全文获取类型
收费全文 | 4814篇 |
免费 | 3篇 |
国内免费 | 11篇 |
专业分类
航空 | 2516篇 |
航天技术 | 1551篇 |
综合类 | 181篇 |
航天 | 580篇 |
出版年
2021年 | 29篇 |
2018年 | 69篇 |
2017年 | 36篇 |
2016年 | 40篇 |
2014年 | 72篇 |
2013年 | 99篇 |
2012年 | 106篇 |
2011年 | 159篇 |
2010年 | 101篇 |
2009年 | 182篇 |
2008年 | 191篇 |
2007年 | 114篇 |
2006年 | 87篇 |
2005年 | 84篇 |
2004年 | 114篇 |
2003年 | 139篇 |
2002年 | 179篇 |
2001年 | 198篇 |
2000年 | 93篇 |
1999年 | 123篇 |
1998年 | 153篇 |
1997年 | 99篇 |
1996年 | 138篇 |
1995年 | 168篇 |
1994年 | 146篇 |
1993年 | 95篇 |
1992年 | 122篇 |
1991年 | 57篇 |
1990年 | 63篇 |
1989年 | 125篇 |
1988年 | 58篇 |
1987年 | 60篇 |
1986年 | 51篇 |
1985年 | 152篇 |
1984年 | 120篇 |
1983年 | 108篇 |
1982年 | 117篇 |
1981年 | 153篇 |
1980年 | 56篇 |
1979年 | 46篇 |
1978年 | 57篇 |
1977年 | 37篇 |
1976年 | 40篇 |
1975年 | 48篇 |
1974年 | 39篇 |
1973年 | 25篇 |
1972年 | 49篇 |
1971年 | 47篇 |
1970年 | 30篇 |
1969年 | 33篇 |
排序方式: 共有4828条查询结果,搜索用时 31 毫秒
891.
One of the most important problems for performing a good design of the spacecraft attitude control law is connected to its robustness when some uncertainty parameters are present on the inertial and/or on the elastic characteristics of a satellite. These uncertainties are generally intrinsic on the modeling of complex structures and in the case of large flexible structures they can be also attributed to secondary effects associated to the elasticity. One of the most interesting issues in modeling large flexible space structures is associated to the evaluation of the inertia tensor which in general depends not only on the geometric ‘fixed’ characteristic of the satellite but also on its elastic displacements which of course in turn modify the ‘shape’ of the satellite. Usually these terms can be considered of a second order of magnitude if compared with the ones associated to the rigid part of a structure. However the increasing demand on the dimension of satellites due to the presence for instance of very large solar arrays (necessary to generate power) and/or large antennas has the necessity to investigate their effects on their global dynamic behavior in more details as a consequence. In the present paper a methodology based on classical Lagrangian approach coupled with a standard Finite Element tool has been used to derive the full dynamic equations of an orbiting flexible satellite under the actions of gravity, gravity gradient forces and attitude control. A particular attention has been paid to the study of the effects of flexibility on the inertial terms of the spacecraft which, as well known, influence its attitude dynamic behavior. Furthermore the effects of the attitude control authority and its robustness to the uncertainties on inertial and elastic parameters has been investigated and discussed. 相似文献
892.
Magnetospheric signatures of auroral disturbances during the passage of the solar wind’s CIR and sheath regions 总被引:1,自引:0,他引:1
Based on Polar satellite data, the authors have studied the auroral disturbances that arose during the passage by the Earth of compressed plasma regions formed in front of high-speed solar wind streams (the CIR region) and in front of magnetic clouds (the Sheath region). The aurorae observed by the Polar satellite possessed basic signatures of a substorm: a localized onset and expansion toward the pole and westward and eastward. However, in these cases they had a very large size in longitude and latitude and occupied a very large area. All disturbances observed by the Polar satellite during the Sheath and CIR regions of the solar wind in December of 1996, in 1997–1998, and in 2000 were analyzed. Eight events during disturbance development in the ionosphere, when the Geotail satellite was located in the plasma sheet of the magnetospheric tail, were selected. It is shown that in all selected cases some typical signatures of substorm development in the magnetospheric tail were observed, namely: (1) fast plasma flows (flow reversal, i.e., from tailwards to Earthwards) and (2) a sharp decrease of the total pressure, which followed an interval of total pressure increase. One can draw the conclusion that in the CIR and Sheath regions with a high solar wind density, substorm disturbances of a specific type are observed, with large latitudinal and longitudinal size (sometimes occupying the entire polar cap). 相似文献
893.
