全文获取类型
收费全文 | 4476篇 |
免费 | 35篇 |
国内免费 | 18篇 |
专业分类
航空 | 2015篇 |
航天技术 | 1688篇 |
综合类 | 17篇 |
航天 | 809篇 |
出版年
2022年 | 16篇 |
2021年 | 36篇 |
2019年 | 36篇 |
2018年 | 100篇 |
2017年 | 80篇 |
2016年 | 65篇 |
2015年 | 28篇 |
2014年 | 111篇 |
2013年 | 141篇 |
2012年 | 108篇 |
2011年 | 196篇 |
2010年 | 143篇 |
2009年 | 228篇 |
2008年 | 251篇 |
2007年 | 124篇 |
2006年 | 100篇 |
2005年 | 120篇 |
2004年 | 126篇 |
2003年 | 160篇 |
2002年 | 99篇 |
2001年 | 161篇 |
2000年 | 77篇 |
1999年 | 116篇 |
1998年 | 129篇 |
1997年 | 100篇 |
1996年 | 93篇 |
1995年 | 131篇 |
1994年 | 125篇 |
1993年 | 69篇 |
1992年 | 99篇 |
1991年 | 34篇 |
1990年 | 43篇 |
1989年 | 94篇 |
1988年 | 35篇 |
1987年 | 48篇 |
1986年 | 33篇 |
1985年 | 126篇 |
1984年 | 89篇 |
1983年 | 83篇 |
1982年 | 88篇 |
1981年 | 119篇 |
1980年 | 55篇 |
1979年 | 39篇 |
1978年 | 36篇 |
1977年 | 27篇 |
1976年 | 30篇 |
1975年 | 33篇 |
1974年 | 20篇 |
1972年 | 22篇 |
1969年 | 18篇 |
排序方式: 共有4529条查询结果,搜索用时 13 毫秒
81.
A data fusion model consisting of several levels of parallel decision fusions is considered. Global optimization of such a model is discussed to obtain the fusion rules for overall optimal performance. The reliability analysis of the proposed model is carried out to establish its superiority over the existing parallel and serial fusion models 相似文献
82.
T.B. Leyser 《Space Science Reviews》2001,98(3-4):223-328
A high frequency electromagnetic pump wave transmitted into the ionospheric plasma from the ground can stimulate electromagnetic radiation with frequencies around that of the ionospherically reflected pump wave. The numerous spectral features of these stimulated electromagnetic emissions (SEE) and their temporal evolution on a wide range of time scales are reviewed and related theoretical, numerical, and simulation results are discussed. On long (thermal) time scales the SEE constitutes a self-organization of the ionospheric plasma which depends on the interaction of nonlinear processes in a hierarchy of time scales in response to the electromagnetic pumping. Particularly, the appearance of the rich SEE spectrum is associated with the slow self-structuring of the plasma density into a spectrum of magnetic field-aligned density striations. The dependence of the SEE on electron gyroharmonic effects and the presence of density striations suggests that the existence of a magnetic field in the plasma is important for plasma turbulence to dissipate into non-thermal electromagnetic radiation during the long time quasi-stationary state of the turbulence evolution. 相似文献
83.
T.E. Moore M.O. Chandler M.-C. Fok B.L. Giles D.C. Delcourt J.L. Horwitz C.J. Pollock 《Space Science Reviews》2001,95(1-2):555-568
The discovery of terrestrial O+ and other heavy ions in magnetospheric hot plasmas, combined with the association of energetic ionospheric outflows with geomagnetic activity, led to the conclusion that increasing geomagnetic activity is responsible for filling the magnetosphere with ionospheric plasma. Recently it has been discovered that a major source of ionospheric heavy ion plasma outflow is responsive to the earliest impact of coronal mass ejecta upon the dayside ionosphere. Thus a large increase in ionospheric outflows begins promptly during the initial phase of geomagnetic storms, and is already present during the main phase development of such storms. We hypothesize that enhancement of the internal source of plasma actually supports the transition from substorm enhancements of aurora to storm-time ring current development in the inner magnetosphere. Other planets known to have ring current-like plasmas also have substantial internal sources of plasma, notably Jupiter and Saturn. One planet having a small magnetosphere, but very little internal source of plasma, is Mercury. Observations suggest that Mercury has substorms, but are ambiguous with regard to the possibility of magnetic storms of the planet. The Messenger mission to Mercury should provide an interesting test of our hypothesis. Mercury should support at most a modest ring current if its internal plasma source is as small as is currently believed. If substantiated, this hypothesis would support a general conclusion that the magnetospheric inflationary response is a characteristic of magnetospheres with substantial internal plasma sources. We quantitatively define this hypothesis and pose it as a problem in comparative magnetospheres. 相似文献
84.
