Browsing by Author "Najmafshar, Maryam"

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    Modeling High-Latitude Ionospheric Scintillations for Radio Occultation GPS Receiver Performance Analysis
    (2017) Najmafshar, Maryam; Skone, Susan; Gao, Yang; O'Keefe, Kyle Patrick Gordon; Knudsen, David J.; Datta-Barua, Seebany
    The performance of a radio occultation GPS receiver carrier tracking loop, subjected to ionospheric scintillations, is studied in this thesis. A physics-based simulation of high-latitude ionospheric disturbances for radio occultation signals has been employed to generate a variety of test cases to assess receiver performance. This scintillation simulator is built on an existing tool and is novel in allowing simulation of high-latitude ionospheric perturbations on multiple frequencies for radio occultation signals. Further, an experimental simulation method is developed which extracts scintillation-induced perturbations directly from real observations and applies them to a nominal GPS signal. Simulation results are validated against scintillation perturbations extracted from real radio occultation signals collected during solar maximum. The simulation approach is found to closely represent the real effects of scintillation. Scintillated signals are analyzed for effectiveness of detrending methods and selection of cutoff frequency for filtering scintillation data and capturing scintillation information. It is determined through spectral analysis that a higher cutoff frequency is required for filtering high-latitude scintillation signals. Also, scintillation measurements are dependent on the method applied for detrending. According to the studies conducted in this thesis, receiver tracking performance depends on both scintillation levels and the receiver mode (tracking versus acquisition). The receiver can track strong scintillation signals provided sufficient time to acquire signals pre-scintillations. Signal acquisition and tracking during scintillations, however, is challenging and might fail for severe scintillation conditions. Also, it is found that the presence of amplitude scintillation alone does not cause significant carrier performance degradation unless it is severe enough to reduce signal power below tracking threshold level leading to loss of lock. Strong phase scintillations, on the other hand, result in noticeable tracking loop performance degradation and measurement errors.