The goal of this thesis is to improve the understanding of the performance of Global Navigation Satellite System (GNSS) receivers that use assistance data provided by cellular networks. A typical example of such a receiver is a mobile phone including a Global Positioning System (GPS) receiver. Using assistance data such as an accurate estimate of the GPS system time is known to improve the availability and the time-tofirst- fix performance of a GNSS receiver. However, the performance depends on the architecture of the cellular network and may vary significantly across networks. This thesis presents three new contributions to the performance analysis of assisted-GNSS receivers in cellular networks. I first introduce a mathematical framework that can be used to calculate a theoretical lower bound of the time-to-first-fix (TTFF) in an assisted-GNSS receiver. Existing methods, for example the flow-graph method, generally focus on calculating the theoretical mean acquisition time of a pseudo-noise signal for one satellite only. I extend these methods to calculate the full probability distribution of the joint acquisition of several satellites, as well as the sequential acquisition of satellites, which is commonly performed in assisted receivers. The method is applied to real measurements made in a multipath fading channel. I next consider time assistance in unsynchronised cellular networks. It is often argued that unsynchronised networks can not provide fine-time aiding since they do not have a common clock, although few experimental results have been reported in the existing literature. I carried out experiments on a GSM network, a second-generation cellular network, in Cambridge, UK, in order to measure the time stability of the synchronisation signals. The results showed a large variability in the time stabilities across different base stations and I evaluated the performance of an ensemble filter that combines the measurements into a single, more accurate, estimate of the universal time. The main contribution is to show that the performance of such a filter is adequate to provide fine-time assistance to a satellite navigation receiver. Finally, I address the positioning performance of an assisted receiver in synchronised cellular networks. Cellular positioning has been often investigated in the literature, but few results on real networks have been presented. Many positioning methods are proprietary and little information about their performance in real networks haven been published publicly. A CDMA2000 cellular network in Calgary, Canada, was used to collect experimental data. The time stability and the synchronisation of the CDMA2000 pilot signals were excellent and were used to evaluate the performance of CDMA2000-based cellular positioning system. I then developed a method to combine the pseudo-range measurements from the GPS signals and the CDMA2000 base stations. I evaluated the performance of positioning in both outdoor and indoor environments, and I analysed the effects and the possible mitigation of non-line-of-sight signals. The main contribution is to show that additional satellite navigation signals can improve the accuracy of cellular positioning beyond what is theoretically expected from the improvement in the geometry.