High Resolution Signal Processing Techniques for Enhancing GPS Receiver Performance
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Despite the major advances in signal processing methods used nowadays, GPS receivers still face substantial challenges, such as multipath, which remains a dominant source of ranging error. The presence of GPS multipath influences the acquisition and tracking modules inside the receiver leading to biased measurements or loss of lock of the GPS satellite signal. Consequently, GPS receivers cannot provide reliable position, velocity and time solutions. The main objective of this thesis is to introduce high resolution signal processing techniques to enhance the performance of the GPS receiver in harsh navigation environments. This research proposes a two-tier approach. The first introduces a robust spectral estimation method to acquire the carrier frequency accurately after the completion of the coarse acquisition of the GPS signals. The second method targets improving the code delay estimation inside the tracking loops of the GPS receiver to mitigate the challenging closely spaced multipath effect. In this research, a SPIRENT GPS simulator is utilized to examine the performance of the proposed methods against the state of the art techniques. The proposed fine acquisition method uses Gram-Schmidt orthogonalization to provide robust spectral estimation of satellite Doppler frequency. The performance of the proposed fine acquisition method is evaluated against both the computational load and the noise effects. The results show that the proposed method outperforms the FFT-based fine acquisition methods resulting in 70% improvement in the acquisition accuracy at low SNR of -15 dB using only short window size of 1 ms. Moreover, this method speeds up the tracking loops in order to correctly lock the carrier Doppler shift frequency. Furthermore, the proposed multipath mitigation technique, which operates in the tracking module, is based on fast orthogonal search to enhance the code delay estimation for GPS receivers. The proposed tracking module results in an average improvement of 32% in the positioning accuracy when compared to other multipath mitigation techniques. This thesis research resulted in computationally efficient GPS acquisition module that can work in low SNR environment for limited resource GPS receivers. In addition, a robust tracking module capable of mitigating challenging GPS multipath effects is also introduced in this thesis.