全文获取类型
| 收费全文 | 2730篇 |
| 免费 | 10篇 |
| 国内免费 | 7篇 |
专业分类
| 航空 | 1272篇 |
| 航天技术 | 958篇 |
| 综合类 | 11篇 |
| 航天 | 506篇 |
出版年
| 2021年 | 21篇 |
| 2019年 | 20篇 |
| 2018年 | 64篇 |
| 2017年 | 39篇 |
| 2016年 | 40篇 |
| 2015年 | 15篇 |
| 2014年 | 61篇 |
| 2013年 | 98篇 |
| 2012年 | 64篇 |
| 2011年 | 98篇 |
| 2010年 | 78篇 |
| 2009年 | 136篇 |
| 2008年 | 143篇 |
| 2007年 | 71篇 |
| 2006年 | 70篇 |
| 2005年 | 72篇 |
| 2004年 | 70篇 |
| 2003年 | 92篇 |
| 2002年 | 48篇 |
| 2001年 | 97篇 |
| 2000年 | 52篇 |
| 1999年 | 66篇 |
| 1998年 | 74篇 |
| 1997年 | 67篇 |
| 1996年 | 70篇 |
| 1995年 | 89篇 |
| 1994年 | 86篇 |
| 1993年 | 43篇 |
| 1992年 | 52篇 |
| 1991年 | 15篇 |
| 1990年 | 24篇 |
| 1989年 | 51篇 |
| 1988年 | 32篇 |
| 1987年 | 23篇 |
| 1986年 | 31篇 |
| 1985年 | 78篇 |
| 1984年 | 71篇 |
| 1983年 | 44篇 |
| 1982年 | 71篇 |
| 1981年 | 71篇 |
| 1980年 | 19篇 |
| 1978年 | 31篇 |
| 1977年 | 14篇 |
| 1975年 | 24篇 |
| 1974年 | 17篇 |
| 1973年 | 16篇 |
| 1972年 | 17篇 |
| 1969年 | 13篇 |
| 1967年 | 13篇 |
| 1966年 | 13篇 |
排序方式： 共有2747条查询结果，搜索用时 0 毫秒
201.
Fabrizio G.A. Gershman A.B. Turley M.D. 《IEEE transactions on aerospace and electronic systems》2004,40(2):510-525
Adaptive beamforming is used to enhance the detection of target echoes received by high frequency (HF) surface wave (HFSW) over-the-horizon (OTH) radars in the presence of spatially structured interference. External interference from natural and man-made sources typically masks the entire range-Doppler search space and is characterized by a spatial covariance matrix that is time-varying or nonstationary over the coherent processing interval (CPI). Adaptive beamformers that update the spatial filtering weight vector within the CPI are likely to suppress such interference most effectively, but the intra-CPI antenna pattern fluctuations result in temporal decorrelation of the clutter which severely degrades subclutter visibility after Doppler processing. A robust adaptive beamformer that effectively suppresses spatially nonstationary interference without degrading subclutter visibility is proposed here. The proposed algorithm is computationally efficient and suitable for practical implementation. Its operational performance is evaluated using experimental data recorded by the Iluka HFSW OTH radar, located near Darwin in far north Australia. 相似文献
202.
Application of digital cross-correlation spectroscopy to the spectra of the W Serpentis binaries SX Cas and RX Cas has allowed an accurate determination of the orbits and rotations of the (mass-losing) K-subgiant secondary components. The distortion of the primary radial-velocity curves due to the influence of the prominent accretion disks in these systems has been modelled to first order. This enables us to estimate k 1, and thereby the mass ratio q ≈ 0.30, to within ～ ± 20%. The absolute radii of the secondaries are derived independently from the observed rotations and periods, assuming synchronous rotation. They show that the stars fill their Roche lobes, or at least very nearly so. Rough fits to the available photometry shows both primaries to be unevolved mid-B stars; that in RX Cas appears completely obscured by the disk. Preliminary spectroscopic data for W Ser and W Cru show some promise for similar analyses of these systems. 相似文献
203.
Shats S. Bobrovsky B.Z. Shaked U. 《IEEE transactions on aerospace and electronic systems》1988,24(6):670-677
The problem of minimum variance discrete-time state estimation of a continuous-time double integrator via noisy continuous-time measurements is considered. The error covariance matrices of this estimation are calculated and analyzed. The relations between these covariance matrices and the error covariance matrix of the optimal continuous-time filter are obtained, and a way for determining the required sampling period is proposed. A commonly used approximated model is investigated; it is shown to be inappropriate unless a specific improvement is introduced in the model 相似文献
204.
