Urban road transport navigation: performance of the global positioning system after selective availability

Satellite navigation systems have a potential to support multi-modal transport navigation requirements. In road transport, the global positioning system (GPS) is currently supporting a wide variety of in-car navigation and transport telematics systems. The performance of GPS has in the past been limited by the artificial degradation of the signal through the process of selective availability (SA). With SA operational, the instantaneous horizontal positional accuracy was 100 m 95% of the time. Additional infrastructure was used with the differential concept (where range errors are determined at a known location and transmitted to users) to improve this to the level of a few metres. The US Government on 1 May 2000 removed SA. This paper presents the results of a study to assess and characterise the post-SA performance of GPS for positioning vehicles in urban areas. This is an important functionality of advanced transport telematics systems that aim to address everyday problems associated with road transport, particularly in urban areas. The performance assessment addresses, in varying levels of detail, the issues of service coverage, positioning accuracy, integrity and availability of service. Comparison is made with the results of a previous study conducted when SA was turned on. The results show an improvement in stand-alone navigation accuracy without SA compared to the period when SA was operational. Furthermore, no significant difference is seen between the level of accuracy achievable with differential positioning and post-SA stand-alone navigation. The parameters that characterise the performance of GPS determined at the analysis stage have been used to specify an architecture for a local navigation system for urban areas.