Meteore, die in die Atmosphäre eindringen, bilden bei hohen Oberflächentemperaturen, die durch Kollisionen mit der umgebenden Luft hervorgerufen werden, einen mehrere Kilometer langen Plasmaschweif aus. An diesem Schweif werden ausgesandte Radarwellen reflektiert und zurückgestreut. Dies führt zu einem charakteristischen Schwingungsverhalten, auch Fresnel Zonen genannt, am Empfänger. Die Überlagerung dieser Wellen ist verantwortlich für die typische Signalform eines Meteors, mit dem abrupten Anstieg und dem exponentiellen Abfall für \"underdense\" Meteore. Mit Hilfe einer Simulation wird der theoretische Zusammenhang zwischen Geschwindigkeit und Signalverlauf demonstriert. Des weiteren wird gezeigt, das die Methode von Baggaley et al. [1997] zur Bestimmung von Meteoreintrittsgeschwindigkeiten auch auf ein Radarinterferometer (SKiYMET) anwendbar ist. Abschliessend werden die gewonnen Ergebnisse mit einem anderen Verfahren sowie der Literatur verglichen. / Meteors, penetrating the earths atmosphere, creating at high surface temperatures, which are caused by collisions with the surrounding air molecules, a several kilometer long plasma trail. The ionized plasma backscatters transmitted radar waves. This leads to characteristic oscillations, called Fresnel zones, at the receiver. The interference of these waves entails the typical signal shape of a underdense meteor with the sudden rise of the signal and the exponential decay. By means of a simulation the theoretical connection between velocity and signal shape is demonstrated. Furthermore it is presented, that the method from Baggaley et al. [1997] for determination of meteor entry velocities is applicable for a radar interferometer (SKiYMET). Finally the results are compared to other radar methods on similar equipment and to other experiments.

127 leaves : ill., appendix / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Thesis (Ph.D.)--University of Adelaide, Dept. of Physics, 1973

Thesis (Ph.D.) -- University of Adelaide, Dept. of Physics, 1973.

Thesis (Ph.D.) -- University of Adelaide, Physics Dept., 1967. / Typescript.

Thesis (M.S.)--University of Wisconsin--Madison, 1954. / Typescript. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references.

This study considers the meteor-burst communication (MBC) environment at three levels. At the lowest level, the trails themselves are studied and analysed. Then individual links are studied in order to determine the data throughput and wait time that might be expected at various data rates. Finally, at the top level, MBC networks are studied in order to provide information on the effects of routing strategies, topologies, and connectivity in such networks. A significant amount of theoretical work has been done in the classification of meteor trails, and the analysis of the throughput potential of the channel. At the same time the issues of wait time on MBC links, and MBC network strategies, have been largely ignored. The work presented here is based on data captured on actual monitoring links, and is intended to provide both an observational comparison to theoretical predictions in the well-researched areas, and a source of base information for the others. Chapter 1 of this thesis gives an overview of the field of meteor-burst communications. Prior work in the field is discussed, as are the advantages and disadvantages of the channel, and current application areas. Chapter 2 describes work done on the classification of observed meteor trails into distinctive 'families'. The rule-based system designed for this task is discussed as well as the eventual classification schema produced, which is far more comprehensive and consistent than previously proposed schemas. Chapter 3 deals with the throughput potential of the channel, based on the observed trails. A comparison to predicted results, both as regards fixed and adaptive data-rates, is made with some notable differences between predicted v results and observed results highlighted. The trail families with the largest contribution to the throughput capacity of the channel are identified. Chapter 4 deals with wait time in meteor-burst communications. The data rates at which wait time is minimised in the links used are found, and compared to the rates at which throughput was optimised. These are found to be very different, as indeed are the contributions of the various trail families at these rates. Chapter 5 describes a software system designed to analyse the effect of routing strategies in MBC networks, and presents initial results derived from this system. Certain features of the channel, in particular its sporadic nature, are shown to have significant effects on network performance. Chapter 6 continues the presentation of network results, specifically concentrating on the effect of topologies and connectivity within MBC networks. Chapter 7 concludes the thesis, highlighting suggested areas for further research as well as summarising the more important results presented. / Thesis (Ph.D.)-University of Natal, Durban, 1991.

No description available.

Historiens Väktare är ett interaktivt läromedel framtaget av Educational Technology Group, som är ett samarbete mellan Linköpings och Lunds universitet. Läromedlet använder olika pedagogiska metoder som lärbara agenter och begreppskartor för att lära ut historia till mellanstadieelever. Läromedlet är utvecklat i ett JavaScript-ramverk som heter Meteor, och det innehåller en rad olika aktiviteter för informationsinhämtning, inlärning och utvärdering. En av dessa aktiviteter är en så kallad begreppskarta i vilken eleven skall, med hjälp av fördefinierade relationer, koppla samman olika historiska begrepp med varandra. Uppdraget bakom detta arbete var att implementera stöttat lärande i aktiviteten med begreppskartan. Stöttat lärande är en pedagogisk metod som handlar om vilken hjälp man skall ge elever för att lösa uppgifter. I detta arbete har tre olika funktioner för stöttat lärande i begreppskartan implementerats; kategorisering av begrepp, stöttande frågor samt förslag på relationer. De tre olika metoderna för stöttat lärande kunde implementeras i Historiens Väktare med hjälp av redan inbyggda funktioner i ramverket och Historiens Väktare utan att några nya beroenden till andra tekniker eller ramverk behövts introduceras.

