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31.
Wideband channel model for aeronautical telemetry 总被引:1,自引:0,他引:1
Rice M. Davis A. Bettweiser C. 《IEEE transactions on aerospace and electronic systems》2004,40(1):57-69
A multipath channel model for wideband aeronautical telemetry links is presented. Channel sounding data were collected at Edwards AFB, CA, at both L-band and lower S-band. Frequency domain analysis techniques were used to evaluate candidate channel models. The channel model is composed of three propagation paths: a line-of-sight direct path and two specular reflections. The first specular reflection is characterized by a relative amplitude of 70% to 96% of the line-of-sight amplitude and a delay of 10-80 ns. This path is the result of "ground bounces" off the dry lake bed at Edwards and is a typical terrain feature at DoD test ranges located in the western United States. The amplitude and delay of this path are defined completely by the flight path geometry. The second path has much lower amplitude and a longer delay. The gain of this path is well modeled as a zero-mean complex Gaussian random variable. The relative amplitude is approximately 2% to 8% of the line-of-sight amplitude. The mean excess delay is 155 ns with an rms delay spread of 74 ns. 相似文献
32.
Those in the space community interested in deploying space solar power (SSP) need to know whether it would make economic sense. This article aims to develop a conceptual model of the economic value of SSP as a source of power to in-space activities, such as spacecraft and space stations. We offer several estimates of the value based on interviews and published data, discuss technological innovations that may compete with or be complementary to SSP, and consider alternative institutional arrangements for government and the private sector to provide SSP. 相似文献
33.
Ernest Davis 《Spatial Cognition & Computation》2013,13(4):315-318
Abstract Some of John Bateman's critiques of my article “Qualitative Spatial Reasoning in Interpreting Text and Narrative” are certainly valid. However, the alternative solutions that he proposes do not seem to me to be improvements. 相似文献
34.
Aristotle Socrates Shane W. Davis Omer Blaes 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2006,38(12):2880-2883
Turbulent Comptonization, a potentially important damping and radiation mechanism in relativistic accretion flows, is discussed. Particular emphasis is placed on the physical basis, relative importance, and thermodynamics of turbulent Comptonization. The effects of metal absorption opacity on the spectral component resulting from turbulent Comptonization is considered as well. 相似文献
35.
Michael Loewenstein David S. Davis 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2008,41(12):1972-1974
We discuss the elemental composition in the hot ISM of elliptical galaxies derived from new and recent X-ray spectral analysis in the context of new phenomenological models of their chemical evolution. Star formation histories, the IMF, the astrophysics of supernovae, and the nature of galactic winds impact the metal content and relative abundances in the hot ISM. We evaluate how X-ray spectroscopy may be utilized to deconstruct how elliptical galaxies, and the stellar populations that compose them, form and evolve, with an emphasis on present and future high resolution spectroscopic analysis. 相似文献
36.
The author defines a phase-only gradient-based adaptive algorithm analogous to the least mean square (LMS) algorithm. Two phase-only perturbation algorithms are then defined. It is shown that the gradient for the perturbation algorithms can be obtained by cross-correlating binary perturbation sequences, that are applied to the adaptive phases, with the resulting instantaneous output power or voltage. It is also shown that a single set of phase-only adaptive weights can be used to simultaneously null interference in multiple output beams. Simulation results are presented for all of the new algorithms. The phase-only perturbation techniques eliminate the need for element level receivers and support low cost retrofitting of adaptive nulling on phased arrays by using conventional beamsteering circuits to apply the adaptive weights 相似文献
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M.E. Wiedenbeck N.E. Yanasak A.C. Cummings A.J. Davis J.S. George R.A. Leske R.A. Mewaldt E.C. Stone P.L. Hink M.H. Israel M. Lijowski E.R. Christian T.T. von Rosenvinge 《Space Science Reviews》2001,99(1-4):15-26
Cosmic-ray isotope observations from NASAs Advanced Composition Explorer (ACE) mission have been used to investigate the composition of cosmic-ray source material. Source abundances relative to 56Fe are reported for eleven isotopes of Ca, Fe, Co, and Ni, including the very rare isotopes 48Ca and 64Ni. Although the source abundances range over a factor 104, most of the ratios to 56Fe are consistent with solar-system values to within 20%. However, there are some notable differences, the most significant being an excess of (70±30)% relative to the solar system for the cosmic-ray source ratio 58Fe/56Fe. The possible association of such an excess with a contribution to the cosmic-ray source from Wolf–Rayet star ejecta is discussed. 相似文献
39.
DORIS is an expert system developed for aiding in the fault diagnosis process. DORIS performs two major tasks: one as an expert consultant to the end-user, and second as an aid to the knowledge engineer via knowledge acquisition tools. DORIS is coded in LISP without third party expert system development tools. System features include a user-friendly interface, knowledge bases, forward and backward chaining inference systems, an explanation system, and a rule maintenance system. DORIS has been applied as a prototype in two areas: fault diagnosis of a Serial Digital Multiplex Assembly, and procedrual control for navigational sequencing. 相似文献
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