全文获取类型
收费全文 | 762篇 |
免费 | 135篇 |
国内免费 | 151篇 |
专业分类
航空 | 402篇 |
航天技术 | 248篇 |
综合类 | 76篇 |
航天 | 322篇 |
出版年
2024年 | 7篇 |
2023年 | 19篇 |
2022年 | 18篇 |
2021年 | 31篇 |
2020年 | 41篇 |
2019年 | 29篇 |
2018年 | 44篇 |
2017年 | 37篇 |
2016年 | 41篇 |
2015年 | 32篇 |
2014年 | 68篇 |
2013年 | 45篇 |
2012年 | 65篇 |
2011年 | 46篇 |
2010年 | 47篇 |
2009年 | 28篇 |
2008年 | 48篇 |
2007年 | 46篇 |
2006年 | 86篇 |
2005年 | 42篇 |
2004年 | 25篇 |
2003年 | 32篇 |
2002年 | 22篇 |
2001年 | 3篇 |
2000年 | 16篇 |
1999年 | 27篇 |
1998年 | 18篇 |
1997年 | 11篇 |
1996年 | 11篇 |
1995年 | 25篇 |
1994年 | 14篇 |
1993年 | 5篇 |
1992年 | 5篇 |
1991年 | 2篇 |
1990年 | 4篇 |
1989年 | 4篇 |
1988年 | 1篇 |
1987年 | 1篇 |
1986年 | 2篇 |
排序方式: 共有1048条查询结果,搜索用时 216 毫秒
451.
基于功率复用的椭圆球面波函数非正交调制方法 总被引:2,自引:0,他引:2
围绕如何降低基于椭圆球面波函数的非正交调制(PSWF-NPSM)系统误码率,降低信号检测复杂度,引入调制信号功率复用思想,提出基于功率复用的椭圆球面波函数非正交调制(PD-PSWF-NPSM)方法。该方法对载波信号按子波带进行功率分配,增加了调制信号最小欧式距离,降低了系统误码率;在接收端,提出基于检测统计量的串行干扰相消信号检测方法,依据不同支路信号间功率差异,对信号进行分离、检测。理论与仿真分析表明,该方法在不降低系统频带利用率、调制信号功率谱、峰均功率比特性前提下,能够有效提高系统误码性能,降低调制信号检测复杂度;与原非正交调制方法相比,当误比特率(BER)为10-5时,所提方法系统误码性能提升约1.7 dB。 相似文献
452.
Douglas Gough 《Space Science Reviews》1998,85(1-2):141-158
Standard solar models, although they are free from the influence of much of the fluid motion that is bound to be present in
the Sun, have been shown by helioseismology to represent the spherically averaged structure of the Sun amazingly well. This
state of affairs has come about after painstaking refinements by a great many people of the pertinent microphysics, including
that which controls the equation of state, the opacity, the nuclear reaction rates and the diffusion that inhibits gravitational
segregation of chemical elements. It has instilled confidence in the modellers in being able to predict the composition of
the solar interior. But there are consequences of the flow, related particularly to redistribution of chemical species, that
can be difficult to identify observationally, yet which may degrade any inferences we might make. Their potential presence
must at least be acknowledged by anyone who tries to asses the reliability of the models. This report summarizes the discussions
in the preceding pages of this volume of the current theoretical and observational status of the subject, pointing to many
of the caveats that have been raised, and attempting at the same time to put them into a seemingly coherent discourse in the
context of our present understanding of the workings of the solar interior.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
453.
A. W. Hood 《Space Science Reviews》1999,87(1-2):79-92
Theories and observations of energy input, heating and acceleration mechanisms in the low corona were presented and discussed.
The main topics of discussion were large-scale solar wind simulations, theoretical heating mechanisms, observational constraints,
confronting theory with observations and observational issues.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
454.
455.
Ovidiu Maris Georgeta Maris 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2005,35(12):2129-2140
The high-speed plasma streams in the solar wind are investigated during the solar cycles nos. 20–22 (1964–1996), separately on the two types of streams according to their solar origin: the HSPS produced by coronal holes (co-rotating) and the flare-generated, in keeping with the classification made in different catalogues. The analysis is performed taking into account the following high-speed stream parameters: the durations (in days), the maximum velocities, the velocity gradients and, the importance of the streams. The time variation of these parameters and the high-speed plasma streams occurrence rate show an 11-year periodicity with some differences between the solar cycles considered. A detailed analysis of the high-speed stream 11-year cycles is made by comparison with the “standard” cycles of the sunspot relative number (Wolf number). The different behaviour of the high-speed stream parameters between even and odd solar cycles could be due to the 22-year solar magnetic cycle. The increased activity of the high-speed plasma streams on the descendant phases of the cycles, regardless of their solar sources, proves the existence of some special local conditions of the solar plasma and the magnetic field on a large scale that allow the ejection of the high energy plasma streams. This fact has led us to the analysis the stream parameters during the different phases of the solar cycles (minimum, ascendant, maximum and, descendant) as well as during the polar magnetic field reversal intervals. The differences between the phases considered are pointed out. The solar cycles 20 and 22 reveal very similar dynamics of the flare-generated and also co-rotating stream parameters during the maximum, descendant and reversal intervals. This fact could be due to their position in a Hale Cycle (the first component of the 22-year solar magnetic cycle). The 21st solar cycle dominance of all co-rotating stream parameters against the 20th and 22nd solar cycle ones, during almost all phases, could be due to the same structure of a Hale Cycle – solar cycle 21 is the second component in a 22-year SC. During the reversal intervals, all high-speed stream parameters have comparable values with the ones of the maximum phases of the cycles even if this interval contains a small part of the descendant branch (solar cycles 20 and 22). 相似文献
456.
