4,133 research outputs found
Digital radio and its application in the HF (2-30 MHz) band
Full text linkThe propagation environment at high frequencies (HF, 2-30 MHz) has a significant impact on the performance of radio systems (especially data communications). However, the ability to communicate information over very long ranges using ionospheric propagation paths without any intermediate infrastructure makes the use of HF attractive for many applications. In order to increase the utility of HF communications there is a strong desire to increase HF data rates. Currently data rates of up to -2400 bps can be reliably achieved in standard 3 kHz HF channel allocations.
Whilst further increases in data rate within the confines of these narrowband frequency allocations is likely, the use of larger bandwidths (contiguous or otherwise) appears to offer potential for much greater throughputs. This requires a greater understanding of the characteristics of wideband channels and also requires transmitting and receiving equipment capable of wideband/multi-channel operation.
New waveforms have been proposed for the transmission of higher data rates in extended channel bandwidths (6 kHz). The results of laboratory measurements and analysis of data collected during on-air trials of a number of 16 kbps waveforms are presented. Analysis indicates that operation over surface wave and benign skywave channels is possible, demonstrating the benefit of exploiting greater channel
bandwidths.
Suitable architectures for the implementation of wideband and multi-channel digital HF radios (software radios) have been investigated. The work presented indicates that it is
now possible, for the first time, to construct high performance, direct sampling wideband digital HF receivers. In such a receiver the entire HF band is digitised and
then all subsequent processing is undertaken digitally. Conceptually this would allow an arbitrary number of channels to be simultaneously received using a single RF frontend and digitiser. With careful design performance comparable with that of the high performance conventional super-heterodyne single channel receivers can be obtained.
A prototype wideband multi-channel digital HF transceiver with this architecture has been implemented and its performance shown to agree with that predicted.
A particular challenge in complex systems such as software radios is the deployment of software across a number of heterogeneous processors. A new asynchronous, eventbased,
processing architecture which employs messaging to allow processing tasks to be effectively distributed across a multiple processors and buses is proposed. It has been
implemented on the digital transceiver platform and its effectiveness has been demonstrated.
A new low-power pulse-compression oblique HF ionospheric sounder, known as WHISPER, has been developed. This sounder has been implemented as a software application on the wideband HF digital transceiver. Waveforms suitable for making wideband (-80 kHz) measurements of the channel time varying complex impulse response have been designed. These have been used to make measurements on a 170 km path in the UK during Spring 2001. The results of these measurements have been analysed and confirm the ability of the sounding instrument to measure the channel scattering function and the amplitude and phase within individual modes. A
number of possible directions for further analysis, pertinent to the design of wideband HF modems, have been proposed
Kinetics of catalytic isomerization of quadricyclane to norbornadiene using near infrared absorption spectroscopy: Conversion rate and diffusion motion in heterogeneous reaction
No full textFinancial analysis, asset allocation, and portfolio construction: Theory & practice
No full textThis material is copyrighted and the author retains all rights. No part of this material may be
reproduced or transmitted in any forms or by any means now known or later devised, or sorted
in a data base or retrieval system, without the prior written permission of HF Consulting. This material has been shared with permission from the author and HF Consulting
Terminal suturing of Gondwana along the southern margin of South China Craton : evidence from detrital zircon U-Pb ages and Hf isotopes in Cambrian and Ordovician strata, Hainan Island
No full textThis work was supported by the National Natural Science Foundation of China (grants 41472086 and 41272120), “111” Project (B08030), the fundamental Research Funds for the Central Universities, China University of Geosciences (Wuhan) (CUG2012019240 and CUG2013019137). The first author also acknowledges China Scholarship Council (grant 201208420001) for supporting his research in the University of St. Andrews. Date of Acceptance: 20/11/2014Hainan Island, located near the southern end of mainland South China, consists of the Qiongzhong Block to the north and the Sanya Block to the south. In the Cambrian, these blocks were separated by an intervening ocean. U-Pb ages and Hf isotope compositions of detrital zircons from the Cambrian succession in the Sanya Block suggest that the unit contains detritus derived from late Paleoproterozoic and Mesoproterozoic units along the western margin of the West Australia Craton (e.g., Northampton Complex) or the Albany-Fraser-Wilkes orogen, which separates the West Australia and Mawson cratons. Thus, in the Cambrian the Sanya Block was not part of the South China Craton but rather part of the West Australian Craton and its environs. In contrast, overlying Late Ordovician strata display evidence for input of detritus from the Qiongzhong Block, which constituted part of the southeastern convergent plate margin of the South China Craton in the early Paleozoic. The evolving provenance record of the Cambrian and Ordovician strata suggests that the juxtaposition of South China and West Australian cratons occurred during the early to mid-Ordovician. The event was linked with the northern continuation of Kuungan Orogeny, with South China providing a record of final assembly of Gondwana.Peer reviewe
