Travelling ionospheric disturbances, or TIDs, are wavelike features propagating in the ionosphere. TIDs are studied for many reasons, such as their effects on GNSS navigation and their connection to natural disasters like earthquakes and tsunamis. Sensitive instruments and reliable techniques are required to accurately image and detect the ionospheric perturbations. The primary purpose of this thesis is to evaluate and advance the capabilities of tomographic methods to image TIDs for scientific purposes, which also has a potential application to GNSS positioning in the presence of TIDs. This thesis therefore also quantifies the potential impact of TIDs on state-of-the-art ionospheric correction services, for e.g. Network Real-Time Kinematic (N-RTK) positioning, that occur when the corrections are interpolated for an approximate rover position. Building on these results, a novel TID-mitigation strategy is also tested. Simulations with different methods of interpolation for N-RTK ionospheric corrections show that TIDs can induce errors large enough to merit attention. For positioning in the presence of TIDs, a weighted least-squares interpolation technique is modified to adapt to estimated TID directions and wavelengths. The new methods are shown, in simulation tests and a case study, to decrease the number of large interpolation errors that may impede fast integer ambiguity resolution. Electron density maps generated by ionospheric tomography can also provide an alternative approach to TID mitigation. The MIDAS (Multi-Instrument Data Analysis System) tomography algorithm is therefore tested to determine its suitability for TID imaging. As an initial test, a case study is presented a where MIDAS is used to image a large-scale TID over North America occurring during a geomagnetic storm. The resulting images are verified with in-orbit measurements and ionosonde soundings. Further tests are conducted with simulated data generated with varying TIDs characteristics and satellite geometries, including geostationary orbits. The results show that most TIDs are reconstructed well by MIDAS, with the smaller MSTIDs a possible exception. Electron density images generated from these results can be used for TID mitigation in positioning, and also aid studies into TID and gravity wave generation mechanisms. Together, the results from studies emphasise the importance of satellite and receiver geometry in TID observation and mitigation. The simulation-based tomography results show the benefits of including geostationary satellite geometry in ionospheric tomography, while results from the LSTID case study and interpolation simulations illustrate the importance of a well distributed ground receiver network.
|Date of Award||18 Nov 2020|
|Sponsors||EU - Horizon 2020|
|Supervisor||Cathryn Mitchell (Supervisor) & Biagio Forte (Supervisor)|