全文获取类型
收费全文 | 2831篇 |
免费 | 7篇 |
国内免费 | 8篇 |
专业分类
航空 | 1292篇 |
航天技术 | 1167篇 |
综合类 | 14篇 |
航天 | 373篇 |
出版年
2021年 | 23篇 |
2019年 | 27篇 |
2018年 | 58篇 |
2017年 | 26篇 |
2016年 | 40篇 |
2015年 | 12篇 |
2014年 | 62篇 |
2013年 | 77篇 |
2012年 | 68篇 |
2011年 | 92篇 |
2010年 | 66篇 |
2009年 | 119篇 |
2008年 | 190篇 |
2007年 | 83篇 |
2006年 | 70篇 |
2005年 | 68篇 |
2004年 | 76篇 |
2003年 | 93篇 |
2002年 | 64篇 |
2001年 | 87篇 |
2000年 | 55篇 |
1999年 | 60篇 |
1998年 | 77篇 |
1997年 | 52篇 |
1996年 | 60篇 |
1995年 | 85篇 |
1994年 | 86篇 |
1993年 | 51篇 |
1992年 | 70篇 |
1991年 | 36篇 |
1990年 | 30篇 |
1989年 | 68篇 |
1988年 | 24篇 |
1987年 | 30篇 |
1986年 | 28篇 |
1985年 | 102篇 |
1984年 | 73篇 |
1983年 | 66篇 |
1982年 | 77篇 |
1981年 | 88篇 |
1980年 | 29篇 |
1979年 | 39篇 |
1978年 | 25篇 |
1977年 | 17篇 |
1976年 | 12篇 |
1975年 | 11篇 |
1974年 | 14篇 |
1973年 | 15篇 |
1972年 | 14篇 |
1970年 | 12篇 |
排序方式: 共有2846条查询结果,搜索用时 15 毫秒
671.
The Magnetospheric Multiscale Magnetometers 总被引:2,自引:0,他引:2
C. T. Russell B. J. Anderson W. Baumjohann K. R. Bromund D. Dearborn D. Fischer G. Le H. K. Leinweber D. Leneman W. Magnes J. D. Means M. B. Moldwin R. Nakamura D. Pierce F. Plaschke K. M. Rowe J. A. Slavin R. J. Strangeway R. Torbert C. Hagen I. Jernej A. Valavanoglou I. Richter 《Space Science Reviews》2016,199(1-4):189-256
The success of the Magnetospheric Multiscale mission depends on the accurate measurement of the magnetic field on all four spacecraft. To ensure this success, two independently designed and built fluxgate magnetometers were developed, avoiding single-point failures. The magnetometers were dubbed the digital fluxgate (DFG), which uses an ASIC implementation and was supplied by the Space Research Institute of the Austrian Academy of Sciences and the analogue magnetometer (AFG) with a more traditional circuit board design supplied by the University of California, Los Angeles. A stringent magnetic cleanliness program was executed under the supervision of the Johns Hopkins University’s Applied Physics Laboratory. To achieve mission objectives, the calibration determined on the ground will be refined in space to ensure all eight magnetometers are precisely inter-calibrated. Near real-time data plays a key role in the transmission of high-resolution observations stored on board so rapid processing of the low-resolution data is required. This article describes these instruments, the magnetic cleanliness program, and the instrument pre-launch calibrations, the planned in-flight calibration program, and the information flow that provides the data on the rapid time scale needed for mission success. 相似文献
672.
An extension to the basic concept of correlation detection as a means of image registration is developed. The technique involves linear spatial preprocessing of the inages to be registered prior to the application of a correlation measure. This preprocessing operation utilizes the spatial correlation within each image and greatly improves the detectability of image misregistration. An analysis of the computational aspects of the algorithm is given. Also, results of a computer simulation to evaluate the technique are given. 相似文献
673.
674.
The problem of stabilizing a tumbling satellite prior to attitude control is discussed. The method of Hamilton-Jacobi theory is used to derive an explicit closed-loop control law for the problem. 相似文献
675.
