全文获取类型
收费全文 | 7825篇 |
免费 | 50篇 |
国内免费 | 41篇 |
专业分类
航空 | 3940篇 |
航天技术 | 2760篇 |
综合类 | 92篇 |
航天 | 1124篇 |
出版年
2021年 | 56篇 |
2018年 | 87篇 |
2017年 | 79篇 |
2016年 | 51篇 |
2014年 | 138篇 |
2013年 | 201篇 |
2012年 | 168篇 |
2011年 | 276篇 |
2010年 | 204篇 |
2009年 | 282篇 |
2008年 | 371篇 |
2007年 | 213篇 |
2006年 | 219篇 |
2005年 | 220篇 |
2004年 | 161篇 |
2003年 | 243篇 |
2002年 | 159篇 |
2001年 | 249篇 |
2000年 | 170篇 |
1999年 | 207篇 |
1998年 | 225篇 |
1997年 | 177篇 |
1996年 | 218篇 |
1995年 | 257篇 |
1994年 | 242篇 |
1993年 | 151篇 |
1992年 | 174篇 |
1991年 | 98篇 |
1990年 | 94篇 |
1989年 | 191篇 |
1988年 | 90篇 |
1987年 | 85篇 |
1986年 | 87篇 |
1985年 | 249篇 |
1984年 | 201篇 |
1983年 | 173篇 |
1982年 | 172篇 |
1981年 | 242篇 |
1980年 | 78篇 |
1979年 | 58篇 |
1978年 | 73篇 |
1977年 | 69篇 |
1976年 | 55篇 |
1975年 | 84篇 |
1974年 | 53篇 |
1973年 | 55篇 |
1972年 | 74篇 |
1971年 | 57篇 |
1970年 | 55篇 |
1969年 | 52篇 |
排序方式: 共有7916条查询结果,搜索用时 15 毫秒
61.
J H Bieging 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1995,15(3):3-14
Stars in their late stages of evolution often shed matter in the form of a cool wind which is molecular in composition. These winds are a major source of replenishment of the interstellar gas and dust, so they furnish a large part of the raw materials for new generations of stars and planets. The chemistry of the circumstellar envelope depends strongly on the photospheric abundances of the elements, especially C and O. If C/O > 1, a rich organic chemistry is observable in the microwave and mm-wavelength emission lines of the reactions products. This paper reviews the observational evidence for the presence of organic molecules and their formation pathways in circumstellar envelopes, with special emphasis on rotational spectra at microwave and millimeter wavelengths. 相似文献
62.
63.
64.
Image exploitation technology approaches have generally focused on the detection and spatial analysis of stationary groups of objects on the ground using various sensors. While spatial arrangement is clearly necessary in analyzing military formations, it is usually not sufficient. Typically the arrangement must be examined within some context in order to interpret a pattern of deployment. For moving objects the spatial arrangement of the group relative to the direction of motion is key to recognizing the formation. By examining ground moving target indicator (MTI) radar data over time, motion can be inferred and used to establish a context for interpreting the spatial arrangement of the data. New techniques that exploit the multitemporal nature of MTI data are described. The first is a space-time clustering technique that locates compact groups of objects that persist in time. The technique Is an application of Marr and Hildreth's edge detection methodology to the dual problem of region segmentation, or more accurately, volumetric segmentation of space-time. The second technique is based on the use of the Hough transform for recognizing moving formations such as columns, wedges, and lines abreast by analyzing the shape of clustered MTI detections (specifically the orientation of linear arrangements within the group) with respect to their direction of motion. Preliminary results from simulated MTI data sets are presented 相似文献
65.
L.B. Tsirulnik T.V. Kuznetsova V.N. Oraevsky 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1997,20(12):2369-2372
A new method of nonlinear spectral analysis (called the method of global minimum: MGM), based on the best presentation (in sense of minimal squares) of a given time data set as a sum of sinusoids whose frequencies, amplitudes and phases are to be determined, has been used to find periodicities in annual Wolf sunspot numbers (W) during the period 1700–1995. The possible future behaviour of the 11-year solar cycle (based on an extrapolation of the calculated model) is also presented. The main characteristics of the 23rd solar cycle are as follows: the W maximum occurs about 2004, with a peak of nearly 220. An unusually large value of W will occur during the 23rd cycle, which should be characterised by the longest maximum, specifically, W will be greater than 100 during the 11-year period from 1997 to 2007. The first sharp rise will occur during the period 1996–1998, the second sharp during 2002–2004. The main features of the 24 year cycle are as follows: the next minimum in W, associated with the 24th solar cycle, should occur in the year 2008 and the maximum in 2014. W is expected to peak at about 180. The minimum value for the 25th year cycle is expected to occur in the year 2019. It is shown that the accuracy of these predictions depends, first of all, on the extrapolation of the hyperlong harmonic of the calculated polyharmonic model fit of observed annual sunspot numbers during the period 1700–1995. The error bars in the definition of the maximum and minimum epochs can be as large as two years. 相似文献
66.
