| Summary: | Radio waves in the Very Low Frequency (VLF) band (3-30kHz) are reflected from the lowest part of the ionosphere, the D-region. Due to their relative stability and low attenuation they are utilized for navigation, timing and frequency comparisons. Any changes in D-region electron density structure are known to influence the phase velocity of VLF waves. Both regular diurnal and irregular behaviour have been extensively reported. A comparison of theoretical techniques is presented, demonstrating the influence of changes in radio frequency, ground conductivity, geomagnetic path azimuth and latitude. Described in this thesis is an analysis of the experimentally determined performance, within the U.K., of the VLF navigation system 'Omega'. The system relies on the stability of the phase velocity of the transmitted signals to provide a navigation fix. Both diurnal and seasonal variations in signal phase are investigated while the effects of modal interference and ionospheric disturbances are noted. In order to reduce errors resulting from regular variations in phase velocity, a prediction technique has been developed by the Omega Navigation System Operations Detail (ONSOD). An assessement of the validity of ONSOD predictions within the U.K. is presented and it is apparent that a number of differences exist between them and the experimental data. Major errors are evident at dawn over certain near north-south paths and marked differences may occur between data collected at particular sites. The relevance of these differences to Differential Omega is noted and an investigation into the performance of an Omega/Satellite combined system is conducted. The variations within the U.K. are not completly accounted for by changes in propagation path length although theoretical investigations indicate the importance of changes in electron density structure along the whole path. It is evident that the sunrise schedules of the ONSOD prediction program require modification.
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