The present work was initially aimed at providing an explanation for some of the phenomena that occur in the ionosphere at sunrise. The approach that was taken was to determine the changes that take place on a theoretical model of the ionosphere and then to compare these with observations. A prerequisite for this approach was a theoretical model that would show, among other things, a bifurcation of the F layer during daytime without making unjustified arbitrary assumptions. The absence of such a model, particularly as far as non-equilibrium conditions are concerned, resulted in the present attempt to provide such a model for the F1 region.

This dissertation reports research into the theory and practical application of linear frequency modulated ionospheric sounding, as an alternative to the more usual technique of pulse modulation. A comparison of this technique with that of conventional pulse sounders is given, based on the concepts of matched filters and ambiguity functions for both modulations. A theory is developed to relate the group range and phase velocity of the ionospheric target to the phase and frequency of the difference signal at the receiver output. A method is then described whereby the group range and phase velocity of the reflection point as well as the amplitude, arrival angle and polarisation mode of the reflected energy can be measured. A description of the implementation of the technique is given together with some initial results. Finally, some suggestions for improvements are given

The wavefronts of high frequency (HF) radio waves received after reflection from the ionosphere exhibit both spatial non-linearities and temporal variations which limit the performance of large aperture receiving arrays. The objective of this investigation was to measure the phase and amplitude of ionospherically propagated signals in order to relate these parameters to the reflection process. This thesis describes the design and construction of a large aperture multi-element array and its implementation for wavefrot investigations. The hardware and software developed to control the equipment and to record the measurements are described. The procedures required to verify the performance of the experimental system are discussed and results are presented which demonstrate the accuracy of the measurements. The array was utilised for studies of signals received from several transmitters situated throughout Western Europe. The results obtained demonstrate the widely different behaviour of signals received over the various propagation paths and these have been related to the modal content of the received signals. Limited periods existed during which a single ionospheric mode was received and data corresponding to this condition have been compared with those which would be expected if the signal consisted of both a specular component and a cone of diffracted rays. This model is unable to explain the experimental results. Numerical models of the received signal were therefore developed. Results of these and comparisons with experimental results suggest that the measured parameters can be explained by the existence of a specular component with a varying direction of arrival (DOA), plus some contribution from random components. The experimental results indicate that the random or diffracted components normally contribute less than 10% of the received power in a single moded signal.

The wavefronts of high frequency (HF) radio waves received after reflection from the ionosphere exhibit both spatial non-linearities and temporal variations which limit the performance of large aperture receiving arrays. The first objective of this investigation was to measure the phase and amplitude of ionospherically propagated signals at several widely spaced antennas in order to relate these parameters to the reflection process. From the amplitudes and phases measured at pairs of spaced antennas, the direction of arrival (DOA) of the signal in both azimuth and elevation was determined. Furthermore. by combining the DOA and reflection height measurements the transmitter location can be estimated from a single receiving site. The second objective of this study was to investigate the ability of the system to determine DOA and transmitter locations correctly. Two seven element antenna arrays were employed with maximum apertures of 1526 m and 294 m respectively. The associated multi-channel receiving and data logging equipment is described together with a pulsed sounding system employed for mode identification. Signals received from several European transmitters exhibited widely differing behaviour and this was interpreted in terms of their modal content. For predominantly single moded signals the observations indicate that the diffracted components normally contribute less than 10% of the received power, moreover the DOA varies in both azimuth and elevation by approximately 1-2° over time periods of several minutes. The use of the smaller array for DF and SSL applications is discussed in detail. In particular, the performance of the system was severly affected by multi-moded propagation. Techniques were developed for recognising periods of single moded propagation, when accurate measurements are to be expected. Good position fixes were obtained when measurements were restricted to these periods provided accurate reflection height information was also available.