J. F. Crifo 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1991,11(12):155-160
A radiative hydrodynamic model of comet P/Halley is used to investigate quantitatively the constraints imposed onto the Comet loss rate in solids by both the in-situ data and the remote sensing data. In particular, for the first time, the in-situ data are used to fit composite spectra extending from near-IR wavelengths to the microwave region. Also, for the first time the uncertainty affecting the ejection velocity of large grains is taken into account. The results suggest that only future rendez-vous studies of the
grain density
in the vicinity of a comet would be able to provide definitely reliable values of the comet loss rate. 相似文献
67.
T. J. Ponman A. M. Read 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1993,13(12):361-364
Using data from the Wide Field Camera EUV all-sky survey, we have established upper limits to the EUV flux from a sample of 30 bright, nearby, non-active spiral galaxies. These galaxies were chosen to be those most likely to be detected in the EUV on the basis of (i) low interstellar absorption within our own galaxy, (ii) brightness in other wavebands, (iii) high star formation activity, and (iv) proximity. The derived EUV upper limits are restrictive, and establish for the first time that the EUV flux escaping from galaxies does not constitute a major component of their bolometric luminosity, and in particular that it cannot be the sink for the energy injected into the interstellar medium by supernova explosions, as had been suggested following the failure to detect this power in the X-ray band. 相似文献
68.
M. F. Kessler H. J. Habing 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1993,13(12):485-494
The INFRARED SPACE OBSERVATORY (ISO), a project of the European Space Agency (ESA), will make various astronomical observations in the wavelength range of 2 to 200 μm. Two-thirds of its observing time will be available to guest observers via the traditional route of proposal submission. ISO will be launched in 1995 by an Ariane 4 from Kourou (Guyana) and be brought into a very elongated orbit with a 24hr period of which 16 hr can be used for astronomical observations. The payload module is essentially a large cryostat with a tank that will be filled with over 2000 litres of superfluid helium to keep the instruments cold (2 to 4 K) during an expected lifetime of 18 months. Inside the dewar, there is a Ritchey-Chretien telescope with a primary mirror of diameter 60cm. Four instruments, each provided by a different PI consortium, share the ISO focal plane. These instruments are : a photometer, a camera and two spectrometers — one for the wavelength range 2–45 μm and the other for 45–180 μm. 相似文献
69.
Eagle-Picher Energy Products (EPEP) has been manufacturing and testing large lithium ion cells (up to 100-Ah) for several years. Recently, work has focused on the testing of different chemistries at variable temperatures and designing and fabricating 100-Ah cylindrical cells. For the aircraft application the largest concern is irreversible capacity loss at elevated temperatures (70°C). In contrast, for the aerospace application shelf-life and cycle life are critical. EPEP has found that the major contributor to the loss in low temperature performance due to high temperature testing, was the positive electrode. EPEP discuss recent results of variable temperature cycling and 100-Ah cell performance 相似文献
70.
Bacon B.J. Ostroff A.J. Joshi S.M. 《IEEE transactions on aerospace and electronic systems》2001,37(4):1373-1383
A modified derivation of nonlinear dynamic inversion provides the theoretical underpinnings for a reconfigurable control law for aircraft that have suffered combinations of actuator failures, missing effector surfaces, and aerodynamic changes. The approach makes use of acceleration feedback to extract information pertaining to any aerodynamic change and thus does not require a complete aerodynamic model of the aircraft. The control law does require feedback of effector positions to accommodate actuator dynamics. Both accelerometer and rate gyro failure detection and isolation (FDI) systems are implemented, allowing up to three independent failures for each FDI system as long as they are in different axes. Nonlinear simulation results show that the FDI systems improve the robustness to accelerometer/rate gyro uncertainties. An advanced tailless aircraft model is used to demonstrate the concepts. The simulation includes accelerometer and rate gyro noise and bias, failures due to accelerometers, rate gyros, and actuators, and modeled missing surfaces that cause airplane aerodynamic changes 相似文献