Tanvir Islam Miguel A. Rico-RamirezDawei Han Prashant K. Srivastava 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2012
The separation of rain types in convective and stratiform regimes has long been a goal in microwave remote sensing of precipitation research. In this essence, a dual polarized radar based indexing scheme that provides information on convective and stratiform (C/S) rain regimes has been presented in correspondence with advanced microwave scanning radiometer – earth observing system (AMSR-E) GSFC profiling algorithm estimate of convective rain percentage. The dual polarized radar based C/S indexing scheme first retrieves the normalized gamma drop size distribution parameters, median volume drop diameter (D0) and concentration parameter (Nw), from dual polarized radar measurements ZH and ZDR, representing reflectivity and differential reflectivity respectively, by means of the genetic programming approach. Next, the C/S rain index is calculated based on the formulation of an empirical relation in Nw–D0 domain. The scheme has been inspected and applied on measurements from the S-band Chilbolton dual polarized radar. A considerable number of “coincident” cases from the radar and the AMSR-E observations are investigated. It has been revealed that the dual polarized radar based C/S rain indexing is in a similar pattern with the AMSR-E GSFC profiling algorithm estimate of convective rain percentage. Generally, as C/S rain index value increases, which signifies a stratiform to convective trend, the AMSR-E convective rain percentage also increases. 相似文献
676.
Mcdevitt J. B. Harrison D. R Lockman W. K. 《IEEE transactions on aerospace and electronic systems》1966,(1):2-12
A measurement system for obtaining pressure and heat-transfer data from models flying freely in hypersonic tunnel streams is described. Test models are either released from suspension threads or launched by low-velocity pneumatic launchers and have internal miniaturized frequency-modulated telemetry units. This technique permits the historically desired measurement of support-free afterbody pressure and heat-transfer rates. Base heating and pressure measurements for a cone and a sphere in free flight are included to illustrate application. 相似文献
677.
It is well known that the position of a stationary radar can be estimated by a single moving observer from angle and/or frequency measurements taken passively at different points in its trajectory. Depending on the measurement set different localization methods result: the bearing method (BM), the frequency method (FM), and the combined method (CM). Previous studies analyze the three methods on the basis of constant emitter frequencies. However, radars with constant emitter frequencies are not realistic. Frequency drift and frequency hopping have to be taken into account. Therefore, to include radars the emitter model has been extended by drift and frequency hopping in this study, and the estimation methods have been reanalyzed. The effects of the model extensions become transparent in a parametric Cramer-Rao (CR) analysis. The results obtained in this way are confirmed by Monte Carlo simulations taking maximum likelihood (ML) as estimation procedure 相似文献
678.
Altwegg K. Ehrenfreund P. Geiss J. Huebner W.F. Levasseur-Regourd A.-C. 《Space Science Reviews》1999,90(1-2):373-389
A major objective of the workshop was to learn about the chemical composition, physical structure, and thermodynamic conditions
of the outer parts of the solar nebula where comets formed. Here we sum up what we have learned from years of research about
the molecular constituents of comet comae primarily from in situ measurements of Comet 1P/Halley and remote sensing of Comets 1P/Halley, Hale-Bopp (C/1995 O1), and Hyakutake (C/1996 B2).
These three bright comets are presumably captured Oort cloud comets. We summarize the analyses of these data to predict the
composition of comet nuclei and project them further to the composition, structure, and thermodynamic conditions in the nebula.
Near-future comet missions are directed toward less active short-period Jupiter-family comets. Thus, future analyses will
afford a better understanding of the diversity of these two major groups of comets and their respective regions of origin
in the solar or presolar nebula.
We conclude with recommendations for determining critical data needed to aid in further analyses. Results of the workshop
provide new guidelines and constraints for modeling the solar nebula.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
679.
Importance sampling is a technique which can significantly reduce the number of Monte Carlos necessary to accurately estimate the probability of low-probability of occurrence events (e.g., the probability of false alarm PF associated with a given detection scheme). A new technique called the Chernoff Importance Sampling Method is introduced. It is shown that the number of required Monte Carlos can be reduced by a factor of a Chernoff-like bound on P F. In addition, techniques for choosing the multiplying factor of the distorted variance method (the most common method used in importance sampling) are presented 相似文献
680.
The investigation of the volatile material in the coma of comets is a key to understanding the origin of cometary material,
the physical and chemical conditions in the early solar system, the process of comet formation, and the changes that comets
have undergone during the last 4.6 billion years. So far, in situ investigations of the volatile constituents have been confined
to a single comet, namely P/Halley in 1986. Although, the Giotto mission gave only a few hours of data from the coma, it has
yielded a surprising amount of new data and has advanced cometary science by a large step. In the present article the most
important results of the measurements of the volatile material of Halley's comet are summarized and an overview of the identified
molecules is given. Furthermore, a list of identified radicals and unstable molecules is presented for the first time. At
least one of the radicals, namely CH2, seems to be present as such in the cometary ice.
As an outlook to the future we present a list of open questions concerning cometary volatiles and a short preview on the next
generation of mass spectrometers that are being built for the International Rosetta Mission to explore the coma of Comet Wirtanen.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献