全文获取类型
收费全文 | 2261篇 |
免费 | 10篇 |
国内免费 | 11篇 |
专业分类
航空 | 1096篇 |
航天技术 | 817篇 |
综合类 | 10篇 |
航天 | 359篇 |
出版年
2021年 | 22篇 |
2018年 | 38篇 |
2017年 | 22篇 |
2016年 | 26篇 |
2015年 | 11篇 |
2014年 | 48篇 |
2013年 | 57篇 |
2012年 | 50篇 |
2011年 | 92篇 |
2010年 | 64篇 |
2009年 | 92篇 |
2008年 | 98篇 |
2007年 | 53篇 |
2006年 | 43篇 |
2005年 | 58篇 |
2004年 | 70篇 |
2003年 | 72篇 |
2002年 | 37篇 |
2001年 | 57篇 |
2000年 | 39篇 |
1999年 | 47篇 |
1998年 | 67篇 |
1997年 | 42篇 |
1996年 | 67篇 |
1995年 | 79篇 |
1994年 | 55篇 |
1993年 | 49篇 |
1992年 | 64篇 |
1991年 | 30篇 |
1990年 | 19篇 |
1989年 | 49篇 |
1988年 | 23篇 |
1987年 | 20篇 |
1986年 | 21篇 |
1985年 | 83篇 |
1984年 | 53篇 |
1983年 | 53篇 |
1982年 | 60篇 |
1981年 | 72篇 |
1980年 | 21篇 |
1979年 | 26篇 |
1978年 | 26篇 |
1977年 | 26篇 |
1976年 | 20篇 |
1975年 | 20篇 |
1974年 | 20篇 |
1973年 | 11篇 |
1972年 | 14篇 |
1970年 | 18篇 |
1969年 | 19篇 |
排序方式: 共有2282条查询结果,搜索用时 125 毫秒
711.
A problem f requently encountered by radar systems analysts is the evaluation of the "double-threshold" or M out of N detection process. Detection probabilities of this process are binomially distributed, making it difficult to obtain exact results for large values of the number of samples and for low probabilities of false alarm. In this paper, the M out of N detection algorithm is defined and detection performance is calculated for the special cases of the nonfluctuating target and Swerling cases I and 11 for false alarm probabilities of 106, 10-8 and 10-10. 相似文献
712.
GPS is an innovative technique for an electronic road pricing (ERP) system. By using GPS technology, ERP systems will be extremely efficient and cost-effective. This paper describes the next generation of ERP in Singapore and proposes a GPS based multisensor positioning system for ERP vehicles. Multisensor fusion algorithms are presented for the integration system. The integrated positioning system (IPS) developed in this paper makes it possible to realize GPS based ERP with a simple composition and low cost 相似文献
713.
Norman F. Ness 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1981,1(8):171-176
Jupiter and Saturn are two of the more “exotic” planets in our solar system. The former possesses its own system with 15 satellites in orbit about the parent planet. Saturn has a uniquely well developed and distinctive ring system of particulate matter and also at least 11 satellites, including the largest one amongst all the planets, Titan, with a radius of 2900 km ± 100 km. In the decade of the 70's, the USA launched 4 unmanned spacecraft to probe these giant planets in-situ with a suite of highly advanced instrumentation. Four separate encounters have occurred at Jupiter: 1. Pioneer 10 in December 1973 2. Pioner 11 in December 1974 3. Voyager 1 in March 1979 4. Voyager 2 in July 1979 The characteristics of these trajectories is shown in Table I. Thus far, only a single encounter of Saturn has occurred, that by Pioneer 11 in September 1979. Future encounters of Saturn by Voyager spacecraft will occur in mid-November 1980 and late-August 1981. It is the purpose of this talk to summarize what is presently known about the magnetic fields of these planets and the characteristics of their magnetospheres, which are formed by interaction with the solar wind. 相似文献
714.
F. Frontera F. Fuligni E. Morelli G. Ventura 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1981,1(11):107-110
The hard X-ray latitude effect, as measured by two X-ray scintillator detectors, is reported in the range of McIlwain shell parameter L from 1.4 to 2.6 (earth radius units). The data analysis method used to measure the flux from weak X-ray sources is also given. 相似文献
715.
four widely used self-oscillating dc-to-square-wave parallel inverters which employ an inductor-capacitor tuned network to determine the oscillation frequency are reduced to a common equivalent RLC network. The techniques of singular-point analysis and state-plane interpretations are employed to describe the steady-state and transient ient behavior of these circuits and to elucidate the three possible modes of operation: quasi-harmonic, relaxation, and discontinuous. Design guidelines are provided through a study of the influence of circuit parameter variations on the characteristics of oscillation and on frequency stability. Several examples are provided to illustrate the usefulness of this analysis when studying such problems as transistor emitter-to-base junction breakdown during oscillations and the design of starting circuits to insure self-excited oscillations in these inverters. 相似文献
716.
