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981.
The review contains the important achievements in dynamics of the galactic disks. Among them there are I. New structures
discovered recently: • giant vortices (including giant anticyclone in the Solar vicinity); • slow bar; • inner oscillating
structure within spiral arms similar that of enveloped soliton; • chaotic streamlines in the velocity field of the gaseous
disk of a real galaxy. II. New collective phenomina discovered recently: • new overreflection instability initiating ‘mini-spiral’
in the innermost central parsec of Galaxy; • large-scale convection caused by nonlinear interaction of density wave with
disk gas; • non-kolmogorovian spectrum of weak turbulence corresponding to the observed one in the • Solar vicinity. III. New
methods worked out recently: • reconstruction of full three-dimensional vector field of gas velocity from the observed line-of-
• sight velocity field; • observational test for verification of the wave-nature of the spiral arms; • observational test
to distinguish two types of vertical motions: warp and z-motions in the • density wave; • derivation of correct system of two-dimensional dynamical equations from the initial three-
• dimensional one.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
982.
Anne B. Underhill 《Space Science Reviews》1963,1(4):749-761
Measurements of the shape of the ultraviolet spectrum from B stars are compared with the theoretical spectra predicted from a homogeneous series of eight model atmospheres which are known to be close to a state of radiative equilibrium and to give a good representation of the ordinarily observed spectral region. The broad-band photometer measurements of Byram, Chubb, and Friedman in the region 1314 indicate that the stars become brighter in the ultraviolet as their temperature increases. The theoretical spectra reproduce this trend. However, the theoretical spectra are about three times as bright at 1314 relative to their brightness at 5560 as is observed.The spectral observations at 50Å resolution of Stecher and Milligan of six absorption-line stars are compared in detail with theoretical spectra. The observed shape of the spectrum is reproduced well by the models from 2600 to longer wavelengths. At wavelengths shorter than 2600 Å, the observed fluxes from B stars are less than the predicted fluxes. At 2000 the deficiency is between a factor two and a factor four. The spectrum of Canis Majoris is observed to have a different shape from that found for four other early-type stars. In the case of Canis Majoris the deficiency at 2000 is about a factor 13.The proper manner in which to compare theory and observation is discussed and some astrophysical terminology is explained. Theoretical fluxes, , are given in Table 1 for eight early B type model atmospheres at wavelengths between the Lyman limit and 6251. These fluxes have been computed without consideration of the opacity due to line blanketing. It is shown that line blanketing can probably account for the differences noted between predicted and observed ultra-violet spectra of B stars. It is not necessary at present to invoke unusual sources of opacity in the stellar atmosphere or in the space between the star and the earth in order to explain the observations. Spectra of B stars in the 2000 region at sufficient resolution to show the line spectrum would clarify the problem. 相似文献
983.
The objectives of this paper are threefold. The first is to obtain a sampled data model of a gated phase-locked loop. Data from measurements made on an actual loop constructed in the laboratory are included, and the good agreement between measured and calculated results serves as a verification of the model. Second, with the aid of the model, some of the basic operating properties of the system will be presented. Third, comparisons will be made between gated and continuous loop operation. 相似文献
984.
C. M. Lisse M. F. A’Hearn T. L. Farnham O. Groussin K. J. Meech U. Fink D. G. Schleicher 《Space Science Reviews》2005,117(1-2):161-192
As comet 9P/Tempel 1 approaches the Sun in 2004–2005, a temporary atmosphere, or “coma,” will form, composed of molecules
and dust expelled from the nucleus as its component icy volatiles sublimate. Driven mainly by water ice sublimation at surface
temperatures T > 200 K, this coma is a gravitationally unbound atmosphere in free adiabatic expansion. Near the nucleus (≤ 102 km), it is in collisional equilibrium, at larger distances (≥104 km) it is in free molecular flow. Ultimately the coma components are swept into the comet’s plasma and dust tails or simply
dissipate into interplanetary space. Clues to the nature of the cometary nucleus are contained in the chemistry and physics
of the coma, as well as with its variability with time, orbital position, and heliocentric distance.
The DI instrument payload includes CCD cameras with broadband filters covering the optical spectrum, allowing for sensitive
measurement of dust in the comet’s coma, and a number of narrowband filters for studying the spatial distribution of several
gas species. DI also carries the first near-infrared spectrometer to a comet flyby since the VEGA mission to Halley in 1986.
This spectrograph will allow detection of gas emission lines from the coma in unprecedented detail. Here we discuss the current
state of understanding of the 9P/Tempel 1 coma, our expectations for the measurements DI will obtain, and the predicted hazards
that the coma presents for the spacecraft.
An erratum to this article is available at . 相似文献
985.
Zarnecki J.C. Leese M.R. Garry J.R.C. Ghafoor N. Hathi B. 《Space Science Reviews》2002,104(1-4):593-611
The design and performance of the Surface Science Package (SSP) on the Huygens probe are discussed. This instrument consists
of nine separate sensors that are designed to measure a wide range of physical properties of Titan's lower atmosphere, surface,
and sub-surface. By measuring a number of physical properties of the surface it is expected that the SSP will be able to constrain
the inferred composition and structure of the moon's near-surface environment. Although the SSP is primarily designed to sense
properties of the surface, some of its sensors will also make measurements of the atmosphere along the probe's entry path
and will complement the data gathered by other experiments on the Huygens probe.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
986.
