Thesis (M. Sc.)--University of Hong Kong, 1967. / Mimeographed.

This thesis aims to answer the question, “What physical process dominates the formation of plasma irregularities, capable of directly or indirectly causing GPS L1 band scintillation, in polar cap plasma patches during magnetic storm conditions?.” A novel modelling technique utilising an ionospheric imaging algorithm is developed and used to elucidate the relative importance of the two most commonly discussed processes. These are the Gradient Drift Instability (GDI) and turbulence induced by electric field mapping to the ionosphere from the magnetosphere. The results show that in magnetic storm conditions, at times the GDI process is dominant, but that at other times turbulence may be as significant as the GDI in determining how the plasma within a polar cap patch behaves, possibly more so. This in turn suggests that further study of the turbulence process is necessary in order to fully understand how big a role it plays in causing GPS L1 band scintillation in the polar cap. The success of the modelling technique developed here shows the utility of ionospheric imaging as a tool for understanding physical problems of the ionosphere; efforts to improve it and to apply it in other contexts would be worthwhile.

Radio Occultation (RO) is a remote sensing technique which uses Global Navigation Satellite System (GNSS) signals tracked by a Low-Earth Orbit (LEO) satellite to sound the earth's atmosphere both in low (troposphere, stratosphere) and high (ionosphere) altitudes. GNSS-RO provides global coverage and SI traceable measurements of atmospheric data with high-vertical resolution. Refractivity, dry temperature, pressure and water vapour profiles retrieved from RO measurements have a relevant contribution in Numerical Weather Prediction (NWP) systems and in climate-monitoring. Due to the partial propagation through the ionosphere, a systematic bias is added to the lower atmospheric data product. Most of this contribution is removed by a linear combination of data for two frequencies. In climatology studies, one can apply a second-order correction - so called κ-correction - which relies on a priori information on the conditions in the ionosphere. However, both approaches do not remove high-order terms in the error due to horizontal gradient and earth's geomagnetic fields. The remaining residual ionospheric error (RIE) and its systematic bias in RO atmospheric data is a well-known issue and its mitigation is an open research topic. In this licentiate dissertation, the residual ionospheric error after the standard correction is evaluated with computational simulations using a wave optics propagator (WOP). Multiple Phase Screen (MPS) method is used to simulate occultation events in different ionospheric scenarios, e.g. quiet and disturbed conditions. Electron density profiles (EDP) assumed in simulations are either defined by analytical equations or measurements. The disturbed cases are modelled as small-scale irregularities within F-region in two different ways: as sinusoidal fluctuations; and by using a more complex approach, where the irregularities follow a single-slope power-law that yields moderate to strong scintillation in the signal amplitude. Possible errors in MPS simulations assuming long segment of orbit and ionosphere are also evaluated. The results obtained with the sinusoidal disturbances show minor influence in the RIE after the standard correction, with the major part of the error due to the F-region peak. The implementation of the single-slope power-law is validated and the fluctuations obtained in simulation show good agreement to the ones observed in RO measurements. Finally, an alternative to overcome limitations in MPS simulations considering occultations with long segment of orbit and ionosphere is introduced and validated. The small-scale irregularities modelled in F-region with the power-law can be added in simulations of a large dataset subjected to κ-correction, in order to evaluate the RIE bending angle and the consequences in atmospheric parameters, e.g. temperature. / NRPF-3, Rymdstyrelsen, 241/15

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Ionospheric scintillations can cause significant amplitude and/or phase fluctuations of GNSS signals. This work presents analysis results of scintillation effects on the new GPS L5 signal based on data collected using a real-time scintillation monitoring and data collection system at HAARP, Alaska. The data collection setup includes a custom narrow band front end that collects GPS L1, L2 IF samples and two reconfigurable USRP2 based RF front ends to collect wideband GPS L5 and GLONASS L1 and L2 signals. The results confirm that scintillation has a stronger impact on GPS L2 and L5 signals than on the L1 signal. Our preliminary results also show that carrier phase and amplitude scintillations on each signal are highly correlated. The amplitude and carrier phase scintillation are also correlated among the three signals. In this study, a multi-constellation multi-band GNSS software receiver has been developed based on USRP2, a general purpose radio platform. The C++ class-based software receiver were developed to process the IF data for GPS L1, L2C, and L5 and GLONASS L1 and L2 signals collected by the USRP2 front end. The front end performance is evaluated against the outputs of a high end custom front end driven by the same local oscillator and two commercial receivers, all using the same real signal sources. These results demonstrate that the USRP2 is a suitable front end for applications, such as ionosphere scintillation studies. Another major contribution of this work is the implementation of a Vector tracking loop (VTL) for robust carrier tracking. The VTL is developed based on the extended Kalman filter (EKF) with adaptive covariance matrices. Both scalar tracking loop (STL) and VTL are implemented. Once an error in the scalar loop is detected, the results from the VTL are used to assist the STL. The performance of the VTL is compared with the traditional STL with three different data sets: raw GPS RF data with short signal outages, RF data with strong scintillation impacts collected during the last solar maximum, and high dynamic data with long interval signal outages from a GPS simulator. The results confirm the performance improvement of the VTL over scintillation impacts and show that the VTL can maintain signal lock during long intervals of signal outage if the satellite ephemerides are available and the pseudorange estimation is within one code chip accuracy. The dynamic performance improvement of the VTL is verified as well. The results show the potential of robust tracking based on VTL during scintillation and interference. / Ph. D.

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