The integration of GPS into mobile telephones enables a potentially vast array of new applications ranging from consumer products to safety of life and security applications. In the United States, Enhanced-911 regulations have been a major catalyst for this deployment while in Europe, the commercial potential of location-based services is driving it. However, these new applications as well as the mobile phone environment itself pose significant GPS challenges. These include low cost implementation in restricted spaces on the mobile phone unit and reliable operation in a broad range of environments. These challenges and the availability of mobile communication itself spawned the concept of assisted GPS (AGPS) in which the network assists the receiver to perform various functions. This paper reports on the fundamental signal acquisition and tracking capability of an AGPS receiver under weak signal conditions as well as the impact of different types of aiding acquisition and tracking performance. A SiRFLoc™ evaluation kit is used to investigate performance. Tests are conducted using a hardware simulator and results are analysed in terms of time-to-first-fix (TTFF) and position accuracy. It is found that an AGPS receiver provides a 13 dB improvement in acquisition sensitivity over a comparable high sensitivity receiver operating in unaided mode. The accuracy of timing, the initial reference position and the associated uncertainty of the initial position all have an impact on the TTFF.