全文获取类型
收费全文 | 6071篇 |
免费 | 28篇 |
国内免费 | 12篇 |
专业分类
航空 | 2731篇 |
航天技术 | 2008篇 |
综合类 | 29篇 |
航天 | 1343篇 |
出版年
2021年 | 59篇 |
2019年 | 39篇 |
2018年 | 169篇 |
2017年 | 135篇 |
2016年 | 133篇 |
2015年 | 57篇 |
2014年 | 152篇 |
2013年 | 188篇 |
2012年 | 180篇 |
2011年 | 244篇 |
2010年 | 199篇 |
2009年 | 290篇 |
2008年 | 321篇 |
2007年 | 185篇 |
2006年 | 135篇 |
2005年 | 176篇 |
2004年 | 163篇 |
2003年 | 199篇 |
2002年 | 139篇 |
2001年 | 217篇 |
2000年 | 103篇 |
1999年 | 120篇 |
1998年 | 152篇 |
1997年 | 105篇 |
1996年 | 131篇 |
1995年 | 195篇 |
1994年 | 155篇 |
1993年 | 84篇 |
1992年 | 122篇 |
1991年 | 42篇 |
1990年 | 57篇 |
1989年 | 121篇 |
1988年 | 50篇 |
1987年 | 48篇 |
1986年 | 48篇 |
1985年 | 159篇 |
1984年 | 141篇 |
1983年 | 123篇 |
1982年 | 116篇 |
1981年 | 158篇 |
1980年 | 47篇 |
1979年 | 43篇 |
1978年 | 41篇 |
1977年 | 40篇 |
1976年 | 30篇 |
1975年 | 47篇 |
1974年 | 33篇 |
1973年 | 23篇 |
1972年 | 43篇 |
1971年 | 28篇 |
排序方式: 共有6111条查询结果,搜索用时 15 毫秒
931.
The reliability of the station-keeping activator subsystem of a geostationary satellite using electric thrusters is analyzed by modeling the probabilistic behavior of the system as a homogeneous Markov process. The method is applied to the particular case of a 400-kg satellite using 1.5-mN cesium contact electric thrusters. lt is shown that certain system configurations offer a mass improvement without loss of reliability. 相似文献
932.
A study has been made of the maximum theoretical accuracy in the angular location of a radiating object that is achievable by using a planar or linear array. The elements are assumed to have identical radiation patterns and the complex voltages observed at their ports are assumed to be subject to phase measurement errors, having normal probability density. An optimum scheme for the statistical extraction of the parameters defining the direction is established. It consists of combining the observed phases linearly with weights depending upon the element locations. It is shown that the presence of thermal noise, for sufficiently high signal-to-noise ratio, does not change the structure of the estimator. Comparison with conventional multiple interferometric techniques indicates the superiority of the proposed scheme. Finally, a limited numerical study on a small linear array vertically located on a reflecting terrain is performed. Although in such a situation the scheme proposed is not the theoretical optimum, it leads to errors that, for most directions of the target, are smaller than those found for the same array when using conventional multiple interferometer techniques. 相似文献
933.
Computerized Model Demonstrating Magnetic Submarine Localization 总被引:1,自引:0,他引:1
A modet is described which consists of an aircraft with flux gate magnetometers mounted on the wing tips and a submarine containing a magnet to simulate the induced and permanent fields of an actual submarine. Equations are developed which enable a minicomputer sampling the magnetometers to calculate the location, depth, and heading of the submarine as the aircraft travels towards it. The real time plotted results are presented. 相似文献
934.
lt is possible that usable combination codes can be obtained if Barker and Huffman sequences are chosen as the inner and outer codes. lt is shown that an improvement in either energy efficiency or time sidelobe structure will result either from a modification of a Huffman sequence or from a modification of a Barker sequence. Results are given for a combination code of length 91, with inner and outer codes of length 7 and 13. 相似文献
935.
