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101.
The US space programme continues to face difficulties in restructuring to meet existing political and fiscal conditions, in part because the basic structure and focus of the programme derive from the tensions of the Cold War. This paper argues that the world's new, more complicated political and economic conditions present a challenge for US policy makers that can be met with a renewed focus on the pursuit of science and the public good, within the context of international economic competition and political and scientific cooperation.  相似文献   
102.
Richardson potential is an phenomenological interquark interaction taking care of two aspects of QCD, namely the asymptotic freedom and the confinement. The original potential has a scale parameter having value 400 MeV and is well tested in hadronic property calculations. This potential was then used in strange star calculation. Strange stars are very compact stars composed of strange quark matter, i.e. a very high density strange quark phase consisting of deconfined u, d and s quarks. Here the value of the scale parameter was taken as 100 MeV. The argument was that for a deconfined quark system like a strange star, the scale parameter may have a value quite different from that used in hadronic sector. To remove this discrepancy we introduced two scale parameters in the potential, one for the asymptotic freedom part and the other for the confining part. With suitable values of the parameters, this modified potential has been successfully used in both baryonic property and strange star calculations. The Equation of States obtained with the modified potential are also used to obtain mass–radius relations for the strange stars.  相似文献   
103.
The radio frequency (RF) susceptibility characteristics of two commercial Global Positioning System (GPS) receivers were evaluated. A first-order analysis was performed to predict the receiver susceptibility thresholds based on the receiver sensitivity and processing gain. The receiver susceptibility thresholds in the post-acquisition mode were then measured for various interference signal frequencies and modulations. Both receivers exhibited very low susceptibility thresholds to in-band continuous wave (CW) signals. In addition, both receivers could be over-driven with an out-of-band signal. In this state the receivers indicated acceptable figures of merit despite loss of satellite signal lock  相似文献   
104.
As space faring nations consider manned and unmanned missions to the Moon, there is a growing need to develop high fidelity lunar regolith simulants that can accurately reproduce the properties and behavior of lunar regolith. Such simulants will be employed to verify the performance of equipment, mechanisms, structures and processes to be used on the lunar surface. One of the significant limitations of current terrestrial-based simulants, such as the popular mare simulant, JSC-1A, is the lack of agglutinates. This paper investigates the production of a lunar mare agglutinate simulant based on JSC-1A. A modified plasma processing technique was used to expose the JSC-1A regolith simulant to high temperatures and transform it to predominantly a glassy phase. Detailed characterization results are presented to confirm that the agglutinate simulant material produced during this investigation reasonably satisfies the primary requirements of an agglutinate simulant such as amorphous/crystalline content, particle size, morphology, vesicular structure, chemistry, and presence of nanophase elemental Fe.  相似文献   
105.
The International Terrestrial Reference Frame (ITRF), as a realization of the International Terrestrial Reference System (ITRS), is represented by a set of station positions and linear velocities. They are intended to be used as regularized coordinates to which some corrections should be added to access instantaneous coordinates. The latest ITRS realization is the ITRF2005, which has integrated time series of station positions to form long-term solutions for the four space geodetic techniques. Currently, a purely linear model is used to parameterize station displacements in the estimation process, plus occasional discontinuities in case of earthquakes or equipment changes. However the input data have been derived without applying surface loading models and so surface loading effects are supposed to be embedded in the coordinates as measured quantities. We evaluate the effect of applying a posteriori loading corrections, which include the effect of atmospheric, non-tidal ocean, and continental water loading, to time series of positions estimated from Satellite Laser Ranging (SLR), Very Long Baseline Interferometry (VLBI), and Global Positioning System (GPS) data. We notice that they reduce about 50% or more of the annual signals in the translation and scale parameter time series of the SLR and VLBI techniques, except in SLR Z translation. In general, the estimated secular frame definition is negligibly affected and estimated positions and velocities are not significantly modified for stations that have accumulated a large number of observations. A multi-technique combination of such derived frames allows concluding that, for some cases, loading model corrections might degrade co-located station coordinates almost as much as they benefit them. However, most significant improvement of the estimated secular coordinates is observed for stations with less than 100 estimated positions as demonstrated with a multi-technique combination.  相似文献   
106.
    
The determination of the detailed performance of an MHD-augmented high-enthalphy shock tunnel requires the simultaneous measurment of a large number of aerodynamic, electrical, and electromagnetic parameters in a test time interval of the order of several hundred microseconds. In the feasibility study currently being conducted in our laboratory of such a device, an extensive measuring system was set up and evaluated, and is being used to acquire facility performance data. This paper describes this measuring system, discusses the modifications and adaptations applied to make the various components of the system operable and compatible, and gives illustrative examples of the performance of the system.  相似文献   
107.
108.
  总被引:1,自引:0,他引:1  
Gravity waves are prominent in the polar region of the terrestiral thermosphere, and can be excited by perturbations in Joule heating and Lorents force due to magnetospheric processes. We show observations from the Dynamics Explorer-2 satellite to illustrate the complexity of the phenomenon and review the transfer function model (TFM) which has guided our interpretation. On a statistical basis, the observed atmospheric perturbations decrease from the poles toward the equator and tend to correlate with the magnetic activity index, Ap, although individual measurements indicate that the magnetic index is often a poor measure of gravity wave excitation. The theoretical models devised to describe gravity waves are multifaceted. On one end are fully analytical, linear models which are based on the work of Hines. On the other end are fully numerical, thermospheric general circulation models (TGCMs) which incorporate non-linear processes and wave mean flow interactions. The transfer function model (TFM) discussed in this paper is between these two approaches. It is less restrictive than the analytical approach and relates the global propagation of gravity waves to their excitation. Compared with TGCMs, the TFM is simplified by its linear approximation; but it is not limited in spatial and temporal resolution, and the TFM describes the wave propagation through the lower atmosphere. Moreover, the TFM is semianalytical which helps in delineating the wave components. Using expansions in terms of spherical harmonics and Fourier components, the transfer function is obtained from numerical height integration. This is time consuming computationally but needs to be done only once. Once such a transfer function is computed, the wave response to arbitrary source distributions on the globe can then be constructed in very short order. In this review, we discuss some numerical experiments performed with the TFM, to study the various wave components excited in the auroral regions which propagate through the thermosphere and lower atmosphere, and to elucidate the properties of realistic source geometries. The model is applied to the interpretation of satellite measurements. Gravity waves observed in the thermosphere of Venus are also discussed.  相似文献   
109.
    
This paper examines the debate within the USA over how to meet the perceived competition from the successful European Ariane launcher and the loss of US market share for space launch services that it represented. In particular, it explores the origins of the 1983 Reagan Administration policy to turn over expendable launch vehicle production and operation to private industry. The Administration's other decisions to: (1) use the Space Shuttle to fly all government payloads, and (2) allow NASA to market Space Shuttle services commercially, conflicted with this commercialization policy. These policies effectively caused US industry to delay entry into the international competition for launch services until after the loss of the Space Shuttle Challenger in January 1986.  相似文献   
110.
    
The fifth workshop of the EOPOLE project1 was held in Seville, Spain, 17–19 January 2000, with the purpose of forming an opinion on European Earth observation data archiving policy. This report discusses the growing size and importance of EO data archives and presents the main conclusions of the workshop. The case is made for the establishment of a European centre for data archiving and users are urged to become more involved in archiving policy.  相似文献   
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