D. Koschny V. Dhiri K. Wirth J. Zender R. Solaz R. Hoofs R. Laureijs T.-M Ho B. Davidsson G. Schwehm 《Space Science Reviews》2007,128(1-4):167-188
ESA’s Rosetta mission was launched in March 2004 and is on its way to comet 67P/Churyumov-Gerasimenko, where it is scheduled
to arrive in summer 2014. It comprises a payload of 12 scientific instruments and a Lander. All instruments are provided by
Principal Investigators, which are responsible for their operations.
As for most ESA science missions, the ground segment of the mission consists of a Mission Operations Centre (MOC) and a Science
Operations Centre (SOC). While the MOC is responsible for all spacecraft-related aspects and the final uplink of all command
timelines to the spacecraft, the scientific operations of the instruments and the collection of the data and ingestion into
the Planetary Science Archive are coordinated by the SOC. This paper focuses on the tasks of the SOC and in particular on
the methodology and constraints to convert the scientific goals of the Rosetta mission to operational timelines. 相似文献
85.
We describe a numerical integral-projection method used by the authors for the approximate solution of systems of interrelated two-dimensional linear boundary-value problems in mechanics of composite shell systems. The method is based on discretization in each shell substructure of a two-dimensional problem along one of coordinates using a projection-grid variant of the Galerkin-Petrov method and its subsequent transformation to a system of ordinary differential equations; by integration and introduction of sought functions as unknown derivatives, the system is reduced to a system of integral equations being solved by the method of mechanical quadratures. The method is characterized by the fact that its application requires no additional conditions of conformity with discretization parameters of substructures being mated. 相似文献
86.
Rice M. Oliphant T. Haddadin O. McIntire W. 《IEEE transactions on aerospace and electronic systems》2007,43(4):1484-1495
This paper describes data-aided signal level and noise variance estimators for Gaussian minimum shift keying (GMSK) when the observations are limited to the output of a filter matched to the first pulse-amplitude modulation (PAM) pulse in the equivalent PAM representation. The estimators are based on the maximum likelihood (ML) principle and assume burst-mode transmission with known timing and a block of L0 known bits. While it is well known that ML estimators are asymptotically unbiased and efficient, the analysis quantifies the rate at which the estimators approach these asymptotic properties. It is shown that the carrier phase, amplitude, and noise variance estimators are unbiased and can achieve their corresponding Cramer-Rao bounds with modest combinations of signal-to-noise ratio and observation length. The estimates are used to estimate the signal-to-noise ratio. It is shown that the mean squared error performance of the ratio increases with signal-to-noise ratio while the mean squared error performance of the ratio in decibels decreases with signal-to-noise ratio. Simulation results are provided to confirm the accuracy of the analytic results. 相似文献
87.
A. G. Yahnin I. V. Despirak A. A. Lubchich B. V. Kozelov N. P. Dmitrieva M. A. Shukhtina H. K Biernat 《Space Science Reviews》2006,122(1-4):97-106
Although the auroral substorm has been long regarded as a manifestation of the magnetospheric substorm, a direct relation
of active auroras to certain magnetospheric processes is still debatable. To investigate the relationship, we combine the
data of the UV imager onboard the Polar satellite with plasma and magnetic field measurements by the Geotail spacecraft. The
poleward edge of the auroral bulge, as determined from the images obtained at the LHBL passband, is found to be conjugated
with the region where the oppositely directed fast plasma flows observed in the near-Earth plasma sheet during substorms are
generated. We conclude that the auroras forming the bulge are due to the near-Earth reconnection process. This implies that
the magnetic flux through the auroral bulge is equal to the flux dissipated in the magnetotail during the substorm. Comparison
of the magnetic flux through the auroral bulge with the magnetic flux accumulated in the tail lobe during the growth phase
shows that these parameters have the comparable values. This is a clear evidence of the loading–unloading scheme of substorm
development. It is shown that the area of the auroral bulge developing during substorm is proportional to the total (magnetic
plus plasma) pressure decrease in the magnetotail. These findings stress the importance of auroral bulge observations for
monitoring of substorm intensity in terms of the magnetic flux and energy dissipation. 相似文献
88.
