This paper presents the results of a study carried out in the Greater London area to assess
and characterise the performance of the Global Positioning System (GPS) for vehicle
positioning in urban areas. The performance assessment addresses, in varying levels of detail,
the issues of service coverage, positioning accuracy, integrity and availability of service. The
project was supported by Racal Survey Limited and is part of the wider research at Imperial
College in the area of architectural design and testing of Advanced Transport Telematics
(ATT) systems. The results highlight the shortcomings of GPS as the sole means for in-car
navigation in urban areas and details the temporal and spatial considerations to be taken
into account in the process of designing an integrated positioning system capable of meeting
navigation requirements placed on ATT systems.