共查询到20条相似文献,搜索用时 15 毫秒
1.
The Pade approximation (PA) method is used to analyze the detection performance of single and multiple pulse radar systems operating in K-distributed clutter and thermal noise. Simple approximations for false-alarm and detection probabilities are obtained, using lower order moments for the detection decision statistic. Both envelope and squaring detector laws are considered, with noncoherent integration, for independent and correlated K clutter. The target is assumed to be pulse-to-pulse Rayleigh fading. The methods are a substantial application of the PA methods we have previously published 相似文献
2.
Space Science Reviews - The general problem of diffusive transport and acceleration of energetic charged particles is considered. The transport of solar-flare particles, solar modulation of... 相似文献
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Space Science Reviews - The magnetospheric research community has long sought the capability to view the Sun-Earth system in a global way and to probe concurrently the microphysical details of key... 相似文献
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Ground-based VLF observations show evidence that strong whistler-mode waves in the magneto-sphere are often stimulated by harmonic radiation from electrical power transmission lines. These stimulated emissions sometimes dominate the wave activity in the kHz range. A VLF transmitter at Siple, Antarctica has been used to simulate these power line effects with ~ 0.5 W radiated power at a given frequency. Occurrence statistics of power line effects are also summarized. 相似文献
5.
Dynamic range improvement in magnetic resonance imaging (MRI) by nonlinear amplitude compression is proposed. Quantization noise reduction by a factor of 10 to 20 is confirmed by a numerical simulation, provided an appropriate nonlinear transform is chosen. The piecewise-linear transform is the best nonlinear transform studied so far. Implementing amplitude compression requires no additional hardware in a usual MRI system if the low spatial frequency lines of the magnetic resonance (MR) signal are selectively prescanned through an attenuator. The method will be particularly effective in 3-D imaging of large objects. MR angiography, and imaging of solids 相似文献
6.
Chiu Y. T. Cornwall J. M. Fennell J. F. Gorney D. J. Mizera P. F. 《Space Science Reviews》1983,35(3):211-257
Space Science Reviews - Observations and theoretical interpretations of auroral plasma distributions have led to a spectacular advance, in the latter part of the 1970's, in understanding the... 相似文献
7.
Li F. Liu H. Vaccaro R.J. 《IEEE transactions on aerospace and electronic systems》1993,29(4):1170-1184
Subspace based direction-of-arrival (DOA) estimation has motivated many performance studies, but limitations such as the assumption of an infinite amount of data and analysis of individual algorithms generally exist in these performance studies. The authors have previously proposed a unified performance analysis based on a finite amount of data and achieved a tractable expression for the mean-squared DOA estimation error for the multiple signal classification (MUSIC). Min-Norm, estimation of signal parameters using rotational invariance techniques (ESPRIT), and state-space realization algorithms. However, this expression uses the singular values and vectors of a data matrix, which are obtained by the highly nonlinear transformation of the singular value decomposition (SVD). Thus the effects of the original data parameters such as numbers of sensors and snapshots, source coherence and separations were not explicitly analyzed. The authors unify and simplify this previous result and derive a unified expression based on the original data parameters. They analytically observe the effects of these parameters on the estimation error 相似文献
8.
A. Balogh J. A. Gonzalez-Esparza R. J. Forsyth M. E. Burton B. E. Goldstein E. J. Smith S. J. Bame 《Space Science Reviews》1995,72(1-2):171-180
Between its launch in October 1990 and the end of 1993, approximately 160 fast collisionless shock waves were observed in the solar wind by the Ulysses space probe. During the in-ecliptic part of the mission, to February 1992, the observed shock waves were first caused mainly by solar transient events following the solar maximum and the reorganisation of the large scale coronal fields. With the decay in solar activity, relatively stable Corotating Interaction Regions (CIRs) were observed betwen 3 and 5.4 AU, each associated with at least one forwardreverse shock pair. During the out-of-ecliptic phase of the orbit, from February 1992 onwards, CIRs and shock pairs associated with them continued to dominate the observations. From July 1992, Ulysses encountered the fast solar wind flow from the newly developed southern polar coronal hole, and from May 1993 remained in the unipolar magnetic region associated with this coronal hole. At latitudes beyond 30°, CIRs were associated almost exclusively with reverse shocks only. A comprehensive list of shock waves identified in the magnetic field and solar wind plasma data from Ulysses is given in Table 1. The principal characteristics were determined mainly from the magnetic field data. General considerations concerning the determination of shock characteristics are outlined in the Introduction. 相似文献
9.
