Towards practical autonomous deep-space navigation using X-Ray pulsar timing     

Setnam Shemar  George Fraser  Lucy Heil  David Hindley  Adrian Martindale  Philippa Molyneux  John Pye  Robert Warwick  Andrew Lamb 《Experimental Astronomy》2016,42(2):101-138

We investigate the feasibility of deep-space navigation using the highly stable periodic signals from X-ray pulsars in combination with dedicated instrumentation on the spacecraft: a technique often referred to as ‘XNAV’. The results presented are based on the outputs from a study undertaken for the European Space Agency. The potential advantages of this technique include increased spacecraft autonomy and lower mission operating costs. Estimations of navigation uncertainties have been obtained using simulations of different pulsar combinations and navigation strategies. We find that the pulsar PSR B1937?+?21 has potential to allow spacecraft positioning uncertainties of ~2 and ~5 km in the direction of the pulsar after observation times of 10 and 1 h respectively, for ranges up to 30 AU. This could be achieved autonomously on the spacecraft using a focussing X-ray instrument of effective area ~50 cm² together with a high performance atomic clock. The Mercury Imaging X-ray Spectrometer (MIXS) instrument, due to be launched on the ESA/JAXA BepiColombo mission to Mercury in 2018, is an example of an instrument that may be further developed as a practical telescope for XNAV. For a manned mission to Mars, where an XNAV system could provide valuable redundancy, observations of the three pulsars PSR B1937?+?21, B1821-24 and J0437-4715 would enable a three-dimensional positioning uncertainty of ~30 km for up to 3 months without the need to contact Earth-based systems. A lower uncertainty may be achieved, for example, by use of extended observations or, if feasible, by use of a larger instrument. X-ray instrumentation suitable for use in an operational XNAV subsystem must be designed to require only modest resources, especially in terms of size, mass and power. A system with a focussing optic is required in order to reduce the sky and particle background against which the source must be measured. We examine possible options for future developments in terms of simpler, lower-cost Kirkpatrick-Baez optics. We also discuss the principal design and development challenges that must be addressed in order to realise an operational XNAV system.  相似文献   

Lode–gold mineralization in the Tanami region, northern Australia     

David L. Huston  Leon Vandenberg  Andrew S. Wygralak  Terrence P. Mernagh  Leon Bagas  Andrew Crispe  Alexis Lambeck  Andrew Cross  Geoff Fraser  Nick Williams  Kurt Worden  Tony Meixner  Bruce Goleby  Leonie Jones  Pat Lyons  David Maidment 《Mineralium Deposita》2007,42(1-2):175-204

The Tanami region of northern Australia has emerged over the last two decades as the largest gold-producing region in the Northern Territory. Gold is hosted by epigenetic quartz veins in sedimentary and mafic rocks, and by sulfide-rich replacement zones within iron formation. Although limited, geochronological data suggest that most mineralization occurred at about 1,805–1,790 Ma, during a period of extensive granite intrusion, although structural relationships suggest that some deposits predate this period. There are three main goldfields in the Tanami region: the Dead Bullock Soak goldfield, which hosts the world-class Callie deposit; The Granites goldfield; and the Tanami goldfield. In the Dead Bullock Soak goldfield, deposits are hosted by carbonaceous siltstone and iron formation where a late (D₅) structural corridor intersects an early F₁ anticlinorium. In The Granites goldfield, deposits are hosted by highly sheared iron formation and are interpreted to predate D₅. The Tanami goldfield consists of a large number of small, mostly basalt-hosted deposits that probably formed at a high structural level during D₅. The D₅ structures that host most deposits formed in a convergent structural regime with σ ₁ oriented between E–W and ENE–WSW. Structures active during D₅ include NE-trending oblique thrust (dextral) faults and ESE-trending (sinistral) faults that curve into N- to NNW-trending reverse faults localized in supracrustal belts between and around granite complexes. Granite intrusions also locally perturbed the stress field, possibly localizing structures and deposits. Forward modeling and preliminary interpretations of reflection seismic data indicate that all faults extend into the mid-crust. In areas characterized by the N- to NW-trending faults, orebodies also tend to be N- to NW-trending, localized in dilational jogs or in fractured, competent rock units. In areas characterized by ESE-trending faults, the orebodies and veins tend to strike broadly east at an angle consistent with tensional fractures opened during E–W- to ENE–WSW-directed transpression. Many of these deposits are hosted by reactive rock units such as carbonaceous siltstone and iron formation. Ore deposition occurred at depths ranging from 1.5 to 11 km from generally low to moderate salinity carbonic fluids with temperatures from 200 to 430°C, similar to lode–gold fluids elsewhere in the world. These fluids are interpreted as the product of metamorphic dewatering caused by enhanced heat flow, although it is also possible that the fluids were derived from coeval granites. Lead isotope data suggest that lead in the ore fluids had multiple sources. Hydrogen and oxygen isotope data are consistent with both metamorphic and magmatic origins for ore fluids. Gold deposition is interpreted to be caused by fluid unmixing and sulfidation of host rocks. Fluid unmixing is caused by three different processes: (1) CO₂ unmixing caused by interaction of ore fluids with carbonaceous siltstone; (2) depressurization caused by pressure cycling in shear zones; and (3) boiling as ore fluids move to shallow levels. Deposits in the Tanami region may illustrate the continuum model of lode–gold deposition suggested by Groves (Mineralium Deposita 28:366–374, 1993) for Archean districts.  相似文献   

