GLOBAL POSITIONING SYSTEMS IN THE TIME-DOMAIN - HOW USEFUL A TOOL FORINTELLIGENT VEHICLE-HIGHWAY SYSTEMS

Much of the research and development work in intelligent vehicle-highw ay systems (IVHS) relies on the availability of methods for locating a nd monitoring vehicles (e.g. ''probe vehicles'') in real time across a road network. This paper considers the use of the global positioning system (GPS) as one method for obtaining information on the position, speed and direction of travel of vehicles. It reports the results of a series of field studies, in which real-time GPS data were compared to data collected by an instrumented vehicle, under a range of physical and traffic conditions. The field studies and consequent data analysis provide a picture of the reliability and usefulness of GPS data for t raffic monitoring purposes, and hence the possibilities for the use of GPS in IVHS projects. The use of GPS receivers tailored for mobile ap plications, and able to provide direct observations of vehicle speed a nd travel direction, coupled with database management using geographic information systems (GIS) software, was found to provide a reliable a nd efficient system for vehicle monitoring. Field data collection unde r ''ideal'' GPS conditions indicated that accurate speed and position data were readily obtained from the GPS. Under less favourable conditi ons (e.g. in downtown networks), data accuracy decreased but useful in formation could still be obtained. In addition, the conditions and sit uations under which GPS data errors could be expected were noted. The finding that it is possible to relate standard GPS signal quality indi cators to increased errors in speed and position provides an enhanced degree of confidence in the use of the GPS system for real-time traffi c observations.