S. Yashiro N. Gopalswamy P. Mäkelä S. Akiyama W. Uddin A.K. Srivastava N.C. Joshi R. Chandra P.K. Manoharan K. Mahalakshmi V.C. Dwivedi R. Jain A.K. Awasthi N.V. Nitta M.J. Aschwanden D.P. Choudhary 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2014
Active region NOAA 11158 produced many flares during its disk passage. At least two of these flares can be considered as homologous: the C6.6 flare at 06:51 UT and C9.4 flare at 12:41 UT on February 14, 2011. Both flares occurred at the same location (eastern edge of the active region) and have a similar decay of the GOES soft X-ray light curve. The associated coronal mass ejections (CMEs) were slow (334 and 337 km/s) and of similar apparent widths (43° and 44°), but they had different radio signatures. The second event was associated with a metric type II burst while the first one was not. The COR1 coronagraphs on board the STEREO spacecraft clearly show that the second CME propagated into the preceding CME that occurred 50 min before. These observations suggest that CME–CME interaction might be a key process in exciting the type II radio emission by slow CMEs. 相似文献
894.
Assessment of precision in ionospheric electron density profiles retrieved by GPS radio occultations
P. Alexander A. de la Torre R. Hierro P. Llamedo 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2014
The Constellation Observing System for Meteorology Ionosphere and Climate (COSMIC) is a six satellite radio occultation mission that was launched in April 2006. The close proximity of these satellites during some months after launch provides a unique opportunity to evaluate the precision of Global Positioning System (GPS) radio occultation (RO) retrievals of ionospheric electron density from nearly collocated and simultaneous observations. RO data from 30 consecutive days during July and August 2006 are divided into ten groups in terms of daytime or nighttime and latitude. In all cases, the best precision values (about 1%) are found at the F peak height and they slightly degrade upwards. For all daytime groups, it is seen that electron density profiles above about 120 km height exhibit a substantial improvement in precision. Nighttime groups are rather diverse: in particular, the precision becomes better than 10% above different levels between 120 and 200 km height. Our overall results show that up to 100–200 km (depending on each group), the uncertainty associated with the precision is in the order of the measured electron density values. Even worse, the retrieved values tend sometimes to be negative. Although we cannot rely directly on electron density values at these altitudes, the shape of the profiles could be indicative of some ionospheric features (e.g. waves and sporadic E layers). Above 200 km, the profiles of precision are qualitatively quite independent from daytime or latitude. From all the nearly collocated pairs studied, only 49 exhibited a difference between line of sight angles of both RO at the F peak height larger than 10°. After analyzing them we find no clear indications of a significant representativeness error in electron density profiles due to the spherical assumption above 120 km height. Differences in precision between setting and rising GPS RO may be attributed to the modification of the processing algorithms applied to rising cases during the initial period of the COSMIC mission. 相似文献
895.
S.A. Washburn S.R. Blattnig R.C. Singleterry S.C. Westover 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2014
The use of active radiation shielding designs has the potential to reduce the radiation exposure received by astronauts on deep-space missions at a significantly lower mass penalty than designs utilizing only passive shielding. Unfortunately, the determination of the radiation exposure inside these shielded environments often involves lengthy and computationally intensive Monte Carlo analysis. In order to evaluate the large trade space of design parameters associated with a magnetic radiation shield design, an analytical model was developed for the determination of flux inside a solenoid magnetic field due to the Galactic Cosmic Radiation (GCR) radiation environment. This analytical model was then coupled with NASA’s radiation transport code, HZETRN, to account for the effects of passive/structural shielding mass. The resulting model can rapidly obtain results for a given configuration and can therefore be used to analyze an entire trade space of potential variables in less time than is required for even a single Monte Carlo run. Analyzing this trade space for a solenoid magnetic shield design indicates that active shield bending powers greater than ∼15 Tm and passive/structural shielding thicknesses greater than 40 g/cm2 have a limited impact on reducing dose equivalent values. Also, it is shown that higher magnetic field strengths are more effective than thicker magnetic fields at reducing dose equivalent. 相似文献
896.
