillations of the magnetic field lines in the heated region and giving rise to fd artificial variations and magnetic pulsations. In the case of continuous action of the powerful HF transmitter ionospheric waves are sometimes observed with periods 12–25 min, typical of medium-scale travelling ionospheric disturbances. 相似文献
997.
Suresh M.S. Subrahmanyam A. Sathyanarayana P. Seth R. 《Aerospace and Electronic Systems Magazine, IEEE》1995,10(4):26-29
There has been a debate about the need for reconditioning nickel/hydrogen batteries in geosynchronous satellites. A study was done as part of life cycling, to determine the necessity of reconditioning and its effect on the cell performance. A 36 Ah nickel/hydrogen cell was put on a GEO simulated cycling at 15°C without reconditioning up to four eclipse seasons. The effect of reconditioning on the fifth and sixth eclipse seasons was studied. The study has conclusively proven the need for reconditioning and has shown the benefits of a high rate reconditioning. It has also been possible to draw some conclusions about the effect of a long duration trickle charge on the positive electrode 相似文献
998.
R D Lorenz 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1995,15(3):317-320
Some of the aspects of methane precipitation on Titan are considered. In particular, descent velocities are computed. It is found that raindrops fall much slower than on Earth. Additionally, the maximum size of raindrops on Titan is over 9 mm, compared with under 6 mm on Earth. The composition of drops will vary with altitude. Implications of these properties for Titan and the Huygens mission are considered. 相似文献
999.
W. J. Raitt G. A. Berg D. C. Thompson S. Sazykin 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1995,15(12):79-82
During the initial period of high voltage biasing of the SPEAR-3 sounding rocket payload, it was observed that electrical breakdown occurred in the gas surrounding the rocket. The breakdown occurred almost all the way to apogee of 289 km on the upleg, but did not recur on the downleg until the payload reached an altitude of 100 km. It is suggested that this behavior can be attributed to payload outgassing on the upleg leading to abnormally high gas pressure near the payload skin. Consideration of a modified Paschen discharge process with varying pressure along the discharge path was found to be consistent with the results. 相似文献
1000.
A demonstration program is described: Weapons System Open Architecture (WSOA) - funded jointly by the Air Force Research Laboratory (AFRL), DARPA, and the Open Systems Joint Task Force (OSJTF). WSOA provides an open systems "bridge" between legacy embedded mission systems and off-board C3I sources and systems. This "bridge" is used to support Internet-like connectivity between command and attack nodes. The foundation of this bridge is the creation of a Common Object Request Broker Architecture (CORBA) layer over Link 16. In addition, application of quality of service techniques and resource management technologies will ensure the timely exchange and processing of mission critical information by both attack and command nodes in even the most time-sensitive situations. 相似文献