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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
41

Anisotropic turbulence and stochastic arrays of three-dimensional scatterers in the simulation of transhorizon tropospheric propagation

Duvoisin, Paul Frank, January 1969 (has links)
Thesis (Ph. D.)--University of Wisconsin--Madison, 1969. / Typescript. Vita. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references.
42

RAKE resolution analysis and models of refractive index fluctuations for tropospheric scatter propagation

Derbort, Herman Joseph, January 1970 (has links)
Thesis (Ph. D.)--University of Wisconsin--Madison, 1970. / Typescript. Vita. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references.
43

Estimation of the spectral moments and quadrature cross-correlation function and their application to troposcatter systems

Gupta, Ashok Kumar, January 1900 (has links)
Thesis--University of Wisconsin--Madison. / Typescript. Vita. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references (leaves 262-268).
44

An investigation of oblique incidence propagation of radio pulses between Grahamstown and Durban

Nadasen, Arunajallam January 1968 (has links)
This thesis describes the investigation carried out on the propagation of radio pulses of frequency 4.73 Mc/s between Grahamstown and Durban. The thesis is divided into two sections - A and B. Section A consists of two chapters. The introductory chapter gives a brief account of how the existence of the ionosphere came to be known. Then follows a description of the different layers of ionization and a review of the theories that have been propounded on the formation of these layers. Chapter 2 deals with the apparatus which includes the transmitter in Grahamstown and the receiving apparatus in Durban. The receiving apparatus comprises: i) a superheterodyne receiver whose gain was high (between 130 and 140 dbs); (ii) a time delay calibrator which could measure time differences of 100 μsec fairly accurately; (iii) a 310 A Tektronix oscilloscope; (iv) a continuously running 35 mm recording camera. Section B is made up of three chapters and is concerned with the actual analysis of the data recorded. The theory of propagation of radio waves in the ionosphere is discussed in Chapter 3. The effects of the magnetic field are neglected since it is found that the error introduced would not make the results unacceptable. Chapter 4 contains the analysis of the data recorded. One summer day and one winter day are discussed in detail in order to obtain the pattern of the diurnal variations for both summer and winter. Some interesting phenomena are also dealt with. An attempt to do ray tracing was successful and the paths followed by a Pedersen and a lower ray from Grahamstown to Durban have been drawn. New topics for further research are discussed in Chapter 5. There are two appendices. Appendix I gives the time delays of all the pulses recorded and their possible identifications. An overall picture of the propagation via the various layers throughout the day (both for summer and for winter is presented in Appendix II.
45

Correlation of geomagnetic field changes with ionospheric motions determined by a doppler technique

Lewis, Trevor John January 1964 (has links)
Standard measurements of the geomagnetic field at the earth's surface are correlated with doppler measurements. The doppler shift in the frequency of a radio wave travelling in the ionosphere measures the change in refractive index with respect to time, integrated along the wave's path. For a vertical path the integrated change in electron density is measured, and the ionosphere's vertical motion can be deduced, using simplifying assumptions. A definite correlation between short period (less than four minutes) geomagnetic field changes and the doppler shift has been found. For longer period changes, evidence for a correlation is not so definite. The magnitude of the ratio of the doppler shift (in a frequency of 4 or 5 Mc.) to the observed geomagnetic field change for short period events is between 0.1 and 0.4 cps/ɣ and for longer events, 0.05 to 0.15 cps/ɣ. Sometimes the doppler shift is proportional to ΔH or ΔD. However no relation that would hold in general to predict the doppler shift from the observed geomagnetic changes was found. Apart from the diurnal variation in the doppler shift, three types of uncorrelated events were found. The first was a continuous variation occurring during the daytime which has been observed by many workers. The second was a large travelling ionospheric disturbance which followed a sudden change in the geomagnetic field, and thus might be considered indirectly to be correlated. The last type was a train of large amplitude irregular oscillations with periods from about 3 to 0.5 minutes, the longer periods appearing first. The problem of determining ionospheric motions from the doppler shift is very complex, and in order to make any progress, certain simplifying assumptions must be made which cannot be completely justified. By assuming also that the observed long period geomagnetic field changes are caused by overhead currents, and by using average ionospheric conductivities, the electric field in the ionosphere is calculated, the values ranging between 10⁻⁷ and 10⁻⁶ e.s.u. For shorter period geomagnetic field changes, the doppler shift is larger (approximately by a factor of 3) than the value which has previously been calculated by assuming that overhead currents cause the geomagnetic changes. / Science, Faculty of / Earth, Ocean and Atmospheric Sciences, Department of / Graduate
46

The Effect of Digital Elevation Model Resolution on Wave Propagation Predictions at 24Ghz

Rose, Scott Michael 09 May 2001 (has links)
Digital Elevation Models (DEMs) are computer-generated representations of the earth's surface. These surfaces can be used to predicted Line-of-Sight (LOS) radio propagation. DEM resolution can affect the results of this prediction. This study examines the effect of DEM resolution on accuracy by comparing varied resolution terrain data for a portion of Blacksburg, Virginia using the prediction of ESRI's ArcView® viewshed algorithm. Results show that resolutions between one-meter and thirty-meters have little effect on the aggregate accuracy of the viewshed. / Master of Science
47

