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Wireless underground sensor networks: Research challenges 总被引:5,自引:0,他引:5
This work introduces the concept of a Wireless Underground Sensor Network (WUSN). WUSNs can be used to monitor a variety of conditions, such as soil properties for agricultural applications and toxic substances for environmental monitoring. Unlike existing methods of monitoring underground conditions, which rely on buried sensors connected via wire to the surface, WUSN devices are deployed completely belowground and do not require any wired connections. Each device contains all necessary sensors, memory, a processor, a radio, an antenna, and a power source. This makes their deployment much simpler than existing underground sensing solutions. Wireless communication within a dense substance such as soil or rock is, however, significantly more challenging than through air. This factor, combined with the necessity to conserve energy due to the difficulty of unearthing and recharging WUSN devices, requires that communication protocols be redesigned to be as efficient as possible. This work provides an extensive overview of applications and design challenges for WUSNs, challenges for the underground communication channel including methods for predicting path losses in an underground link, and challenges at each layer of the communication protocol stack. 相似文献
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Concluding remarks With this brief exposition of the areas covered in this special double issue on Eurotra, we shall now conclude our introduction. We hope that the volume will achieve its goals, as outlined above, at least to some extent. Certainly, much more could be said about a wide range of topics which have been covered in the lifetime or the project so far. For instance, these article, mainly written by linguists, deliberately neglect the software implementation and environment aspects of the prototype Eurotra Translation System, as well as a number of peripheral tools and components of the system that are available to the user, such as lexical data bases, text-handling mechanisms and the like. The reader interested in aspects of this kind is referred to Raw et al. 1989 for a very brief introduction, and to the Eurotra software team at the Commission of the European Communities (DG XIII, Luxembourg) for more details. However, one goal that we do hope to come close to achieving is to give a fair overview of the Eurotra linguistic theory of translation and the mainstream and sideline formalisms expressing variant versions of it. If we have come anywhere near achieving that, our gratitude is due to the numerous Eurotra colleagues who have supported us in preparing this volume, and to the editor of this journal. 相似文献
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METHODS FOR BRAZING CERAMIC AND METAL-CERAMIC JOINTS 总被引:5,自引:0,他引:5
Owing to improved manufacturing processes technical ceramics on the basis of oxide as well as nonoxide ceramics have gained an increasing technical potential. Among other joining techniques brazing has proved to be the most flexible joining process that can easily be adapted to various ceramic-metal-combinations. In order to induce a wetting reaction there are two different approaches. The more complicated process encircles a pre-metallization process and a subsequent brazing process. The less sophisticated process is the 'active brazing process' where special brazing alloys are employed that are able to wet ceramic base materials. Although both joining processes are very flexible there are restrictions regarding the filler metals to be used, the premetallization process and the actual (active-) brazing process. 相似文献
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A method for the determination of carbon black fillers in peroxide vulcanizates or sulfur vulcanizates of natural rubber (NR, cis-1,4-polyisoprene) by metathesis degradation with 1-octene was developed using the catalyst WCl6? (C2H5)3Al2Cl3. The carbon black was separated from the reaction mixture by centrifuging and determined by weighing. In this connection, investigations were also performed with the catalyst WCl6? C2H5Al2Cl3 which was less suitable for filler determination. The rate of degradation of crosslinked NR with (E)-4-octene was measured. Metathesis reactions of 2-methyl-2-pentene (low-molecular-weight model compound of NR) with 7-tetradecene showed the importance of double bond shift for the degradation of NR. 相似文献
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The causes of degradation of aquatic systems are often complex and stem from a variety of human influences. Comprehensive, multimetric biological indices have been developed to quantify this degradation and its effect on aquatic communities, and measure subsequent recovery from anthropogenic stressors. Traditionally, such indices have concentrated on small‐to medium‐sized streams. Recently, however, the Ohio River Fish Index (ORFIn) was created to assess biotic integrity in the Ohio River. The goal of the present project was to begin developing a companion Ohio River multimetric index using benthic macroinvertebrates. Hester–Dendy multiplate samplers were used to evaluate benthic macroinvertebrate assemblages in relation to a gradient of water quality disturbance, represented by varying distances downstream of industrial and municipal wastewater outfalls in the Ohio River. In August 1999 and 2000, samplers were set every 100 m downstream of outfalls (12 outfalls in 1999, 22 in 2000) for 300–1000 m, as well as at upstream reference sites. Candidate metrics (n = 55) were examined to determine which have potential to detect changes in water quality downstream of outfalls. These individual measures of community structure were plotted against distance downstream of each outfall to determine their response to water quality disturbance. Values at reference and outfall sites were also compared. Metrics that are ecologically relevant and showed a response to outfall disturbance were identified as potentially valuable in a multimetric index. Multiple box plots of index scores indicated greater response to outfall disturbance during periods of low‐flow, and longitudinal river‐wide trends. Evaluation of other types of anthropogenic disturbance, as well as continued analysis of the effects of chemical water quality on macroinvertebrate communities in future years will facilitate further development of a multimetric benthic macroinvertebrate index to evaluate biotic integrity in the Ohio River. Copyright © 2007 John Wiley & Sons, Ltd. 相似文献
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Structured document storage and refined declarative and navigational access mechanisms in HyperStorM 总被引:2,自引:0,他引:2
Klemens Böhm Karl Aberer Erich J. Neuhold Xiaoya Yang 《The VLDB Journal The International Journal on Very Large Data Bases》1997,6(4):296-311
The combination of SGML and database technology allows to refine both declarative and navigational access mechanisms for
structured document collection: with regard to declarative access, the user can formulate complex information needs without
knowing a query language, the respective document type definition (DTD) or the underlying modelling. Navigational access is
eased by hyperlink-rendition mechanisms going beyond plain link-integrity checking. With our approach, the database-internal
representation of documents is configurable. It allows for an efficient implementation of operations, because DTD knowledge
is not needed for document structure recognition. We show how the number of method invocations and the cost of parsing can
be significantly reduced.
Edited by Y.C. Tay. Received April 22, 1996 / Accepted March 16, 1997 相似文献
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