全文获取类型
| 收费全文 | 9323篇 |
| 免费 | 38篇 |
| 国内免费 | 20篇 |
专业分类
| 航空 | 4309篇 |
| 航天技术 | 3267篇 |
| 综合类 | 32篇 |
| 航天 | 1773篇 |
出版年
| 2021年 | 97篇 |
| 2019年 | 58篇 |
| 2018年 | 201篇 |
| 2017年 | 132篇 |
| 2016年 | 139篇 |
| 2015年 | 69篇 |
| 2014年 | 238篇 |
| 2013年 | 279篇 |
| 2012年 | 277篇 |
| 2011年 | 413篇 |
| 2010年 | 291篇 |
| 2009年 | 449篇 |
| 2008年 | 504篇 |
| 2007年 | 277篇 |
| 2006年 | 224篇 |
| 2005年 | 253篇 |
| 2004年 | 234篇 |
| 2003年 | 298篇 |
| 2002年 | 195篇 |
| 2001年 | 304篇 |
| 2000年 | 170篇 |
| 1999年 | 205篇 |
| 1998年 | 256篇 |
| 1997年 | 163篇 |
| 1996年 | 220篇 |
| 1995年 | 270篇 |
| 1994年 | 259篇 |
| 1993年 | 163篇 |
| 1992年 | 206篇 |
| 1991年 | 88篇 |
| 1990年 | 75篇 |
| 1989年 | 195篇 |
| 1988年 | 88篇 |
| 1987年 | 84篇 |
| 1986年 | 92篇 |
| 1985年 | 241篇 |
| 1984年 | 189篇 |
| 1983年 | 166篇 |
| 1982年 | 179篇 |
| 1981年 | 282篇 |
| 1980年 | 80篇 |
| 1979年 | 73篇 |
| 1978年 | 70篇 |
| 1977年 | 55篇 |
| 1975年 | 66篇 |
| 1974年 | 53篇 |
| 1973年 | 54篇 |
| 1972年 | 57篇 |
| 1971年 | 52篇 |
| 1970年 | 52篇 |
排序方式： 共有9381条查询结果，搜索用时 15 毫秒
71.
In radars that achieve a high subclutter visibility by coherent processing over several pulses, a serious problem appears in the form of blind Dopplers, or ?speeds,? at which target detection is impossible. Of the possible methods of eliminating these blind speeds, the most basic one that is employed when the performance requirements are high involves the use of several PRF's. These PRF's are chosen so that coverage is obtained at any Doppler with at least one PRF. The problem faced by the radar designer is to select the set of PRF's and the pulse numbers for each PRF so that the search frame time is minimized. This paper evolves a systematic method for the design of the blind-speed elimination scheme. A formalized approach is offered that shows the possible combinations of wavelength, PRF, and pulse number and the tradeoffs involved, without introducing the confusion ordinarily associated with multiparameter choices. 相似文献
72.
Paulikas G.A. Blake J.B. Imamoto S.S. 《IEEE transactions on aerospace and electronic systems》1975,(6):1138-1144
The Aerospace Corporation energetic electron-proton spectrometer operating on Applications Technology Satellite-6 (ATS-6) detects energetic electrons in four channels between 140 keV and greater than 32 MeV, and measures energetic protons in five energy channels between 2.3 and 80 MeV and energetic alpha particles in three channels between 9.4 and 94 MeV. After more than a year of operation in orbit, the experiment continues to return excellent data on the behavior of energetic magnetospheric electrons as well as information regarding the fluxes of solar protons and alpha particles. 相似文献
73.
M R Sims C T Pillinger I P Wright J Dowson S Whitehead A Wells J E Spragg G Fraser L Richter H Hamacher A Johnstone N P Meredith C de la Nougerede B Hancock R Turner S Peskett A Brack J Hobbs M Newns A Senior M Humphries H U Keller N Thomas J S Lingard T C Ng 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1999,23(11):1925-1928
The aim of the proposed Beagle 2 small lander for ESA's 2003 Mars Express mission is to search for organic material on and below the surface of Mars and to study the inorganic chemistry and mineralogy of the landing site. The lander will have a total mass of 60kg including entry, descent, and landing system. Experiments will be deployed on the surface using a robotic arm. It will use a mechanical mole and grinder to obtain samples from below the surface, under rocks, and inside rocks. Sample analysis by a mass spectrometer will include isotopic analysis. An optical microscope, an X-ray spectrometer and a Mossbauer spectrometer will conduct in-situ rock studies. 相似文献
74.
