全文获取类型
收费全文 | 3632篇 |
免费 | 23篇 |
国内免费 | 7篇 |
专业分类
航空 | 1903篇 |
航天技术 | 1200篇 |
综合类 | 12篇 |
航天 | 547篇 |
出版年
2021年 | 30篇 |
2019年 | 27篇 |
2018年 | 123篇 |
2017年 | 76篇 |
2016年 | 49篇 |
2015年 | 26篇 |
2014年 | 67篇 |
2013年 | 85篇 |
2012年 | 81篇 |
2011年 | 126篇 |
2010年 | 95篇 |
2009年 | 138篇 |
2008年 | 189篇 |
2007年 | 95篇 |
2006年 | 78篇 |
2005年 | 95篇 |
2004年 | 87篇 |
2003年 | 113篇 |
2002年 | 70篇 |
2001年 | 126篇 |
2000年 | 73篇 |
1999年 | 83篇 |
1998年 | 108篇 |
1997年 | 70篇 |
1996年 | 95篇 |
1995年 | 131篇 |
1994年 | 102篇 |
1993年 | 66篇 |
1992年 | 91篇 |
1991年 | 34篇 |
1990年 | 39篇 |
1989年 | 86篇 |
1988年 | 43篇 |
1987年 | 37篇 |
1986年 | 35篇 |
1985年 | 96篇 |
1984年 | 92篇 |
1983年 | 82篇 |
1982年 | 82篇 |
1981年 | 94篇 |
1980年 | 27篇 |
1979年 | 27篇 |
1978年 | 29篇 |
1977年 | 27篇 |
1976年 | 19篇 |
1975年 | 36篇 |
1974年 | 25篇 |
1973年 | 18篇 |
1972年 | 33篇 |
1971年 | 18篇 |
排序方式: 共有3662条查询结果,搜索用时 687 毫秒
991.
Flinn ED 《Aerospace America》2002,40(4):18-19
Researchers are perfecting the use of ceramic photocells for retinal implantation. The work is being done at the Space Vacuum Epitaxy Center in Houston, TX. The photocells are the results of experiments with oxide detectors conducted in space using the Wake Shield Facility. Artificial retinas are constructed of 100,000 microscopic ceramic detectors attached to a polymer film, which disintegrates after implantation. Initially, four arrays will be implanted, totalling 400,000 detectors per eye. If successful, two additional arrays would be implanted. Human trials are expected to begin in 2002. 相似文献
992.
Sonkin VD Kozlovskaya IB Zaitseva VV Bourchick MV Stepantsov VI 《Acta Astronautica》1998,43(3-6):291-311
Perspectives of long-term space programs make it necessary to develop autonomous computer expert system for crew-members physical state control. The purpose of the work--to develop a set of objective formalizable physiological indices of working capacity suitable for reliable algorithmization of physical state control. Investigations were performed in on-earth microgravity simulation (3- and 7-day dry immersion, 6 subjects; 4-month antiorthostatic hypokinesy, 10 subjects) with volunteers' participation as well with 34 members of MIR-station expeditions during flights. Model exercise investigations were made also with 20 young male volunteers to evaluate the validity of different physical state indices. A set of indices was found which, being simple enough for measuring, performs to get satisfactory adequate evaluations of current organism physical state in long-term real or simulated microgravity. It was proved that some ergometric indices along with heart rate derivatives could reflect real working ability even better than traditional characteristics of organism energy systems state. 相似文献
993.
Calculations of total dose and dose equivalent as a function of time since the start of the event are presented for four of the major solar particle events that occurred during the period from August to December 1989. Results are presented for exposures to the skin, ocular lens and bone marrow shielded by a nominal thickness of aluminum shielding, comparable to that provided by a spacesuit. The calculated curves of organ dose and dose equivalent versus time are parameterized using a Weibull functional form for the fitting equation. The fitting parameters are determined using least squares regression techniques. These results provide a useful starting point for the development of methods to predict the cumulative doses and times to reach various dose limits from a limited number of dose measurements early in a solar particle event. 相似文献
994.
The NASA Performance Assessment Workstation was used to assess cognitive performance changes in eight males subjected to seventeen days of 6 degrees head-down bed rest. PAWS uses six performance tasks to assess directed and divided attention, spatial, mathematical, and memory skills, and tracking ability. Subjective scales assess overall fatigue and mood state. Subjects completed training trials, practice trials, bed rest trials, and recovery trials. The last eight practice trials and all bed rest trials were performed with subjects lying face-down on a gurney. In general, there was no apparent cumulative effect of bed rest. Following a short period of performance stabilization, a slight but steady trend of performance improvement was observed across all trials. For most tasks, this trend of performance improvement was enhanced during recovery. No statistically significant differences in performance were observed when comparing bed rest with the control period. Additionally, fatigue scores showed little change across all periods. 相似文献
995.
