Implementation of a baseband GPS receiver

Abstract

The Global Positioning System (GPS) is a space-based radio navigation system which was originally developed for military use. However, GPS receivers have become more popular in recent years and are now incorporated in mobile phone electronics and also in navigation systems, namely in automotive, marine and aerospace equipment. This thesis focuses on digital signal processing (DSP) performed by a typical L 1-hand Cl A-code receiver. Such baseband processing mainly involves local carrier and pseudo-random noise (PRN) code generation, converting the incoming GPS signal down to baseband, correlating the resulting baseband signal with the locally generated codes, determining the available satellites and subsequently choosing at least four of them, phase-aligning the local and incoming codes of the selected satellites, and finally, detecting and demodulating the navigation messages from the individual selected satellites. Thus, this dissertation presents the design and implementation of various DSP and communication blocks which are responsible for such baseband processing. In this work, perfect carrier phase recovery is assumed. These baseband digital modules are implemented on an FPGA. Such approach allows additional flexibility in the implementation when compared to the ASIC approach, due to the chip's reprogrammability feature. The functionality of each module is tested and verified. A GPS satellite signal modulation model is also developed to test and simulate the baseband processor for different scenarios, for example for multiple satellite transmissions and in the presence of noise. From these tests, it is noted that as the number of parallel transmissions is increased or when the SNR is decreased, the correlation between the incoming and local codes of the visible satellites is reduced. The minimum SNR value that the designed baseband processor can tolerate is -10 dB. For SNR values greater than or equal to this value, it is guaranteed that navigation messages from the different selected satellites are decoded correctly. This only holds if automatic gain control is adopted. If no gain control is used, the minimum SNR value that the processor can tolerate is greater than -10 dB. Good noise performance is crucial in GPS receivers since, in practice, the noise power level is significantly greater than the actual GPS signal power level.