全文获取类型
收费全文 | 19115篇 |
免费 | 39篇 |
国内免费 | 126篇 |
专业分类
航空 | 10244篇 |
航天技术 | 5627篇 |
综合类 | 244篇 |
航天 | 3165篇 |
出版年
2021年 | 157篇 |
2018年 | 273篇 |
2017年 | 159篇 |
2016年 | 180篇 |
2014年 | 434篇 |
2013年 | 516篇 |
2012年 | 446篇 |
2011年 | 661篇 |
2010年 | 492篇 |
2009年 | 846篇 |
2008年 | 865篇 |
2007年 | 449篇 |
2006年 | 431篇 |
2005年 | 436篇 |
2004年 | 470篇 |
2003年 | 545篇 |
2002年 | 493篇 |
2001年 | 615篇 |
2000年 | 367篇 |
1999年 | 465篇 |
1998年 | 460篇 |
1997年 | 336篇 |
1996年 | 412篇 |
1995年 | 483篇 |
1994年 | 457篇 |
1993年 | 364篇 |
1992年 | 342篇 |
1991年 | 249篇 |
1990年 | 242篇 |
1989年 | 418篇 |
1988年 | 210篇 |
1987年 | 242篇 |
1986年 | 245篇 |
1985年 | 644篇 |
1984年 | 526篇 |
1983年 | 413篇 |
1982年 | 493篇 |
1981年 | 622篇 |
1980年 | 246篇 |
1979年 | 190篇 |
1978年 | 189篇 |
1977年 | 146篇 |
1976年 | 156篇 |
1975年 | 190篇 |
1974年 | 181篇 |
1973年 | 161篇 |
1972年 | 188篇 |
1971年 | 148篇 |
1970年 | 143篇 |
1969年 | 147篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
951.
Optimal speckle reduction in polarimetric SAR imagery 总被引:9,自引:0,他引:9
Speckle is a major cause of degradation in synthetic aperture radar (SAR) imagery. With the availability of fully polarimetric SAR data, it is possible to use the three complex elements (HH , HV , VV ) of the polarimetric scattering matrix to reduce speckle. The optimal method for combining the elements of the scattering matrix to minimize image speckle is derived, and the solution is shown to be a polarimetric whitening filter (PWF). A simulation of spatially correlated, K -distributed, fully polarimetric clutter is then used to compare the PWF with other, suboptimal speckle-reduction methods. Target detection performance of the PWF, span, and single-channel |HH |2 detectors is compared with that of the optimal polarimetric detector (OPD). A novel, constant-false-alarm-rate (CFAR) detector (the adaptive PWF) is as a simple alternative to the OPD for detecting targets in clutter. This algorithm estimates the polarization covariance of the clutter, uses the covariance to construct the minimum-speckle image, and then tests for the presence of a target. An exact theoretical analysis of the adaptive PWF is presented; the algorithm is shown to have detection performance comparable with that of the OPD 相似文献
952.
Cascaded detector for multiple high-PRF pulse Doppler radars 总被引:1,自引:0,他引:1
A postdetection design methodology for a multiple high-pulse-repetition frequency (PRF) pulse Doppler radar has been developed. The postdetection processor consists of an M out of N detector where range and target ambiguities are resolved, followed by a square-law detector which enhances the minimum signal-to-noise (S/N) power-ratio per pulse burst performance. For given probabilities of false alarm and detection, formulas are derived from which the three thresholds associated with the cascaded detector can be found. Fundamental tradeoffs between the minimum S/N required, number of ghosts, and the number of operations (NOPs) that the cascaded detector must perform are identified. It is shown that the NOPs and the number of ghosts increase and the minimum S/N required decreases as the binary M out of N detector passes more detections to the square-law detector 相似文献
953.
954.
S. Hayakawa J. Kuijpers J. Kleczek Boris Valnicek L. Kresak H. R. De Ruiter 《Space Science Reviews》1991,57(1-2):189-192
955.
The proposed KAAD (knowledge-based automated air defense) system demonstrates a man-machine environment for airspace defense systems. When the unknown aircraft is hostile, a threat rating and response methods are generated by the system. It serves as a double-check decision-making system for a war control center. In addition to this application, the KAAD system can also be a useful tool as a training program for the war controller. The capabilities of the system are limited due to the shortage of knowledge resources. It requires communications among war controllers and air fighter pilots to organize a practical knowledge base. It is shown that the KAAD system can be combined with an automated ATC (air traffic control) system to become a practical system for air defense applications 相似文献
956.
A white light coronagraph was launched into orbit aboard the space shuttle OV 103 (Discovery) on 7 April 1993. This device was one of two instruments included in the SPARTAN 201-1 payload, a completely autonomous sub-satellite deployed from the shuttle for a period of about 47 hours. The WLC system is an externally occulted coronagraph system which incorporates a rotating half-wave plate polarimeter, and the image data is used to infer the brightness, the polarized brightness and the degree of polarization of the white light emission from the solar corona. These data are in turn used to infer estimates of the K- and F-coronal brightnesses and density distributions. We shall present preliminary results of the electron density estimate in the coronal streamer and hole region and describe the methods employed.affiliated to USRA 相似文献
957.
B. Holback S. -E. Jansson L. Åhlén G. Lundgren L. Lyngdal S. Powell A. Meyer 《Space Science Reviews》1994,70(3-4):577-592
The Wave Experiment, F4, on the Swedish/German satelliteFreja, is designed to measure the electric wave fields up to 4 MHz, the magnetic wave fields up to 16 kHz and the plasma density and its relative variations up to 2 kHz. Six wave signals and four density probe signals can be measured simultaneously. The wave forms of all signals are transmitted to ground without any analysis onboard. The limited TM allocation does not allow continuous sampling of the wave signals, so normally the measurements are made in snapshots of various lengths dependent on sampling frequency, etc. Continuous sampling can be made for shorter time periods by using a 6 Mbyte memory as a buffer. 相似文献
958.
Profiles of the visible Fe X (6374 Å) coronal emission line as a function of height above the limb were obtained out to 1.16 solar radii in a coronal hole using the NSO/Sacramento Peak Observatory Coronagraph, Universal Spectrograph and a CCD camera. These are the first coronal line profiles obtained as a function of height in a coronal hole from the ground. Analysis of the line widths suggests a large component of nonthermal broadening which increases with height ranging from 40 to 60 km/s, depending upon the assumed temperature or thermal component of the profile. 相似文献
959.
Numerical simulations of energy depositions in the middle and upper solar chromosphere result in ejection of chromospheric material into the corona and heating of the chromospheric gas. These simulations may be capable of describing some of the features seen by the soft X-ray telescope on board theYohkoh satellite. 相似文献
960.
Bhanu B. Das S. Symosek P. Snyder S. Roberts B. 《IEEE transactions on aerospace and electronic systems》1994,30(3):709-721
Range measurements to objects in the world relative to mobile platforms such as ground or air vehicles are critical for visually aided navigation and obstacle detection/avoidance. An approach is presented that consists of a synergistic combination of two types of passive ranging method: binocular stereo and motion stereo. We show a new way to model the errors in binocular and motion stereo in conjunction with an inertial navigation system (INS) and derive the appropriate Kalman filter to refine the estimates from these two stereo ranging techniques. We present results using laboratory images that show that refined estimates can be optimally combined to give range values which are more accurate than any one of the individual estimates from binocular and motion stereo. By incorporating a blending filter, the approach has the potential of providing accurate, dense range measurements for all the pixels in the field of view (FOV) 相似文献