VLF propagation studies based on phase comparison records /

Teso, William A. January 1964 (has links)
No description available.
48

The mid-latitude ionosphere under quiet geomagnetic conditions: propagation analysis of SuperDARN radar observations from large ionospheric perturbations

De Larquier, Sebastien 23 December 2013 (has links)
The Earth's ionosphere is a dynamic environment strongly coupled to the neutral atmosphere, magnetosphere and solar activity. In the context of this research, we restrict our interest to the mid-latitude (a.k.a., sub-auroral) ionosphere during quiet geomagnetic conditions. The Super Dual Auroral Radar Network (SuperDARN) is composed of more than 30 low-power High Frequency (HF, from 8-18 MHz) Doppler radars covering the sub-auroral, auroral and polar ionosphere in both hemispheres. SuperDARN radars rely on the dispersive properties of the ionosphere at HF to monitor dynamic features of the ionosphere. Though originally designed to follow auroral expansion during active periods, mid-latitude SuperDARN radars have observed ground and ionospheric scatter revealing several interesting features of the mid-latitude ionosphere during periods of moderate to low geomagnetic activity. The past 7 years' expansion of SuperDARN to mid-latitudes, combined with the recent extended solar minimum, provides large-scale continuous views of the sub-auroral ionosphere for the first time. We have leveraged these circumstances to study prominent and recurring features of the mid-latitude ionosphere under quiet geomagnetic conditions. First, we seek to establish a better model of HF propagation effects on SuperDARN observations. To do so, we developed a ray-tracing model coupled with the International Reference Ionosphere (IRI). This model is tested against another well established ray-tracing model, then optimized to be compared to SuperDARN observations (Chapter 2). The first prominent ionospheric feature studied is an anomaly in the standard ionospheric model of photo-ionization and recombination. This type of event provides an ideal candidate for testing the ray-tracing model and analyzing propagation effects in SuperDARN observations. The anomaly was first observed in ground backscatter occurring around sunset for the Blackstone, VA SuperDARN radar. We established that it is related to an unexpected enhancement in electron densities that leads to increased refraction of the HF signals. Using the ray-tracing, IRI model, and measurements from the Millstone Hill Incoherent Scatter Radar (ISR), we showed that this enhancement is part of a global phenomenon in the Northern Hemisphere, and is possibly related to the Southern Hemisphere's Weddell Sea Anomaly. We also tested a potential mechanism involving thermospheric winds and geomagnetic field configuration which showed promising results and will require further modeling to confirm (Chapter 3). The second ionospheric feature was a type of decameter-scale irregularity associated with very low drift velocities. Previous work had established that these irregularities occur throughout the year, during nighttime, and equatorward of both the auroral regions and the plasmapause boundary. An initial analysis suggested that the Temperature Gradient Instability (TGI) was responsible for the growth of such irregularities. We first used our ray-tracing model to distinguish between HF propagation effects and irregularity occurrence in SuperDARN observations. This revealed the irregularities to be widespread within the mid-latitude ionosphere and located in the bottom-side F-region (Chapter 4). A second study using measurements from the Millstone Hill ISR revealed that TGI driven growth was possible but only in the top-side F-region ionosphere. We found that initial growth may occur primarily at larger wavelengths, with subsequent cascade to decameter-scale with coupling throughout the F-region (Chapter 5). In summary, the research conducted during this PhD program has established a robust method to analyze quiet-time SuperDARN observations. It also furthered our physical understanding of some prominent features of the mid-latitude ionosphere. It leaves behind a flexible ray-tracing model, multiple online tools to browse SuperDARN data, and a thorough and growing Space Science API providing access to multiple datasets, models and visualization tools. / Ph. D.
49

Phase-space analysis of wave propagation in homogeneous dispersive and dissipative media

Hoc, Ngo Dinh January 1983 (has links)
A phase-space asymptotic approach to wave propagation in homogeneous dispersive and dissipative media is discussed which has several advantages by comparison to conventional techniques, such as the stationary phase method, ordinary ray tracing, etc. This approach, which is based on the wave-kinetic theory [1,2], is used to examine in detail three types of one-dimensional canonic dispersive and dissipative media: cubic dispersive and quadratic dissipative, cubic dispersive and quartic dissipative, quintic dispersive and quartic dissipative. Purely dissipative media are also investigated. The analysis is also carried out using standard Fourier techniques for comparison purposes. For an arbitrary medium, exact solutions are impossible. Approximations must be made which give rise to new basic functions defined in integral form. The method of steepest descents [3], the WKB method [4], the method of dominant balance [4] and the FORMAC73 language [5] are utilized to find asymptotic series for these functions. / M.S.
50

RF propagation model for direct sequence spread spectrum communication systems

Thomas, Phillip Andre 01 January 1998 (has links)
No description available.

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