全文获取类型
收费全文 | 2276篇 |
免费 | 4篇 |
国内免费 | 3篇 |
专业分类
航空 | 1227篇 |
航天技术 | 827篇 |
综合类 | 13篇 |
航天 | 216篇 |
出版年
2018年 | 25篇 |
2017年 | 22篇 |
2014年 | 24篇 |
2013年 | 47篇 |
2012年 | 28篇 |
2011年 | 59篇 |
2010年 | 47篇 |
2009年 | 57篇 |
2008年 | 126篇 |
2007年 | 38篇 |
2006年 | 32篇 |
2005年 | 46篇 |
2004年 | 69篇 |
2003年 | 72篇 |
2002年 | 37篇 |
2001年 | 55篇 |
2000年 | 49篇 |
1999年 | 27篇 |
1998年 | 75篇 |
1997年 | 53篇 |
1996年 | 62篇 |
1995年 | 69篇 |
1994年 | 85篇 |
1993年 | 51篇 |
1992年 | 69篇 |
1991年 | 29篇 |
1990年 | 33篇 |
1989年 | 70篇 |
1988年 | 24篇 |
1987年 | 29篇 |
1986年 | 50篇 |
1985年 | 87篇 |
1984年 | 54篇 |
1983年 | 60篇 |
1982年 | 56篇 |
1981年 | 68篇 |
1980年 | 34篇 |
1979年 | 27篇 |
1978年 | 25篇 |
1977年 | 24篇 |
1975年 | 24篇 |
1974年 | 24篇 |
1973年 | 24篇 |
1972年 | 21篇 |
1971年 | 31篇 |
1970年 | 17篇 |
1969年 | 25篇 |
1968年 | 18篇 |
1967年 | 20篇 |
1966年 | 19篇 |
排序方式: 共有2283条查询结果,搜索用时 219 毫秒
791.
Pulford G.W. La Scala B.F. 《IEEE transactions on aerospace and electronic systems》2002,38(2):367-377
Two algorithms are derived for the problem of tracking a manoeuvring target based on a sequence of noisy measurements of the state. Manoeuvres are modeled as unknown input (acceleration) terms entering linearly into the state equation and chosen from a discrete set. The expectation maximization (EM) algorithm is first applied, resulting in a multi-pass estimator of the MAP sequence of inputs. The expectation step for each pass involves computation of state estimates in a bank of Kalman smoothers tuned to the possible manoeuvre sequences. The maximization computation is efficiently implemented using the Viterbi algorithm. A second, recursive estimator is then derived using a modified EM-type cost function. To obtain a dynamic programming recursion, the target state is assumed to satisfy a Markov property with respect to the manoeuvre sequence. This results in a recursive but suboptimal estimator implementable on a Viterbi trellis. The transition costs of the latter algorithm, which depend on filtered estimates of the state, are compared with the costs arising in a Viterbi-based manoeuvre estimator due to Averbuch, et al. (1991). It is shown that the two criteria differ only in the weighting matrix of the quadratic part of the cost function. Simulations are provided to demonstrate the performance of both the batch and recursive estimators compared with Averbuch's method and the interacting multiple model filter 相似文献
792.
J Liao G Liu O Monje G W Stutte D M Porterfield 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2004,34(7):1579-1584
Numerous spaceflight experiments have noted changes in the roots that are consistent with hypoxia in the root zone. These observations include general ultrastructure analysis and biochemical measurements to direct measurements of stress specific enzymes. In experiments that have monitored alcohol dehydrogenase (ADH), the data shows this hypoxically responsive gene is induced and is associated with increased ADH activity in microgravity. These changes in ADH could be induced either by spaceflight hypoxia resulting from inhibition of gravity mediated O2 transport, or by a non-specific stress response due to inhibition of gravisensing. We tested these hypotheses in a series of two experiments. The objective of the first experiment was to determine if physical changes in gravity-mediated O2 transport can be directly measured, while the second series of experiments tested whether disruption of gravisensing can induce a non-specific ADH response. To directly measure O2 bioavailability as a function of gravity, we designed a sensor that mimics metabolic oxygen consumption in the rhizosphere. Because of these criteria, the sensor is sensitive to any changes in root O2 bioavailability that may occur in microgravity. In a KC-135 experiment, the sensor was implanted in a moist granular clay media and exposed to microgravity during parabolic flight. The resulting data indicated that root O2 bioavailability decreased in phase with gravity. In experiments that tested for non-specific induction of ADH, we compared the response of transgenic Arabidopsis plants (ADH promoted GUS marker gene) exposed to clinostat, control, and waterlogged conditions. The plants were grown on agar slats in a growth chamber before being exposed to the experimental treatments. The plants were stained for GUS activity localization, and subjected to biochemical tests for ADH, and GUS enzyme activity. These tests showed that the waterlogging treatment induced significant increases in GUS and ADH enzyme activities, while the control and clinostat treatments showed no response. This work demonstrates: (1) the inhibition of gravity-driven convective transport can reduce the O2 bioavailability to the root tip, and (2) the perturbation of gravisensing by clinostat rotation does not induce a nonspecific stress response involving ADH. Together these experiments support the microgravity convection inhibition model for explaining changes in root metabolism during spaceflight. 相似文献
793.
