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271.
Most nonlinear high-resolution bearing estimators are unstable in the presence of correlated noise, system phase errors, and multipath arrivals because they inadvertently place too much emphasis on unstable eigenvectors of the cross-sensor correlation matrix. For moderately correlated noise there will be sufficiently many stable eigenvectors to resolve and localize discrete sources. A method is given (the "stable nonlinear method" or SNLM) whereby reweighting of the eigenvectors is achieved implicitly, without actual calculation of the eigenvectors. This SNLM is compared with Capon's maximum likelihood method (MLM) in simulations of correlated noise, partially correlated signals, and phase errors, and is shown to provide good stability in the cases considered. 相似文献
272.
Summary Two co-existing plutonic rocks (diorite and granodiorite) were studied from an intrusion of Variscan age in the Raztocna Valley
– Nízke Tatry Mountains, Western Carpathians. Geochemical analyses of major and trace elements constrain a volcanic arc as
emplacement environment and give the first hints of a mixture of two magmatic end-members: the so-called Prasivá granodiorite
and the Raztocna diorite. The 87Sr/86Sr(0) ratios vary between 0.7075 and 0.7118, the ε Nd(0) values range from −1.4 to −5.0. Common Pb isotopes reveal a dominant crustal source with minor influences from a mantle and
a lower crustal source.
Modelling based on Sr and Nd isotope data and using three component mixing calculations indicates that mixing of 2/3 of upper
mantle material with 1/3 upper crustal material can produce the isotopic composition of the Raztocna diorite. Very minor amounts
of lower crust were incorporated in the diorite. For the Prasivá granodiorite, the mixing ratio of upper mantle and upper
crust is similar, but a lower crustal reservoir contributed about 5–10% of the source material. 相似文献
273.
274.
275.
Erosion of man-made, forestry drainage channels occurs when the plough cuts through the surface peat layer into the underlying erodible mineral soil. A procedure is developed, based on hydraulic considerations, which will allow the drainage engineer to design stable drainage networks in upland forestry plantations. An example design chart is given for an erodible sandy loam type soil. 相似文献
276.
Based on our 1997 observations with a CCD camera and narrow-band filters attached to the 1-m Assy Obsev vatory telescope, we extensively study the latitudinal variations in limb darkening and normal albedo on Jupiter's disk at wavelengths of 387, 445, 502, 619, and 702 nm. In addition, we carried out observations in 1998 with broad-band red, green, and blue filters. Apart from a general regularity—the increase in darkening coefficient with normal albedo of Jupiter's cloud cover—there is an appreciable scatter of darkening coefficients for the same albedo, which is most pronounced in the ultraviolet. This scatter may result from differences in the optical depth of the aerosol haze above the clouds. The lack of any wavelength dependence of the darkening coefficients is confirmed for Jupiter's polar regions, while at other latitudes, they decrease with decreasing wavelength. 相似文献
277.
A. El-Bassuny Alawy 《Astrophysics and Space Science》2001,277(3):473-495
A new approach is proposed and developed to handle pre-processed CCD frames in order to identify stellar images and derive
their relevant parameters. The present method relies on: 1) identifying stellar images and assigning approximate positions
of their centres using artificial intelligence techniques, 2) accurate determination of the centre coordinates applying an
elementary statistical concept and 3) estimating the image peak intensity as a stellar magnitude measure employing a simple
numerical analysis approach. The method has been coded for personal computer users. A CCD frame of the star cluster M67 was
adopted as a test case. The results obtained are discussed in comparison with the DAOPHOTII ones and the corresponding published
data. Exact coincidence has been found between both results except very few cases. These exceptions have been discussed in
view of both methods' bases and the cluster plates. It has been realized that the method suggested exhibits very simple, extremely
fast and high precise approach in stellar CCD photometry domain. Moreover, it is more capable for handling blended and distorted
stellar images than the DAOPHOTII. These characteristics show the usefulness of the present method in some astronomical applications
such as auto-focusing and auto-guiding sensing approaches beside the main purpose, viz. stellar photometry.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
278.
Bjerrum-Niese C. Bjorno L. Pinto M.A. Quellec B. 《Oceanic Engineering, IEEE Journal of》1996,21(2):143-149
The paper discusses the development of a simulation tool to model high data-rate acoustic communication in shallow water. The simulation tool is able to generate synthetic time series of signals received at a transducer array after transmission across a shallow-water communication channel. The simulation tool is suitable for testing advanced signal processing techniques for message recovery. A channel model has been developed based on the physical aspects of the acoustic channel. Special emphasis has been given to fluctuations of the signal transmission caused by time-varying multipath effects. At shorter ranges, the temporal variations are dominated by acoustic scattering from the moving sea surface. Therefore, the channel model produces a coherence function which may be interpreted as a time-varying reflection coefficient for the surface scattered acoustical path. A static, range-independent ray model identifies the significant multipaths, and the surface path is modulated with the time-varying reflection coefficient. The advantages and limitations of the channel model are discussed and assumptions necessary to overcome the limitations are emphasised. Based on the assumptions, an algorithm has been developed and implemented to model how a binary message will be modulated when transmitted by a transducer, is distorted in the channel and finally is received by a transducer array 相似文献
279.
I.A. Crawford 《Astrophysics and Space Science》2003,285(3-4):661-675
I review the recent discoveries of variable interstellar absorption lines, widely taken to imply the existence of very small
scale(10–100 AU) structure in the interstellar medium. Possible origins of this structure are discussed, and attention is
drawn to the fact that most known examples seem to be associated with interstellar shells and bubbles of various kinds. On
the other hand, in at least one case (κ Velorum),the variation appears to occur in the supposedly quiescent cold neutral medium,
consistent with earlier studies of small scale structure in atomic hydrogen probed at radio wavelengths. Further work is urgently
required to determine the prevalence, mode of formation, and the physical and chemical state of these enigmatic structures.
Only then will their implications for our wider understanding of the interstellar medium become apparent.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
280.
The previously proposed (Rodin, 2002) method for calculating the microphysical properties of spatially inhomogeneous rarefied aerosol media with mixing using the lowest-order moments of the size distribution is generalized to particle coagulation. We show that when the problem is formulated in terms of moments, all of the solutions admitted by the stochastic coagulation equation lie within a narrow range whose boundaries can be determined by means of quadratic programming. We discuss the choice of an optimal solution within this range and compare the moment method with the results of our computations by the classical finite-difference method using a model of photochemical aerosols in Titan's atmosphere as an example. The moment method allows the efficiency of microphysical computations to be significantly increased by using precomputed low-dimension interpolation tables. It can be used to construct self-consistent models for the globular circulation of planetary atmospheres. 相似文献