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Both the high latitude ionosphere and mid --latitude trough region are essentially inhomogeneous and non-stationary media containing a multitude of large-scale irregularities. Electron density gradients associated with these irregularities form a diverse range of tilted reflection surfaces for high frequency (HF) radio waves. There are also a large number of smaller scale inhomogeneities that scatter the signal.;A series of measurements in the polar cap, auroral and sub-auroral zones of the Earth have been made by researchers at the University of Leicester over a number of years. It have been established that HF radio signals propagating through the high latitude ionosphere often arrive at the receiver over paths well displaced from the great circle direction, sometimes by up to 100°. Another common feature of high latitude HF propagation is the large Doppler and delay spreads imposed on the signal. These together with directional spread of the received signal energy are important parameters to be considered in the design of communication system and the associated signal processing methods.;The main outcome of the project described in this thesis is the design of a model of the high latitude ionosphere providing numerical investigation of off-great circle propagation of HF signals based on three-dimensional ray tracing. A large number of simulations were carried out for different types of propagation paths, and a comparison of the model results with observations indicates that the model is capable of reproducing the main features of HF signals propagating in the high latitude ionosphere.;A major outcome of the ray-tracing simulations is that paths other than those subject to experimental investigation can be assessed. It is anticipated that the results of this research will be incorporated into prediction tools for forecasting the effects of off-great circle propagation on any path impinging on the northerly ionosphere.

Previous studies on anomalous, over-sea propagation have been either focused on single links employing space/antenna diversity or on point-to-multipoint links, usually involving single frequency. Measurements on two co-linear, trans-horizon paths (50km and 140km long) over the English Channel have been made over periods in excess of a year in order to investigate the propagation characteristics of VHF and UHF signals propagating over the sea. The setup comprises a transmitter located on Jersey and receivers on Alderney and Portland. Signal strengths, meteorological factors within the lowest 1km as well as their mutual relationships have been studied. Signal strength enhancements have been observed on both paths, primarily in the late afternoon and evening periods, in the spring and summer months. These enhancements occur for different percentages of time between 12% and 21%. It was observed that the enhancements at both receiving sites and both frequencies may/may not be concurrent, leading to a probability of interference. The values of median lapse rate of refractivity in lowest 1km of atmosphere, effective earth radius factor and surface refractivity significantly less than those used by ITU have been observed. Refractivity gradients indicative of super-refraction and ducting are observed between heights of 52m and 84m for considerable amounts of time. Different current propagation models have been used to predict the median propagation loss values, which do not always clearly point out the dominant propagation mechanisms. This study has made available further results regarding enhanced signal strength events, has improved the values of some critical parameters linked to tropospheric propagation and has identified certain trends relating weather to signal level enhancements. These issues bear direct relevance to radio propagation in marine and coastal areas.

Deployment of commercial satellite networks at higher frequency bands such as the Ka and V -band has become essential to meet an increasing demand for high data rate multimedia services. Satellite systems operating at high frequency bands offer large bandwidth and are able to provide better quality of service for broadcasting. However, ability to render these services can be severely impaired by rain induced attenuation. Hence, this thesis presents time diversity (TD) as a highly promising link restoration scheme that makes for efficient use of higher frequency bands without imposing unrealistically high fixed propagation margins on the satellite link. Accurate knowledge of rain attenuation distribution is necessary for a proper design of a TD model to operate with required service availability. Two years of beacon measurements of long term attenuation data have been analysed to characterise rain attenuation at a selected location such as Sparsholt, based on seasonal and annual distributions. It is observed that cumulative distributions of rain attenuation increase in seasonal order of summer, autumn, winter and spring. The summer months are characterised by highest rain attenuation, perhaps owing to a prevalence of convective rainfall which is usually intense and short-lived. Hence, the seasonal variation can be exploited to improve the link availability. Furthermore, analyses have revealed significant variations in TD performance at 20, 40 and 50 GHz. Similar to rain attenuation, the summer months yielded the highest TD gain. The seasonal analyses also led to the observation that cumulative distributions of TD gain decrease along the trend of gain magnitudes, following the seasonal order of summer, autumn, winter and spring. The seasonal behaviour of TD gain can be exploited to estimate the optimum gain applicable for all seasons to improve the link availability. The analyses also led to derivations of models to estimate optimum delay as a function of the rain event depth and event duration respectively. Models have demonstrated the variability between optimum TD gains and the TD gains achieved using other values of delay and fade depth. Based on reviews of existing TD models, a novel model to statistically predict TD was derived using 2 years of ITALSAT beacon measurements. The new model was validated by comparing the TD gain predicted at 50 GHz from measured TD gain at 40 GHz with the TD gain at 50 GHz predicted using existing empirical models, example the Fukuchi et al and Matricciani models. The performance measure provided average root mean squared errors of 13%, 36% and 35% respectively. Also, comparison of the TD model with the actual measurements yielded a performance measure of <5% for link frequencies 20, 40 and 50 GHz. Statistical performance of a designed V-band (50/40 GHz) link is evaluated through link power budget analysis. A measure of the overall link performance based on the overall carrier to noise ratio was observed. The analysis was based on Satellite beacon measurements and 4 diversity delays (10s, 30s, 60s, and 180s) using two diversity combining methods, namely the selection combining (SC) and maximal ratio combining (MRC). The MRC method provided the best joint overall carrier to noise ratio. Finally, a near real-time implementation of TD at V-band was simulated, to demonstrate the efficacy of the technique in fade mitigation. Time diversity was applied to impaired signal frames, that is, signals with overall carrier to noise ratio lower than the 9.6 dB threshold due to rain attenuation. At the receiver the original and delayed channels were combined using SC and MRC. TD implemented using MRC showed significant reductions in outage seconds, leading to BER as low as 4.82x 10-7. Furthermore, it was found that SC provided little benefit, whereas MRC yielded a substantial reduction in outage seconds, sometimes in excess of 50% depending on event dynamics. Hence, from these simulations, TD is revealed as being an efficient fade mitigation technique.