On financial statements modelling and fundamental analysis
No full textThis material is copyrighted and the author retains all rights. No part of this material may be
reproduced or transmitted in any forms or by any means now known or later devised, or sorted
in a data base or retrieval system, without the prior written permission of HF Consulting. This material has been shared with permission from the author and HF Consulting
The mathematics of statistical modelling: Abstract to specific
No full textThis material is copyrighted and the author retains all rights. No part of this material may be
reproduced or transmitted in any forms or by any means now known or later devised, or sorted
in a data base or retrieval system, without the prior written permission of HF Consulting. This material has been shared with permission from the author and HF Consulting
High field strength element systematics and Lu-Hf & Sm-Nd garnet geochronology of orogenic eclogites
Full text linkConcerning the Bulk Silicate Earth (BSE), the depleted mantle and the continental crust are thought to balance the budget of refractory and lithophile elements, resulting in complementary trace element patterns. However, the two high field strength elements (HFSE) Niob and Tantal appear to contradict this mass balance. All reservoirs of the silicate Earth exhibit subchondritic Nb/Ta ratios, possibly as a result of Nb depletion. The two HFSE Zr and Hf on the other hand seem not to be fractionated between the silicate reservoirs. They show more or less chondritic Zr/Hf ratios. In this study a series of orogenic eclogites from different localities was analyzed to determine their HFSE concentrations and to contribute to the question if eclogites could form a hidden reservoir to account for the mass imbalance of the BSE. The results show that the orogenic eclogites have subchondritic Nb/Ta ratios and near chondritic Zr/Hf ratios. The investigated eclogites show no fractionation of Nb/Ta ratios and no enrichment of Nb compared to e.g. MOR-basalts, the likely precursor of these rocks. With an average Nb/Ta ratio of 14.9 these eclogites could not balance the differences between BSE and chondrite. Additionally, with an average Nb/Ta ≈ MORB they also cannot balance the small differences in the Nb/Ta of the crust and the mantle. LA-ICPMS analyses of rutiles in these eclogites reveal a zonation of Nb/Ta ratios in this mineral, with rutile cores having higher Nb/Ta than rutile rims. As a consequence, Laser Ablation data of rutiles have to be evaluated carefully and cannot necessarily reflect a bulk rock Nb and Ta composition, although over 90% of these elements reside in rutile.Die kontinentale Kruste und der Mantel sind die beiden wichtigsten geochemischen Reservoire der Silikaterde (Bulk Silicate Earth, BSE). Dementsprechend verhalten sie sich bezüglich ihrer Gehalte an refraktären lithophilen Elementen relativ zu Chondrit in der Regel komplementär. Dies gilt z.B. für viele Seltene Erden (REE) und Sr-Nd Isotope (HOFMANN, 1988; HOFMANN, 1997). Die High Field Strength Elemente (HFSE) Zr, Nb, Hf und Ta werden weitgehend ebenfalls als Spurenelemente betrachtet, die in chondritischer Häufigkeit in der Silikaterde vertreten sind und ebensolche Elementverhältnisse aufzeigen. Es zeigt sich allerdings, dass sowohl die Nb/Ta Verhältnisse der kontinentalen Kruste und des verarmten Mantels, als auch die gesamte Silikaterde, subchondritisch sind, ein Phänomen, das als "Nb-Paradox" bekannt ist (BARTH et al., 2000; KAMBER and COLLERSON, 2000; RUDNICK et al., 2000; MÄNKER et al., 2003; RAPP et al., 2003). Nb und Ta sind zwei HFSE mit sehr ähnlichen geochemischen Eigenschaften und wurden lange Zeit als "geochemische Zwillinge" betrachtet, die sich während der Entwicklung des Kruste-Mantel Systems identisch verhalten. Erste Ideen um die Massenbilanz der Silikaterde auszugleichen legen ein superchondritisches Reservoir nahe, welches im Mantel zurückgehalten wird, und somit zu der heutigen Fraktionierung der Nb/Ta Verhältnisse zwischen BSE und dem chondritischen Wert geführt hat. Ein solches Reservoir kann in Form subduzierter ozeanischer Kruste vorliegen, deren Restit als Eklogit in den Mantel absinkt (MCDONOUGH, 1991; RUDNICK et al., 2000). Diese Betrachtungen basieren auf einem chondritischen Nb/Ta Verhältnis von 17.6, welches aus Analysen von Chondriten (Allende und Murchison) und Eukriten (Vesta) hergeleitet wurde (WEYER et al., 2002). MÄNKER et al. (2003) dagegen haben zur Berechnung des chondritischen Nb/Ta einen deutlich größeren Datensatz zugrundegelegt und kommen auf ein höheres Verhältnis von 19.9. Es wurden zudem von diesen Autoren einige kohlige Chondrite der CV-Gruppe (z.B. Allende) aussortiert, da diese eine ungewöhnliche Anreicherung von Nb-verarmten Einschlüssen zeigten. Dieser neue und höhere Wert für das - IV - chondritische Nb/Ta Verhältnis wird in dieser Arbeit verwendet, da er diesen durch den größeren Datensatz besser repräsentiert. ..
