Design and simulation of a coherent GPS receiver

Naser Farhan Abdullah Al-Falahi
Dr. Samir S. Al-Attar
Dr.Khaldoon A. Ghaidan

There are a growing number of applications worldwide that require regularly updated vehicle position and sometimes orientation. For example, applications based on in-car navigation systems require that the navigation computer knows the position of the car on the earth. In many cases, this requirement can be met by using the Global Positioning System (GPS). Although originally developed for the military, the GPS has proven to be an invaluable service for a multitude of civilian applications. Each application demands specific performance from the GPS receiver, and the associated requirements often vary widely. This work introduces a digital coherent receiver design and simulation for the Global Positioning System (GPS)/Standard Positioning Service (SPS). The technology adopted by this receiver depends on the idea of Digital Phase Tracking Loops and Digital Code Tracking Loops. This work involves a kind of ranging technique called pseudo noise code ranging (PN Code ranging). By measuring the time delay between transmitted and locally generated codes, a receiver can determine his/her range. The determination of the satellite's ranges is an important parameter in the determination of their orbits. The principle behind GPS is the measurement of distance (or _range_) between the receiver and the satellites. The satellites also tell us exactly where they are in their orbits through the navigation message (Ephemeris). The proposed system was designed and simulated with Matlab toolboxes. The system was tested with AWGN channel in two modes: first in the acquisition mode and second in the tracking mode, and it was also tested with Doppler Effect to establish the system performance. The results of the testing were also shown. The results prove that the system is capable of operating with a signal-to-noise ratio SNR of 4dB for the acquisition mode, and -25dB for the tracking mode depending on the Code Tracking Loop performance. There is an improvement in Phase Tracking Loop performance compared to traditional phase locked loops of 4dB for this system in acquisition mode and an improvement of 6.5dB in tracking mode without any phase slipping, and the reason behind that is the use of Spread Spectrum signals.