Jack J. Lissauer 《Space Science Reviews》2005,116(1-2):11-24
Models of the origins of gas giant planets and ‘ice’ giant planets are discussed and related to formation theories of both
smaller objects (terrestrial planets) and larger bodies (stars). The most detailed models of planetary formation are based
upon observations of our own Solar System, of young stars and their environments, and of extrasolar planets. Stars form from
the collapse, and sometimes fragmentation, of molecular cloud cores. Terrestrial planets are formed within disks around young
stars via the accumulation of small dust grains into larger and larger bodies until the planetary orbits become well enough
separated that the configuration is stable for the lifetime of the system. Uranus and Neptune almost certainly formed via
a bottom-up (terrestrial planet-like) mechanism; such a mechanism is also the most likely origin scenario for Saturn and Jupiter. 相似文献
457.
We derive electron temperature and density as a function of height up to 0.2 R⊙ above the limb in polar coronal holes, using five EUV data sets recorded by the SOHO Coronal Diagnostic Spectrometer between
July 1997 and February 1998. Radial T and N distributions, averaged in a 2° to 10° range of position angles, are the same
above the North and South coronal holes. They do not show any time variability over a period of seven months. Polar plumes
are found to have lower electron temperature and higher density than the interplume lanes. The electron density slope suggests
that the proton temperatures are twice as high as the electron temperatures.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
458.
L. Strachan Y-K. Ko A. V. Panasyuk D. Dobrzycka J. L. Kohl M. Romoli G. Noci S. E. Gibson D. A. Biesecker 《Space Science Reviews》1999,87(1-2):311-314
We constrain coronal outflow velocity solutions, resolved along the line-of-sight, by using Doppler dimming models of H I Lyman alpha and O VI 1032/1037 Å emissivities obtained with data from the Ultraviolet Coronagraph Spectrometer (UVCS) on SOHO. The local emissivities, from heliocentric heights of 1.5 to 3.0 solar radii, were determined from 3-D reconstructions of line-of-sight intensities obtained during the first Whole Sun Month Campaign (10 August to 8 September 1996). The models use electron densities derived from polarized brightness measurements made with the visible light coronagraphs on UVCS and LASCO, supplemented with data from Mark III at NCAR/MLSO. Electron temperature profiles are derived from 'freezing-in' temperatures obtained from an analysis of charge state data from SWICS/Ulysses. The work concentrates on neutral hydrogen outflow velocities which depend on modeling the absolute coronal H I Lyα emissivities. We use an iterative method to determine the neutral hydrogen outflow velocity with consistent values for the electron temperatures derived from a freezing-in model. 相似文献
459.
简要介绍了数字频率调制和数字伺服在国产激光抽运小型铯束管频率标准装置中的应用 ,包括实现方法、硬件框图、软件流程 ,及其在应用中的一些特点。为下一阶段工作打下基础。 相似文献
460.
D. A. Brain 《Space Science Reviews》2006,126(1-4):77-112
The solar wind at Mars interacts with the extended atmosphere and small-scale crustal magnetic fields. This interaction shares
elements with a variety of solar system bodies, and has direct bearing on studies of the long-term evolution of the Martian
atmosphere, the structure of the upper atmosphere, and fundamental plasma processes. The magnetometer (MAG) and electron reflectometer
(ER) on Mars Global Surveyor (MGS) continue to make many contributions toward understanding the plasma environment, thanks
in large part to a spacecraft orbit that had low periapsis, had good coverage of the interaction region, and has been long-lived
in its mapping orbit. The crustal magnetic fields discovered using MGS data perturb plasma boundaries on timescales associated
with Mars' rotation and enable a complex magnetic field topology near the planet. Every portion of the plasma environment
has been sampled by MGS, confirming previous measurements and making new discoveries in each region. The entire system is
highly variable, and responds to changes in solar EUV flux, upstream pressure, IMF direction, and the orientation of Mars
with respect to the Sun and solar wind flow. New insights from MGS should come from future analysis of new and existing data,
as well as multi-spacecraft observations. 相似文献