Theoretical concepts related to decision-making, group dynamics, and communication processes are reviewed. A psychological distancing model of electronic media is described. Issues related to the role of electronic media in networking decision-makers are highlighted. Several laboratory studies that show some of the effects video, audio, and computer teleconferencing can have upon group interaction patterns and productivity are described. It is found that increasing communication richness does not guarantee improved group performance. Task demands as well as individual difference factors can affect the way groups use available media. It is concluded that existing theory is not yet sophisticated enough to account for all of the results observed. A more comprehensive theory of communication processes is needed that specifically addresses group-to-group interaction processes 相似文献
10.
E. J. Smith M. Neugebauer A. Balogh S. J. Bame R. P. Lepping B. T. Tsurutani 《Space Science Reviews》1995,72(1-2):165-170
The radial component of the magnetic field at Ulysses, over latitudes from –10° to –45° and distances from 5.3 to 3.8 AU, compares very well with corresponding measurements being made by IMP-8 in the ecliptic at 1AU. There is little, if any, evidence of a latitude gradient. Variances in the field, normalized to the square of the field magnitude, show little change with latitude in variations in the magnitude but a large increase in the transverse field variations. The latter are shown to be caused by the presence of large amplitude, long period Alfvénic fluctuations. This identification is based on the close relation between the magnetic field and velocity perturbations including the effect of anisotropy in the solar wind pressure. The waves are propagating outward from the Sun, as in the ecliptic, but variance analysis indicates that the direction of propagation is radial rather than field-aligned. A significant long-period component of 10 hours is present. 相似文献
11.
B. E. Goldstein M. Neugebauer J. T. Gosling S. J. Bame J. L. Phillips D. J. Mccomas A. Balogh 《Space Science Reviews》1995,72(1-2):113-116
We report observations of radial and latitudinal gradients of Ulysses plasma parameters. The solar wind velocity increased rapidly with latitude from 0° to 35°, then remained approximately constant at higher latitudes. Solar wind density decreased rapidly from 0° to 35° of latitude, and also was approximately constant beyond that latitude. The mass flux similarly decreased away from the equator (but less than the density), whereas the momentum flux was relatively constant. The radial gradient of the entropy at high latitude indicated a value for the polytrope index of about 1.72 (close to adiabatic); the in-ecliptic estimates of radial gradients for temperature and entropy may be biased by temporal variation. A striking increase in the alpha particle-proton velocity difference with latitude is found. 相似文献
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13.
