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排序方式: 共有236条查询结果,搜索用时 328 毫秒
181.
182.
Jason W. Barnes Lawrence Lemke Rick Foch Christopher P. McKay Ross A. Beyer Jani Radebaugh David H. Atkinson Ralph D. Lorenz Stéphane Le Mouélic Sebastien Rodriguez Jay Gundlach Francesco Giannini Sean Bain F. Michael Flasar Terry Hurford Carrie M. Anderson Jon Merrison Máté ádámkovics Simon A. Kattenhorn Jonathan Mitchell Devon M. Burr Anthony Colaprete Emily Schaller A. James Friedson Kenneth S. Edgett Angioletta Coradini Alberto Adriani Kunio M. Sayanagi Michael J. Malaska David Morabito Kim Reh 《Experimental Astronomy》2012,33(1):55-127
We describe a mission concept for a stand-alone Titan airplane mission: Aerial Vehicle for In-situ and Airborne Titan Reconnaissance (AVIATR). With independent delivery and direct-to-Earth communications, AVIATR could contribute to Titan science either alone or as part of a sustained Titan Exploration Program. As a focused mission, AVIATR as we have envisioned it would concentrate on the science that an airplane can do best: exploration of Titan??s global diversity. We focus on surface geology/hydrology and lower-atmospheric structure and dynamics. With a carefully chosen set of seven instruments??2 near-IR cameras, 1 near-IR spectrometer, a RADAR altimeter, an atmospheric structure suite, a haze sensor, and a raindrop detector??AVIATR could accomplish a significant subset of the scientific objectives of the aerial element of flagship studies. The AVIATR spacecraft stack is composed of a Space Vehicle (SV) for cruise, an Entry Vehicle (EV) for entry and descent, and the Air Vehicle (AV) to fly in Titan??s atmosphere. Using an Earth-Jupiter gravity assist trajectory delivers the spacecraft to Titan in 7.5 years, after which the AVIATR AV would operate for a 1-Earth-year nominal mission. We propose a novel ??gravity battery?? climb-then-glide strategy to store energy for optimal use during telecommunications sessions. We would optimize our science by using the flexibility of the airplane platform, generating context data and stereo pairs by flying and banking the AV instead of using gimbaled cameras. AVIATR would climb up to 14?km altitude and descend down to 3.5?km altitude once per Earth day, allowing for repeated atmospheric structure and wind measurements all over the globe. An initial Team-X run at JPL priced the AVIATR mission at FY10 $715M based on the rules stipulated in the recent Discovery announcement of opportunity. Hence we find that a standalone Titan airplane mission can achieve important science building on Cassini??s discoveries and can likely do so within a New Frontiers budget. 相似文献
183.
A. J. Tatem H. G. Lewis P. M. Atkinson M. S. Nixon 《International journal of geographical information science》2013,27(7):647-672
Land cover class composition of remotely sensed image pixels can be estimated using soft classification techniques increasingly available in many GIS packages. However, their output provides no indication of how such classes are distributed spatially within the instantaneous field of view represented by the pixel. Techniques that attempt to provide an improved spatial representation of land cover have been developed, but not tested on the difficult task of mapping from real satellite imagery. The authors investigated the use of a Hopfield neural network technique to map the spatial distributions of classes reliably using information of pixel composition determined from soft classification previously. The approach involved designing the energy function to produce a ‘best guess’ prediction of the spatial distribution of class components in each pixel. In previous studies, the authors described the application of the technique to target identification, pattern prediction and land cover mapping at the sub-pixel scale, but only for simulated imagery. We now show how the approach can be applied to Landsat Thematic Mapper (TM) agriculture imagery to derive accurate estimates of land cover and reduce the uncertainty inherent in such imagery. The technique was applied to Landsat TM imagery of small-scale agriculture in Greece and largescale agriculture near Leicester, UK. The resultant maps provided an accurate and improved representation of the land covers studied, with RMS errors for the Landsat imagery of the order of 0.1 in the new fine resolution map recorded. The results showed that the neural network represents a simple efficient tool for mapping land cover from operational satellite sensor imagery and can deliver requisite results and improvements over traditional techniques for the GIS analysis of practical remotely sensed imagery at the sub pixel scale. 相似文献
184.
Kristin M. Stock Rob Atkinson Chris Higgins Mark Small Andrew Woolf Keiran Millard 《International journal of geographical information science》2013,27(2):231-252
The use of a semantically rich registry containing a Feature Type Catalogue (FTC) to represent the semantics of geographic feature types including operations, attributes and relationships between feature types is required to realise the benefits of Spatial Data Infrastructures (SDIs). Specifically, such information provides a more complete representation of the semantics of the concepts used in the SDI, and enables advanced navigation, discovery and utilisation of discovered resources. The presented approach creates an FTC implementation in which attributes, associations and operations for a given feature type are encapsulated within the FTC, and these conceptual representations are separated from the implementation aspects of the web services that may realise the operations in the FTC. This differs from previous approaches that combine the implementation and conceptual aspects of behaviour in a web service ontology, but separate the behavioural aspects from the static aspects of the semantics of the concept or feature type. These principles are demonstrated by the implementation of such a registry using open standards. The ebXML Registry Information Model (ebRIM) was used to incorporate the FTC described in ISO 19110 by extending the Open Geospatial Consortium ebRIM Profile for the Web Catalogue Service (CSW) and adding a number of stored queries to allow the FTC component of the standards‐compliant registry to be interrogated. The registry was populated with feature types from the marine domain, incorporating objects that conform to both the object and field views of the world. The implemented registry demonstrates the benefits of inheritance of feature type operations, attributes and associations, the ability to navigate around the FTC and the advantages of separating the conceptual from the implementation aspects of the FTC. Further work is required to formalise the model and include axioms to allow enhanced semantic expressiveness and the development of reasoning capabilities. 相似文献
185.