W.N. Brandt C. Vignali D.P. Schneider D.M. Alexander S.F. Anderson F.E. Bauer X. Fan G.P. Garmire S. Kaspi G.T. Richards 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2004,34(12):2478-2485
X-ray studies of high-redshift (z > 4) active galaxies have advanced substantially over the past few years, largely due to results from the new generation of X-ray observatories. As of this writing X-ray emission has been detected from nearly 60 high-redshift active galaxies. This paper reviews the observational results and their implications for models of the first massive black holes, and it discusses future prospects for the field. 相似文献
794.
795.
J B Robertson J M Eaddy J O Archambeau G B Coutrakon D W Miller M F Moyers J V Siebers J M Slater J F Dicello 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1994,14(10):271-275
We have studied radiation effects utilizing the new 250 MeV Synchrotron at Loma Linda University Medical Center. In this paper we present the data collected for the survival of Chinese hamster lung (V79) cells, that were irradiated with a beam of mixed energy protons up to 200 MeV. The RBE for protons, when compared to 60Co gamma rays, ranged from a low of 1.2 at the high energy portion of the field to 1.3+ at the low energy portion of the field. These results are consistent with the measured lineal energy (microdosimetric) spectra. 相似文献
796.
The objective of this primarily tutorial item is to describe a general model for the observable data and the appropriate data processing involved in sensing rigid target fields with coherent radars. Any number of radars may be involved, and the scene and each radar may be in any kind of motion-with no restrictions on motion through resolution cells during the coherent processing time of the radars. The motions are assumed to be known. To some extent motion parameters can be estimated from the radar data, e.g., by adaptive parameter adjustments in the data processing; however, this subject is beyond the scope of this discussion. In large measure, the analysis in this item highlights the central conceptual result obtained by J.L. Walker as described in [1] -a major work in radar theory. 相似文献
797.
798.
NASA-ISC requires avionics platforms capable of serving a wide range of applications in a cost-effective manner. In part, making the avionics platform cost effective means adhering to open standards and supporting the integration of COTS products with custom products. Inherently, operation in space requires low power, mass, and volume while retaining high performance, reconfigurability, scalability, and upgradability. The Universal Mini-Controller (UMC) project is based on a modified PC/104-Plus architecture while maintaining full compatibility with standard COTS PC/104 products. The architecture consists of a library of stackable building block modules, which can be mixed and matched to meet a specific application. A set of NASA developed core building blocks, (e.g. processor card, analog input/output card, high level analog card, and a Mil-Std-1553 card) were constructed to meet critical functions and unique interfaces 相似文献
799.
R.C. Canfield T.A. Gunkler H.S. Hudson L.W. Acton J.W. Leibacher A.L. Kiplinger 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1982,2(11):145-149
We have observed the flare of 1980 May 7 1456 UT with several Solar Maximum Mission instruments, in coordination with the Sacramento Peak Observatory Vacuum Tower Telescope. From the X-ray data we determine the total amount of plasma at T > 2 × 106 K, commonly attributed to chromospheric evaporation. From Hα we have determined the amount of plasma that has been evaporated from the chromosphere. We find that enough material has been evaporated from the chromosphere to account for the X-ray plasma. Taken together, the Hα, soft and hard X-ray images suggest that chromospheric evaporation is driven both by flare accelerated electrons, during the impulsive phase, and conduction, during the thermal phase. 相似文献
800.
Among the major objectives of NASA's program of space exploration is a better understanding of the origin and evolution of the solar system. Crucial to this objective is the study of comets, which are thought to be the most primitive, pristine bodies remaining in the solar system. The importance of the study of comets has led NASA to plan a mission to rendezvous with comet Tempel 2 in 1997. Critical to the understanding of comets will be measurements of the nucleus material to determine its elemental and isotopic composition, its mechanical properties, and its thermal state and properties. This paper describes a proposal for a Comet Nucleus Penetrator to accomplish these measurement goals. The Comet Nucleus Penetrator will implant instruments into the comet's nucleus beneath a probable volatile-depleted surface mantle into material more representative of the bulk composition of the nucleus. 相似文献