Ti和Ti/Ni/Ti连接钨与铜及其合金的界面结合机制与接头强度 总被引:4,自引:0,他引:4
采用热 力学模拟试验机Gleeble1500D作加热设备,用Ti片和Ti箔 Ni片 Ti箔复合中间层扩散连接钨与铜及铜合金CuCrZr。结果表明,当用Ti片连接钨与铜,连接温度下Ti与Cu反应但未转化成液相时,则反应层由具有一定脆性的多层化合物组成,接头强度偏低;当Ti片通过共晶反应转化成液相且大部分液相被挤出连接区时,接头强度显著提高,最高达220MPa。用Ti Ni Ti复合中间层连接钨与CuCrZr时,结合界面是通过Ti分别与Ni、W及Cu相互扩散并反应生成多层化合物和固溶体而形成的;与Ti片连接钨与铜的接头形成相似,连接过程中Ti箔未转化成液相时接头强度偏低,Ti箔转化成液相时接头强度明显提高。 相似文献
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The Gravity field and steady-state Ocean Circulation Explorer (GOCE) satellite, currently planned to he launched in the course
of 2006, will require a precise drag compensation and a fine attitude control along the Local Orbiting Reference Frame (LORF)
of a polar Sun-synchronous low orbit, allowing the Earth gravity field to be recovered with unprecedented accuracy by post-processing
the scientific telemetry. To this aim, the spectral density of the spacecraft linear and angular accelerations must be limited
below 0.025
respectively, in the frequency range from 5 mHz to 0.1 Hz, the gradiometer measurement bandwidth. In the same range, the
orientation errors of the spacecraft in the LORF and of the LORF in the inertial frame must be kept below 10
. The Drag-Free Mode, encharged of drag-free and attitude control (DFAC) during measurement phases, determines the spacecraft
state vector using a very precise gradiometer, one large Field-of-View Star Tracker and a Satellite-to-Satellite Tracking
Instrument. Force and torque commands are actuated by two assemblies of thrusters: a single ion-thruster acting along the
orbital direction, a set of eight micro-thrusters acting along the other five degrees of freedom. To cover every mission scenario,
other control modes have been studied and designed: the Coarse Pointing Mode dedicated to rate damping and Sun acquisition,
the Fine Pointing Mode handling the transition to Drag-Free Mode and the Ultimate Safe Mode, a survival operative mode to
improve mission reliability. Results presented in this paper give a positive perspective on the solidity of the current DFAC
design.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
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V: SEA LEVEL: Benefits of GRACE and GOCE to sea level studies 总被引:1,自引:0,他引:1
The recently published Third Assessment Reports of the Intergovernmental Panel on Climate Change have underlined the scientific
interest in, and practical importance of past and potential future sea level changes. Space gravity missions will provide
major benefits to the understanding of the past, and, thereby, in the prediction of future, sea level changes in many ways.
The proposal for the GOCE mission described well the improvements to be expected from improved gravity field and geoid models
in oceanography (for example, in the measurement of the time-averaged, or ‘steady state’, ocean surface circulation and better
estimation of ocean transports), in geophysics (in the improvement of geodynamic models for vertical land movements), in geodesy
(in positioning of tide gauge data into the same reference frame as altimeter data, and in improvement of altimeter satellite
orbits), and possibly in glaciology (in improved knowledge of bedrock topography and ice sheet mass fluxes). GRACE will make
many important steps towards these ‘steady state’ aims. However, its main purpose is the provision of oceanographic (and hydrological
and meteorological) temporally-varying gravity information, and should in effect function as a global ‘bottom pressure recorder’,
providing further insight into the 3-D temporal variation of the ocean circulation, and of the global water budget in general.
This paper summaries several of these issues, pointing the way towards improved accuracy of prediction of future sea level
change.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
19.
Aerodynamic scaling to free flight conditions: Past and present 总被引:1,自引:0,他引:1
This report summarizes some of the problems when wind tunnel data should be scaled to free flight conditions. The main challenges in performing this extrapolation is how model support, wall interference, aeroelastic effects and a potentially lower Reynolds number in the wind tunnel should be corrected. A historical review of scale effects is presented showing wind tunnel to flight discrepancies of different types of aircraft configurations. An overview of scaling methodologies and Reynolds number effects are presented and discussed. Some modern approaches where computational fluid dynamics (CFD) are used, together with wind tunnel testing, in order to identify scaling phenomena are presented as well. 相似文献
20.
We investigate ways of accurately simulating the propagation of energetic charged particles over small times where the standard
Monte Carlo approximation to diffusive transport breaks down. We find that a small-angle scattering procedure with appropriately
chosen step-lengths and scattering angles gives accurate results, and we apply this to the simulation of propagation upstream
in relativistic shock acceleration.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献