186.
187.
Research on the transmission of infectious diseases amongst the human population at fine scales is limited. At this level,
the dynamics of humans are determined by their social activities and the physical condition of the environment. Raster space
AB (activity bundle) simulation is a method to simulate humans' contacts within a space under the framework of an individual
space–time activity-based model (ISTAM). The parameters of the raster space AB simulation were explored. For static ABs, the
results show the relations between proportion infected and proportion occupied, number of index cases, size of AB, size of
cell, ratio of AB and ratio of cell, respectively due to the different spatial distributions of individuals. The most important
parameters were number of index cases, size of AB and proportion occupied. For dynamic ABs, analysis shows that movement frequency
is more important than movement proportion. An example application of raster space AB simulation shows that this method can
be used effectively to quantify the infection risk (proportion infected) at the within-AB level. This research can further
the understanding of transmission process at fine scales and is beneficial to the design and testing of control measures.
相似文献
188.
Pollution, turbidity and coral bleaching history, as well as coral diversity, vary along the Tanzanian coastline. Prior to this study, it was not known whether exposure to such environmental variation might have influenced the diversity and distribution of Symbiodinium along this coastline. Such information can provide insight into whether Tanzanian reef-building corals develop adaptation to current trends of climate change. Here, 10 reef-building coral samples were collected from different reefs along the Tanzanian coast with different micro-environments and bleaching histories. The ITS-2 region of ribosomal DNA was employed in the characterisation of Symbiodinium harboured by reef-building corals. DGGE fingerprints and DNA sequences showed that most coral species host a single Symbiodinium type, which is maintained throughout the coast regardless of local environmental differences. We present polymorphic symbioses in Acropora spp., Millepora sp. and Galaxea fascicularis in mainly turbid and warm environments as a sign of adaptation to harsh conditions. However, such adaptation may not provide marked resistance to bleaching because Acropora is a bleaching-susceptible genus. 相似文献
189.
This paper demonstrates the performance of a new design of atmospheric pressure flow-tube reactor coupled with an ion trap mass spectrometer. The reactor is designed for studying gas-phase reactions with reaction times in the range from 0.2 to 2 s. The system provides a simple, fast and robust method for introducing the reactant mixtures and for identifying the products of gas-phase reactions. As a practical example of application, we used the flow reactor to study the reaction of O3 with 2,3-dimethyl-2-butene. Good agreement was obtained between the flow tube results and previously reported studies for the identification of major products and measurement of their yields. This design has the potential to be used to study the atmospheric ozonolysis reactions of unsaturated species at realistic pressures and temperatures. 相似文献
190.
Summary. A summary is presented of the use of stress relaxation testing in studies of the mechanical behaviour of materials with emphasis on the underlying assumptions of the method. It is shown that relaxation testing is a powerful experimental method which could profitably be used in future studies of the mechanical properties of geological materials to augment, but not to replace, the more conventional creep and constant strain rate tests.
Stress relaxation data can be analysed to determine important parameters in equations describing (a) laws of flow and fracture, and (b) dislocation dynamics. The variation of the first derivative of any stress relaxation curve with stress can be used to establish the applied stress sensitivity to strain rate and the values of activation parameters such as 'activation volume'. Such parameters may be used to constrain the inference of the dominant deformation mechanism. This account is illustrated by reference to some recent experiments on polycrystalline, synthetic galena (lead sulphide) and pre-faulted cylinders of Tennessee sandstone deformed over a wide range of environmental conditions. The accurate sensing of the second derivative of a relaxation curve is required to establish parameters relating to dislocation dynamics, e.g. partition between thermal and athermal components of applied stress, and the stress dependence of dislocation velocity. The determination of such parameters is illustrated with new experiments on single crystals of calcite. 相似文献
Stress relaxation data can be analysed to determine important parameters in equations describing (a) laws of flow and fracture, and (b) dislocation dynamics. The variation of the first derivative of any stress relaxation curve with stress can be used to establish the applied stress sensitivity to strain rate and the values of activation parameters such as 'activation volume'. Such parameters may be used to constrain the inference of the dominant deformation mechanism. This account is illustrated by reference to some recent experiments on polycrystalline, synthetic galena (lead sulphide) and pre-faulted cylinders of Tennessee sandstone deformed over a wide range of environmental conditions. The accurate sensing of the second derivative of a relaxation curve is required to establish parameters relating to dislocation dynamics, e.g. partition between thermal and athermal components of applied stress, and the stress dependence of dislocation velocity. The determination of such parameters is illustrated with new experiments on single crystals of calcite. 相似文献