Pergamon Transp. Res.-C. Vol. 3, No. 4, pp. 193-209, 1995 Copyright (3 1995 Elsevier Science Ltd Printed in Great Britain. AI1 rights reserved 0968-090X/95 $9.50+ 0.00 0968-09OX(95)OOOOG2 GLOBAL POSITIONING SYSTEMS IN THE TIME DOMAIN: HOW USEFUL A TOOL FOR INTELLIGENT VEHICLE-HIGHWAY SYSTEMS? R. ZITO, G. D’ESTE and M. A. P. TAYLOR Transport Systems Centre, University of South Australia. The Levels, South Australia 5095, Australia zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPO (Received 31 January 1995) Abstract -- Much of the research and development work in intelligent vehicle-highway systems (IVHS) relies on the availability of methods for locating and 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 traffic monitoring purposes, and hence the possibilities for the use of GPS in IVHS projects. The use of GPS receivers tailored for mobile applications, and able to provide direct observations of vehicle speed and travel direction, coupled with database manage- ment using geographic information systems (GIS) software, was found to provide a reliable and efficient system for vehicle monitoring. Field data collection under “ideal” GPS conditions indi- cated that accurate speed and position data were readily obtained from the GPS. Under less favourabie conditions (e.g. in downtown networks), data accuracy decreased but useful infor- mation could still be obtained. In addition, the conditions and situations under which GPS data errors could be expected were noted. The finding that it is possible to relate standard GPS signal quality indicators to increased errors in speed and position provides an enhanced degree of confidence in the use of the GPS system for real-time traffic observations. INTRODUCTION Much of the research and development work in IVHS relies on the availability of methods for locating and 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 traffic monitoring purposes, and hence the possibilities for the use of GPS in IVHS projects. The GPS consists of some 24 satellites encircling the earth at inclined orbits of 60”. There are six terrestrial control stations that update the satellites with new information as it comes to hand (see Fig. 1 for a schematic representation of the GPS system). GPS is owned and maintained by the U.S. Department of Defense, and is available worldwide to any user who has a GPS receiver. The basic output from a receiver is the x, y and z coordinates for a moving or stationary object, at possible update rates of the order of once/s. Figure 1 shows the three segments that make up the GPS system, with the user segment being the final segment where the GPS data can be used in many different appli- cations, such as transport planning, management, control and scheduling, and hence can play a potentially important role in IVHS. 193