Abstract
Drifting ionospheric irregularities in the form of inhomogeneities in the electron density distribution can affect the propagation of Global Navigation Satellite System (GNSS) signals causing fluctuations in amplitude and phase. Such fluctuations in amplitude and phase of the signals with shorter periods arise from small-scale irregularities and are known as scintillation. Typically, scintillation is accompanied by temporal fluctuations in Total Electron Content (TEC): that is, fluctuations in the phase of the signals with longer periods arising from large-scale irregularities. Large-scale irregularities tend to cascade into small-scale irregularities owing to instability mechanisms.Whilst this is the case for the equatorial ionosphere, at high-latitudes, such energy cascade does not seem to be as developed. Consequently, mainly phase scintillation tends to be detected on GNSS at auroral and polar latitudes. TEC fluctuations can be used as a proxy to indicate the presence of phase fluctuations induced by large-scale irregularities at high-latitudes. Scintillation and TEC fluctuations can significantly impact the performance of GNSS particularly in equatorial latitudes and high-latitudes ionosphere. Scintillation can be modelled by means of propagation through phase-changing screens which are thin irregular layers across a propagation path used to approximate a stochastic medium between a transmitter and a receiver.
The thesis aims to characterise the spatial distribution of electron density irregularities along profiles in the E and F layer transverse to GPS ray paths and their impact on GPS signals at high-latitudes. To achieve this, in 2018 and 2019, multi-instrument experimental campaigns were designed and conducted involving the European Incoherent SCATter (EISCAT)/EISCAT Svalbard Radar (ESR) UHF radars and geodetic GNSS receiver stations of relevance at the auroral and polar latitudes. It is worthy to clearly mention here that the data collected from these experiments were utilised in the research.
EISCAT UHF/ESR radar beams were used to infer the distribution of electron density irregularities along hypothetical phase screens in which EISCAT UHF/ESR electron density profiles were measured along hypothetical phase screens alternately intersecting GNSS ray paths at different ionospheric shell heights. In this investigation, representative case studies were considered in the auroral and polar ionospheres. Whilst auroral irregularities seemed to be originated mainly by particle precipitation, polar irregularities could arise from polar patches and/or particle precipitation.
The effects of electron density irregularities distributed along phase screens across GNSS ray paths of relevance were investigated in terms of temporal fluctuations in TEC and positioning performance. These experiments provide information about the outer scale of the phase screens and the likely spatial distance over which auroral and polar irregularities distribute. The electron density distribution along a given phase screen, that is whether it is symmetrical or not, and the origin of the phase screens were also considered. The results provide insight into the impact of adverse space weather conditions on real-time and post-processing positioning applications at auroral and polar latitudes.
| Date of Award | 8 Sept 2021 |
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| Original language | English |
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| Supervisor | Biagio Forte (Supervisor) & Cathryn Mitchell (Supervisor) |