Mitigation of scintillation effects on GPS satellite positioning

• Main Author: Mokhtar, Mohd Hezri
• Published: University of Leeds 2012
• Subjects: 910.285
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.612610

Ionospheric scintillation causes significant effects on GPS signals such as amplitude fading and fast phase changes that can lead to cycle slips resulting in error in the positioning determination. In a worst case scenario, it will lead to the loss of lock of the PLL in the receiver. The study of amplitude and phase scintillation on GPS signals, especially in the high and low latitude regions where the scintillation can be severe, as well as the prediction and mitigation of these effects on the GPS positioning, has been conducted in the research work presented in this thesis. The tracking error at the output of the PLL limits the accuracy of the range measurements which the receiver uses to compute the position. Determination of tracking error variance is accomplished using Conker's model (Conker et al.,2003) which requires the spectral parameters p and T. Three methods to accurately determine the tracking error variance by making use of scintillation indices from high sample rate scintillation data are investigated. These all find the spectral parameters (p and T) required by the Conker model. The first method determines the spectral parameters p and T from the PSD of phase, obtained from the detrended high sample rate phase data, through performing the FFT (Fast Fourier Transform. The second method (Strangeways, 2009). finds these parameters from the scintillation indices using an approximation model of the phase and amplitude spectra together with an estimated Fresnel frequency, thus without the need to perform FFTs and also useful when high sample rate data is not available. The third method filters the detrended high sample rate phase scintillation data employing five band pass filters to allow an approximate phase spectrum to be obtained. The determination of the spectral parameters is accomplished after detrending the time series of the phase and intensity of the received signal which removes trends due to the moving satellite. Different methods for performing this detrending (3 for amplitude and 2 for phase) are compared. The application of the method to estimate the Fresnet frequency by performing a cubic fit to the amplitude spectrum (required for checking the accuracy of the method of finding the spectral parameters from the scintillation indices) is also presented. Finally comparison is made between the three methods of determining spectral parameters in order to determine their relative accuracies and their respective appropriate areas of usefulness and validity. The mitigation of the scintillation effects has been accomplished by observing the scintillation level on the paths of the signals from every satellite received simultaneously. The accuracy of GPS positioning estimation has shown the improvement by using the weighting measurement based on the tracking jitter variance for each of the satellites used to calculate the position.