A.B. Akopova 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1998,21(12):1809-1812
We propose to study the radiation environment on board different flight vehicles: cosmos-type satellites, orbital stations, Space Shuttles and civil (sonic and supersonic) aircraft. These investigations will be carried out with single type of passive detector, namely, nuclear photoemulsions (NPE) with adjustable threshold of particle detection within broad range of linear energy transfer (LET) that is done by means of the technique of selective development of NPE exposed in space.
These investigations will allow one to determine:
- • integral spectra of LET of charged particles of cosmic ray (CR) over a wide range from 2.0 to 5×104 MeV/cm in biological tissue;
• differential energy spectra of fast neutrons (1–20 MeV);
• estimation of absorbed and equivalent doses from charged and neutral component CR;
• charge and energy spectra of low energy nuclei (E≤100 MeV) with Z≥2 having in view the extreme hazard radiation to biological objects and microelectronic schemes taken on board inside and outside of these different flight vehicles with exposures from several days to several months.
The investigation of radiation environment on board the airplanes depending on the flight parameters will be conducted using emulsions of different sensitivity without any controlling of threshold sensitivity (Akopova et al., 1996). The proposed detector can be used in the joint experiments on the new International Cosmic Station “Alpha”. 相似文献
75.
P A Craven M J Rycroft 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1996,18(1-2):45-49
Beyond the magnetic influence of the Earth, the flux of galactic cosmic radiation (GCR) represents a radiological concern for long-term manned space missions. Current concepts of radiation quality and equivalent dose are inadequate for accurately specifying the relative biological \"efficiency\" of low doses of such heavily ionising radiations, based as they are on the single parameter of Linear Energy Transfer (LET). Such methods take no account of the mechanisms, nor of the highly inhomogeneous spatial structure, of energy deposition in radiation tracks. DNA damage in the cell nucleus, which ultimately leads to the death or transformation of the cell, is usually initiated by electrons liberated from surrounding molecules by the incident projectile ion. The characteristics of these emitted \"delta-rays\", dependent primarily upon the charge and velocity of the ion, are considered in relation to an idealised representation of the cellular environment. Theoretically calculated delta-ray energy spectra are multiplied by a series of weighting algorithms designed to represent the potential for DNA insult in this environment, both in terms of the quantity and quality of damage. By evaluating the resulting curves, and taking into account the energy spectra of heavy ions in space, a relative measure of the biological relevance of the most abundant GCR species is obtained, behind several shielding configurations. It is hoped that this method of assessing the radiation quality of galactic cosmic rays will be of value when considering the safety of long-term manned space missions. 相似文献
76.
A. Kiraga Z. Kos V. N. Oraevsky V. C. Dokukin S. A. Pulinets 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1995,15(12):21-24
There were several cases when modulated electron beam had been injected from APEX satellite into an otherwise unmodified ionospheric plasma. The beams were formed from 2 μs pulses repeated at a rate of 40kHz. Injections took place in the altitude range 400–1100km over Europe. The onboard receiver connected to the dipole antenna swept the frequency interval 1–10 MHz in 1 sec. Due to a relatively narrow receiver bandwidth (15 kHz), moderate frequency step (50 kHz), slow changes of fn and fc (local electron plasma and gyro frequencies) fine structure of excited emissions was detected. In many cases, a very prominent doublet could be convincingly identified as a (fn,fu=sqrt(fn*fn+fc*fc)) band, with fn practically equal to the local plasma frequency in the unperturbed ionosphere. Determination of plasma frequency is of prime importance in analysis of complex structures composed from various harmonics fn,fc,fu. Complexity is manifested as coalescing of various harmonics with maxima shifted from nominal frequencies or splitting them into components due to mixing of local and propagation effects. Despite the existence of strong emissions on frequencies characteristic for the unperturbed ionosphere, very strong broadcasting transmissions are frequently cut off, even when the beam is directed upward. Most typical spectra are discussed. 相似文献
77.