Statement of work: In essence the research work was to focus on the development of an ionospheric model suitable for real time HF frequency prediction and direction finding applications. The modelling of the ionosphere had to be generic in nature, sufficient to ensure that the CSIR could simultaneously secure commercial competitiveness in each of the three niche market areas aforementioned, while requiring only minimal changes to software architecture in the case of each application. A little research quickly showed that the development of an ionospheric model capable of driving a HFDFSSL system in "real time" would result in one having to make only slight re-structuring of the software to facilitate application of the same model in the areas of real time frequency prediction and spectrum management. The decision made at the outset of the project to slant the research toward the development of a model best suited for HF direction finding applications is reflected in the avenues followed during the course of the modelling process

Thesis (M.Sc.)--University of Adelaide, Dept. of Physics, 1969. / Candidate's reply (16 leaves) in pocket.

The occurrence of mid-latitude spread F (SF) over South Africa has not been extensively studied since the installation of the DPS-4 digisondes at Madimbo (30.88◦E, 22.38◦S), Grahamstown (33.32◦S, 26.50◦E) and Louisvale (28.51◦S, 21.24◦E). This study is intended to quantify the probability of the occurrence of F region disturbances associated with ionospheric spread F (SF) and L-band scintillation over South Africa. This study used available ionosonde data for 8 years (2000-2008) from the three South African stations. The SF events were identified manually on ionograms and grouped for further statistical analysis into frequency SF (FSF), range SF (RSF) and mixed SF (MSF). The results show that the diurnal pattern of SF occurrence peaks strongly between 23:00 and 00:00 UT. This pattern is true for all seasons and types of SF at Madimbo and Grahamstown during 2001 and 2005, except for RSF which had peaks during autumn and spring during 2001 at Madimbo. The probability of both MSF and FSF tends to increase with decreasing sunspot number (SSN), with a peak in 2005 (a moderate solar activity period). The seasonal peaks of MSF and FSF are more frequent during winter months at both Madimbo and Grahamstown. In this study SF was evident in ∼ 0.03% and ∼ 0.06% of the available ionograms at Madimbo and Grahamstown respectively during the eight year period. The presence of ionospheric irregularities associated with SF and scintillation was investigated using data from selected Global Positioning System (GPS) receiver stations distributed across South Africa. The results, based on GPS total electron content (TEC) and ionosonde measurements, show that SF over this region can most likely be attributed to travelling ionospheric disturbances (TIDs), caused by gravity waves (GWs) and neutral wind composition changes. The GWs were mostly associated with geomagnetic storms and sub-storms that occurred during periods of high and moderate solar activity (2001-2005). SF occurrence during the low solar activity period (2006-2008)can probably be attributed to neutral wind composition changes.

A full solar eclipse is going to be visible from a range of states in the contiguous United States on August 21, 2017. Since the atmosphere of the Earth is charged by the sun, the blocking of the sunlight by the moon may cause short term changes to the atmosphere, such as density and temperature alterations. There are many ways to measure these changes, one of these being ionospheric scintillation. Ionospheric scintillation is rapid amplitude and phase fluctuations of signals passing through the ionosphere caused by electron density irregularities in the ionosphere. At mid-latitudes, scintillation is not as common of an occurrence as it is in equatorial or high-altitude regions. One of the theories that this paper looks into is the possibility of the solar eclipse producing an instability in the ionosphere that will cause the mid-latitude region to experience scintillations that would not normally be present. Instabilities that could produce scintillation are reviewed and altered further to model similar conditions to those that might occur during the solar eclipse. From this, the satellites that are being used are discuses, as is hardware and software tools were developed to record the scintillation measurements. Although this work was accomplished before the eclipse occurred, measurement tools were developed and verified along with generating a model that predicted if the solar eclipse will produce an instability large enough to cause scintillation for high frequency satellite downlinks. / Master of Science

published_or_final_version / Physics / Master / Master of Science

Thesis (M. Sc.)--University of Hong Kong, 1968. / Mimeographed.