Bird M.K. Dutta-Roy R. Heyl M. Allison M. Asmar S.W. Folkner W.M. Preston R.A. Atkinson D.H. Edenhofer P. Plettemeier D. Wohlmuth R. Iess L. Tyler G.L. 《Space Science Reviews》2002,104(1-4):613-640
A Doppler Wind Experiment (DWE) will be performed during the Titan atmospheric descent of the ESA Huygens Probe. The direction
and strength of Titan's zonal winds will be determined with an accuracy better than 1 m s−1 from the start of mission at an altitude of ∼160 km down to the surface. The Probe's wind-induced horizontal motion will
be derived from the residual Doppler shift of its S-band radio link to the Cassini Orbiter, corrected for all known orbit
and propagation effects. It is also planned to record the frequency of the Probe signal using large ground-based antennas,
thereby providing an additional component of the horizontal drift. In addition to the winds, DWE will obtain valuable information
on the rotation, parachute swing and atmospheric buffeting of the Huygens Probe, as well as its position and attitude after
Titan touchdown. The DWE measurement strategy relies on experimenter-supplied Ultra-Stable Oscillators to generate the transmitted
signal from the Probe and to extract the frequency of the received signal on the Orbiter. Results of the first in-flight checkout,
as well as the DWE Doppler calibrations conducted with simulated Huygens signals uplinked from ground (Probe Relay Tests),
are described. Ongoing efforts to measure and model Titan's winds using various Earth-based techniques are briefly reviewed.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
996.
Geometric accuracy in airborne SAR images 总被引:1,自引:0,他引:1
Blacknell D. Freeman A. Quegan S. Ward I.A. Finley I.P. Oliver C.J. White R.G. Wood J.W. 《IEEE transactions on aerospace and electronic systems》1989,25(2):241-258
Uncorrected across-track motions of a synthetic aperture radar (SAR) platform can cause both a severe loss of azimuthal positioning accuracy in, and defocusing of, the resultant SAR image. It is shown how the results of an autofocus procedure can be incorporated in the azimuth processing to produce a fully focused image that is geometrically accurate in azimuth. Range positioning accuracy is also discussed, leading to a comprehensive treatment of all aspects of geometric accuracy. The system considered is an X -band SAR 相似文献
997.
Linearly combined order statistic (LCOS) constant false-alarm rate (CFAR) detectors are examined for efficient and robust threshold estimation applied to exponentially distributed background observations for improved detection. Two optimization philosophies have been employed to determine the weighting coefficients of the order statistics. The first method optimizes the coefficients to obtain efficient estimates of clutter referred to the censored maximum likelihood (CML) and best linear unbiased (BLU) CFAR detectors. The second optimization involves maximizing the probability of detection under Swerling II targets and is referred to as the most powerful linear (MPL) CFAR detector. The BLU-CFAR detector assumes no knowledge of the target distribution in contrast to the MPL-CFAR detector which requires partial knowledge of the target distribution. The design of these CFAR detectors and the probability of detection performance are mathematically analyzed for background observations having homogeneous and heterogeneous distributions wherein the trade-offs between robustness and detection performance are illustrated 相似文献
998.
Ion-induced nucleation has been suggested to be a potentially important mechanism for atmospheric aerosol formation. Ions
are formed in the background atmosphere by galactic cosmic rays. A possible connection between galactic cosmic rays and cloudiness
has been However, the predictions of current atmospheric nucleation models are highly uncertain because the models are usually
based on the liquid drop model that estimates cluster thermodynamics based on bulk properties (e.g., liquid drop density and
surface tension). Sulfuric acid (H2SO4) and water are assumed to be the most important nucleating agents in the free troposphere. Measurements of the molecular
thermodynamics for the growth and evaporation of cluster ions containing H2SO4 and H2O were performed using a temperature-controlled laminar flow reactor coupled to a linear quadrupole mass spectrometer as well
as a temperature-controlled ion trap mass spectrometer. The measurements were complemented by quantum chemical calculations
of the cluster ion structures. The analysis yielded a complete set of H2SO4 and H2O binding thermodynamics extending from molecular cluster ions to the bulk, based on experimental thermodynamics for the small
clusters. The data were incorporated into a kinetic aerosol model to yield quantitative predictions of the rate of ion-induced
nucleation for atmospheric conditions. The model predicts that the negative ion-H2SO4-H2O nucleation mechanism is an efficient source of new particles in the middle and upper troposphere. 相似文献
999.
Urkowitz [1]has discussed the detection of a deterministic signal of unknown structure in the presence of flat, band-limited, Gaussian noise of known power density. That analysis is extended here to the case where jamming or other conditions preclude knowledge of the noise power density. The chi-square statistic of Urkowitz is replaced with Fisher's variance-ratio statistic, using a separate set of noise samples to estimate the unknown noise power density. Curves are given to show the additional degradation of perform over that due to ignorance of the signal structure, caused by ignorance of the noise power density. 相似文献
1000.
Capraro C.T. Capraro G.T. Bradaric I. Weiner D.D. Wicks M.C. Baldygo W.J. 《IEEE transactions on aerospace and electronic systems》2006,42(3):1080-1099
Many practical problems arise when implementing digital terrain data in airborne knowledge-aided (KA) space-time adaptive processing (STAP). This paper addresses these issues and presents solutions with numerical implementations. In particular, using digital land classification data and digital elevation data, techniques are developed for registering these data with radar return signals, correcting for Doppler and spatial misalignments, adjusting for antenna gain, characterizing clutter patches for secondary data selection, and ensuring independent secondary data samples. These techniques are applied to select secondary data for a single-bin post-Doppler STAP algorithm using multi-channel airborne radar measurement (MCARM) program data. Results with the KA approach are compared with those obtained using the standard sliding window method for choosing secondary data. These results illustrate the benefits of using terrain information, a priori data about the radar, and the importance of statistical independence when selecting secondary data for improving STAP performance 相似文献