AbstractWith the increase of human demands the development of science and technology, the Global Navigation Satellite System (GNSS) has become an increasingly vital technology. Signals broadcast by GNSS are affected by various errors during propagation, where the ionosphere is one of the largest error sources. Though the first order ionospheric error can be mitigated through the method of ionosphere-free combination, it is still hard to globally predict and mitigate the effect of ionospheric scintillation, a phenomenon caused by the irregularities in the ionosphere. Scintillation can lead to fluctuations in signal phase and strength, thus resulting in measurement errors, unreliable signal tracking performance, cycle slips, and even losses of lock. These effects are particularly significant in applying Precise Point Positioning (PPP), a positioning technique where the reference station is not required. The scintillation effect has been significantly mitigated based on scintillation parameters S4 and σф in past methods, generated with high-rate GNSS data (typically at 50 Hz). However, the availability of high-rate data is less, thus limiting the global scintillation research. This leads to the purpose of this thesis, using parameters obtained with the low-rate data to mitigate the scintillation effect. The parameters are Multipath Parameter (MP) and rate of change of Total Electron Content Index (ROTI) in this thesis, which can be computed from 1/30 Hz data.
First, the research investigated the relationship between the parameters obtained from low-rate data (i.e., MP and ROTI), and scintillation parameters S4, σф. Both temporal and spatial relationships were evaluated based on the statistical tools Structural Similarity (SSIM), Pearson Correlation Coefficient (CC), and variograms. Based on the results, it was suggested that MP and ROTI are spatiotemporally correlated with S4 and σф during scintillation events. However, it was uncertain whether the observed scintillation events were caused by the real scintillation or false alarms due to the multipath effect. Thus, an integrated methodology to distinguish the scintillation event from multipath was developed in the second research part of this thesis. According to the results, all the detected scintillation events are real, and the methodology has more functionality than past methods, where the hybrid event can be identified for the first time and scintillation parameters are not necessary. Finally, a method to mitigate the effects of scintillation on PPP is developed based on MP, ROTI, S4 and σф, where the scintillation parameters are optional. Three strategies are proposed: satellite removal, observation removal, and weighting. The results show that the performances of observation removal and weighting strategies are comparable to or even better than those of past methods. The highest improvements obtained using the two strategies are 91.7% and 93.1%, respectively. Furthermore, MP and ROTI can mitigate scintillation more substantially than S4 and σф sometimes, and vice versa.
|Date of Award||Nov 2022|
|Supervisor||Nicholas Hamm (Supervisor), Sreeja V. Veettil (Supervisor) & Craig Hancock (Supervisor)|
- Ionospheric delay
- Ionospheric scintillation
- Error mitigation