N. Panafidina U. Hugentobler H. Krásná R. Schmid M. Seitz 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2019,63(1):51-62
Within the analysis of space geodetic observations, errors of the applied subdaily Earth rotation model can induce systematic effects in different estimated parameters. In this paper, we focus on the impact of the subdaily Universal Time (UT1) model on the celestial pole offsets (CPO) estimated from very long baseline interferometry (VLBI) observations. We provide a mechanism that describes the error propagation from the subdaily UT1 into the daily CPO.In typical 24-h VLBI sessions the observed quasars are well distributed over the sky. But the observations, if looked at from the Earth-fixed frame, are not homogeneously distributed. The amount of observations performed in different terrestrial directions shows an irregularity which can be roughly compared to the case where the observations are collected in only one Earth-fixed direction. This peculiarity leads to artefacts in VLBI solutions, producing a correlation between the subdaily variations in UT1 and the position of the celestial pole. As a result errors in diurnal terms of the subdaily UT1 model are partly compensated by the estimated CPO. We compute for each 24-h VLBI session from 1990 until 2011 the theoretical response of the CPO to an error in the subdaily UT1 by setting up a least-squares adjustment model and using as input the coordinates of the observed quasars and observation epochs. Then real observed response of the estimated CPO derived from the VLBI session solutions is compared to the predicted one. A very good agreement between the CPO values estimated from VLBI and the predicted values was achieved. The presented model of error propagation from the subdaily UT1 into the daily CPO allows to predict and explain the behaviour of CPO estimates of VLBI solutions computed with different subdaily Earth rotation models, what can be helpful for testing the accuracy of different subdaily tidal models. 相似文献
89.
Degradation monitoring in Silvo-pastoral systems: A case study of the Mediterranean region of Turkey
Orkan Ozcan G.A. Aksu E. Erten N. Musaoglu M. Cetin 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2019,63(1):160-171
To identify policies that will promote positive effects and mitigate negative ones of grazing is a major challenge in the Silvo-pastoral system. This paper presents the role of examining land-cover change trajectories by remote sensing imagery in grazing policy monitoring. The study was conducted for Duzlercami forest ecosystem located in the Mediterranean geographical region of Turkey and administrated by the General Directorate of Forestry (GDF) of the Ministry of Forestry and Water Affairs. Time series land-cover datasets from Landsat images between 1988 and 2016 were collected and classified. To link the conversions among trajectories and grazing policy, class level landscape metrics derived from the classified images were used. To validate the approach, yearly grazing-plans managed by GDF and populations of livestock were used. Results of this research have indicated that even though there is a yearly grazing plan, overgrazing can happen on the pilot site, and it can be easily identified by the destruction of woody vegetation. The notable correlation (r2?=?0.89) between degraded woody vegetation and cattle population has occurred in the last 30?years in the landscape, and Landsat imagery can effectively support the grazing policy mapping and monitoring. 相似文献
90.
J.T. Rudd D.M. Oliveira A. Bhaskar A.J. Halford 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2019,63(1):317-326
In this paper, we investigate temporal and spatial magnetosphere response to the impact of interplanetary (IP) shocks with different inclinations and speeds on the Earth’s magnetosphere. A data set with more than 500 IP shocks is used to identify positive sudden impulse (SI+) events as expressed by the SuperMAG partial ring current index. The SI+ rise time (RT), defined as the time interval between compression onset and maximum SI+ signature, is obtained for each event. We use RT and a model suggested by Takeuchi et al. (2002) to calculate the geoeffective magnetospheric distance (GMD) in the shock propagation direction as a function of shock impact angle and speed for each event. GMD is a generalization of the geoeffective magnetosphere length (GML) suggested by Takeuchi et al. (2002), defined from the subsolar point along the X line toward the tail. We estimate statistical GMD and GML values which are then reported for the first time. We also show that, similarly to well-known results for RT, the highest correlation coefficient for the GMD and impact angle is found for shocks with high speeds and small impact angles, and the faster and more frontal the shock, the smaller the GMD. This result indicates that the magnetospheric response depends heavily on shock impact angle. With these results, we argue that the prediction and forecasting of space weather events, such as those caused by coronal mass ejections, will not be accurately accomplished if the disturbances’ angles of impact are not considered as an important parameter within model and observation scheme capabilities. 相似文献