全文获取类型
收费全文 | 8538篇 |
免费 | 21篇 |
国内免费 | 24篇 |
专业分类
航空 | 4233篇 |
航天技术 | 2995篇 |
综合类 | 38篇 |
航天 | 1317篇 |
出版年
2021年 | 65篇 |
2019年 | 52篇 |
2018年 | 144篇 |
2017年 | 89篇 |
2016年 | 86篇 |
2014年 | 182篇 |
2013年 | 229篇 |
2012年 | 206篇 |
2011年 | 289篇 |
2010年 | 204篇 |
2009年 | 339篇 |
2008年 | 428篇 |
2007年 | 223篇 |
2006年 | 210篇 |
2005年 | 236篇 |
2004年 | 190篇 |
2003年 | 274篇 |
2002年 | 169篇 |
2001年 | 288篇 |
2000年 | 156篇 |
1999年 | 203篇 |
1998年 | 237篇 |
1997年 | 179篇 |
1996年 | 220篇 |
1995年 | 278篇 |
1994年 | 259篇 |
1993年 | 160篇 |
1992年 | 192篇 |
1991年 | 113篇 |
1990年 | 84篇 |
1989年 | 199篇 |
1988年 | 97篇 |
1987年 | 105篇 |
1986年 | 85篇 |
1985年 | 263篇 |
1984年 | 210篇 |
1983年 | 181篇 |
1982年 | 201篇 |
1981年 | 262篇 |
1980年 | 92篇 |
1979年 | 72篇 |
1978年 | 74篇 |
1977年 | 66篇 |
1976年 | 63篇 |
1975年 | 75篇 |
1974年 | 56篇 |
1973年 | 58篇 |
1972年 | 69篇 |
1971年 | 60篇 |
1970年 | 59篇 |
排序方式: 共有8583条查询结果,搜索用时 15 毫秒
621.
Belter S.E. Williams C.R. Bass S.C. 《IEEE transactions on aerospace and electronic systems》1976,(6):698-705
The application of 4-bit microprocessors to the communication of the control and status signals found in an advanced aircraft instrument landing system is presented. The advantages as well as some of the peculiarities of such an approach are described. Among the benefits, one must include the possibility of a very convenient and understandable human interface. 相似文献
622.
R. S. Polidan 《Space Science Reviews》1989,49(3-4):85-94
In this paper we present new results from the Voyager ultraviolet spectrometers and the IUE spacecraft on V356 Sgr and Lyr. The V356 Sgr observations cover, in detail, two eclipses and include one IUE high dispersion SWP image. During both eclipses the total strength of the UV emission lines were found to be invariant. Also, an uneclipsed UV continuum was detected at wavelengths shorter than 1600 Å. The IUE high dispersion SWP spectrum revealed that the emission lines are extremely broad, almost symmetrical lines with weak, slightly blue shifted absorption components. No evidence of carbon is seen in the emission or absorption spectrum of V356 Sgr in eclipse. A model for the origin of the circumstellar matter in this binary system is presented. The Voyager ultraviolet observations of Lyr show a strong far-UV continuum that is detectable down to 912 Å The far-UV continuum flux level was variable on time scales shorter than the orbital period and displayed no obvious orbital modulation or eclipses. The spectral shape of the far-UV continuum closely resembles that of a UX UMa type cataclysmic variable. On 16 August 1985 an rapid brightening of the far-UV continuum was observed which was also reminiscent of cataclysmic variables. Analysis of the Lyr data suggest that the central object must be small, with a radius on the order of 1 R or less.This work is supported by NASA Grants NAGW-587 and NAG5-441. 相似文献
623.
A design method is proposed for a class of nonparametric truncated sequential detectors. These detectors test nonparametric statistics against two parallel linear boundaries with an abrupt truncation at some sample size. The proposed method obtains the asymptotic relative efficiencies (ARE) of these tests with respect to their corresponding fixed-sample-size (FSS) tests in terms of some parameters of the tests. There parameters are then chosen to optimize the ARE. This (asymptotically) optimal set of parameters is used to design the thresholds of the sequential tests. Numerical results are obtained and design examples are presented, using the sum of the signs of the observations as the test statistic. The method can be used for nonparametric sequential detectors and for robust and parametric sequential detectors as well 相似文献
624.
High-frequency-link power conversion and distribution based on a resonant inverter has been recently proposed. The design of several topologies is reviewed and a simple approximate design procedure is developed for the phase-controlled parallel-loaded resonant inverter. This design procedure seeks to ensure the expected benefits of resonant conversion and is verified by data from a laboratory 2.5 kVA, 20-kHz converter. A simple phasor analysis is introduced as a useful approximation for design purposes. Experimental results show that under transient conditions such as load short-circuit, a reversal of the expected commutation sequence is possible. This should be accounted for in the design of the power circuit, or prevented by the design of the controller 相似文献
625.
Frequency estimation techniques for high dynamic trajectories 总被引:7,自引:0,他引:7
Vilnrotter V.A. Hinedi S. Kumar R. 《IEEE transactions on aerospace and electronic systems》1989,25(4):559-577
A comparison is presented of four different estimation techniques applied to the problem of continuously estimating the rapidly varying parameters of a sinusoidal signal, observed in the presence of additive noise. Frequency estimates are emphasized, although phase and/or frequency rate are also estimated by some of the algorithms. These parameters are related to the velocity, position, and acceleration of the maneuvering receiver or transmitter. Estimated performance at low carrier-to-noise ratios and high dynamics is investigated for the purpose of determining the useful operating range of an approximate maximum likelihood estimator, an extended Kalman filter, a cross-product automatic frequency loop and a phase-locked loop. Numerical simulations are used to evaluate performance while tracking a common trajectory exhibiting high dynamics 相似文献
626.
Moyer L.R. Morgan C.J. Rugger D.A. 《IEEE transactions on aerospace and electronic systems》1989,25(4):584-587
An exact expression for the bistatic resolution-cell area (A B) is developed for the special case in which either the transmit or receive antenna has a broad azimuth beamwidth or is omnidirectional. Quantitative examples are presented to illustrate the variation of A B with location. A comparison is made with the performance of a commonly used approximation formula 相似文献
627.
628.
Allen M.R. Katz S.L. Urkowitz H. 《IEEE transactions on aerospace and electronic systems》1989,25(5):689-700
Long-term integration is defined as integration, perhaps interrupted, over time periods long enough for targets to move through volumes in space resolvable by the radar. Because the motion of the target is unknown prior to detection, long-term integration must be performed along multiple paths representing plausible target paths. The geometry of such a set of integration paths affects detection performance in several ways. The simplest implementation of long-term integration, using constant radial velocity paths, is investigated. The effects of path geometry on detection is quantified and optimized for a target whose motion is nearly radial but otherwise unknown 相似文献
629.
McCormick W.S. Tsui J.B.Y. Bakkie V.L. 《IEEE transactions on aerospace and electronic systems》1989,25(5):729-732
The instantaneous frequency measurement (IFM) receiver is capable of measuring the center frequency of single frequency pulses over a wide range (bandwidth) of center frequencies. Because of various constraints, the frequency resolution requirement results in long correlator delay times that reduce the single correlator bandwidth. A large bandwidth can be achieved only if two or more correlators are used. The problem of estimating frequency is then reduced to the simultaneous congruence problem of number theory. A design procedure is presented for solving the congruence problem for a given amount of noise protection, a stated frequency resolution, a minimum bandwidth, and a fixed level of precision (bits) in the IFM receiver 相似文献
630.
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