This thesis is concerned with the observation and study of meteors with a narrow beam VHF radar operated by the University of Adelaide at the Buckland Park research station, in particular the study of the structure and characteristics of meteor showers and the geocentric speeds of meteors. There have been several observations of meteors with the radar previously (Steel & Elford 1991, Cervera 1996, Taylor et al. 1996), but this is the first with an automated data analysis directed to a systematic study of the properties of meteor showers. The Buckland Park VHF radar offers significant advantages over the wide beam radars traditionally used for meteor observation. The narrow beam, while reducing the collecting area of the radar, allows observations of much lower electron line densities than a wide beam radar of similar power. It also allows the determination of meteor shower radiants by the use of the radar response function. Pulse repetition frequencies of up to 2000 Hz allow excellent time resolution, and the ability to record in-phase and quadrature data allows the phase information to be used. This phase information is important as it allows the use of the phase information to accurately determine radial wind drifts, and the atmospheric speeds of meteoroids. During 1998, 1999 and 2000, observations were made of a number of meteor showers and the sporadic background. These showed that the η-Aquarid meteor shower was active in these years, and the Orionid and the Leonid showers were detected in 1999. Analysis of the η-Aquarid activity revealed multiple peaks which show that the shower is produced by at least four distinct "filaments", subsets of the meteoroid stream which produces the shower. Not only does the stream have spatial structure, containing groups of particles in different orbits, but also the presence or absence of the peaks in a random fashion from day to day shows that the filaments are themselves made up of clumps of particles. The radar response function is developed and used to determine radiants for the four η-Aquarid filaments. Evidence is given of a significant number of meteors detected at heights which are above the "radar ceiling", a height at which the theoretical initial radius attenuation factor is near zero for radars operating at the frequency of the Buckland Park VHF Radar, and underdense echoes should be impossible to detect. Investigation showed that over 60 % of meteor trails at heights above the ceiling (105 km) were underdense. Not only does the expected meteor height distribution extend up to 130 km, but also another distribution, peaking at 145 km is uncovered. Diffusion coefficients estimated from the decay of echoes are compared to theoretical calculations. There is a general agreement, but a number of meteor trails show slower diffusion than expected. This is attributed to the effect of the Earth's magnetic field. Three methods are used to determine meteoroid speeds using the phase data, each applying to a different type of meteor echo, and in combination, speeds could be determined for over 90 % of all meteor echoes. The first, the pre-t0 method can be applied to transverse meteor echoes with great success, although it may underestimate the speed of weak echoes with speeds under 15 km s-¹. Using the Cauchy approximations to the Fresnel integrals allows speed determination from head echoes which were aliased near the t0-point. Meteor trails which form at a small angle to the boresight of the radar beam are called "down-the-beam" echoes, and a new method is developed to determine the meteoroid speed and deceleration from these. The speed measurements of meteors detected during the η-Aquarids show a strong peak in the distribution at 66 km s-¹, as well as a smaller peak at 50 km s-¹, which may due to a minor shower. The sporadic background shows a broad peak at 25 km s-¹, with a smaller peak at 58 km s-¹. Distributions of the speed of meteors in the sporadic background show good agreement with previous observations (McCrosky & Posen 1961, Nilsson 1962, Elford et al. 1995, Cervera 1996), with the exception of meteoroid speeds smaller than 15 km s-¹, which can be underestimated by the pre-t0 technique. / Thesis (Ph.D.)--Physics and Mathematical Physics, 2002.

This thesis presents the results from investigations into various features of the dynamics of the middle atmosphere. Wind measurements from the meteor radar on Ascension Island and temperatures measured by the Aura MLS instrument are used to characterise Ultra-Fast Kelvin Waves (UFKW) in the MLT-region. Rainfall rates from TRMM are used as a proxy for latent heat release in an investigation of the excitation of UFKW, and results are compared with predictions of the Kyushu-GCM. Amplitudes and vertical wavelengths are determined, as are the accelerations resulting from wave dissipation. Intra-seasonal oscillations (ISOs) are observed in the wave amplitudes and accelerations, and oscillations with the same periods are observed in the rainfall and MLT-region winds, suggesting that UFKW play a role in carrying the tropospheric ISOs to higher regions. The seasonal and interannual variability of the tidal field over Ascension Island is investigated. Amplitudes, phases and vertical wavelengths of the diurnal and semidiurnal tides are reported on. Our observations of tidal parameters are compared with the predictions of the extended Canadian Middle Atmosphere Model (eCMAM) and the Whole Atmosphere Community Climate Model (WACCM). Correlations between tidal amplitudes, the stratospheric Quasi-Biennial Oscillation (QBO) and the El Nin ̃o Southern Oscillation (ENSO) are discussed. The Hocking (2005) method is adapted to recover monthly-mean gravity-wave momentum fluxes and wind variances either side of the Drake passage gravity-wave hotspot. The ability of the method to recover momentum fluxes over each radar are tested by sampling a series of specified (known) wave fields of increasing complexity with the actual meteor distributions, and comparing the recovered momentum fluxes with the specified values. The analysis is then applied to the real data to obtain climatologies of the MLT-region variances and momentum fluxes for a composite year of the 2008 to 2012 data.