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Results from radio and optical observations of natural and artificially enhanced geospace plasmas are presented. Firstly, naturally occurring plasma phenomena was studied by means of the radio and optical ground-based instruments and the results are compared with the in-situ satellite measurements. Observations of auroral absorption combined with auroral optical data provide information about characteristic energies of precipitating electrons. The energies recorded using the ground-based instruments are compared with the DMSP satellite data recordings during geomagnetic conjunctions. Results of this investigation indicate there is good agreement between the satellite Maxwellian spectra and the characteristic energies derived from the ground-based measurements when there is no ion precipitation and geomagnetic condition is moderate. Secondly, artificially enhanced ionospheric plasma by means of high power HF radio waves have been studied theoretically and experimentally. Radio observations indicate persistent descending of pump-induced enhancement in the UHF ion-line backscatter power. This effect has been modeled using electron temperature and density perturbation equations. It is concluded that a maximum electron temperature enhancement of up to ∼5700 K was achieved on average. Possible mechanisms and improvements to the model are discussed. The artificial optical rings were produced by pump-induced accelerated electrons. The optical emissions’ descent in height during each Heater-on cycle have been estimated by height triangulation from bi-static camera observations. The spatial evolution of the optical emissions, radar ion-line enhancements and stimulated electromagnetic emissions (SEE) spectra all show a similar altitude morphology. Typically, the optical height is close and below that of the radar backscatter, both of which are above the SEE generation altitude. Various radio and optical signatures of the artificially enhanced plasma are discussed. There is evidence that upper hybrid and Langmuir waves may act simultaneously to accelerate electrons causing the artificial optical emissions.

This thesis is concerned with the statistical studies of VHF/UHF radio wave propagation over the sea path at the limits of line-of-sight range. The objective is to provide a set of data that leads to the understanding of the characteristic of VHF/UHF radio wave over the sea path. A series of experiments were conducted using two paths of around 33 and 48 km across the English Channel. These two paths are between fixed land-based locations that provide an unobstructed condition. This allows a prolonged period of data collection under several sea states and atmospheric conditions without the heavy expenses of ship borne trial. The statistic studies showed that the high signal strength variation observed at both receiving sites are the results due to ducting and super-refraction. It occurred around 43 to 76% and 31 to 48% of the total time (percentage of days) during summer 2001 and 2002 respectively. In comparison, the total time was below 10% during winter period. Across the Jersey-Alderney path (48 km), high fading phenomenon was observed which is a result due to interference fading between the diffracted and troposcattered signal. The statistics showed that it occurred at around 35 to 55% of the total times during summer with an average fading range of around 10 and 7 dB during autumn and summer respectively, with an average fading period of around 7 seconds. The results from simulation showed that when the VHF/UHF signal reaches the radio horizon, the dominant propagating mechanism is smooth earth diffraction. Beyond the radio horizon, the attenuation rate increases dramatically and at a certain distance (depending on the frequency, antenna height and seasonal condition), the diffracted signals will be weaken and the troposcatter effect will become the dominant propagating mechanism.

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