V A Sarychev V V Sazonov A S Zlatorunsky S F Khlopina A D Egorov V I Somov 《Acta Astronautica》1980,7(6):719-730
The problem considered in this paper is the investigation of the properties of a mass-meter, i.e. the device for determining the mass of cosmonaut's body under zero-gravity conditions. The estimates of accuracy of mass measurement by this device are given, and the results of measuring the masses of cosmonauts' bodies on the Salyut 5 and 6 orbital stations are presented. 相似文献
936.
D. Cunnold 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1984,4(6):47-56
Stratospheric ozone observations by the SAGE and SBUV satellite instruments in March and April, 1979 have been analyzed. All SAGE profiles have been smoothed vertically over 8 km to provide some compatibility with the SBUV vertical resolution. Comparing the zonal mean ozone mixing ratios against smoothed LIMS profiles, it is inferred that SAGE is systematically overestimating ozone by approximately 20% at tropical latitudes at pressures lower than 5 mb and that SBUV is underestimating ozone by approximately 15% at 50–70° latitude at 10 mb. A comparison of the longitudinal variations of ozone by SBUV and SAGE is made and the detectability of planetary waves in ozone is emphasized. The uncorrelated portion of the SAGE variances are found to be approximately consistent with the SAGE noise model. Based on the correlated variances, the amplitudes of the smoothed SAGE planetary waves in ozone are found to be the same, on average, as in the SBUV experiment at mid-latitudes between 1 and 10 mb. Planetary wave detectability is illustrated during two several day periods at mid-latitudes and a persistent and theoretically-consistent relationship between ozone and temperature is noted. These examples, however, indicate that differences between ozone planetary wave amplitudes derived from the two sensors may occur when there is a strong vertical gradient in wave amplitude. 相似文献
937.
The problem of bearing estimation for active systems is examined from the point of view of the generalized wideband ambiguity function (GAF). The maximum likelihood (ML) estimator is derived and its local and global properties are discussed. A structure is proposed which searches the three-dimensional ambiguity surface in two stages first, in range-Doppler, and then, in bearing with the goal of reducing search complexity when utilizing highly resolvent waveforms. Comparisons are made between the ML estimators and structures utilizing phase information which generate closed form estimators. The beneficial results of full bandwidth utilization are discussed in terms of both local and global properties of the GAF. 相似文献
938.
The relationships of type Pi (broadband) pulsations to various other substorm-related phenomena are reviewed. Several of the more popular mechanisms for the origin of Pi activity are discussed in the light of the observations. There is only one mechanism in sight that tentatively accounts for observed characteristics of Pi 1–2 activity at auroral oval and polar cap latitudes and that is the three-dimensional current loop mechanism. If two or more mechanisms are involved in the generation of Pi noise, then it is possible that the garden-hose overstability and/or a drift Alfvén wave mechanism operating in the plasma sheet contribute to the observed pulsations.The common feature of all Pi 1–2 events is not the presence of temporal precipitation pulsations but the presence of an E-region, suggesting that enhanced conductivity and E-region currents are required. Pi activity appears to be closely related to unsteady convection in progress. Pi data promise to provide useful information on convection and field-aligned and ionospheric currents. 相似文献
939.
Ramachandra K.V. Srinivasan V.S. 《IEEE transactions on aerospace and electronic systems》1977,(4):419-423
An X, Y, Z Kalman tracking filter is described and its steady state characteristics are analytically determined when the radar sensor meaures range, bearing, and elevation (?, ?, ?) at uniform intervals of time, T seconds. The relationship between the quantities measured by the sensor (?, ?,?) and the Cartesian coordinate system (X, Y, Z) is explicitly considered. 相似文献
940.
M.J. Smith J.B. Gregory A.H. Manson C.E. Meek R. Schminder D. Ku¨rschner K. Labitzke 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1982,2(10):173-176
The global developments of the four stratospheric events (~20–50 km) are described, using balloon, satellite and rocket data. Winds data for heights of 60–95 km, derived from L.F. drift (52°N, 15°E; Europe) and M.F. radar (52°N, 107°W; Canada), are then compared with the stratospheric data. There is clear evidence that the preliminary planetary waves often penetrate strongly to ~90 km, and that mean wind reversals also occur. However, there are dramatic differences between European and Canadian mesospheric responses. 相似文献