Astoria Fan sediments, DSDP site 174, Cascadia Basin: Hf-Nd-Pb constraints on provenance and outburst flooding
No full textThe Astoria submarine fan, located off the coast of Washington and Oregon, has grown throughout the Pleistocene from continental input delivered by the Columbia River drainage system. Enormous floods from the sudden release of glacial lake water occurred periodically during the Pleistocene, carrying vast amounts of sediment to the Pacific Ocean. Deep Sea Drilling Project (DSDP) site 174, located on the southern distal edge of the Astoria Fan, is composed of 879 m of terrigenous sediment. The section is divided into two major units separated by a distinct seismic discontinuity: an upper, turbidite fan unit (Unit I), and an underlying finer-grained unit (Unit II). Both units have overlapping ranges of Nd and Hf isotope compositions, with the majority of samples having εNd values of - 7.1 to - 15.2 and εHf values - 6.2 to - 20.0; the most notable exception is the uppermost sample in the section, which is identical to modern Columbia River sediment. Nd depleted mantle model ages for the site range from 2.0 to 1.2 Ga and are consistent with derivation from cratonic Proterozoic source regions, rather than Cenozoic and Mesozoic terranes proximal to the Washington-Oregon coast. The Astoria Fan sediments have significantly less radiogenic Nd (and Hf) isotopic compositions than present day Columbia River sediment (εNd = - 3 to - 4; [Goldstein, S.J., Jacobsen, S.B., 1987. Nd and Sr isotopic systematics of river water suspended material: implications for crustal evolution. Earth. Planet. Sci. Lett. 87, 249-265.]), and suggest that outburst flooding, tapping Proterozoic source regions, was the dominant sediment transport mechanism in the genesis and construction of the Astoria Fan. Pb isotopes form a highly linear 207Pb/204Pb-206Pb/204Pb array, and indicate the sediments are a binary mixture of two disparate sources with isotopic compositions similar to Proterozoic Belt Supergroup metasediments and Columbia River Basalts. The combined major, trace and isotopic data argue that outburst flooding was responsible for depositing the majority (top 630 m) of the sediment in the Astoria Fan. © 2006 Elsevier B.V. All rights reserved
Cross-relaxation dynamics on anomalous saturation processes in low pressure supersonic cw HF chemical laser amplifier
No full textIt is assumed that both translational and rotational nonequilibrium cross-relaxations play a role simultaneoulsy in low pressure supersonic cw HF chemical laser amplifier. For two-type models of gas flow medium with laminar and turbulent flow diffusion mixing, the expressions of saturated gain spectrum are derived respectively, and the numerical calculations are performed as well. The numerical results show that turbulent flow diffusion mixing model is in the best agreement with the experimental result
Late Cenozoic evolution of the Nicobar Fan - Insights from U-Pb-Hf in Zircon
No full textInternational audienceLate Cenozoic deep-sea deposits of the Bengal – Nicobar Fan recorded the events of Himalayan accretion during the India – Asia collision. IODP Expedition 362 – The Sumatra Subduction Zone – recovered a sedimentary section at the Nicobar Fan that comprises Early Miocene to Holocene strata. This work aims to identified potential sources and the evolution of the fluvial system that fed into the Nicobar Fan during that interval. For this, we analyzed zircon U-Pb-Hf in seven samples from sandy and silty turbidite deposits of U1480 and U1481 Sites. U-Pb zircon ages range from 22 to 3358 Ma and εHf −33.7 to +15, originated from Central and Eastern Himalayan, Lhasa Block, peninsular India and Indo Burma Ranges, the Ganges – Brahmaputra rivers drained areas. Our data suggest a significant reorganization of source areas since the Late Miocene. Rapid uplift of the Himalayan syntaxis in the Late Miocene to Pleistocene exposed distinct source rocks, altering the provenance signature and enhancing sediment delivery towards the sink. This is evidenced by gradually switched in first cycle zircons in the Nicobar Fan from the Lhasa Block and Gangdese Batholith to the Bomi-Chayu and Lohit Batholiths. Sediment provenance shifted since the Pliocene, with an increase in Ganges-derived Archean peninsular India, Cambro-Ordovician granites and Greater Himalaya orthogneisses at the Nicobar Fan. We attribute this shift to the rise of the Shillong Plateau, which projected the Brahmaputra delta to the west towards the Ganges river. This rise of the Shillong Plateau and the sea level fall at the Plio-Pleistocene enabled the merge of the Brahmaputra and Ganges deltas for the first time. Our data suggest that the Nicobar Fan was majorly supplied by the Brahmaputra River during the Tortonian, with an only minor contribution from the Ganges river. Following that, an increase in sediment contribution from the Ganges River is observed
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