In August, 1981, the Westerbork Synthesis Radio Telescope was used for 4 h to search for narrowband pulsing radio beacons in the direction of the Galactic Center. By using both the spatial discrimination and temporal stability available to an interferometric measurement, weak intermittent signals can be detected even in the face of the strong, naturally caused radiation from this region. A radio beacon within our bandwidth, centered on the 21 cm neutral hydrogen line, would be recognizable if it had a repetition period between 40 sec and 1/2 h. The rms sensitivity to point sources was approximately 50 mJy/cycle, and the detection limit was 500 mJy/cycle. The limit degrades for pulse widths < 0.02s. No repetitive signals were found. For a swept, narrow-band radio beacon constrained to the Galactic Disk (beamwidth = 0.02 rad), our detection limit corresponds to a transmitter power of 10(11) MW at the Galactic Center. 相似文献
78.
Middlebtook R.D. Kimble S.G. 《IEEE transactions on aerospace and electronic systems》1976,(2):162-172
The High Energy Astronomy Observatory (HEAO) main bus regulator contains a redundant error amplifier (REA) and a 12-section sequenced shunt transconductance amplifier (STA) in which not more than three and not less than one shunt section are in an active control mode regardless of a failure in any one of the twelve sections. The part of the work concerned with the shaping of the frequency response of the REA and STA minor feedback loops and of the teolator major feedback loop is described. Emphasis is on the application of a general design technique embodying a "design-analyze-measure iteration loop," in which a first-cut design and analysis is corrected ard improved after experimental observation of discrepancies and deficiencies. In particular, an incipient high-frequency oscillation was thereby reliably eliminated. It is demonstrated that analysis techniques involving a minimum of algebra can provide full quantitative information on an efficient and reliable system design. 相似文献
79.
Devajyoti Dutta Sanjay Sharma G.K. Sen B.A.M. Kannan S. Venketswarlu R.M. Gairola J. Das G. Viswanathan 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2011
By using a Doppler Weather Radar (DWR) at Shriharikota (13.66°N & 80.23°E), an Artificial Neural Network (ANN) based technique is proposed to improve the accuracy of rain intensity estimation. Three spectral moments of a Doppler spectra are utilized as an input data to an ANN. Rain intensity, as measured by the tipping bucket rain gauges around the DWR station, are considered as a target values for the given inputs. Rain intensity as estimated by the developed ANN model is validated by the rain gauges measurements. With the help of a developed technique, reasonable improvement in the estimation of rain intensity is observed. By using the developed technique, root mean square error and bias are reduced in the range of 34–18% and 17–3% respectively, compared to Z–R approach. 相似文献
80.
A. Guharay P.P. Batista R.A. Buriti 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2021,67(10):3125-3133
An evident signature of a least studied quasi-90-day oscillation is found in the winds and tides in the MLT from an equatorial station, São João do Cariri (7.4°S, 36.5°W). The oscillation is found to appear mainly in certain intervals with small but appreciable seasonal (fourth harmonic of annual oscillation) contribution. The maximum amplitude of the oscillation is found to be around 10 m/s in the zonal wind. The enhancement peak of the oscillation exhibits downward movement indicating a plausible role of upward moving waves/tides in carrying its imprint from below to the MLT. Similar oscillation feature in the tropospheric zonal wind and ozone may imply its lower atmospheric origin as a component of the intraseasonal oscillation (ISO) that moves upward by modulating the tides. Subsequently, the propagating tides (mainly semidiurnal) are enhanced by the ozone in the stratosphere through absorption of solar UV radiation and finally manifest the oscillation in the MLT. Consistency of the present findings with the past investigations are observed in some aspects of the oscillation, whereas existing mismatches in others are believed to be due to geophysical variability depending on space and time among various locations on the globe. 相似文献