I.L. Babich V.F. BoretskijA.N. Veklich R.V. Semenyshyn 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2014
Optical emission and linear laser absorption spectroscopy techniques were used in investigation of plasma with copper and silver admixture. The method of selection of spectral lines and spectroscopic data with the aim of diagnostics of multicomponent air plasma with two metal vapors admixture was developed. Energy level populations behavior on the Boltzmann plot were used for Cu I and Ag I spectroscopic data selection. In this way the selection of spectroscopic data for some of Cu I and Ag I lines was realized. Stark broadening parameters of Cu I and Ag I were examined. Experimentally obtained temperature and electron density radial distributions were used in the calculation of plasma composition in the assumption of local thermodynamic equilibrium. Linear laser absorption spectroscopy was used to examine the state of plasma. 相似文献
897.
R. Stamm R. Hammami I. Hannachi H. Capes L. Godbert-Mouret M. Koubiti Y. Marandet J. Rosato 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2014
Stochastic processes provide flexible and fast calculations for modeling dynamical interactions between an atom and charged particles. We use a stochastic renewal process for the plasma microfield being the cause of Stark broadening. The accuracy and improvement possibilities of Lyman profiles calculations with a renewal process are analyzed by comparing to ab initio simulations for ion broadening only. Stochastic processes may also be applied to out of equilibrium plasmas. We present our first results for the effect of Langmuir waves on a line broadened by electrons only, and for the changes of atomic populations submitted to strong temperature fluctuations. 相似文献
898.
A.J. de Abreu P.R. Fagundes M. Gende O.S. Bolaji R. de Jesus C. Brunini 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2014
The responses of the ionospheric F region using GPS–TEC measurements during two moderate geomagnetic storms at equatorial, low-, and mid-latitude regions over the South American and African sectors in May 2010, during the ascending phase of solar cycle 24, are investigated. The first moderate geomagnetic storm studied reached a minimum Dst value of −64 nT at 1500 UT on 02 May 2010 and the second moderate geomagnetic storm reached a minimum Dst value of −85 nT at 1400 UT on 29 May 2010. In this paper, we present vertical total electron content (VTEC) and phase fluctuations (in TECU/min) from Global Positioning System (GPS) observations from the equatorial to mid-latitude regions in the South American and African sectors. Our results obtained during these two moderate geomagnetic storms from both sectors show significant positive ionospheric storms during daytime hours at the equatorial, low-, and mid-latitude regions during the main and recovery phases of the storms. The thermospheric wind circulation change towards the equator is a strong indicator that suggests an important mechanism is responsible for these positive phases at these regions. A pre-storm event that was observed in the African sector from low- to the mid-latitude regions on 01 May 2010 was absent in the South American sector. This study also showed that there was no generation or suppression of ionospheric irregularities by storm events. Therefore, knowledge about the suppression and generation of ionospheric irregularities during moderate geomagnetic storms is still unclear. 相似文献
899.
The use of system models in the EuroMoon spacecraft design 总被引:2,自引:0,他引:2
R. C. Parkinson 《Acta Astronautica》1999,44(7-12):437-443
900.
Diffusive shock acceleration is the theory of particle acceleration through multiple shock crossings. In order for this process to proceed at a rate that can be reconciled with observations of high-energy electrons in the vicinity of the shock, and for cosmic rays protons to be accelerated to energies up to observed galactic values, significant magnetic field amplification is required. In this review we will discuss various theories on how magnetic field amplification can proceed in the presence of a cosmic ray population. On both short and long length scales, cosmic ray streaming can induce instabilities that act to amplify the magnetic field. Developments in this area that have occurred over the past decade are the main focus of this paper. 相似文献