Reid D.B. Gesing W.S. Mcwilliam B.N. Gibson J.R. 《IEEE transactions on aerospace and electronic systems》1983,(5):751-760
An airborne system has been developed for charting shallow coastal and inland waters. The primary components of this system are an aerial survey camera, a profiling laser radar, an analytical stereo plotter, and a multisensor track recovery system (TRS). The TRS comprises a gimbaled inertial navigation system and a number of auxiliary sensors which acquire redundant position and attitude information. The sensor data are combined postmission using a U-D factorized Kalman filter and modified Bryson-Frazier smoother to compute accurate estimates of the orientation parameters of the survey camera at the times of film exposure. These parameters are used to position each overlapping pair of photographs on the analytical plotter to form a stereo image and corresonding analytical stereomodel from which water depth measurements are made. Flight trial results demonstrate that the TRS can achieve radial position and attitude accuracies which exceed 1 m and 2 arcmin root mean square (rms), respectively, and that this level of performance is sufficient to enable water depth measurements to be made to an accuracy of better than 0.65 m (rms). 相似文献
205.
A brief summary of the main results of magnetospheric ion composition measurements in general is first presented. PROGNOZ-7 measurements in the nightside plasma mantle are then described and analyzed. Some of the results are the following: In the nightside mantle not too far from midnight the properties of the mantle are sometimes consistent with the open magnetosphere model. However during most magnetic storm situations O+ ions appear in the mantle in large proportions and with high energies. The acceleration process affecting the ions has been found in several cases to give equal amounts of energy to all ions independent of mass. Along the flanks of the magnetosphere the flow of the plasma is often low or absent. The O+ content is high (up to 20%) and the energy spectrum of both ions and electrons may be very hot, even up to the level of the ring current plasma in the keV range.The O+ content in the plasma mantle is positively correlated with the magnetospheric activity level. The mantle, however, does not appear to be the dominating source for the storm time ring current. Direct acceleration of ionospheric ions onto the closed field lines of the plasma sheet and ring current is most likely the main source. The magnetopause on the nightside and along the flanks of the magnetosphere appears to be a fairly solid boundary for mantle ions of ionospheric origin. This is especially evident during periods with high geomagnetic activity, when the mantle is associated with fairly strong fluxes of O+ ions.An interesting observation in most of the mantle passages during geomagnetically disturbed periods is the occurrence of intense, magnetosheath like, regions deep inside the mantle. In some cases these regions with strong antisunward flow and with predominant magnetosheath ion composition was observed in the innermost part of the mantle, i.e. marking a boundary region between the lobe and the mantle. These magnetosheath penetration events are usually associated with strong fluxes of accelerated ionospheric ions in nearby parts of the mantle. Evanescent penetration regions with much reduced flow properties are frequently observed in the flank mantle. 相似文献
206.
Rijsdijk F.B. van der Spek G.A. 《IEEE transactions on aerospace and electronic systems》1978,(2):226-236
For a planar-array antenna with a monopulse feed horn, this study describes a simple algorithm for the determination of the direction of target echoes. Antenna pattern measurements of the array indicate that the direction sines of a received wavefront can be independently obtained with one simple relation between a normalized difference channel output and a direction sine. This paper determines the accuracy of the algorithm. 相似文献
207.
The prime purpose of this paper is to describe the characteristics and application of high powered Zener diodes as transient suppressors for transients emanating in ground vehicle and aircraft power supplies. Information is presented on voltage transients as specified in MIL-STD-704 for aircraft and MIL-STD-1275(AT) for military ground vehicles and the ability of advanced high power Zener diodes in suppressing the levels of voltages specified therein. A complete transient suppressor capable of operating externally to the equipment intended for protection is also described. 相似文献
208.
Cosmic Research - An Erratum to this paper has been published: https://doi.org/10.1134/S0010952521120017 相似文献
209.
An iteration method for airfoil shape completing using the pressure coefficient distribution specified on its sought part is proposed. The incompressible flow viscosity is taken into account by the boundary layer model, the iteration process is constructed with the use of G.Yu. Stepanov’s idea. The solution algorithm is compiled and a set of numerical calculations is carried out. It is shown that the method proposed offers advantages over the well-known numerical-analytical scheme of solving mixed inverse boundary-value problems of aerohydrodynamics in the case of viscous incompressible fluid. 相似文献
210.
G.L. Smith K.J. Priestley N.G. Loeb B.A. Wielicki T.P. Charlock P. Minnis D.R. Doelling D.A. Rutan 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2011
The Clouds and Earth Radiant Energy System (CERES) project’s objectives are to measure the reflected solar radiance (shortwave) and Earth-emitted (longwave) radiances and from these measurements to compute the shortwave and longwave radiation fluxes at the top of the atmosphere (TOA) and the surface and radiation divergence within the atmosphere. The fluxes at TOA are to be retrieved to an accuracy of 2%. Improved bidirectional reflectance distribution functions (BRDFs) have been developed to compute the fluxes at TOA from the measured radiances with errors reduced from ERBE by a factor of two or more. Instruments aboard the Terra and Aqua spacecraft provide sampling at four local times. In order to further reduce temporal sampling errors, data are used from the geostationary meteorological satellites to account for changes of scenes between observations by the CERES radiometers. 相似文献