全文获取类型
收费全文 | 5149篇 |
免费 | 16篇 |
国内免费 | 15篇 |
专业分类
航空 | 2235篇 |
航天技术 | 1694篇 |
综合类 | 26篇 |
航天 | 1225篇 |
出版年
2021年 | 51篇 |
2019年 | 25篇 |
2018年 | 195篇 |
2017年 | 145篇 |
2016年 | 125篇 |
2015年 | 51篇 |
2014年 | 145篇 |
2013年 | 175篇 |
2012年 | 156篇 |
2011年 | 237篇 |
2010年 | 175篇 |
2009年 | 265篇 |
2008年 | 267篇 |
2007年 | 162篇 |
2006年 | 105篇 |
2005年 | 150篇 |
2004年 | 147篇 |
2003年 | 164篇 |
2002年 | 112篇 |
2001年 | 159篇 |
2000年 | 72篇 |
1999年 | 91篇 |
1998年 | 117篇 |
1997年 | 80篇 |
1996年 | 103篇 |
1995年 | 148篇 |
1994年 | 108篇 |
1993年 | 75篇 |
1992年 | 100篇 |
1991年 | 39篇 |
1990年 | 38篇 |
1989年 | 84篇 |
1988年 | 33篇 |
1987年 | 36篇 |
1986年 | 35篇 |
1985年 | 141篇 |
1984年 | 105篇 |
1983年 | 93篇 |
1982年 | 96篇 |
1981年 | 142篇 |
1980年 | 43篇 |
1979年 | 42篇 |
1978年 | 37篇 |
1977年 | 39篇 |
1976年 | 32篇 |
1975年 | 29篇 |
1974年 | 29篇 |
1972年 | 27篇 |
1970年 | 22篇 |
1969年 | 24篇 |
排序方式: 共有5180条查询结果,搜索用时 625 毫秒
861.
The availability and ease of programming for many computer aided design (CAD) languages permits their use where, formerly, bread boarding or analytical means would have been employed to solve specific circuit analysis problems. One of many available CAD languages is the electronic circuit analysis program (ECAP). The ECAP language is divided into three sections, dc-analysis. acanalysis, and transient analysis. The three modes of ECAP together with fast computer access provide a helpful tool for the design engineer. Following is a typical problem of oscillatory transients encountered in line-type modulators. The solution to the problem is a model of sufficient correlation with the observed oscillation as an aid to find a network which will critically damp the oscillation. 相似文献
862.
The leakage reactance of the machine is expressed as an equation involving slot leakage, end-connection leakage, differential leakage, stator-halves leakage, and tooth-top leakage reactance. This leakage reactance equation, or objective function, is then minimized under suitable constraints. Geometric programming is used to carry through the optimization procedure. A computer program is developed and applied to find the optimum leakage reactance for a 95 kVA, 208 V, wye-connected, 40 800 r/min aerospace alternator, using 8-, 10-, and 12-pole machines. 相似文献
863.
Space radiation dosimetry with active detections for the scientific program of the second Bulgarian cosmonaut on board the Mir space station. 总被引:1,自引:0,他引:1
J V Dachev TsPMatviichuk YuN Semkova R T Koleva B Boichev P Baynov N A Kanchev P Lakov P T Ivanov YaJTomo V M Petrov V I Redko V I Kojarinov R Tykva 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1989,9(10):247-251
A dosimetry-radiometry system has been developed at the Space Research Institute of the Bulgarian Academy of Science to measure the fluxes and dose rates on the flight of the second Bulgarian cosmonaut. The dosimetry system is designed for monitoring the different space radiations, such as solar cosmic rays, galactic cosmic rays and trapped particles in the earth radiation belts. The system consists of a battery operated small size detector unit and a "read-write" and telemetry microcomputer unit. The sensitivity of the instrument (3.67 x 10(-8) rad/pulse) permits high resolution measurements of the flux and dose rate along the track of the Mir space station. We report our initial results for the period of the flight between the 7th and 17th June 1988. 相似文献
864.
