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It is well known that a global positioning system (GPS) receiver needs to `see` at least four satellites to provide a three-dimensional fix solution. But in difficult environments such as an urban canyon, the number of `visible` satellites is often not enough. Wireless fidelity (WiFi) signals have been utilised for positioning mainly based on the fingerprinting technology. However, the accuracy of WiFi positioning outdoors is from several tens of metres to more than one hundred metres. This study proposes a new methodology to integrate WiFi positioning technology and GPS to improve positioning accuracy in urban canyons. When only two GPS satellites are visible, the pseudorange observations can be used to generate a time difference of arrival (TDOA) measurement. The TDOA generates a hyperboloid surface which can be intersected with the surface of the Earth and shows the possible location of the user on that line of position. Integrating this method with the WiFi fingerprinting technology can increase the positioning performance significantly. The test results show that the positioning accuracy can be improved by more than 50% if the new method can be applied.