Lateglacial and Holocene relative sea‐level changes and first evidence for the Storegga tsunami in Sutherland,Scotland          下载免费PDF全文

Antony J. Long  Natasha L. M. Barlow  Sue Dawson  Jon Hill  James B. Innes  Catherine Kelham  Fraser D. Milne  Alastair Dawson 《第四纪科学杂志》2016,31(3):239-255

We reconstruct one of the longest relative sea‐level (RSL) records in north‐west Europe from the north coast of mainland Scotland, using data collected from three sites in Loch Eriboll (Sutherland) that we combine with other studies from the region. Following deglaciation, RSL fell from a Lateglacial highstand of +6?8 m OD (Ordnance Datum = ca. mean sea level) at ca. 15 k cal a BP to below present, then rose to an early Holocene highstand and remained at ca. +1 m OD between ca. 7 and 3 k cal a BP, before falling to present. We find no evidence for significant differential Holocene glacio‐isostatic adjustment between sites on the north‐west (Lochinver, Loch Laxford), north (Loch Eriboll) and north‐east (Wick) coast of mainland Scotland. This suggests that the region was rapidly deglaciated and there was little difference in ice loads across the region. From one site at the head of Loch Eriboll we report the most westerly sedimentary evidence for the early Holocene Storegga tsunami on the Scottish mainland. The presence of the Storegga tsunami in Loch Eriboll is predicted by a tsunami wave model, which suggests that the tsunami impacted the entire north coast of Scotland and probably also the Atlantic coastline of north‐west Scotland.

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The Nolans Bore rare-earth element-phosphorus-uranium mineral system: geology,origin and post-depositional modifications     

David L. Huston  Roland Maas  Andrew Cross  Kelvin J. Hussey  Terrence P. Mernagh  Geoff Fraser  David C. Champion 《Mineralium Deposita》2016,51(6):797-822

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52nd Photogrammetric Week     

Ian  Dowman  Clive  Fraser  Josep-Lluis  Colomer  Andrew  Morris 《The Photogrammetric Record》2010,25(129):79-83

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A new technique for evaluating mesospheric momentum balance utilizing radars and satellite data     

D. J. Frame  B. N. Lawrence  G. J. Fraser  R. A. Vincent  A. Dudhia 《Annales Geophysicae》2000,18(4):478-484

A new method for evaluating momentum balance in the mesosphere using radar and satellite data is presented. This method is applied to radar wind data from two medium frequency installations (near Adelaide, Australia and Christchurch, New Zealand) and satellite temperature data from the Improved Stratospheric and Mesospheric Sounder (ISAMS). Because of limitations in data availability and vertical extent, the technique can only be applied to evaluate the momentum balance at 80 km above the radar sites for May 1992. The technique allows the calculation of the residual terms in the momentum balance which are usually attributed to the effects of breaking gravity waves. Although the results are inconclusive above Adelaide, this method produces values of zonal and meridional residual accelerations above Christchurch which are consistent with expectation. In both locations it is apparent that geostrophic balance is a poor approximation of reality. (This result is not dependent on a mismatch between the radar and satellite derived winds, but rather is inherent in the satellite data alone.) Despite significant caveats about data quality the technique appears robust and could be of use with data from future instruments.  相似文献   

The influence of vegetation cover on evapotranspiration atop waste rock piles,Elk Valley,British Columbia     

Erin M. Nicholls  Gordon B. Drewitt  Stephanie Fraser  Sean K. Carey 《水文研究》2019,33(20):2594-2606

Surface mining in the Elk Valley, British Columbia, involves removing vegetation, soil, and rock to access underlying metallurgical coal. Subsequent waste rock is placed into adjacent valleys, frequently burying headwater streams. Due to their coarse texture, waste rock piles increase infiltration and percolation, increasing solute transport and concentration of geochemicals in downstream surface waters. Previous research suggests that weathering solutes are transport limited, and it is hypothesized that revegetation will enhance evapotranspiration (ET) and reduce percolation through the waste rock, potentially reducing loading. This study examined the surface‐atmosphere water and energy exchanges using the eddy covariance technique for three waste rock surfaces with different levels of reclamation: (a) an ~25‐year‐old mixed coniferous forest, (b) a grass site, and (c) bare waste rock. Measurements were taken from May to October in 2013 and 2014. Soil moisture and matric suction were measured to 1‐m depth. Sap flow at the forested site was measured to partition transpiration from total ET. In all years, ET rates were greatest at the forested site, followed by the grass cover and lowest at the bare waste rock site. Growing season ET rates at the forest were 56% higher than grass in 2013 and 35% higher in 2014. At the vegetated sites, climate was the main driver of ET, with high radiation, and warm and dry conditions enhancing fluxes. Maximum ET at these sites corresponded with peak growing season, with vegetation increasing both transpiration and rainfall interception. At the bare rock site, ET was weakly related to atmospheric conditions, and ET rates briefly increased during periods following rainfall when near‐surface soil moisture was enhanced. Transpiration comprised 29% of overall ET at the forest site from late July to early October. Results suggest that vegetation establishment can be incorporated into mine reclamation plans to enhance ET rates and limit percolation, potentially reducing downstream geochemical loads.  相似文献   