Nonparametric Radar Extraction Using a Generalized Sign Test 总被引:3,自引:0,他引:3
A nonparametric procedure used in a constant false alarm rate (CFAR) radar extractor for detecting targets in a background of noise with unknown statistical properties is described. The detector is based on a generalization of the well-known two-sample sign test and thus requires a set of reference noise observations in addition to the set of observations being tested for signal presence. The detection performance against Gaussian noise is determined for a finite number of observations and asymptotically, for both nonfluctuating and pulse-to-pulse Rayleigh fluctuating target statistics. It is noted that the performance loss, as compared to the optimum parametric detector, depends critically on the number of reference noise observations available when the number of hits per target is not large. In the same case a much larger loss is also found for a pulse-to-pulse fluctuating target even though the asymptotic loss is the same as for a nonfluctuating target. A comparison is finally made with a detector based on the Mann-Whitney test, which usually is considered to be one of the better nonparametric procedures for the two-sample case. 相似文献
987.
A hysteresis current-controlled pulsewidth modulation (PWM) scheme is simple and robust in switching control, but it is subject to not constant switching frequency. However, since its harmonic spectrum is not randomly distributed like those yielded by the random PWM (RPWM) scheme, the annoying harmonic effects still exist. A random switching control strategy is proposed here to let the harmonic spectrum of the hysteresis current-controlled PWM inverter be randomly distributed. First, the effect of varying band on the harmonic distribution characteristic of a hysteresis current-controlled PWM scheme is analyzed. Then a random hysteresis PWM scheme and its quantitative design procedure are proposed. Finally, a robust spectrum shaping technique is devised such that the desired harmonic distribution pattern can be achieved. In the resulting frequency spectrum, the magnitudes of lower frequency harmonics are shaped and reduced. Hence the inverter output with a better harmonic attribute is obtained. Validity of the proposed random hysteresis PWM scheme is verified by some simulated and measured results 相似文献
988.
Two-state tracking filters are considered for seeker applications. Estimation of the line-of-sight (LOS) rate is the main purpose and equations of motion for the LOS are discussed and simplified for design. An observer approach is applied, the filter transfer functions are given and the gain is designed by pole placement. Also a measurement time delay is taken into account. The filter properties are investigated for deterministic LOS rotations, which is a case of practical interest, corresponding to target maneuvers. The influence of measurement noise is also considered 相似文献
989.
G. Gloeckler E. C. Roelof K. W. Ogilvie D. B. Berdichevsky 《Space Science Reviews》1995,72(1-2):321-326
Proton phase space densities in the solar wind frame from suprathermal velocities 10 km s–1 to 30,000 km s–1 (0.5 eV–5 MeV) were derived from combined SWICS and HISCALE measurements when Ulysses was at 5 AU and –24° heliolatitude. The period (19–23 January 1993) encompasses a forward/reverse shock pair (20 January, 0500 UT and 22 January, 0300 UT). Strong evidence is found for shock acceleration of pickup protons from interstellar hydrogen at all energies measured. 相似文献
990.
THE CLUSTER ION SPECTROMETRY (CIS) EXPERIMENT 总被引:5,自引:0,他引:5
H. RÈME J. M. Bosqued J. A. Sauvaud A. Cros J. Dandouras C. Aoustin J. Bouyssou Th. Camus J. Cuvilo C. Martz J. L. MÉDALE H. Perrier D. Romefort J. Rouzaud C. D'Uston E. MÖBIUS K. Crocker M. Granoff L. M. Kistler M. Popecki D. Hovestadt B. Klecker G. Paschmann M. Scholer C. W. Carlson D. W. Curtis R. P. Lin J. P. Mcfadden V. Formisano E. Amata M. B. Bavassano-CATTANEO P. Baldetti G. Belluci R. Bruno G. Chionchio A. Di Lellis E. G. Shelley A. G. Ghielmetti W. Lennartsson A. Korth H. Rosenbauer R. Lundin S. Olsen G. K. Parks M. Mccarthy H. Balsiger 《Space Science Reviews》1997,79(1-2):303-350
The Cluster Ion Spectrometry (CIS) experiment is a comprehensive ionic plasma spectrometry package on-board the four Cluster spacecraft capable of obtaining full three-dimensional ion distributions with good time resolution (one spacecraft spin) with mass per charge composition determination. The requirements to cover the scientific objectives cannot be met with a single instrument. The CIS package therefore consists of two different instruments, a Hot Ion Analyser (HIA) and a time-of-flight ion COmposition and DIstribution Function analyser (CODIF), plus a sophisticated dual-processor-based instrument-control and Data-Processing System (DPS), which permits extensive on-board data-processing. Both analysers use symmetric optics resulting in continuous, uniform, and well-characterised phase space coverage. CODIF measures the distributions of the major ions (H+, He+, He++, and O+) with energies from ~0 to 40 keV/e with medium (22.5°) angular resolution and two different sensitivities. HIA does not offer mass resolution but, also having two different sensitivities, increases the dynamic range, and has an angular resolution capability (5.6° × 5.6°) adequate for ion-beam and solar-wind measurements. 相似文献