F.J. Bonn P.J. Howarth 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1982,2(8):151-154
This report summarizes a major paper reviewing Canadian work using Landsat imagery for monitoring the environmental effects of the development of both renewable and non-renewable resources. Landsat has definite advantages for studying resources. In particular, the data are able to show changes in surface cover indicating in many cases development or the exploitation of resources. Monitoring of areas and rates of change over time is possible. Outlined in the paper are examples related to three aspects of resource development. First is the use of Landsat for updating topographic maps for areas where recent developments have taken place. The second aspect is forestry, and examples related to the detection and measurement of clear cuts and recording of areas affected by spruce budworm are reported. Finally, studies of landscape changes produced by mining activities are presented. 相似文献
717.
F.B. McDonald Z. Fujii P. Ferrando B. Heber A. Raviart H. Kunow N. Lal R. Mller-Mellin G. Wibberenz R. McGuire C. Paizis 《Space Science Reviews》2001,97(1-4):321-325
The combination of Voyager 1 (77.9 AU, 34.4° N) and Voyager 2 (61.2 AU, 24.5° S) at moderate heliolatitudes in the distant heliosphere and Ulysses with its unique latitudinal surveys in the inner heliosphere along with IMP 8 and other satellites at 1 AU constitutes a
network of observatories that are ideally suited to study cosmic rays over the solar minimum of cycle 22 and the onset of
solar activity and the long term cosmic ray modulation of cycle 23. Through 2000.7 there have been three well-defined step
decreases in the cosmic ray intensity at 1 AU with the cumulative effect being in good agreement with the net decrease in
cycle 21 at a comparable time in the solar cycle. Over this period the intensity changes at Ulysses are similar to those at 1 AU. In the distant heliosphere the initial decreases appear to be smaller than those at 1 AU. However
the full effects of the interplanetary disturbances producing the most recent and largest step decrease in the inner heliosphere
have not yet reached V-2.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
718.
B. C. Boland S. F. T. Engstrom B. B. Jones R. W. P. McWhirter P. C. Thonemann R. Wilson 《Space Science Reviews》1972,13(4-6):639-641
Observations of the solar spectrum have been made between 1200–2200 with high spectral resolution. The results were obtained with an all-reflecting echelle spectrograph carried by a stabilized Skylark rocket launched in April 1970. Measurements of the profiles of a number of emission lines due to Siii, Cii, Siiii and Civ formed in the temperature range 104-105 K, indicate ion energies which are considerably in excess of the electron temperatures derived from the ionization balance. Since the ion/electron relaxation time is very short the observed ion energies cannot correspond to an ion temperature and hence a non-thermal mechanical energy component exists in the transition zone.It is postulated that the non-thermal energy component represents the actual mechanical energy responsible for the heating of the corona, and, that, it is propagated as an acoustic wave. On this basis and with a preliminary estimate of the reflection from the transition zone, a flux of 3 × 105 erg cm -2 s -1 is established as entering the corona. This value is in agreement with estimates of the total energy loss from the corona due to conduction, radiation and the solar wind, thus establishing a gross energy balance.Theoretical calculations are currently underway to establish the physical nature of the atmosphere which would result from such a propagating flux. At the present time this has been carried out for an atmosphere in hydrostatic equilibrium and the energy balance equation solved. A preliminary temperature structure which results is shown in Figure 1, together with the derived distribution in electron density. This gives a corona of the right temperature and density but the observed structure deviates in detail from those derived from an analysis of the solar XUV spectrum. 相似文献
719.
The Widrow-Hoff least mean square (LMS) algorithm based on the method of steepest descent is conditionally stable. A modified algorithm is given which is unconditionally stable, capable of better performance when used in adaptive filter processing, and provides a more realistic means for simulating the Applebaum-Howells adaptive loop. 相似文献
720.
G. Szejwach T.N. Sletten A.F. Hasler 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1982,2(6):161-164
The feasibility of determining cirrus “emissivity” from combined stereoscopic and infrared satellite observations in conjunction with radiosounding data is investigated for a particular case study. Simultaneous visible images obtained during SESAME-1979 from two geosynchronous GOES meteorological satellites were processed on the NASA/Goddard interactive system (AOIPS) and were used to determine the stereo cloud top height ZC as described by Hasler [1]. Iso-contours of radiances were outlined on the corresponding infrared image. Total brightness temperature TB and ground surface brightness temperature TS were inferred from the radiances. The special SESAME network of radiosoundings was used to determine the cloud top temperature TCLD at the level defined by ZC. The “effective cirrus emissivity” NE where N is the fractional cirrus cloudiness and E is the emissivity in a GOES infrared picture element of about 10 km × 10 km is then computed from TB, TS and TCLD. 相似文献