RoboNet‐II: Follow‐up observations of microlensing events with a robotic network of telescopes     

Y. Tsapras  R. Street  K. Horne  C. Snodgrass  M. Dominik  A. Allan  I. Steele  D.M. Bramich  E.S. Saunders  N. Rattenbury  C. Mottram  S. Fraser  N. Clay  M. Burgdorf  M. Bode  T.A. Lister  E. Hawkins  J.P. Beaulieu  P. Fouqu  M. Albrow  J. Menzies  A. Cassan  D. Dominis‐Prester 《Astronomische Nachrichten》2009,330(1):4-11

RoboNet‐II uses a global network of robotic telescopes to perform follow‐up observations of microlensing events in the Galactic Bulge. The current network consists of three 2 m telescopes located in Hawaii and Australia (owned by Las Cumbres Observatory) and the Canary Islands (owned by Liverpool John Moores University). In future years the network will be expanded by deploying clusters of 1 m telescopes in other suitable locations. A principal scientific aim of the RoboNet‐II project is the detection of cool extra‐solar planets by the method of gravitational microlensing. These detections will provide crucial constraints to models of planetary formation and orbital migration. RoboNet‐II acts in coordination with the PLANET microlensing follow‐up network and uses an optimization algorithm (“web‐PLOP”) to select the targets and a distributed scheduling paradigm (eSTAR) to execute the observations. Continuous automated assessment of the observations and anomaly detection is provided by the ARTEMiS system (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

Diffraction theory modeling of near-forward radio wave scattering from particle clusters     

Fraser S. Thomson  Essam A. Marouf 《Icarus》2009,204(1):290-302

We investigate the use of diffraction theory as an alternative to solving the full electromagnetic problem of near-forward scattering by a collection of dielectric spheres of arbitrary spacing and orientation. We define an amplitude screen, which is constructed by projecting the shadows of a cluster of spheres onto a plane perpendicular to the wave vector of the incident radio wave. The far-field (Fraunhofer) diffraction pattern of the amplitude screen is then computed and compared with the Mie-theoretic result. We show that for EM scattering from a cluster of electrically large spheres—both singly-sized and belonging to a size distribution—there is excellent agreement between the exact Mie solution and its diffraction theory approximation when near-forward scattering is the angular range of interest. This excellent agreement holds over a broad range of particle separation and orientation configurations relative to the incidence direction. It is also achieved at a much reduced computational cost compared with an exact solution of the electromagnetic interaction problem. Fortified by these results, the authors have applied diffraction theory to the analysis of Cassini radio occultation data, thereby detecting fine-scale structure in Saturn’s rings [Thomson, F.S., Marouf, E.A., Tyler, G.L., French, R.G., Rappoport, N.J., 2007. Periodic microstructure in Saturn’s rings A and B. Geophys. Res. Lett. 34, L24203].  相似文献   

Mapping the vulnerability of crop production to drought in Ghana using rainfall, yield and socioeconomic data     

Philip Antwi-Agyei  Evan D.G. Fraser  Andrew J. DougillLindsay C. Stringer  Elisabeth Simelton 《Applied geography (Sevenoaks, England)》2012,32(2):324-334

This study evaluates new multi-scale, multi-indicator methods for assessing the vulnerability of crop production to drought at a national and regional scale. It does this by identifying differences across and within ten regions of Ghana, a country that faces many climate and crop production challenges typical of sub-Saharan Africa. In particular, we illustrate how a quantitative national and regional study is a critical first step in assessing differences in the drought sensitivity of food production systems and show how such an assessment enables the formulation of more targeted district and community level research that can explore the drivers of vulnerability and change on a local-scale. Finally, we propose methodological steps that can improve drought sensitivity and vulnerability assessments in dynamic dryland farming systems where there are multiple drivers of change and thresholds of risk that vary in both space and time. Results show that the vulnerability of crop production to drought in Ghana has discernible geographical and socioeconomic patterns, with the Northern, Upper West and Upper East regions being most vulnerable. Partly, this is because these regions have the lowest adaptive capacity due to low socioeconomic development and have economies based on rain-fed agriculture. Within these regions we find considerable differences between districts that can be explained only partly by socioeconomic variables with further community and household-scale research required to explain the causes of differences in vulnerability status. Our results highlight that national and regional scale multi-indicator vulnerability assessments are a vital (and often ignored) first step in assessing vulnerability across a large area. These inputs can guide both local-level research and also demonstrate the need for region-specific policies to reduce vulnerability and to enhance drought preparedness within dryland farming communities.  相似文献