This review paper is a survey of infrared astronomy up to early 1969. The techniques and photometric standards are mentioned briefly, and results cover solar, lunar, and planetary observations. Point sources and extended sources both within and beyond the Galaxy are included, ending with the problem of cosmic background radiation. It is concluded that great progress will be possible when large infrared telescopes are placed above the atmosphere in orbit for extended periods of time.This work was conducted under the McDonnell Douglas Corporation Independent Research and Development Program. 相似文献
865.
A two-element adaptively phased array withelement gains of 18 dB was assembled andevaluated at 136 MHz. The gain improvementof the system over that of a single element wasmeasured while tracking ATS-C. The measuredgain improvement fell within 0.5 dB ofthe theoretical maximum 90 percent of thetime, corresponding to a normalized improvementof 2.5 dB for the two-element array. Athorough analysis of the AGC weighting processused in the system was made with particularemphasis placed on the effects of gainand noise figure variations between arraychannels. 相似文献
866.
867.
F. Fárník S. Fischer L. Křivský B. Valníček O.B. Likin V.N. Lutsenko 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1981,1(13):243-246
During the observation of solar cosmic rays on the Prognoz 6 and Helios 1 and 2 spacecrafts, several events with anomalous composition of accelerated particles (higher abundance of 3-He or Fe nuclei) occurred. We found seven such events from the period September to December 1977 for which data from the Prognoz 6 solar X-ray photometer are available. This material together with published optical and radio data from terrestrial observatories enabled us to identify more reliably the source flares and describe their characteristics. It turned out that the character of X-ray emission accompanying the emission of accelerated particles with anomalous composition shows no pronounced difference from other flares. No correlation has been found among the ratio 3-He/4-He and the angular distance between the field lines connected with the source flare and the place of observation. If a solar flare with anomalous ratio 3-He/4-He appears in a given active region, this region will probably produce other anomalous events. 相似文献
868.
Several forms of sequential hypothesis testing algorithms are described and their performance as classification algorithms for automatic target recognition is evaluated and compared. Several forms of parameteric algorithms, as well as a sequential form of a useful nonparametric algorithm are considered. The primary focus is the design of algorithms for automatic target recognition that produce maximally reliable decisions while requiring, on the average, a minimum number of backscatter measurements. The tradeoffs between the average number of required measurements and the error performance of the resulting algorithms are compared by means of Monte-Carlo simulation studies 相似文献
869.
An overview is given of the Productivity, Reliability, Availability, and Maintainability (PRAM) Project 00-256, which aims to integrate the Digital Multimedia Information System (DMIS) into the Reconnaissance Module Automatic Test System (RMATS). Benefits to be derived from the incorporation of the DMIS and return on investment for the US Air Force are discussed. The process used to obtain approval and funding support for this advanced maintenance aiding and data collection system is described 相似文献
870.
P S Haskins J E McKisson A G Weisenberger D W Ely T A Ballard C S Dyer P R Truscott R B Piercey A V Ramayya D C Camp 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1992,12(2-3):461-464
The Shuttle Activation Monitor (SAM) experiment was flown on the Space Shuttle Columbia (STS-28) from 8-13 August, 1989 in a 57 degrees, 300 km orbit. One objective of the SAM experiment was to determine the relative effect of different amounts of shielding on the gamma-ray backgrounds measured with similarly configured sodium iodide (NaI) and bismuth germante (BGO) detectors. To achieve this objective twenty-four hours of data were taken with each detector in the middeck of the Shuttle on the ceiling of the airlock (a high-shielding location) as well as on the sleep station wall (a low-shielding location). For the cosmic-ray induced background the results indicate an increased overall count rate in the 0.2 to 10 MeV energy range at the more highly shielded location, while in regions of trapped radiation the low shielding configuration gives higher rates at the low energy end of the spectrum. 相似文献